Download - Origin of the bathochromically shifted optical spectra of meso-tetrathien-2?- and 3?-ylporphyrins as compared to meso-tetraphenylporphyrin

Origin of the bathochromically shifted optical spectra of meso-tetrathien-20- and 30-ylporphyrins as compared to meso-tetraphenylporphyrin

Christian Bruckner,*a Paul C. D. Foss,b James O. Sullivan,b Ryan Pelto,b

Matthias Zeller,cRobert R. Birge

aand Guy Crundwell*

b

Received 4th January 2006, Accepted 16th March 2006

First published as an Advance Article on the web 6th April 2006

DOI: 10.1039/b600010j

The UV-Vis and fluorescence spectra of free base and diprotonated meso-tetrathien-2 0-yl-

porphyrins are, when compared to the spectra of meso-tetra-phenyl- or even -thien-30-yl-

porphyrins, characterized by surprisingly large red-shifts. A comparison of the optical spectra and

the computed rotational barriers for these meso-aryl-substituted porphyrins and a detailed con-

formational analysis of the single crystal X-ray structure of a diprotonated meso-tetrathien-2 0-yl-

porphyrin suggest that the origin of the altered electronic properties of meso-tetrathien-20-yl-

porphyrins are mainly due to the contribution of conformations in which the thienyl groups

adopt idealized co-planar arrangements with the porphyrin ring. These conformations allow an

efficient extension of the porphyrinic p-system through conjugation. We synthesized a meso-

tetrathien-20-ylporphyrin with methyl groups in the o-position, thus preventing the formation of

conformers with co-planar thienyl groups and a corresponding thien-20-ylporphyrin with methyl

substituents in a distal position that possesses the same steric requirements for thienyl group

rotation as the parent compound, to conclusively deduce the influence of the conformers on the

electronic structure. A MNDO-PSDCI computation of their optical spectra further supports our

key hypothesis. DFT computations of the total energies of the hypothetical diprotonated thien-20-yl-

porphyrin conformer with perpendicular thienyl groups and the conformer containing near-co-

planar thienyl groups quantify the resonance stabilization energy. Our results support and

complement recent photophysical and theoretical studies by Gupta and Ravikanth and Friedlein

et al. on thien-20-yl-substituted core-modified porphyrins and [meso-tetra(50-bromothien-20-yl)-

porphyrinato]Zn(II), respectively.

1 Introduction

meso-Tetraarylporphyrins, such as the parent compound

meso-tetraphenylporphyrin (TPP, 1), are the most popular

synthetic porphyrins. They are widely used in the design of

model compounds of naturally occurring porphyrinic cofac-

tors, were incorporated into synthetic light harvesting devices

or form the platform for potential drugs for the photodynamic

treatment of cancer.1 The popularity of meso-tetraarylpor-

phyrins arises from their well developed and straightforward

syntheses and the availability of a large number of aryl-

functionalized derivatives.2 The vast majority of meso-tetra-

arylporphyrins carry meso-phenyl-based substituents, though

meso-pyridinium-based porphyrins have been studied for their

ability to bind to DNA.3

It is long known that substituents on the meso-phenyl

groups generally modulate the electronic properties of the

porphyrin chromophore only to a small to even diminishing

extent.4 This is because the phenyl groups are, in the low

energy conformation, arranged in an idealized orthogonal

fashion with respect to the mean plane of the porphyrin. Steric

interaction between the o-phenyl and the b-hydrogens on the

porphyrin prevent co-planarity and significant p-conjugationbetween the two aromatic systems (Fig. 1).

However, the rotational barrier for o-H substituted phenyl

groups is low enough that phenyl rotation at room tempera-

ture is observed.5 Only meso-phenylporphyrins carrying larger

substituents than hydrogen in at least one of the o-positions

allow for the observation and separation of atropisomers.6,7

Few meso-aryl-substituted porphyrinic macrocycles that

carry aryl groups other than phenyl groups have become

known, among them systems with meso-furyl,8 -pyrazolyl,7,9

-imidazoyl,10 azulenyl-,11 or -pyrrolyl12 groups. In addition, a

number of reports describing porphyrins and heteroporphyr-

ins carrying meso-thienyl substituents have appeared in the

recent literature.13,14 Two alternate methods for the synthesis

of meso-thienylporphyrins are available.13,15 Porphyrins with

meso-tetrathien-20- or 30-yl moieties have been used in the

design of light harvesting and energy-transfer molecules,16,17

the electrochemical construction of ultrathin films of quasi-2D

porphyrin polymers,18 the realization of organic semiconduc-

tor devices,19 supramolecular architectures with extended

aDepartment of Chemistry, University of Connecticut, Storrs, CT06269-3060, USA. E-mail: [email protected]; Fax: +01 860486 2981; Tel: +01 860 486 2743

bDepartment of Chemistry, Central Connecticut State University,New Britain, CT 06050-2490, USA. E-mail:[email protected]; Fax: +01 860 832 2704; Tel: +01 860832 2682

cDepartment of Chemistry, Youngstown State University, OneUniversity Plaza, Youngstown OH, 44555-3663, USA

2402 | Phys. Chem. Chem. Phys., 2006, 8, 2402–2412 This journal is �c the Owner Societies 2006

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

electronic states20 and the study of rotational barriers in

peripherally crowded porphyrins21. Further, meso-tetrathien-

20-yl groups were incorporated into dithiaporphyrins that

proved to be potent phototoxins.22

A surprisingly pronounced modulation of the electronic

structure of the chromophore by the meso-thienyl substituents

was reported most recently:23 They are all characterized by

bathochromically shifted UV-Vis spectra when compared to

the corresponding meso-phenyl derivatized systems. For in-

stance, the bands in the UV-Vis spectrum of meso-tetrathien-

30-ylporphyrin (3) are about 4–6 nm bathochromically shifted

as compared to those of TPP (1) and those of the thien-20-yl-

porphyrin (2a) are yet 5–8 nm further bathochromically

shifted as compared to 3.24 The extent of the bathochromic

shifts for lSoret and lmax of the UV-Vis spectra can also be

related to the number and type (thien-20- or 30-yl groups)

attached to core-modified porphyrins (21-thia-, 21,23-dithia-,

21-oxa- and 21,23-dioxa-porphyrins).8,23 Thien-20-yl substitu-

ents provided about twice as large a shift (B3–4 nm per

thienyl group) as a thien-30-yl substituents (B2 nm per thienyl

group).8,23 The electronic modulation brought about by the

thienyl groups was also measured by an anodic shift of the

electrochemical potential of their ZnII and CuII complexes,

with the complexes of the thien-20-ylporphyrin exhibiting a

larger shift than the isomeric thien-30-ylporphyrin com-

plexes.24

These shifts were initially attributed to inductive effects.13

Gupta and Ravikanth have most recently corrected this overly

simplistic view by deducing that the electronic modulation is

due to greater p-localization of thien-20-ylporphyrins because

of a stronger resonance interaction between the porphyrin

macrocycle and the thienyl groups.23 Their conclusion was

based on the study of the photophysical properties (UV-Vis

and fluorescence spectra, fluorescence yields and lifetimes) of a

number of thien-20-yl-substituted porphyrins and N3S, N3O

and N2S2 heteroporphyrins carrying varying numbers of thien-

20-yl groups. Further, Friedlein et al. demonstrated through

quantum-chemical calculations the contributions of thien-20-yl

groups to the MOs of the zinc(II) complex of meso-tetra-

(50-bromothien-20-yl)porphyrin.20

In this contribution we further refine the picture of the

origin of the altered electronic properties of thien-20- and 30-yl-

porphyrins. We provide independent experimental evidence

that they are indeed due to a larger contribution of the

conformation in which the thienyl groups adopt an idealized

co-planar arrangement to the porphyrin ring as compared to

meso-tetraphenylporphyrins. We base our conclusions on a

molecular modeling study of the energetics of the rotation of

the meso-aryl substituents (phenyl, thien-20- and 30-yl groups)

and an investigation of the optical properties of methyl-

substituted and non-substituted meso-thien-20-ylporphyrins,

meso-thien-30-ylporphyrin and their dications, in comparison

to the properties of TPP and its dication. The findings are

backed by a computation of the UV-Vis spectra that estimate

the energetics of the resonance contribution. Further, the

crystal structure of the diprotonated meso-tetra(50-methyl-

thien-20-yl)porphyrin [2b2H]2+, as its bis-trifluoroacetate salt,

provides structural data supporting our conclusions.

2 Experimental

2.1 Synthesis

2.1.1 General. 1H NMR and 13C NMR spectra were

referenced to residual solvent peaks. ESI mass spectra were

recorded at the University of Connecticut, whereas high

resolution FAB mass spectra were provided by the Mass

Spectrometry Facility, Department of Chemistry and Bio-

chemistry, University of Notre-Dame, USA. Elemental ana-

lyses were provided by Numega Resonance Labs Inc., San

Diego, CA, USA. The analytical TLC plates were aluminium

backed Silicycle ultra pure silica gel 60, 250 mm; preparative

TLC plates (500 mm silica gel on glass) and the flash column

silica gel (standard grade, 60 A, 32–63 mm) used were pro-

vided by Sorbent Technologies, Atlanta, GA, USA.

All reagents were used as received. meso-Tetraphenylpor-

phyrin (1), meso-tetrathien-20-ylporphyrin (2a) and meso-

tetrathien-30-ylporphyrin (3) were prepared as described pre-

viously,13 and purified by column chromatography and crys-

tallization until analytically pure samples were obtained.

2.1.2. meso-Tetra(50-methylthien-20-yl)porphyrin, (2b). 5-

Methylthiophene-2-carboxaldehyde (4.60 g, 3.65 � 10�2

mol) and pyrrole (2.50 mL, 3.65 � 10�2 mol) in 250 mL of

propionic acid were heated to reflux. Over the course of 30

min, the formation of a black precipitate was observed. The

mixture was cooled and filtered. The filter cake was recrystal-

lized from CH2Cl2/MeOH and purified by column chromato-

graphy (silica/CH2Cl2). Evaporation of the solvent yielded 2b

as a purple, crystalline solid in 6.0% yield (380 mg). Rf = 0.82

(silica-CH2Cl2/50% pet. ether); 1H NMR (CDCl3, 400 MHz,

d): 9.10 (s, 4H), 7.66 (d, 3J = 3 Hz, 2H), 7.13 (d, 3J = 3 Hz,

2H), 2.76 (s, 6H), �2.62 (br s, 1H) ppm; 13C NMR (CDCl3,

100 MHz, d): 142.8, 140.6, 131.0–133.5 (m), 134.0, 128.2,

124.8, 113.0, 15.8 ppm; UV-Vis and fluorescence spectral data,

see Table 2; HR-MS (FAB+ of MH+, NBA) m/z calcd for

C40H31N4S4: 695.9548, found: 695.1461; Anal. Calcd for

C40H31N4S4: C, 69.13; H, 4.35; N, 8.06. Found: C, 69.03; H,

4.34; N, 8.00.

Fig. 1 Illustration of the steric interaction between the o-hydrogens

on the meso-phenyl group and the flanking b-hydrogens in a meso-

phenyl-substituted porphyrin, preventing a co-planar arrangement of

the two planar systems.

This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 2402–2412 | 2403

2.1.3. meso-Tetra(30-methylthien-20-yl)porphyrin, (2c). Pre-

pared according to the procedure described for 2b from 3-

methylthiophene-2-carboxaldehyde as a purple, crystalline

solid in 9.3% yield (590 mg). Rf = 0.64 (silica-CH2Cl2/50%

pet. ether); 1H NMR (CDCl3, 400 MHz, d): 9.10 (s, 1H) 8.91

(s, 7H), 7.71 (d, 3J = 4 Hz, 2H), 7.32 (d, 3J = 4 Hz, 2H),

1.99–2.15 (m, 6H), �2.63 (br s, 1H) ppm; 13C NMR (CDCl3,

100 MHz, d): 137.1–137.2 (m), 136.8, 131–132 (br m),

128.7–128.8 (m), 125.8–129.5 (m), 123.6, 111.5–111.7 (m),

16.3, 15.3, 15.2 ppm; UV-Vis and fluorescence spectral data,

see Table 2; HR-MS (FAB+ of MH+, NBA) m/z calcd for

C40H31N4S4: 695.9548, found: 695.1461; Anal. Calcd for

C40H31N4S4: C, 69.13; H, 4.35; N, 8.06. Found: C, 69.09; H,

4.41, N, 8.12.

2.2 Computational methods

2.2.1 Computation of the rotational barrier of 1, 2a and 3.

The energy of the porphyrin system was calculated as a

function of rotation of a single aryl group using density

functional methods [B3LYP/6-31G(d)]. A hypothetical por-

phyrin with a single meso-aryl group was created and the

adiabatic surface was calculated by minimizing all coordinates

as a function of the dihedral angle of the aryl group which was

scanned in increments of 51. The barriers calculated by this

method are only approximate because electrostatic and steric

contributions from the other aryl groups are ignored.

Test calculations at the energy extrema including all of the

aryl groups suggest we are underestimating the barrier in the

fully substituted porphyrin by about 10% in all three cases

studied. However, our use of a limited basis set [6-31G(d)]

tends to overestimate barriers by about the same amount so

we anticipate our surfaces are realistic though not rigorously

quantitative.

2.2.2 Molecular orbital calculations of excited state proper-

ties. We used MNDO-PSDCI molecular orbital theory to

calculate the energies and one-photon properties of the excited

singlet state manifold.25,26 All calculations were based on a

ground state geometry generated using density functional

methods [B3LYP/6-31G(d)] as discussed in the text. The

configuration interaction was carried out using full single

and double CI over the 8 highest energy occupied p orbitals

and the 8 lowest energy unoccupied p orbitals. The resulting

basis set consisted of the 64 single excitations and 2080 double

excitations. Our methods and procedures are identical to those

used in our previous study of porphyrins and chlorins.25

2.2.3 Single crystal structure determination of [2b2H]2+ .

(triflate)2. A plate of [2b2H]2+ � (triflate)2, grown by slow

evaporation of a TFA (trifluoroacetic acid) solution of 2b,

was used for the single crystal X-ray diffraction analysis. The

sample displayed homogenous birefringence when viewed

between the crossed polars of a polarizing microscope. The

X-ray intensity data were measured on a Bruker SMART

APEX CCD area detector system equipped with a graphite

monochromator and a Mo Ka fine-focused sealed tube gen-

erator (l = 0.71073 A) operated at 2.0 kW power.

A total of 1212 frames of data were collected with a scan

width of 0.31 in o and an exposure time of 15 s per frame.27

The frames were integrated with the Bruker SAINT software

package using a narrow-frame integration algorithm.28 Ana-

lysis of data showed no decay during data collection. Data

were corrected for absorption effects using the multi-scan

technique of SADABS.29 Pertinent absorption correction

statistics are given in Table 1. The structure was solved and

refined using SHELXTL (Version 6.1) using dual-space recy-

cling methods in direct methods solution and using full-matrix

least-squares refinement on F2.30 For details of the refinement,

see Table 1 and section 3.10.

3 Results and discussion

3.1 Synthesis and characterization of meso-tetrathien-20- and

30-yl-porphyrins

The known and novel porphyrins 1, 2a–c and 3 subject to this

study were synthesized from pyrrole and the appropriate

arylaldehydes using the Adler method, as described previously

(Scheme 1).13 The products were chromatographed and re-

crystallized until analytically pure materials were obtained.

Their analytical and spectroscopic data established their

structure. The 1H NMR spectra of the 50-methyl derivative

2b differ from those of the known derivative 2a by a simplified

peak pattern assigned to the hydrogens in the thien-20-yl

moiety and a signal for the methyl group. The corresponding

signals in the 1H NMR spectrum of the novel isomeric

structure 2c are also expected to be two doublets in a 1 : 1

ratio and a singlet for the methyl group. However, the signals

recorded were, due to the presence of atropisomers, more

Table 1 Data collection and refinement parameters [2b2H]2+ �(triflate)2

CSD deposition number CCDC 272182Color/shape Translucent dark blue plateCrystal size/mm 0.60 � 0.60 � 0.20Chemical formula C40H28N4S4(CF3CO2)2Formula weight/g mol�1 923.02Temperature/K 100(2)Crystal system MonoclinicSpace group I2 (alternative setting of C2)Unit cell dimensions a = 16.2521(7) A

b = 16.2521(7) Ac = 15.5357(7) Aa = b = g = 901

Z, Volume/A3 4, 4103.5(3)Density (calculated)/g cm�3 1.492F000 1896Max. & min. transmission 0.835 and 0.940y Range for data collection/1 1.77–28.28 (Mo Ka)Reflections measured 21340Index ranges �21 Z h Z 21

�20 Z k Z 20�20 Z l Z 20

Independent reflections 9813 [Rint = 0.0258]Coverage of independent reflections/% 96.8Data/restraints/parameters 9813/1438/848Goodness-of-fit on F2 1.162D/smax 2.157R indices [for 4061, I > 2s(I)]a R1 = 0.0775, wR2 = 0.1824R indices (all data)a R1 = 0.0777, wR2 = 0.1825Largest diff. peak & hole 0.422 and �0.322 eA�3

a Weighting scheme: w = 1/[s2(F2O) + [(0.0470P)2 + 11.4425P] where

P = (F2O) + 2F2

C/3


complex. The observation of atropisomers for o-substituted

meso-arylporphyrins is well known and finds its explanation in

the much higher energy barrier for aryl rotation as compared

to o-hydrogen substituted systems.7,31 The occurrence of

atropisomers is also observed in the 13C NMR of this

compound.

3.2 Optical properties of the free base 2- and

3-thienyl-porphyrins

The UV-Vis spectra of the free base compounds under in-

vestigation are shown in Fig. 2 (solid traces). As noted above,

Soret and side bands observed for the free base thien-30-

ylporphyrin 3 are slightly bathochromically shifted as com-

pared to the spectrum of TPP (1). The spectra of the thien-20-yl

derivatives 2a–c are even more shifted, whereby the effects due

to the presence of the methyl groups in 2b and 2c depend on

the position of the methyl group. Methyl substitution of the

o-position (30-position) in 2b has a small hypsochromic effect

as compared to 2a (�1 nm) while substitution of the 50-

position results in a 4–5 nm bathochromic shift (see Table 2

for a complete listing of the spectral data).

The normalized fluorescence spectra of the free base por-

phyrins 1–3 are shown in Fig. 3A and are listed in Table 2.32

They follow generally the trends of the UV-Vis spectra deli-

neated above but also feature some significant differences. The

two-band spectrum typical for a regular porphyrin is broa-

dened to a single band spectrum for the two thien-20-ylpor-

phyrins not carrying a methyl group in the o-position, 2a and

2b. All the above observations complement those for a series

of thien-2-yl-substituted heteroporphyrins reported by Gupta

and Ravikanth.23

A comparison of the absorption spectra of themeso-thien-2-

yl-substituted porphyrin 2a relative to the sterically equivalent

furan-2-yl-substituted porphyrin is illustrative of the generally

large electronic effect of five-membered heterocyclic meso-

susbtituents. In meso-tetra(furan-2-yl)porphyrin, the Soret

band and lmax are, at 433 and 670 nm, 10 and 9 nm bath-

ochromic shifted compared to those of 2a, respectively.8 The

corresponding meso-pyrroly-2-yl is not known. However, its

sterically more demanding (and therefore not directly compar-

able) N-isopropylpyrrol-2-yl-substituted12 derivative possesses

an UV-spectrum that lies between that of 2a and that of the

pyrrol-2-yl derivative.

3.3 Computed rotational barriers for meso-phenyl-,

-thien-20- and 30-ylporphyrins

What are the likely reasons for the observed effect in the

optical spectra of the thienylporphyrins? As noticed before,23

the steric requirements for a 5-membered meso-thienyl ring are

smaller than those of a 6-membered meso-phenyl group. In

addition, a 2-thienyl group lacks one o-H-b-interaction. Thisshould allow for a much more facile aryl-rotation for thien-20-yl-

porphyrins and a somewhat reduced barrier for rotation for

the thien-30-yl-substituted system, a supposition confirmed

and quantified by computation. The computed lowering of

the rotational barrier of 2a compared to those found in 3 and 1

is shown in Fig. 4.

As expected, the computed rotational barriers for the thien-

20-yl and thien-30-yl groups are lower. They are about 50 and

75%, respectively, of that of a phenyl group.33 This suggests

that the UV-Vis spectrum of the thienylporphyrins may be

influenced by a larger contribution of more co-planar con-

formers at thermal equilibrium, facilitating p-overlap between

the two aromatic systems. In fact, computations by Friedlein

et al. indicate that below a dihedral angle between the thio-

phene and porphyrin rings of 601, a strong coupling between

the two aromatic systems is observed, with a computed

HOMO that is extended over the entire molecule.20

Scheme 1 Synthesis, structure and numbering system of meso-tetra-arylporphyrins 1–3.

Table 2 UV-Vis and fluorescence spectral data of the unprotonated (X in CH2Cl2, trace Et3N) and protonated ([X2H]2+, in CH2Cl2/5% TFA)porphyrins investigateda

Porphyrin lmax Soret/nm, (log e) lmax side bands/nm, (log e) Fluorescence lmax/nmb Stoke shift/nm

1 417 (5.70) 514 (4.42) 549 (3.65) 588 (3.34) 647 (3.33) 651, 715 4[12H]2+ 437 (5.69) 654 (4.79) 696 422a 426 (5.59) 523 (4.25) 558 (3.70) 594 (3.48) 661 (3.40) 670, 720 (sh) 9[2a2H]2+ 459 (5.55) 718 (4.83) 795 772b 430 (5.61) 526 (4.27) 569 (4.09) 597 (3.88) 665 (3.83) 674 (br) 9[2b2H]2+ 465 (5.45) 763 (4.99) 828 652c 425 (5.58) 520 (4.31) 557 (3.85) 597 (3.83) 660 (3.57) 666, 725 6[2c2H]2+ 462 (5.50) 700 (4.71) 753 533 421 (5.67) 519 (4.39) 556 (4.22) 594 (4.04) 653 (4.05) 661, 723 8[32H]2+ 446 (5.66) 680 (4.85) 731 51

a See Fig. 2 (UV-Vis) and 3 (fluorescence) for the full traces of the spectra. b Excitation at the respective lSoret.


The computed rotational barriers at room temperature are,

however, still too high to speak of ‘free’ rotation but the rate

of rotation is fast on the NMR timescale, as the absence of the

observation of atropisomers for 2a and 2b (or 3) indicate.

Thus, in the nonprotonated case, the slight differences ob-

served in the UV-Vis for 2a–c and 3 can be rationalized using

inductive, steric and conjugation effects. The thien-20-yl sub-

stituent provides a larger electron-density and allows for a

larger degree of conjugation with the porphyrinic p-systemthan the thien-30-yl moiety since it can adopt a more co-planar

conformation. Hence the spectrum of 2a is bathochromically

shifted compared to that of 3. The 50-methylthien-20-yl-system

2b has the same steric requirements as 2a but is more electron-

rich, red-shifting its spectrum. The 30-methylthien-20-yl system

2c is at least as electron-rich as 2b but the o-methyl groups

prevent the same degree of co-planarity as in the sterically

unimpeded isomer 2c or 2a. Hence, its spectrum is the most

blue-shifted of the thien-20-ylporphyrins investigated.

3.4 Solid state structures of the free base meso-tetra-50-

methylthien-20-ylporphyrin 2b

In all the solid state structures of meso-thienyl group contain-

ing porphyrins, such as free base meso-tetrakis-2-thienylpor-

phyrin (2b),34 the Zn(II)35 and Cu(II)36 complexes of 2a and in

examples of meso-tetrathien-20- and 30-yldithiaporphyrins,14

the thienyl groups adopt the typical, near-perpendicular posi-

tion. The average dihedral angle of 621 between the thienyl

and porphyrin mean plains observed for 2b is also within the

60–801 angle typically observed in meso-tetraarylporphyrins

(Fig. 5A).34 Evidently, the thienyl groups are not small enough

to assume in the crystal a low-energy co-planar conformation

with the porphyrin ring. Though the o-carbon-b-carbon dis-

tances are slightly longer than those observed in the crystal

structure of 1 (Table 3), suggesting a larger conformational

freedom, the Cortho–Cipso–Cmeso–Ca dihedral angles are com-

parable.

3.5 Optical properties of the diprotonated meso-thien-20- and

30-ylporphyrins

Protonation of a porphyrin can be brought about by treatment

with TFA. The UV-Vis spectra of the protonated compounds

under investigation are shown in Fig. 2 (dashed traces). Upon

protonation, the trends observed in the shifts of the spectra of

the non-protonated thienylporphyrins are amplified. For com-

parison, protonation of 1 red-shifts lmax by 5 nm and lSoretshifts 20 nm. The corresponding shift of lmax in thien-30-

ylporphyrin 3 is 25 nm for the Soret band and 27 nm for

lmax. Protonation of 2a leads to a red-shift of the Soret band

Fig. 2 UV-Vis spectra of chromophores investigated. Solid traces: Free base forms, X, in CHCl3 with a trace (o0.1%) of Et3N. Dashed traces:

Protonated forms, [X2H]2+ in CHCl3/5% TFA.


of 33 nm and lmax shifts 57 nm. Also, the half-height width of

the lmax bands broadens from [12H]2+, to [32H]2+, to

[2a2H]2+, a sign that indicates an increasingly distorted

chromophore. Again, the position of the methyl groups on

the thien-20-yl rings has a large and distinct effect. The lSoret ofthe diprotonated thien-20-ylporphyrin substituted with a 30-

methyl group ([2c2H]2+) exhibits only a 3 nm bathochromic

shift compared to the corresponding lSoret of the non-methy-

lated species [2a2H]2+. Further, lmax of [2c2H]2+ is 18 nm

blue-shifted as compared to that of [2a2H]2+, to a value that is

between the values observed for [2aH]2+ and [32H]2+. On the

other hand, the Soret band and lmax of the diprotonated 50-

methyl-substituted porphyrin [2c2H]2+ are 28 and 109 nm

bathochromically shifted, respectively, when compared to the

corresponding bands of [12H]2+ and also surpass those

of [2a2H]2+.

The normalized fluorescence spectra of the diprotonated

porphyrins 1–3 are shown in Fig. 3B and are listed in Table 2.

They follow the trends of the UV-Vis spectra and also show a

magnified effect of the methyl-substitution. It is of note that

the thienylporphyrins generally exhibit larger Stoke shifts than

1, an effect particularly pronounced for the diprotonated

species (Table 2). Here, the Stoke’s shift for the thienylpor-

phyrins is 20–35 times the 2 nm shift measured for [12H]2+.

The largest shifts are observed for the two thien-20-ylporphy-

rins not carrying a methyl group in the ortho-position,

[2a2H]2+ and [2b2H]2+.

The trends are matching those noted by Gupta and Ravi-

kanth for thien-20-yl-porphyrins and -heteroporphyrins.14,23 In

addition, these authors also reported diminished fluorescence

quantum yields and fluorescence lifetimes for these systems, an

observation they attributed to the heavy-atom effect of the

added sulfur atom in the conjugated thien-20-yl groups.

3.6 Solid state structure of the diprotonated meso-tetra

(50-methylthien-20-yl)porphyrin [2b2H]2+ bistriflate salt

The solid state structure of the diprotonated 50-methylthien-

20-ylporphyrin [2b2H]2+ is shown in Fig. 5B. It is well known

that protonation leads to a deplanarization of the porphyrin

macrocycle due to transannular steric and electrostatic repul-

sions exerted by the four central hydrogens.37,38 Alternate

pyrrolic units are canted up- and downward, resulting in a

saddled conformation of the chromophore. This deformation

mode is also observed here, albeit in an extreme form.

The results of a normal-coordinate structural decomposi-

tion (NSD) analysis of the two independent molecules in the

lattice of [2b2H]2+ � 2(triflate) (MolA and MolB) and those of

three other representative diprotonated TTP structures

([12H]2+ �FeCl�4 �Cl�, [12H]2+ � 2ClO�4 and [12H]2+ � 2HSO�4 )

in comparison to the computed structures (see below) are

shown in Table 3. An NSD analysis breaks down the out-of-

plane distortion of the porphyrinic macrocycle with respect to

the six lowest frequency out-of-plane normal coordinates and,

thus, qualifies and quantifies the macrocycle deformation.39–41

Both, MolA and MolB of [2b2H]2+ � 2(triflate) are, like the

three other [12H]2+ structures, largely saddled, with a slight

Fig. 3 Normalized fluorescence spectra of the porphyrins investi-

gated (uncorrected). Conditions: Excitation wavelengths were the

respective lSoret listed in Table 2. A: CH2Cl2 with a trace of Et3N;

B: CH2Cl2 with 5% TFA.

Fig. 4 Adiabatic barriers to internal rotation of a single meso-phenyl,

-thien-20-yl or -thien-30-yl group attached to a porphyrin macrocycle.

See Experimental section for the details of the computation.

Fig. 5 A: Stick representation of the single crystal X-ray structures of

2b in the free base state.34 View angle was chosen to highlight the 621

twist angle between the mean plane of the thienyl groups and the

macrocycle. B: Stick representation of the single crystal X-ray struc-

tures of [2b2H]2+ � 2CF3COO�, side view. Counter anions omitted for

clarity and only one of the two conformers (MolA) observed in the

crystal is shown (for a full description of this structure, see below).


ruffled contribution. MolA is significantly more distorted from

planarity than MolB or any other diprotonated TPP structure,

while MolB much resembles the structure of [12H]2+

� 2(ClO�4 ). The observed differences in the conformations of

the diprotonated porphyrins or the computed structure are

likely a result of varying interactions with the anions (or, for

the computed ‘gas phase’ structure, the absence of any anion

interaction). The two different types of interactions of the

triflate anions with [2b2H]2+ MolA and MolB are further

investigated below.

The saddled conformation alleviates—but not removes—

the steric hindrance between the o-hydrogens on the meso-aryl

group, allowing the aryl groups to assume a much more co-

planar position relative to the mean of the porphyrin macro-

cycle (measured as the Cortho–Cipso–Cmeso–Ca dihedral angle).

The extremely saddled porphyrin structure of

[2b2H]2+ � 2(triflate) MolA allows the thienyl groups to lie in

a more co-planar fashion than in any other structure. Yet, due

to the smaller size of the thienyl group, the resulting o-C-b-Cdistance is still larger than in any other diprotonated structure,

again indicating the smaller size of the thienyl group and,

inferred, a lower barrier of rotation around the Cmeso–Cipso

axis. All in all, the dramatically saddled conformation and the

low dihedral angle between the thienyl- and porphyrin chro-

mophore, allowing for an efficient p-overlap, provide a struc-

tural proof for the spectroscopic findings that are discussed in

the following chapter.

3.7 The rationalization of the trends observed in the UV-Vis

spectra of the diprotonated thienylporphyrins

The influences of the type and position of phenyl substituents,

the degree of saturation of the macrocycle and the nature of

the central metal on the rotational barrier of meso-tetraphe-

nylporphyrins is well investigated for its metal complexes.5

Free base systems have not been investigated in that detail

because an axial ligand in metalloporphyrins provides a simple

means to introduce the face differentiation that distinguishes

the o-phenyl hydrogens on one phenyl group by NMR and

enables the determination of the rotational barriers. Never-

theless, the factors derived for metalloporphyrins can be

projected onto the free base systems. Steric effects of the o-

substituent, flexibility of the porphyrinic macrocycle and

electronic stabilization of the co-planar transition state are

the main factors controlling the height of the barrier of phenyl

rotation. Applied to the thienylporphyrins investigated here,

the following picture emerges: The flexibility of the chromo-

phore is identical in all cases. Electron-donating substituents

(such as the methyl groups in 2b or 2c as compared to 2a)

increase the rate of rotation but o-methyl groups (such as

those present in 2c) effectively block mechanically any rota-

tion, likely even in the diprotonated case. Neither compound

carries substituents that increase the rate of rotation by

providing a favorable resonance structure with the porphyrin

in the co-planar transition state.

We assume that the diprotonated macrocycle allows free

rotation for all but the o-methylated porphyrin [2c2H]2+.

Modulation of the porphyrin conformation affects its optical

properties.42,43 Hence, the sum of the non-planar conforma-

tion and the near-co-planar aryl groups, the facilitated rota-

tion and the concomitant intermittent p-conjugation of the

thienyl group, inferring a lower result in the observed bath-

ochromically shifted UV-Vis spectra and explain the observed

trends. The spectrum of [2b2H]2+ is bathochromically shifted

as compared to that of [2a2H]2+ because the more electron-

rich 50-methylthienyl group in [2b2H]2+ allows a lower barrier

of rotation and, thus, the contributions of the co-planar

conformer are significantly larger. On the other hand, isomer

[2c2H]2+ is locked with respect to free rotation. Hence its

spectrum is hypsochromically shifted compared to that of

[2a2H]2+, despite the fact that the thien-30-yl group is more

electron rich than the unmethylated thienyl group. The thien-

Table 3 Comparative NSD analysis of the out-of-plane deformations of 2b, [2b2H]2+ � 2 CF3COO�, and a number of comparison structures ofprotonated TPP ([12H]2+),41 and the computed structures for [2b2H]2+.39,40

Porphyrin doopa/A

dsad/A(B2u)

bdruf /A(B1u)

bddom/A(A2u)

bdwav(x)/A(Eg(x))

bdwav(y)/A(Eg(y))

bdpro/A(A1u)

bAveragedC(o)–C(ß)/A

AveragedS–C(ß)/A

AverageCo–Cipso–Cmeso–Ca

dihedralangle(range)/1 Ref.

1 0.390 0.000 0.000 0.000 �0.275 0.185 0.000 3.15 — 61 (61–62) 372b 0.344 0.000 0.000 0.000 0.241 �0.115 0.000 3.24 3.34 62 (60–64) 34, 41[2b2H]2+ � 2(triflate�)-MolA

2.797 �2.671 �0.319 �0.009 0.019 0.113 0.059 3.16 3.17 28 (19–35) Thiswork

[2b2H]2+ � 2(triflate�)-MolB

3.591 �3.520 0.143 0.033 �0.001 0.026 0.030 3.12 3.12 40 (34–46) Thiswork

Computed [2a2H]2+,saddled

3.255 �3.255 �0.036 0.000 0.000 0.000 0.000 3.21 3.41 35 (�) Thiswork

[12H]2+ �FeCl�4 �Cl� 3.204 3.114 �0.754 0.000 0.000 0.000 0.000 3.05 — 36 (35–37) 41[12H]2+ � 2(ClO�4 ) �C6H6

2.743 2.662 0.000 0.181 0.053 0.108 0.014 3.07 — 40 (36–42) 41

[12H]2+ � 2(HSO�4 ) 2.520 2.434 0.000 0.045 0.000 0.031 0.000 3.08 — 41 (39–44) 41Computed [2a2H]2+,domed

0.587 0.000 0.000 0.567 0.000 0.000 �0.045 3.13 3.25 61 (�) Thiswork

a Total out-of-plane deformation. b Deformation in the lowest frequency mode of each symmetry type. The signs of the deformation are

dependent on the relative orientation of the porphyrin C20N4-macrocycle and are, thus, of no direct relevance in the comparison made here.


30-yl group containing system [32H]2+ is, in terms of sterics,

more encumbered than the thien-20-yl-carrying system. Thus,

co-planar conformers are less likely to contribute to the

observed optical spectrum. Therefore, its UV-Vis spectrum is

the least red-shifted. We believe that this rationalization

explains better the large differences observed in the UV-Vis

spectra of the protonated and unprotonated thienylporphyrins

than inductive effects alone. This rationalization further sup-

ports and complements the propositions made previously

using an alternative set of experimental data.23

3.8 Computed conformation of the diprotonated

meso-tetrathien-20-ylporphyrin [2a2H]2+

Is it possible to accurately model the saddled conformation of

the diprotonated thienylporpyrins and is it, based on this

model, possible to compute the observed UV-Vis spectrum

of this macrocycle? The following sections investigated these

questions in equivalent detail.

Density functional methods [B3LYP/6-31G(d)] predict two

low-energy stable conformations for the diprotonated meso-

tetrathien-20-ylporphyrin [2a2H]2+ as shown in Fig. 6. The

higher energy structure has C4 symmetry, a dipole moment of

1.7 D and is characterized by a domed structure with the four

central hydrogen atoms in close contact. The lower energy

structure has S4 symmetry, a bent saddled structure and a zero

dipole moment. The C4 structure is 169 kJ mol�1 higher in

energy due primarily to the interatomic repulsions associated

with the central hydrogen and nitrogen atoms. Although this

structure has a non-zero dipole moment, an observable popu-

lation of C4 species is unlikely even in highly polar solvent.

Unless stated otherwise, all spectroscopic calculations were

carried out on the low-energy S4 structure or a modified

version in which the thienyl rings are forced into orthogonality

with the plane of the porphyrin ring.

An NSD analysis of the computed saddled (S4) structure of

[2a2H]2+ confirms the viability of this calculated structure

(Table 3). The distortion modes of the computed structure lie

closely between those of the experimentally determined con-

formations of [2b2H]2+—MolA and [2b2H]2+—MolB. The

only difference is that the computed structure underestimates

the minor domed, waved and propelling contributions which

are, therefore, likely due to crystal packing effects and the

interaction of the dicationic porphyrin with the triflate counter

anions. Accordingly, the occurrence and extent of these dis-

tortion modes vary widely with the counterions in all the

structures of [12H]2+ listed.

3.9 The effect of thienyl conjugation on the computed

absorption spectrum

The computed lowest-energy conformer of diprotonated tetra-

thienylporphyrin [2a2H]2+ is characterized by a significant

rotation of the thienyl residues into the plane of the porphyrin

ring. In this section we seek to provide a quantitative measure

of the extent this conformation enables a significant resonant

interaction with the porphyrinic p-system that will rationalize

the measured absorption spectra.

As detailed above, the UV-Vis spectrum of [2a2H]2+ is

characterized by a significantly red-shifted Q band at 13.9 kK

(720 nm) while the Q band in diprotonated tetraphenylpor-

phyrin [12H]2+ is observed at 15.5 kK (647 nm). The large

1600 cm�1 red shift is associated with two electronic contribu-

tions. First, this molecule is a diprotonated species which is

known to produce a small red shift of about B150 cm�1

relative to the neutral species.38 The more important contribu-

tion, however, is the ability of the thienyl rings to rotate

partially into the plane of the porphyrin ring, thus enhancing

resonant interaction with the porphyrinic p system. This will

extend the effective size of the p system and yield a red shift.

We quantitatively investigated this contribution by calculating

the electronic transitions for an artificial species in which the

thienyl rings were rotated and locked so that they were

orthogonal to the porphyrin ring. The entire system was, with

the orthogonal rings locked, minimized using density func-

tional methods (B3LYP/6-31G(d)). The resulting structure

had a calculated total energy that was 95 kJ mol�1 (22.7 kcal

Fig. 6 Comparison of the geometries and relative energies of the two

stable conformations of the diprotonated tetra-2-thienyl porphyrin 2a.

The low energy saddled conformation (A) has S4 symmetry and is 169

kJ mol�1 (40.4 kcal mol�1) lower in energy that the domed C4

symmetry conformer (B) in vacuum.


mol�1) higher in energy than the S4 geometry shown in Fig. 6.

Thus, each ring provides approximately 24 kJ mol�1 of

resonant stabilization to the porphyrin macrocycle. No sig-

nificant steric interactions could be identified that could have

contributed to the energy differences of these two conformers.

Next, we carried out MNDO-PSDCI calculations on the

‘orthogonal’ geometry and the results are shown in Fig. 7A.

For purposes of comparison, we plot the calculated transitions

on top of the absorption spectrum from diprotonated meso-

tetraphenylporphyrin, which we project to have a spectrum

similar to one that would be observed upon locking the thienyl

rings into orthogonality. We infer from the minor shift of lmax

that 12H exhibits upon protonation, that the relatively large

dihedral angle of 401 between the meso-phenyl groups and the

diprotonated porphyrin mean plane provides the means of

little if any resonant stabilization of the porphyrin macrocycle

p-system. Previous theoretical studies also indicated this.25

The MNDO-PSDCI calculations provide an excellent perspec-

tive on the thienyl ring effects and calculate a blue shift of

B2700 cm�1 associated with the rotation of the thienyl groups

(compare Fig. 7A and 7B). We consider this an upper limit to

the bathochromic effect associated with the thienyl rings. We

can compare this result with the experimental estimate based

on the [12H]2+ reference system, which yields a blue shift of

about B1400 cm�1, roughly half of what the theory predicts.

But we would expect the [12H]2+ reference system to provide a

smaller blue shift because the phenyl groups do provide only

modest resonant stabilization in the diprotonated species.

Thus the true value lies somewhere between the lower limit

of B1400 cm�1 (model compounds) and the upper limit of

B2700 cm�1 (theory).

3.10 Single crystal structure determination of

[2b2H]2+ . (triflate)2

The crystal structure determination of C40H28N4S4 �(CF3CO2)2 ([2b2H]2+ � (triflate)2) presented many challenges.

The structure closely mimicked a tetragonal cell setting (I-4).

However, close inspection of the collected data suggested a

pseudo-merohedrally twinned monoclinic setting in the space

group C2. To better resemble the metrically tetragonal setting

the alternative inner-centered setting in I2 with the tetragonal

cell parameters listed in Table 1 was chosen. Refinement of the

structure required the application of a combination of racemic

and pseudo-merohedral twinning with a twofold rotational

twinning law about the (1 0 1) direction. The twinning matrix

used in the refinement was 0 0 1, 0 �1 0, 1 0 0, �4 and the final

BASF values for the twinning ratios were determined to be

0.362(5), 0.08(10) and 0.070(5). In this setting, there are two

cations (the protonated porphyrin moieties, MolA and MolB)

and four counter anions (trifluoroacetates) in the asymmetric

unit that are arranged into two groups, each consisting of one

porphyrin dication and two anions. Both cations are located

on a two-fold crystallographic axis and their porphyrin cores

could also be described in the orthorhombic space group I2mm

(alternative setting of the usual Imm2). Considering the entire

unit cell, this higher symmetry is broken by the arrangement of

the thienyl substituents and the trifluoroacetate anions. The

3-methylthienyl substituents are flip disordered. Due to the

steric demands of the methyl substituents, the relative orienta-

tion of the thienyl groups on one independent porphyrin

molecule are dependent on those of the other. The refined

occupancy ratios for the thienyl groups are 0.752(3) to 0.248(3)

and 0.728(3) to 0.272(3), respectively. All four trifluoroacetate

anions are disordered around the two-fold crystallographic

axis of the unit cell and two different types of coordination

modes of the anions are observed. On one hand, one group of

anions is coordinated in a head-on bidentate fashion to MolB,

aligning themselves ideally along a non-crystallographic S4

axis. To MolA, on the other hand, canted anions coordinate in

a monodentate fashion (Fig. 8). The two anions in this

arrangement are therefore related only by a two-fold rotation,

thus breaking the local S4 symmetry of the porphyrin core of

MolA. The crystallographic literature reveals that mono- and

bidentate trifluoroacetate binding to the protonated porphyrin

is commonplace,44 as is flip disorder for thienyl ligands.34

Due to the pronounced disorder and the degree of twinning

and pseudosymmetry observed, a range of constraints was

required in order to achieve a stable refinement. The bond

distances within the 3-methylthienyl substituents were re-

straint to resemble those of the unprotonated 3-methylthie-

nylporphyrin34 and each thienyl substituent was restraint to be

Fig. 7 Comparison of two computed spectra. (A) shows the results of a MNDO-PSDCI calculation of the spectroscopic transitions calculated for

a computed, low-energy S4 conformer of [2a2H]2+ in which the thien-20-yl groups were rotated and locked orthogonal to the plane prior to

minimization superimposed on the absorption spectrum of [12H]2+ in CH2Cl2 + 5% TFA. (B) shows the results of a MNDO-PSDCI calculation

of the low-lying electronic transitions in the lowest-energy conformation of computed [2a2H]2+ (S4 conformer in Fig. 6) superimposed on the

actual spectrum of [2a2H]2+ in CH2Cl2 + 5% TFA. For the experimental details, see Experimental section.


flat. All equivalent bonds within the trifluoroacetate ions were

restrained to be identical. The anisotropic displacement para-

meters of a small number of atoms were restrained to be close

to isotropic (C15C, C20C, S1C, F5, C15B, C1A, C1B) and

finally global SIMU and DELU restraints were applied for all

non-hydrogen atoms. Despite this substantial application of

restraints the overall quality of the structure with an R1 value

for all data of 7.8% and a goodness-of-fit of 1.162 allows a

solid description of the molecular geometry of the protonated

tetrathienylporphyrin. An ORTEP diagram of MolA is shown

in Fig. 9.

CCDC reference number 272182.

For crystallographic data in CIF or other electronic format

see DOI: 10.1039/b600010j

4 Conclusions

Based on the spectroscopic, structural and computational data

presented, we conclude that the single most important factor

in the red shift Q band of thienyl porphyrins is associated with

rotation of the thienyl groups into resonant interaction with

the porphyrin macrocycle p system and we have semi-quanti-

fied the energetics of this interaction. Using a different and

independent set of experimental data, we further support the

conclusions Gupta and Ravikanth recently put forward. They

rested on the study of the photophysical properties of meso-

thien-20-yl substituted heteroporphyrins.23 The results may

point toward a way to generate long-wavelengths absorbing

porphyrinic molecules by co-planarizing 2-thienyl groups

through covalent linkages to the pyrrolic b-positions. We,

and others, have recently demonstrated the feasibility of such

linkages in the tetraphenylporphyrin series.45 Experiments to

this end are currently underway in our laboratories.

Acknowledgements

The authors thank Dr Charles Campana of Bruker AXS for

guidance during the single crystal structure refinement. The

work was supported with funds provided by the donors of the

Petroleum Research Fund (PRF), administered by the Amer-

ican Chemical Society, NSF grant No. 0517782 (to CB) and

NIH grant GM-34548 (to RRB). PCDF thanks the National

Science Foundation for an NSF-REU summer research sti-

pend. RP, JS and GC were supported in part by CCSU

Faculty-Research Grants as well as with CSU-AAUP Re-

search Grants.

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27 SMART: Bruker Axs Inc., 5465 East Cheryl Parkway, Madison,WI 53711-5373, USA, 2000.

28 Saint V-7.06A Bruker Axs Inc., 5465 East Cheryl Parkway,Madison, WI 53711-5373, USA, 2000.

29 SADABS-Bruker Nonius area detector scaling and absorptioncorrection-V2.10 Bruker Axs Inc., 5465 East Cheryl Parkway,Madison, WI 53711-5373, USA, 2000.

30 Shelxtl V-6.14 Bruker Axs Inc., 5465 East Cheryl Parkway,Madison, WI 53711-5373, USA, 2000.

31 M. J. Crossley, L. D. Field, A. J. Forster, M. M. Harding and S.Sternhell, J. Am. Chem. Soc., 1987, 109, 341.

32 The fluorescence quantum yield for 2a is reportedly only 4.2% ofthat of 1, ref. 23.

33 Based on experimentally derived rotational barriers for metallo-porphyrins, we estimate the computed absolute values to be about25–50% too high, though comparison data for free base porphyrindo not exist.

34 M. Belizzi, P. C. D. Foss, R. Pelto, G. Crundwell, C. Bruckner, J.B. Updegraff III, M. Zeller and A.D. Hunter, Z. Kristallogr. - NewCryst. Struct., 2004, 219, 129.

35 B. Purushothaman, B. Varghese and P. Bhyrappa, Acta Crystal-logr., Sect. C, 2001, 57, 252.

36 Y. Diskin-Posner, S. Balasubramanian, G. Kumar Patram and I.Goldberg, Acta Crystallogr., Sect. E, 2001, 57, m346.

37 A. Stone and E. B. Fleischer, J. Am. Chem. Soc., 1968, 90, 2735.38 B. Cheng, O. Q. Munro, H. M. Marques andW. R. Scheidt, J. Am.

Chem. Soc., 1997, 119, 10732.39 (a) W. Jentzen, X.-Z. Song and J. A. Shelnutt, J. Phys. Chem. B,

1997, 101, 1684; (b) X.-Z. Song and J. A. Shelnutt, NSD EngineVersion 3.0, (http://jasheln.unm.edu/jasheln/content/nsd/NSDen-gine/start.htm).

40 J. A. Shelnutt, in The Porphyrin Handbook, ed. K. M. Kadish, K.M. Smith and R. Guilard, San Diego, 2000, vol. 7, ch. 50, pp.167–224.

41 CSD References codes: 1-TPHPORO4; 2b-LAFQIT;[12H]2+ �FeCl4� �Cl�-TPPFEC; [12H]2+ � 2(ClO4

�) �C6H6-RUH-QEQ; [12H]2+ � 2(HSO4

�)-LEXSIQ.42 J. A. Shelnutt, X.-Z. Song, J.-G. Ma, W. Jentzen and C. J.

Medforth, Chem. Soc. Rev., 1998, 27, 31.43 R. E. Haddad, S. Gazeau, J. Pecaut, J.-C. Marchon, C. J.

Medforth and J. A. Shelnutt, J. Am. Chem. Soc., 2003, 125, 1253.44 A CSD search on ‘‘porphyrin’’ and ‘‘trifluoro’’ yields 30 hits with

several hits being distorted porphyrins or molecules containingother types of trifluoro groups. Of these hits, the vast majority oftrifluoroacetate anions coordinate in a monodentate, canted fash-ion to the porphyrin with 1 or 2 anions per cation: CSD referencecodes BASJUA, GOBSOF, KIBLIQ, KIBMAJ, KIBMEN, KIB-PEQ, KIBPEQ 01, LEYFUQ, LEYHIG, QEZKIP, YEVKAL,YEVKIT; only four show bidentate coordination to the porphyrin:CSD reference codes ERACAB, LEYQAH, YEVKOZ, YEV-KUF. Four structures show no coordination at all to the porphyr-in core: CSD reference codes GOBYIF, KIKDEN, MUKRIT,NEXPOV.

45 For representative examples, see: (a) L. Barloy, D. Dolphin, D.Dupre and T. P. J. Wijesekera, J. Org. Chem., 1994, 59, 7976; (b)R. W. Boyle and D. Dolphin, J. Chem. Soc., Chem. Commun.,1994, 2463; (c) S. Richeter, C. Jeandon, N. Kyritsakas, R. Ruppertand H. J. Callot, J. Org. Chem., 2003, 68, 9200; (d) J. R. McCarthy,M. A. Hyland and C. Bruckner,Org. Biomol. Chem., 2004, 2, 1484;(e) H. W. Daniell and C. Bruckner, Angew. Chem., Int. Ed., 2004,43, 1688; (f) S. Fox and R. W. Boyle, Chem. Commun., 2004,1322.


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