on the electronic structure of barrelene-based rigid organic donor-acceptor systems. a comparison of...

Chemical Physics 175 (1993) 357-367 North-Holland

On the electronic structure of barrelene-based rigid organic donor-acceptor systems. A comparison of INDO/S-CI calculations with absorption and fluorescence emission spectra

Thomas Fox, Notker R&h ’ Lehrstuhlfiir Theoretische Chemie, Technische Universitiit Miinchen, D-85747 Garching, Germany

Jose Corks and Hans Heitele * Institutfir Physikalische und Theoretische Chemie, Technische Universitiit Mtinchen, D-85747 Garching, Germany

Received 21 April 1993

We investigate at the INDO/S-CI level the electronic structure of a series of barrelene-based organic donor-acceptor molecules exhibiting photo-induced charge separation even in nonpolar solution. Results for excitation energies, oscillator stmn@s, and excited state dipole moments are compared to static UV absorption and fluorescence emission spectra. Satisfactory agmement is obtained for transition energies and excited state dipole moments ifthe induced part of the dipole moment of the donor-acceptor molecules in solution is properly taken into account. A particular focus lies on the conditions that foster the occurmnce of low- lying charge-transfer states.

1. IntrodtK!tlon

In recent years the quantum-chemical treatment of charge-transfer states and photo-induced charge- transfer processes in organic molecules has attracted enormous interest [ l-31. Although most of the charge-transfer (CT) chemistry happens in the con- densed phase, e.g. in polar solution, the recent dis- covery of photo-induced long-range intramolecular electron transfer in the gas phase [ 4-6 ] provides a sufficient stimulus for an effort to test quantum- chemical methods for such states even for bare molecules. The conditions for the occurrence of long- range electron transfer in a solvent-free isolated molecule were analyzed recently [ 7 1. Ultimately, however, the challenge is to extend these techniques to include the interaction between molecular charge- transfer states and polar solvent molecules characteristic for electron-transfer reactions in solution.

In this paper we investigate a series of organic molecules (fig. 1) based on dibenzobarrelenedicarboxy-

’ Towhom uxrespondence should be addmmed.

methylester 1 whose electronic structure has been discussed previously [ 8 1. The interest in these compounds derives from their relative simplicity which permits the application of advanced quantum-chemical methods and their potential for systematically probing specific structural features. In the series of compounds in fig. 1 the aromatic residue in the naphthalene derivatives 2 and 3 is enhuged compared to compound 1. This change is expected to lower the energy of the excited electronic states centered on the aromatic residues. The latter ones are important for the electron donating properties within the molecules. Additionally, breaking the approximate mirror symmetry in compound 3 strongly affects the interaction with the two carboxymethylester residues re- sponsible for the electron accept@ function of the system. Compounds la and 2a which lack these acceptor groups serve as refmnce systems.

The electronic structure of the excited states of these molecules in the gas phase was c&&ted in the INDO/S-CI approximation [ 91. These calculations were put to test by stationary absorption and fluores-

0301-0104/93/S 06.00 8 1993 Elsevier Science Publishers B.V. All rights reserved.

358 T. Fox et al. /Chemical Physics I75 (1993) 35 7-36 7

Fig. 1. Structure of the barrelene-based donor-acceptor compounds 1-3 and of the corresponding reference molecules la and 2r.

cence spectra in several solvents varying the polarity of the environment.

2. Materials aJld methods

The donor-acceptor molecules l-3 were synthe- sized by refluxing anthracene, tetracene, or benzan- thracene with an excess of acetylene-dicarboxyme- thylester in pxylene solution for several hours. The Diels-Alder products were separated on silica gel/ toluene and purified by repeated recrystallization in methanol and sublimation in vacua. Refluxing compound 1 or 2 with phenyl-vinyl-sulfoxide in chloro- benzene gave the reference compounds la and 2a, respectively. PuriGcation was done by column chromatography on silica gellhexane and reerystal- lization in hexane.

Absorption and fluorescence spectra were re- corded on a Varian 2300 UV-Vis-NIR spectropho-

tometer and a Spex Fluorolog F2 12E spectrofluoro- meter, respectively. All measurements were carried out at room temperature. The sample concentrations were low4 and lo- 5 M. Solvents were of spectro- scopic grade and were used as supplied, The samples proved photochemically stable on the time scale of the measurements.

Extending earlier work on compound 1 [ 81 INDO/ S model calculations [9] have been performed to elucidate the electronic structure and the spectro- scopic properties of the compounds 2 and 3. The ge- ometry of the molecules was derived from molecular mechanics calculations using the program DIS- COVER [ lo]. Average bond lengths were used for symmetry-equivalent bonds. The O-C-O moieties of the acceptor groups were assumed to be rotated by 45” with respect to the plane defined by the carbon atoms of the carboxyl group and the ethylene bridge with the carbonyl oxygen atoms pointing in opposite directions to exhibit a local C2 symmetry [ 8 1,

3. Results and discussion

3. I. UV absorption spectra

The UV absorption spectra of the donor-acceptor compounds l-3 together with those of their respec- tive reference systems la and 2a are shown in fig. 2. Reference compound la exhibits an absorption band with strong vibronic structure with the peak intensity at 280 nm (~=4110 mol-’ cm-‘) attributed to the O-O transition of &,+Si. A much stronger, less structured band is observed at 2 16 nm (r=49700 mol- l cm- l) . These features are retained in the corresponding donor-acceptor compound 1 although the bands are slightly blue-shifted by l-3 nm and the vibronic structure is much weaker. Fmthermore, the intensity of the first band is signifiea.ntly reduced ( c = 3030 mol- ’ cm - 1 ) whereas the second band is even stronger (e=78300 mol-i cm-*) than in la. However, in addition to these features a broad shoulder is observed at about 290 nm (ekz 1200 mol- * cm-’ ) as the lowest excitation in 1. Since the lowest excitation of maleic diesters ties below 250 nm, this shoulder strongly suggests the existence of a new first excited state due to a significant interaction between

T. Fox et al. / Chemical Physics 175 (I 993) 35 7-36 7 359

250 300 350

Wavelength (nm)

250 300 350

Wavelength (nm)

Fig. 2. UV absorption spectra in n-hexane solution (upper spectra: lo-’ M, lower spectra: lo-$ M) and line spectra from lNDO/S-CI calculations. AU transition energies refer to vertical excitations. Full lines: donor-acceptor molecules 13, dashed lines: reference compounds la and 28, respectively.

the aromatic donor moiety and the electron accept- ing ester groups.

The absorption spectrum of reference compound 2a is rather similar to the spectrum of naphthalene with the ‘L,, band at 322 nm (e = 1700 mol-’ cm-’ ) and the ‘L, band with peak intensity at 275 nm (e= 13700 mol-* cm-’ ). The same features are found in the corresponding donor-acceptor compound 2, with less pronounced vibronic structure and somewhat lower intensities. No new absorption bands are observed although the increased absorption

around 300 nm might suggest an additional underly- ing excitation.

The spectrum of the “bent” molecule 3 exhibits absorption bands at similar wavelengths, yet, with much moreintensityat328nm(e=1610mol-‘cm-’)and a red-shifted rise of the second band with a maximum at 288 nm which is less intense (e = 45 10 mol-’ cm-‘) than in 2.

360 T. Foxetd./ChemicaiPhysics173(1993)357-367

0.Q

0.0

0.7

8

f 0.6 0.5

4 0.4

0.3

0.2

0.1

0.0

L c

200 300

Wavelength (nm)

Fig. 2 (continued).

3.2. Fluorescence emission spectra

As argued before, UV absorption spectra indicate a direct transition to a CT state as the lowest excited state only for compound 1. The existence and the position of CT states in the other molecules have to be demonstrated using fluorescence emission spectra.

Emission spectra of the three donor-acceptor compounds in several solvents are depicted in fig. 3. The spectrum of the reference system 2a is included for comparison. Emission from compound la was very weak. Most striking is the complete quenching of the strongly structured “primary” fluorescence similar to the reference compounds in the donor-acceptor molecules 2 and 3. On the other hand, a new, broad fluorescence emission appears whose position strongly depends on the polarity of the solvent which is indic- ative of sizable charge transfer. Fluorescence excitation spectra of the broad band closely follow the UV absorption spectra, i.e. independent of the initially excited state, the molecule very rapidly undergoes intramolecular charge-separation to a new lowest ex- citedCTstateY1.ALippert-Matagaanalysis [12-141 reveals a reasonably linear correlation (see fig. 4a)

(I’ Time-resolved measwments give a lower limit of 5 X 10” s-I for the rate of charge wparation. The lifetime of the red-shifted fluorescence is of the order of nanoseconds and is limited by non- radiativepfwesses [ll].

of the position v, of the CT fluorescence maximum with the solvent parameter Af in eq. ( 1) :

Af= e,-1 1 n2-I --z&i’ 2c,+l (1)

with thestatic dielectric constant c, and the refractive index of the solvent n. The required solvent parameters are compiled in table 1. For the determination of v, the experimental spectra were multiplied by a factor of 2’ and then transformed to a wavenumber scale. A factor of A2 stems from the conversion of counts per wavelength to counts per wavenumber units and a further factor of A3 eliminates the factor v3 in the Einstein coefficient which leads to a distor- tion of the experimental bandshape compared to the “ideal” Franck-Condon bandshape relevant for the following discussion.

In order to infer the likely position of the CT excitation from the position of the corresponding CT fluorescence band we use the relationship between the energies of the band maxima for CT fluorescence En and absorption Eob:

E*,,-En=2A, (2)

where 24 is the Stokes shift. If one assigns the shoulder at 290 run in compound 1 to the direct CT absorption, one obtains A= 0.67 eV for this particular compound in n-hexane solution. In view of the close similarity of the other molecules the same value is

i? Fox et al. /Chemical Physics I75 (1993) 35 7-367 361

212a

250 300 350 400 450 500 550

Wavelength (nm)

600 650 700 750

Fii 3. Absorption spectra in hexane and fluorescence emission spectra of 1,1. ,=280 nm, 2 and 2a (dashed lines), A,=275 mn, and 3, A,= 290 mn, at room temperature. For the numbering of the solvents see table 1.

used for the estimate of the CT absorption band in the other molecules as well. A more detailed analysis of the fluorescence bandshape [ 15 ] supports this procedure. The resulting extrapolated position of the direct CT absorption into the lowest CT state are marked in the absorption spectra in fig. 2 and will be used for comparison with the electronic structure calculations. In 2 and 3 they coincide with the fall-off of the second absorption band which could explain its red-shift compared to the reference compounds. In the same approximation the free energy of the re- laxed CT state is (&+A) above the ground state or

by the free energy of charge-separation AG,,

AG,mEn+A-E,,,, , (3)

below the first, essentially nonpolar excited state with the O-O excitation energy Em. The corresponding values are summarized in table 2.

3.3. Electronic structure calculations

The electronic structure of the CT compounds under study may be elucidated by formally constructing them from a central barrelene unit, bicycle-[ 2.2.2]-

.

362 T. Fox et al. /Chemical Physics 173 (1993) 35 7-36 7

250 300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

Fig. 3 (continued).

2,5,7-octatriene. The donors replace two ethylene moieties in this basic unit, the third ethylene group is substituted by the acceptor groups [ 8 1. Crucial for the following argument is the symmetry behavior of the fragment orbitals with respect to the mirror plane perpendicular to the threefold axis of barrelene. (Of course, the mirror plane is only a local, approximate symmetry element once the barrelene unit is substituted with the donor moieties. ) In tig. 5 we show the well-known [ 16- 18 ] x-orbital spectrum of unsubsti- tuted barrclene together with sketches of the MOs as viewed along one of the three twofold axes’as well as the frontier orbital group of benzene and naphthalene. We note that the occupied frontier orbitals of barrelene are all even with respect to the mirror plane. When naphthalene is “inserted” into barrelene sym- metrically (i.e. via its 2,3 positions), only the second highest occupied MO of naphthalene has the proper nodal structure to take over the role of the corresponding ethylene orbital. Together with the even partner of the degenerate HOMO of benzene and the acceptor orbitals, the HOMO of 2, derived from the HOMO of barrelene, exhibits not only contributions from the donor, but also significant acceptor character. Only 0.2 eV below the HOMO one finds a MO that is purely located on the donor and is odd with respect to the approximate mirror plane. It results from the high-lying HOMO of naphthalene. The un- occupied orbitals of the CT compounds may be analyzed in a similar fashion. In naphthalene, only the

orbital above the LUMO has the proper symmetry to take over the role of a ti-orbital of an ethylene moiety of barrelene. The resulting LUMO of 2 is localized at the C=C bridge and on the carboxyl groups and, therefore, may serve as the acceptor orbital of a CT transition. The next higher MO resulting from the naphthalene LUMO lies only 0.1 eV higher and is essentially of donor character.

The symmetry constraints explained so far are not at work when naphthalene is “inserted” side-on (3) into the central barrelene. All four frontier orbitals of naphthalene depicted in fig. 5 contribute to the occupied and to the virtual orbitals of the corresponding “substituted” barrelene. The HOMO of 3 is about 0.5 eV higher in energy than that of 2 since the re- placement for one of the ethylene II units now comes from the HOMO of naphthalene and thus interacts more strongly than the corresponding orbital of 2. This mechanism is the main reason for the reduced HOMO-LUMO gap in 3 compared to 2. Both the LUMO and the MO above it show significant acceptor contributions. However, in contrast to the LUMO of 2, both orbitals have also large donor contributions from naphthalene. Next in energy follow nearly pure donor orbitals, the virtual orbitals with domi- nant acceptor characteristics lying higher yet. A thor- ough discussion of the MO structure of the compounds l-3 investigated will be presented elsewhere [191.

In table 3 we display for various excited states of 2

T. Fox et al. /Chemical Physics I 75 (1993) 357-36 7 363

17000

16000

15000

0.0 01 0.2 0.3 0.4 0.5

Af

24000

23000

17000

16000

15000

0.0 0.1 02 0.3 0.4 0.5 0.6 07 08 0.9 10

Af

Fig4.(a)Dependenceofthepositionofthemaximumofthc~fluorescenceonthesolventpanuneterAf=(~-1)/(2~+1)-~((n2-l))/ (2n*+ 1) (see eqs. ( 1) and (4) ). (b) Dependence of the position of the maximum of the Cl’ fluorescence on the solvent parameter A~‘=(E,-l)/(c,+2)-f(n*-l)/(n*+2) (seeeqs. (6)and (7)).Forthenumberingofthesolventsseetable 1.

and 3 the calculated vertical transition energies, the since both the HOMO and the LUMO have consid- total state dipole moments as well as the correspond- erable contributions from the bridging ethylene ing oscillator strengths. Also shown is the charge moiety, the occurring charge separation is only mod- transfer to the acceptor entailed in the excitation (as erate. As judged by the changes in the Mulliken measured by the change in Mulliken charge on the charges and the state dipole moments, the next live acceptor moiety C-C plus COOMe). In 2, the lowest higher lying excitations entail only a minor charge calculated transition at 30570 cm-’ is essentially an transfer. At 38 113 cm- ’ we find a pronounced CT intra-donor (D+D) excitation. The next higher ex- state originating from the excitation out of a pure do- citation at 31280 cm-’ is the HOMO-LUMO tran- nor orbital into the LUMO. This state shows a dipole sition. Because a charge of almost 0.4 au is trans- moment of 15.2 D and a charge transfer of more than ferred during this transition, it may be classified as a 0.7 au, clearly entailing a significant amount of charge donor-acceptor -(D-A) CT excitation. However, separation.

364 T. Fox et al. /Chemical Physics 175 (J 993) 35 7-367

Table 1 Static dielectric constants c,, refractive indices n, and the solvent parameters 4f=(e,-1)/(2e,+l)-i(n*-1)/(2n*+l) and Af’~(~,-l))/(4+2)-f(n2-1)/(n2+2)atroomtempcratureoftheseriesofsolventsusedinthisstudy

!Iolvent t n Af Af’

( 1) n-hexane 1.88 1.3749 0.092 0.112 (2 ) tetrachloroethylene 2.24 1.5058 0.116 0.144 ( 3 ) tetmchloromethane 2.24 1.4602 0.119 0.155 (4) pentyl ether 2.77 1.4119 0.171 0.247 (5 ) trichloroethene 3.42 1.4767 0.199 0.305 (6) butyl acetate 5.01 1.3894 0.267 0.454 (7 ) ethyl acetate 6.02 1.3724 0.292 0.512 (8) methylene chloride 8.93 1.4242 0.319 0.598 (9) 1,2dichloroethane 10.37 1.4448 0.324 0.625

( 10) dimethyl sulfoxide 46.45 1.4793 0.374 0.796 ( 11) N,Ndimethyl formamide 36.71 1.4305 0.378 0.793 (12) propylene carbonate 64.92 1.4215 0.387 0.828 ( 13) acetonitrile 35.94 1.3441 0.392 0.815

Table 2 Calculated ( INDO) and experimental (exp) excitation energies and free energies of reaction (in eV ) .

Compound &0(exp) l ) &r(exp) b, AG, =) E,‘(INDo) d,

1 4.45 4.28 -0.84 4.22 2 3.85 4.24 -0.28 3.79 3 3.78 3.91 -0.54 3.72

l ) Electronic origin of the lowest nZ excitation. b, Vertical excitation of the lowest charge transfer transition using eq. (2) for the compounds 2 and 3. ‘) Free energy of charge separation in n-hexane using eq. (3). d, Lowest vertical W excitation. ‘) Vertical excitation of the charge transfer state.

Eor(INDO) e,

3.81 3.88 3.46

--

Fik 5. x-orbital spectrum of barrelene together with a sketch of theMOsasviewedfromavertkalC2axis(A).Alsoshownisthe frontier orbital spectmm of benzene (B) and naphthalene (C).

Due to the smaller HOMO-LUMO gap in 3, the lowest excitations are found at lower energies than those of 2. In 3, the HOMO-LUMO transition at 27800 cm-’ is the lowest excitation and it is accom- panied by a charge transfer of more than 0.4 au. However, due to the characteristics of the participat- ing MOs and indicated also by the relatively small state dipole (5.2 D), again only a moderate charge separation is expected. At higher excitation energies we find a number of states without signikant CT character, the next state with significant CT characteristics is found at 38800 cm-‘.

3.4. Comparison of electronic structure calculations and experimental spectra

The calculated excitation energies and oscillator strengths are illustrated by line spectra superimposed

T. Fox et al. /Chemical Physics I75 (1993) 357-367 365

Table 3 Calculated excited states of the compounds l-3 using INDO single excitations. The charge transfer is measured by the change of the Mulliken populations of tht GC and the COOMe moieties with respect to tlte ground state

state 1 2 3

er@aY cc L cr energy P L CT energy P L CT (cm-‘) (D) (au) (cm-‘) (D) (au) (cm-‘) (D) (au)

1 0 1.55 - 0 1.74 - 0 1.49 - 2 30694 5.39 0.039 0.44 30569 3.06 0.051 -0.04 21856 5.17 0.007 0.44 3 34062 1.18 0.014 0.02 31279 4.39 0.044 0.37 30026 3.31 0.115 -0.02 4 34509 1.74 0.079 0.00 33814 1.59 0.014 0.01 31474 1.78 0.073 -0.02 5 34727 1.02 0.033 0.05 34504 0.97 0.001 0.19 33708 2.57 0.018 0.03 6 36366 2.43 0.017 -0.06 35443 2.93 0.144 -0.06 33972 2.13 0.009 0.06 7 36773 4.79 0.000 0.07 35817 2.24 0.111 -0.04 34872 4.46 0.082 -0.08 8 42516 1.20 0.132 0.05 37940 3.14 0.035 -0.12 37424 4.84 0.105 -0.09 9 42613 8.08 0.016 0.58 38113 15.22 0.087 0.72 38763 5.82 0.045 0.50

10 43793 8.16 0.067 0.55 40294 8.30 0.899 -0.18 40782 1.37 0.111 -0.01

onto the UV excitation spectra in fig. 2. Transitions with significant CT character are marked.

For molecule 1 the INDO calculations predict the lowest excitation at 326 nm with significant CT characteristics [ 8 1. This finding is supported by the pres- ence of the shoulder at 290 nm in the UV spectrum which we attribute to direct charge transfer. The energy of the excitation, however, is calculated too low by about 0.4 eV. The structured band at 280 nm in the UV spectrum coincides with five calculated excitations between 294 nm and 272 nm which entail very little charge transfer. The ratio f,(CT)/ cf,k: 0.3 of the oscillator strength of the CT excitation divided by the sum of the oscillator strengths of excitations between 294 and 272 nm seems to indicate an overestimation of the relative intensity of the CT excitation although the strongly overlapping bands prevent a direct comparison with integrated intensities.

For compound 2 experiment and theory both yield a first excitation with weak CT character with excel- lent agreement in energy. The next higher transition entails a moderate charge transfer of about 0.4 au; the estimate from the emission spectrum as de- scribed before yields 290-300 nm for the direct CT excitation. Thus, the position of the CT absorption as the second excitation is correctly reproduced in INDO/S calculations. The position of two excitations of intradonor (D-+D) type at 282 and 279 nm with an oscillator strength of about 0.11 and 0.14, re-

spectively, are assigned to the broad structured band at 276 nm. A very strong excitation at 248 nm cf, x 0.9 ) suggests an assignment to the absorption band maximum at 239 nm with an extinction coefficient of e = 80000 mol- ’ cm-‘. The second excitation with even stronger CT character (amount of transferred charge 0.7 au ) is most likely buried under the very intense band around 240 nm.

As argued above, the asymmetric annellation of the naphthalene moiety in compound 3 compared to 2 leads to a drastic drop in the energy of the lowest CT excitation to 359 nm although its oscillator strength is very low (j&0.0069). In fact, a significant red- shift of the CT emission is observed (0.3 eV in n- hexane, compared to a calculated difference in CT excitation energies of’0.4 eV). Yet, extrapolation to the corresponding CT absorption yields a CT excitation at about 320 nm which is very close to the first two excitations of essentially intradonor type.

It is common practice to estimate excited state dipole moments & in solution from the slope of the solvatochromic shift of the CT fluorescence [ 12- 14 ] (see fig. 4a). In the point-dipole approximation for the emitting molecule this slope is given by 2&( & - &) la8 where & is the ground state dipole moment and 4 is the effective radius of the molecule. On the assumption that & is negligible compared to k the following equation is obtained:

hvfl=hvfl(O)- y*f.

366 T. Fox et al. /Cht?minrlPhysks 175 (1993) 337-367

An estimate of the effective molecular volume using a density of 1 .O g cm-’ yields uc= 5.0-5.3 A (see table 4) which translates into excited state dipole moments & of about 12-l 6 D for the three compounds (see table 4) in solution. INDO calculations yield almost antiparallel dipole moments in the ground and the lowest charge-separated states resulting in total changes of the dipole moments (A- k) of 6-7 D (see tables3and4).

Yet, in order to compare experimental dipole moments in solution with calculated dipole moments of isolated molecules account has to be taken of the polarixability e of the fluorescent state in solution [ 20- 221. The molecular dipole moment in solution p: and the dipole moment of the isolated molecule pz are related by [ 20-221

(5)

For many practical purposes a polarixability of a,/ aa =0.5 is assumed [ 23-261, resulting in the linear relationship in eq. (6) with the solvent-free excited state dipole moment PZ and the solvent parameter Af’ (es. (7)):

hv”=hvfl(O)- yAf#,

i 1 n*-1 Af&d__- e,+2 2n2+2’

As is shown in fig. 4b the solvatochromic shifts fulfill this linear correlation reasonably well. The resulting values for the dipole moments cc: (see table 4) are significantly lower than those of j4: indicating a six- able contribution of an induced dipole moment to A. In fact, somewhat lower effective radii of &=4.4-

4.8 A would already reconcile experimental values of pip with calculated changes Afi of molecular dipole moments.

Further evidence for a significant polarizability of the donor-acceptor molecules in fig. 1 comes from explicit calculations of the solvatochromic shifts of the vertical excitation of the molecules from the ground state to the lowest charge-separated state. As has been shown [ 81 for compound 1, tic electrostatic solute-solvent interaction leads to an increase of the amount of charge separation by about 30% compared to the bare molecule. However, the corresponding excitation energy is shifted by less than 0.1 eV.

Nevertheless, the applicability of the standard Lippert-Mataga procedure seems to be questionable for these compounds. In fact, an analysis of the electrostatic interaction between the charge distribution of compound 1 in the CT state with a surrounding dielectric continuum [ 271 (see also ref. [ 281 for a related study) reveals a significant contribution of higher order multipole moments to the electrostatic solute-solvent interaction energy. However, the Lippert-Mataga analysis subsumes these additional contributions also under an effective dipolar term and thus leads to an overestimation of the change in dipole moment Ap. This limitation of the Lippert- Mataga analysis possibly accounts for a substantial part of the discrepancy between experimental results and MO calculations [ 27 1.

4. Conclusions

The comparison of electronic structure calculations and absorption and fluorescence emission spectra for the series of barrelene-based rigid donor-ac-

Table 4 So1vatochromic slopes 2(p:)2/aa and 2(&‘/a~ (in cm-‘), effkctive radii 6 (in A) estimated from effective molecular volumes witb a density of p= 1 g cm-‘, experimental excited state dipole moments of the free molecule pt and in solution p:, as well as the difftrences of the calculated dipole moments (R-Y) in the ground and excited states (in debye)

Compound 2(U2/a: 2(kz)‘l~d a0 PZ r: k-Y

1 10800 4130 5.0 1.7 11.6 6.9 2 18300 1180 5.3 10.7 16.4 6.1 3 13800 5690 5.3 9.2 14.3 6.6

T. Fox et al. /Chemical Physics 175 (1993) 357-367 367

ceptor molecules shows that INDO/S-CI calculations for the bare molecules yield excitation energies for transitions entailing little charge transfer within 0.2 eV of experimental values in the nonpolar solvent n- hexane. Calculated charge transfer excitation energies are too low by about 0.3-0.5 eV (see table 1). Nevertheless, INDO/S-CI calculations successfully predict the trends in CT excitation energies observed upon changes in the size of the aromatic donor moiety and the symmetry of the molecule. Experimental excited state dipole moments in solution of 12-16 D exceed calculated values for isolated molecules by a factor of two. This apparent discrepancy is signifl- cantly reduced if the polarizability of the donor-acceptor molecules is taken into account in solution. Furthermore, these examples demonstrate how a ju- dicious manipulation of the HOMO-LUMO characteristics of the substituent donor systems may lead to a first excited state of CT characteristic possibly even for an unsolvated molecule. A detailed theoret- ical account of the construction principles that influ- ence the relative ordering of CT and D-tD excited states will be presented elsewhere [ 191.

Acknowledgement

We are deeply indebted to Professor M.E. Michel- Beyerle, Technische Universitit Mlinchen, for con- tinuing advice and encouragement. This work was supported by the Deutsche Forschungsgemeinschaft, the Bundesministerium fur Forschung und Techno- logic, and the Fonds der Chemischen Industrie.

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on the electronic structure of barrelene-based rigid organic donor-acceptor systems. a comparison of...

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