the förster radius for energy transfer from naphthalene to anthracene in polyesters with...

Macromol. Chem. Phys. 197, 1349-1360 (1996) 1349

The Forster radius for energy transfer from naphthalene to anthracene in polyesters with oxyethylene spacers

Julio Bravo

Departamento de Ingenieria, Universidad Carlos 111 de Madrid, 2891 1 LeganCs, Madrid, Spain

Francisco Mendicuti, Enrique Saiz

Departamento de Quimica Fisica, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain

Wayne L. Mattice*

Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, USA

(Received: June 26, 1995; revised manuscript of August 29, 1995)

SUMMARY Model compounds for a series of polyesters in which 3,6,9, and 12-rotatable CH,-0

and CH,-CH, bonds separate rigid units have been studied using the efficiency of nonradiative singlet energy transfer from naphthalene to anthracene. The model compounds have the structure naph-CO-(OCH,CH,),-OOC-anth with rn of 1 -4,where naph and anth denote 2-naphthyl and 9-anthranyl groups, respectively. This series of compounds covers a larger range of potential separation of the two chromophores than was possible in an earlier study, in which the flexible spacers consisted of 2-6 methylene units. The nonradiative singlet energy transfer has an efficiency that depends on the solvent viscosity and on rn. The theoretical analysis employs a rotational isomeric state model for the conformations of these molecules. The Fdrster radius for transfer from naphthalene to anthracene in this system is 16 + 2 A , and is nearly independent of whether the spacers are constructed from oxyethylene or methylene units.

Introduction

Polymers that have a repeating sequence in which a rigid structure alternates with a flexible spacer can exhibit a variety of properties, depending on the manner in which these two moieties are chosen. The repeating pattern can be abbreviated as Ar-S,, where Ar denotes the rigid structure, S denotes the fundamental unit in the flexible spacer, and rn denotes the number of these fundamental units. Many popular choices for Ar contain a n aromatic ring system that can be electronically excited with ultraviolet radiation. Fluorescence is among the pathways by which the electronically excited aromatic ring system can return to the ground state. The competition between this pathway and alternative nonradiative pathways can be exploited t o obtain information about the manner in which the rigid structures interact with one another in the polymer, because the efficiency of some important competing nonradiative pathways depends

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1350 J. Bravo, F. Mendicuti, E. Saiz, W. L. Mattice

on the distance between pairs of Ar. TWO such pathways, with very different dependences of their efficiencies on this distance, are often exploited in experiments.

The pathway that is sensitive to very short distances is a consequence of the formation of a complex between an excited Ar and another Ar in its ground state. These complexes produce a decrease in the intensity of the fluorescence expected from an isolated excited Ar, and the appearance of a new emission band, red-shifted from the emission band for the isolated chromophores. The complexes can be stabilized by a dipole coupling mechanism (excimer) or by charge transfer (exciplex) '). The red- shifted emission from an excimer is observed when the two Ar groups form a planar sandwich with a very tight spacing*), but the geometric requirement for the exciplex is not quite so severe.

The pathway that is sensitive to longer distances is usually observed when two distinct types of chromophores are present in the chain, such that Ar-S, sequences are interspersed with Ar'-S, sequences. The Ar and Ar' cannot be chosen arbitrarily, but must instead be chosen so that there is an appropriate match of their spectral properties. With this match, they form a Forster pair3s4) in which the excitation initially present in Ar can be nonradiatively transferred to in Ar' over distances that can be an order of magnitude larger than those which produce an excimer. One of the popular pairs of chromophores used for this purpose is naphthalene (as the donor) and anthracene (as the acceptor)').

Recently we presented an experimental and theoretical description of Forster transfer in model compounds which demonstrate that this pair of chromophores can be used to explore interactions on a distance scale of = 16 A in polyesters with methylene spacers6). Here we expand on that work by considering the series of compounds in which the methylene spacers are replaced by oxyethylene spacers. Both series of bichromophoric model compounds have the general structure N-S,-A, where N and A denote 2-naphthoyl and 9-anthranoyl groups, respectively, and m is a small positive integer. Currently, S is oxyethylene (and m covers the range 1 -4), whereas previously S was methylene (and m covered the range 2-6). The present series contains longer spacers, because oxyethylene has more bonds than methylene, and also provides an overlap with the size of the spacers studied earlier. The covalent structure of the present series of compounds is depicted in Fig. 1.

Fig. 1. Structures of the bichromophoric compounds denoted by A # EN

Steady state fluorescence measurements were performed in media of varying viscosity, q, employing mixtures of methanol and ethylene glycol at 298 K. The red- shifted emission expected from an exciplex was not observed. Intramolecular

The Forster radius for energy transfer from naphthalene to anthracene . . . 1351

nonradiative singlet energy transfer from N to A was present, with an efficiency which depends on m and q. The results can be rationalized by a theoretical analysis of the conformations of N-S,-A, using the rotational isomeric state model7). They are consistent with a Forster radius3), R,, for the energy transfer from N to A that is in the range 16 f 2 A , which is similar to the result obtained previously with the series in which the spaces were constructed from methylene groups6). The present experiments are a more severe test of the model, because the longer spacers used in the present work probe distances closer to R,.

Experimental part

The bichromoporic compounds studied here are abbreviated collectively as A # EN. The member of the series with rn oxyethylene groups is denoted by substitution of the numerical value of rn for #. The first member of the series, AlEN, is also contained in the series with spacers constructed from methylene units, which was studied earlier 'I. That series was abbreviated A # MN, and AlEN = A2MN. The A # EN with # > 1 are not present in A # MN. The method below for obtaining A # EN with # > 1 differs slightly from the prior preparation for A # MN6).

The 9-anthranoyl chloride was obtained from the commercial acid (Aldrich) by reaction with an excess of thionyl chloride (Fischer) at room temperature. The mixture was heated gently for 2-3 h under N,, and then the excess thionyl chloride was removed by distillation. Anthranoyl groups were attached to an end of the glycol by refluxing (for 3 h at approximately 40 "C under NJ a stoichiometric amount (1 : 1) of 9-anthranoyl chloride with the glycol in the presence of triethylamine (Aldrich) in distilled chloroform. Once the reaction was completed, the solution was washed several times with water and aqueous sodium bicarbonate, and then again with water. The crude product consists mainly of the monosubstituted glycol, A # EOH, the disubstituted products A # EA, and the hydrolyzed 9-anthranoic acid chloride. Separation was achieved by column chromatography on activated silica gel (Fluka, particle size 0.04-0.063 mm), using chloroform as the eluting solvent. The monosubstituted glycols are liquids at room temperature.

A similar method was used to attach the naphthoyl group to the monosubstituted glycols. An excess of 2-naphthoyl chloride (Aldrich) was added previously to a solution of the monosubstituted glycol in chloroform in the presence of triethylamine, and the mixture was refluxed for 4 h under N,. When the reaction was completed, the solution was washed as described above. The crude product consists mainly of the desired A # EN, a small amount of unreacted monosubstituted glycol, and the hydrolyzed 2-naphthoyl acid chloride. Separation was achieved by column chromatography, as described above. The diester A2EN was recrystallized from a chloroform/methanol mixture. A3EN and A4EN are liquids at room temperature.

The monochromophoric model compounds methyl 9-anthranoate and methyl 2-naphthoate, abbreviated AM and NM respectively, were synthesized previously in a similar way6).

Intermediates and required compounds were characterized using UV, NMR, and TLC in several solvents of different polarity. 'H NMR was performed in CDC1,. Results of NM, AM, and AlEN (=A2MN) were reported previously6). The new compounds and intermediates have A2EOH: 6 = 8.51 (1 H, s), 8.05 (4H, c), 7.52(4H, m), 4.77 (2H, t), 3.92 (2H, t), 3.75 (2H, t), 3.66 (2H, t), 1.92 ( l H , s).

A3EOH: 6 = 8.47 ( l H , s), 8.02 (4H, c), 7.47 (4H, m), 4.74 (2H, t), 3.87 (2H, t), 3.63 (6H, m), 3.53 (2H, m). 2.36 ( l H , s).


A4EOH: 6 = 8.48 (1 H, s), 8.04 (4H, c), 7.48 (4H, m), 4.76 (2H, t), 3.90 (2H, t), 3.58 (12H, m), 2.61 (1 H, s).

A2EN:6= 8.55(1H,s),8.38(lH,s),8.16(2H,d),7.8(6H,m),7.4(6H,m),4.83(2H, t), 4.58 (2H, t), 3.94 (4H, m).

A3EN: 6 = 8.57 (1 H, s), 8.45 (1 H, s), 7.95 (8H, m), 7.48 (6H, m), 4.76 (2H, t), 4.47 (2H, t), 3.8 (8H, m).

A4EN 6 = 8.59 (1 H, s), 8.48 ( 1 H, s), 7.96 (8H, m), 7.50 (6H, m). 4.75 (2H, t), 4.48 (2H, t), 3.89 (2H, t). 3.80 (2H, t), 3.67 (8H, m).

Steady-state fluorescence measurements were performed with an SLM 8 lOOC fluorimeter equipped with double and single monochromators in the excitation and emission paths and a cooled photomultiplier. Polarizers were set to magic-angle conditions. Slits were 4 nm for the excitation and emission paths. Right angle geometry was used, with rectangular 1 cm path cells for measurements in solution at 298 K and cyclindrical 1 mm internal diameter cells at 77 K. Front surface illumination at 30’ was used for measurements performed in the solid poly(methy1 methacrylate) (PMMA) matrix. Absorbances at the wavelength of excitation, 294 nm for naphthoate groups, were in the range of 0.05-0.15, corresponding to concentrations in the range of mol . L - I . Solvent baselines were subtracted from the observed spectra. Emission spectra were corrected for the instrument response.

Methanol (Scharlau HPLC grade), ethylene glycol, ethanol, isopentane and diethyl ether (Aldrich, spectrophotometric grade) were used as solvents. Samples of the compound studied in the rigid matrix of PMMA were prepared by thermal polymerization “in situ” of very dilute solutions of the compounds in distilled methyl methacrylate (MMA, Aldrich) in the absence of an initiator, as described previously’).

-

Experimental results

Absorption spectra

The absorption spectra of A # EN are similar to those reported for A # MN6), and they are also similar to the absorption spectra of an equimolecular mixture of NM and AM. The model compounds NM (or AM) exhibited absorption bands centered around 280 and 334 nm (or 344, 362, and 380 nm), and shoulders a t approximately 292 and 320 nm (or 314 and 332 nm) in methanol a t room temperature. Spectra were displaced slightly to the red in ethylene glycol. These characteristics allow separate excitation of naphthalene (294 nm) and anthracene (362 nm).

Fluorescence measurements

Fig. 2 depicts 3D plots of the emission spectra obtained for A4EN using different excitation wavelengths for A4EN in methanol a t 298 K. Upon excitation where naphthalene (294 nm) or anthracene (362 nm or higher) groups are selectively excited, emission is obtained only from naphthalene groups or anthracene groups, respectively, if comparable measurements are performed with an equimolar mixture of NM and AM6). However, as Fig. 2 shows, A4EN produces a combination of both the naphthalene and anthracene emission bands upon excitation of naphthalene groups at 294 nm. The same qualitative effect is seen with the other three A # EN. The relative intensities (or integrated areas) of these bands depend on the choice of molecule, through m, and

The Forster radius for energy transfer from naphthalene to anthracene t 353

Fig. 2. 3D plot of emission spectra obtained at different excitation wavelengths (in the range 270-420 nm) for A4EN in methanol at 298 K

270

I > c ffl C aJ c - -

300 LOO 500 600 700 A/nm

Fig. 3.

t

I

h c ffl C aJ C - -

300 LOO 500 600 700 A/nm

Fig. 4.

Fig. 3. Normalized emission spectra for A # EN (with # denoted for each curve) and for an equimolar mixture of AM and NM, upon excitation of naphthalene groups at 294 nm in methanol at 298 K

Fig. 4. Normalized emission spectra for A # EN (with # denoted for each curve) and for an equimolar mixture of AM and NM, upon excitation of naphthalene groups at 294 nm in ethylene glycol at 298 K


on the viscosity of the medium, q. There is no evidence for the formation of intramolecular exciplexes, as was also the case with the A # MN studied previously6).

Fig. 3 and 4 depict normalized emission spectra for the four A # EN at 298 K, and for a n equimolar mixture of AM and NM, with excitation at 294 nm in all cases. The spectra depicted in Fig. 3 were obtained in methanol, which has q of approximately 0.55 cP, and the spectra depicted in Fig. 4 were obtained in ethylene glycol, for which q is 16.32 cP. Both sets of spectra are normalized to the maximum intensity of the emission of the equimolecular mixture of AM and NM, which appears around 370 nm. The A # EN show a broad band to the red of the emission from the equimolar mixture of AM and NM. It is centered at 470-80 nm, and is attributed to the emission from anthracene in the A # EN. This band is similar to the one obtained for the 1 : 1 mixture of AM and NM when the excitation is shifted from 294 nm to 362 nm. The appearance of the new band is due to the electronic energy transfer process from naphthalene to anthracene. The intensity of this new band depends on rn and r ] .

Fig. 5 depicts the ratios of the integrated areas under the emission bands from anthracene and naphthalene, denoted I A / I N , as a function of l / q in methanol- ethylene glycol mixtures a t 298 K upon excitation at 294 nm. In general, the values of I A / I N increase as r] decreases. In order of increasing m, the values of I A / I N , as averages for severalmeasurements, are 8.5,21.3,21.7, and 12.4in methanol, and 3,3.8, 3.1 and 3.2 in ethylene glycol. The variation of ID/IM with rn at low r] exhibits a similar behavior in bichromophoric model compounds where N and A are substituted by 1-pyrenoate groups’). The bichromophoric compounds with m = 2 and 3 have the highest values of I,,/IM, which denotes the ratio of the intensities of the emission from the excimer band and monomer band. The dynamic contributions, which depend mainly of the type and length of the spacer, have a comparable influence on both the intramolecular energy transfer and excimer formation processes.

The efficiency of energy transfer x from naphthalene to anthracene was evaluated in the manner described by Guillet 9:

O I I I I I I

(l/q)/cP-’ 0 0.1 0.8 1.2 1.6 2.0

Fig. 5 . Intensity ratio IA/ZN VS. reciprocal viscosity l /q for A # EN in methanoVethy- lene glycol mixtures at 298 K upon excitation of naphthalene groups at 294 nm. The number of oxyethylene spacers is (m) 1, ( + ) 2, (A) 3, and (+) 4 (1 CP = Paas)


1.0

0.9 -

0.8 -

0.7

where qlN and are the quantum yields for fluoroescence from naphthalene and anthracene, respectively, obtained for the equimolecular mixture of AM and NM using excitation at 294 and 362 nm. The quantum yields for fluorescence were obtained for the equimolar mixtures of AM and NM using excitation at 294 nm and 362 nm, and the method described previously10). Tab. 1 shows qlN and qlA for the solvents used in our measurements at 298 K. Fig. 6 depicts the dependence of x on l/r] for A # EN when r] is manipulated by changing the ratio of methanol to ethylene glycol in the solvent at 298 K. At low r], no change in x is observed. In the high viscosity range (higher than approximately 2.5 cP) the values of x decrease as r] increases. At high viscosities,

f + + + + +

++ -

-

0.9 ’.O ?-----I

Fig. 6 . Dependence of x on l /q for AlEN, A2EN, A3EN, and A4EN, reading from top to bottom o( =

efficiency of energy transfer, 4 = dynamic viscosity)


Tab. 1. Quantum yield for fluorescence at 298 K, as deduced from an equimolecular mixture of AM and NM upon excitation at 294 and 362 nm, for different mixtures of methanol and ethylene glycol

Solvent a) @N @ A

1 .oo 0.90 0.80 0.67 0.50 0.33 0.20 0.10 0.00

0.280 0.293 0.315 0.333 0.383 0.424 0.400 0.422 0.417

0.062 0.063 0.068 0.076 0.083 0.098 0.103 0.118 0.130

4.50 4.59 4.64 4.63 4.49 4.21 3.89 3.59 3.24

a) Volume fraction of methanol.

x can be approximated as a linear function of 1 /q. Linear extrapolation to l/q 4 0 gives intercepts of 0.86 k 0.02,0.87 % 0.03,0.84 k 0.02 and 0.84 k 0.02 for m of 1 through 4, respectively. There is no convincing dependence of the extrapolated x on rn in the range covered.

In each solvent the values of x for A # EN are considerably higher than the ones reported previously for A # MN, where the spacers were constructed from methylene groups6). This result suggests a higher dynamic contribution to energy transfer in A # EN, arising because spacers of oxyethylene units are more flexible than spacers of methylene units. For the A # MN (where # = 3, 4, 5 and 6), x was in the range 0.36-0.82 at 293 K6). A particularly dramatic comparison is A3EN 01 = 0.96 t 0.04 at 293 K) with A5MN 01 = 0.48 k 0.12 at 298 K@), which differ only in the substitution of a single methylene group for an ether oxygen atom. The values for x cited here are an average of the ones obtained in two solvents, ethylene glycol and methanol, where a dynamic contribution is possible.

Spectra were also obtained for the A # EN in the rigid environments provided by PMMA at ambient temperature, and by two low temperature glass solvent mixtures at 77 K . These systems have the advantage of severely depressing any dynamic contribution to ZA/ZN, but they suffer from the disadvantage that measurement of the quantum yields for fluorescence is difficult. In absence of QN/QA, only the contribution of ZA/ZN to x can be evaluated. The values of ZA/ZN for the four A # EN in PMMA are 17.2, 18.8, 16.2, and 11.9, in the order of increasing m. These results show monotonic decreases of x as m increases from 2 to 4. The two lower temperature glass solvent mixtures, 2 : 1 (v/v) isopentane :ethanol (and 5 : 5 : 2 (v/v/v) diethyl ether:isopentane:ethanol), giive ZA/ZN of 8.5 (5.3), 1.4 (3.0), 1.5 (2.2), and 2.2 (1.1) for the A # EN, in the order of increasing m. All of these rigid systems find the highest value of ZA/IN at AlEN.


Theoretical analysis and discussion of the energy transfer process

The methods used in the theoretical calculation have been described previously for evaluation of intramolecular excimer f ~ r m a t i o n ~ ~ " - ' ~ ) or energy transfer 6, 8, 13, 14). In brief, the location of rotational isomers, relative energies and geometry for methyl esters of the naphthoic and anthranoic acids were obtained using Sybyl 6.0 (Tripos Associates, St. Louis). The ring systems were coplanar, with the torsion angles connect- ing the ring systems to the ester groups having vnaph = 0" and 180", and vanth = k60" and 280 +- 60". The rotatable single bonds in the flexible spacer (i. e., 0-CH, and CH,-CH,) were allowed to have three rotational isomers located at @ = 180" (t) and k60" (g') that determine the conformational energy of the chain. Each of these rotational isomers was split into three isoenergetic states by allowing displacements of A@ = 0, k20" from the perfectly staggered positions. These nine isomers are denoted by t - , t , t , , g - , g, g,, g:, g - , andg; for the rotational angles of 160, 180,220,280, 300, 320, 40, 60 and 80". All conformational energies'2s13) and geometrical parameters6,14) used in this work are collected in Tabs. 2 and 3. The calculations were performed either by discrete enumeration of all the allowed conformations (AlEN, A2EN, and A3EN) or by Monte Carlo simulation15) (AlEN, A2EN, A3EN and A4EN). A subset of lo7 conformations for AlEN, A2EN, and A3EN was enough to give good agreement between both approaches, and validates the Monte Carlo method used for A4EN.

In order to interpret the dependence on m of the energy transfer process, several terms were evaluated for all four A # EN: (a) the distribution function wR, the

Tab. 2. Energies for the first and second order interactions

Order Atoms Symbol Energy in kcal mol-'

First C*O-CH,CH, EU 0.42

First CH,CH,-OCH, Ea2 0.93 Second c*. . .o EW 0.3 Second CH2.. .O EWI 0.36

First OCH ,-CH,O E d -0.4

Second CH,. . .CH, EW, 00

Tab. 3. Bond lengths (in A ) and bond angles (in degrees)

Bond Length Pair of bonds Angle

1358 J. Bravo, E Mendicuti, E. Saiz, W. L. Mattice

probability function for the distance between the centers of mass of the naphthalene and anthracene ring systems; (b) the distribution of the function wR K’, with K’ being the orientational factor in Forster transfer 16); (c) the probability p R of finding two rings separated by a distance not larger than R , denoted by

(d) the product p R K’ as a function of # (or m), for different values of R , and (e) the efficiency of Forster transfer for several assumed values of R,, obtained according to

@ = C p [ 1 + K2R6/(2 /3)R$ (3)

wherep is the probability of each conformation, R is the distance between the centers of mass of the two chromophores, and Ro is an assumed value for the Forster radius. The results for AIEN have been reported previously‘), because this molecule is identical with A2MN.

The averages of K’ were 0.53, 0.68, 0.72 and 0.70 for # of 1 , 2, 3, and 4, respectively. Only for the first member of the series does K~ deviate strongly from the value of 2/3, which is the expectation for a completely random orientation of chromophores.

Fig. 7 depicts the distribution of the function wR K~ for the four A # EN. This representation is very similar to the one obtained for wR. As expected, as m increases, the maximum of the distribution function occurs at higher separation between the

Fig. 7. Distribution of the function w R ~ * as a function of the distance between centers of chromophores, R, for A # EN. Values of # are: (W 1, (A) 2, (*) 3, and ( + I 4

The Forster radius for energy transfer from naphthalene to anthracene . . .

8 1.1 -

1.0 -

0.9 -

0.8 -

0.7 -

0.6 -

0.5 -

0.1 -

0.3 -

0.2 -

0.1 -

0 ~~

1359

0

R 21.5/23.5 8. 19.5 a 17.5 a

15.5 a

13.5 a

11.5 a

N 1.1

4" 1.0

0.5

0.4

0.3

0.2

0.1

R 21.5/23.5 8. 19.5 a 17.5 a

15.5 a

\ \ 13.58.

\\ 11.5 a

1 2 3 4 5 0 1 2 3 4 5 m m

Fig. 8. Fig. 9.

Fig. 8. Dependence of p R on rn and R for R ranging from 3.5 to 23.5 A (s. text)

Fig. 9. Dependence of p R ic2 on rn and R for R ranging from 3.5 to 23.5 A (s. text)

RO 25 8. 23 a 21 a 19 a 17 8. 15 8.

13 8.

11 8,

Fig. 10. Calculated efficiency of Forster transfer energy @ for several assumptions about the value of the Wrster radius, RO


centers of mass of two chromophores. Figs. 8 and 9 depict p , andp, K* for R ranging from 3.5 to 23.5 A. A value of R higher than approximately 20 A is sufficient to make p , and p R ~ 2 equal to 1 for all four A # EN. However, the values of both functions exhibit a monotonic decrease as m increases if R is larger than approximately 1 1 A and smaller than 18 A. Fig. 10 depicts the calculated efficiency of Forster transfer, @, obtained according to Eq. (3) for several assumed values of R,. The monotonic decrease is seen whenever R , is larger than about 7 A. The dependence of @ on m decreases as the assumed value of R , rises above 7 A. The results are consistent with a value of R , of 16 f 2 A , as estimated previously6).

This research was supported by DGICYT through grant PB91-0166 and by NSF DMR 9220369.

J. B. Birks, “Photophysics of Aromatic Molecules’: Wiley, New York 1970 2, H. Braun, Th. Forster, Ber. Bunsenges. Phys. Chem. 70, 1091 (1966) 3, Th. Forster, Ann. Phys. 2, 55 (1948) 4, I. B. Berlman, “Energy nansfer Parameters ofAromatic Compounds’: Academic, New

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‘@ J. R. Lakowicz, “Principles of Fluorescence Spectroscopy”, Plenum Press, New York

York 1973

Cambridge 1985, p. 245

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Phys. 21, 1087 (1953)

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the förster radius for energy transfer from naphthalene to anthracene in polyesters with...

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