intramolecular formation of excimers in model compounds for polyesters obtained from 2,6-naphthalene...

Intramolecular Formation of Excimers in Model Compounds for Polyesters Obtained from 2,6=Naphthalene Dicarboxylic Acid and Cyclohexanediols

JULIO BRAVO,' FRANCISCO MENDICUTI,' and WAYNE 1. MATTICE3

'Departamento de lngenieria a Industrial, Universidad Carlos I l l , 2891 3 Legads, Madrid, Spain;

and 'Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909 Departamento de Quimica Fisica, Universidad de Alcald, Alcald de Henares, Madrid, Spain;

SYNOPSIS

The fluorescence in dilute solution has been measured as a function of solvent viscosity for four bichromophoric models for polyesters with naphthalene in the rigid aromatic unit and diols derived from cyclohexane as the flexible spacer. The spacers are 1,2-cis-cyclohexanediol, l,Z-truns-cyclohexanediol, a 1 : 2 mixture of 1,3- cis- and 1,3-truns-cyclohexanediols, and a 1 : 2 mixture of 1,4-cis- and 1,4-truns-cyclohexanediols. The shape of the emission spectra for the molecules in this series is less sensitive to the viscosity of the medium than was the case for an analogous series in which a methylene or oxyethylene spacer replaces the cyclohexanediol spacer. The dependence of the excimer emission on the type of spacer is different also in the series in which the rigid units contain naphthalene or benzene. When the rigid units contain naphthalene, excimer formation is maximal if the spacer contains 1,2-trans-cyclohexanediol, but this spacer produces a molecule with a very small tendency for excimer formation in its polymers with terephthalate. A conformational analysis correctly concludes that the spacer most conducive to excimer formation should be l,Z-trans-cyclohexanediol, but it does not identify the correct order of the remaining three bichromophoric model compounds. The problem may reside in the method for taking into account the finite width of the torsional well associated with each rotational isomer. 0 1994 John Wiley & Sons, Inc. Keywords: conformation excimer fluorescence hairpins polyester

INTRODUCTION

Many polymers are constructed from an alternating sequence of rigid units and flexible spacers. The re- peating unit in these polymers can be denoted by A-B,, where A contains the rigid unit, B is the fundamental flexible unit in the spacer, and m is an integer that denotes the number of copies of this fundamental flexible unit in the spacer between successive rigid units. Often the rigid unit contains an aromatic ring system, and the flexible spacer is acyclic. When the aromatic system is capable of forming an excimer, the dependence of the excimer

Journal of Polymer Science: Part B Polymer Physics, Vol. 32,1511-1519 (1994) 0 1994 John Wiley & Sons, Inc. CCC 0sS7-6266/94/081511-09

emission in dilute solution on changes in B and m is determined by the fluorescence lifetime, 7, of the chromophore in A, the viscosity of the solvent, 9, the conformation and dynamics of B,, and the ef- ficiency of intramolecular energy migration from an excited A to another A in the ground state. The dy- namic contribution from B, is suppressed as r/q +

0. The influence of energy migration is eliminated in the bichromophoric model compounds that are denoted by A-B,-A. For this reason bichromophoric model compounds in which A contains a naphthalene ~ n i t l - ~ have been used t o assist in the interpretation of the results obtained with the analogous polymer^.^.^

Most polymers in this class are constructed with acyclic spacers, but that is not always the case. In-

1511

1512 BRAVO, MENDICUTI, AND MATTICE

corporation of aliphatic rings into B has obvious consequences for the flexibility of the spacer, and for the ease with which a “hair-pin” turn can be achieved within the sequence A-B,-A. These hairpins can influence the ordering of nematic polymer^.^ Systems based on cyclohexane provide one of the simplest types of alicyclic rings that can be incor- porated into B. Here we extend our recent study of polyesters derived from terephthalate and spacers containing cyclohexane6 to the series where 2,6- naphthalene dicarboxylate replaces terephthalate. Excimer fluorescence is employed to study the ease with which the polymers can form hair-pins.

In order to avoid ambiguities in the interpretation due to the population of excimers by an intramolecular energy migration mechanism in the polymers, we have synthesized and studied bichromophoric model compounds in which the spacer contains a cyclohexane ring system. The structures of two of the model compounds are depicted in Figure 1, and the abbreviations that will be used for all of the model compounds are presented in Table I. The values of I D / I M , where ID and I M denote the inten- sities of the fluorescence from the excimer and monomer, respectively, are obtained from the steady-state fluorescence in dilute solution, in media of varying viscosity. The interpretation is assisted by a theoretical evaluation of the distribution of conformations accessible to the bichromophoric compounds.

Figure 1. Structures of (top) C2 in the axial - equatorial conformation and (bottom) T2 in the axial -axial conformation.

METHODS

The synthesis was performed in a manner similar to the prior preparation of other bichromophoric model compounds for polyester^.^^^^^ In brief, 2- naphthoyl groups were attached to the ends of the cyclohexanediols through the reaction of a slight excess of 2-naphthoyl chloride ( Aldrich) with the appropriate glycol in the presence of triethylamine in chloroform at room temperature. The glycols used, 1,2-cis-cyclohexanediol, 1,2-trans-cyclohexanediol, a 1 : 2 mixture of 1,3-cis- and 1,3-trans- cyclohexanediol, and a 1 : 2 mixture of 1,4-&- and 1,4-trans-cyclohexanediol, were obtained from Aldrich. A monochromophoric model compound, 2- cyclohexylnaphthoate (cyc N) was synthesized in the same way. The solutions were washed succes- sively with water, aqueous sodium bicarbonate, and excess water. The product was dissolved in chloroform and reprecipitated by the addition of methanol. C2 needed to be purified further by chromatography. Nuclear magnetic resonance (NMR) and infrared (IR) were used for the characterization of the sam- ples.

Steady-state fluorescence measurements were performed with right-angle geometry using a Perkin Elmer LS-5B fluorometer. Slit widths were 10 nm for excitation and 2.5 nm for emission. Polarizers were set for magic angle conditions. Typical absor- bances were about 0.1 at the wavelength of excitation, 292 nm. Solvent baselines were subtracted from each spectrum. The ethylene glycol ( Aldrich, spec- trophotometric grade) and methanol ( Schurlau, GPC grade) were used without further purification.

EXPERIMENT, RESULTS, AND DISCUSSION

The monochromophoric and bichromophoric model compounds exhibit similar absorption spectra in ethylene glycol a t 25”. Peaks are observed near 282 and 334 nm. There are prominent shoulders a t 272, 292, and 324 nm. The excitation spectra show peaks at 282 and 334 nm, but the shoulders observed in the absorption spectra are not resolved with the slit widths employed. The shapes of the excitation spectra do not depend on the wavelength used for emission.

Figures 2 and 3 depict normalized emission spectra for cycN and the four bichromophoric model compounds at 25°C. The spectra depicted in Figure 2 were obtained in methanol, which has a viscosity of approximately 0.55 cp at the temperature of the measurement, and the spectra depicted in Figure 3

EXCIMERS IN MODEL PE COMPOUNDS 1513

Table I. Model Compounds Prepared from 2-Naphthoyl Chloride and a Spacer

Abbreviation Diol Used for Spacer

Abbreviations for the Bichromophoric

c2 1,2-cis-cyclohexanediol T2 1,2- trans-cyclohexanediol CTT3

CTT4

cycN cyclohexanol (monochromorphoric

1 : 2 mixture of 1,3-ciS- and 1,3-

1 : 2 mixture of 1,4-ck- and 1,4- trans-cyclohexanediol

trans-cyclohexanediol

model compound)

were obtained in ethylene glycol, for which the viscosity is 16.32 cp. Both sets of spectra are normalized to the maximum intensity of the emission of cycN, which appears at 368-370 nm. The enhancement in the normalized intensity a t the red of the maximum in the bichromophoric model compounds is attributed to the intramolecular formation of an excimer, as was done p r e v i o u ~ l y . ~ ~ ~ ~ ~ ~ ~ The ratio I D / I M was evaluated as ' ~ ~ 9 ~ 9 '

\\ \ cTT3

h (nm)

Figure 2. Emission spectra in methanol at 25"C, normalized at 370 nm, for cycN and the four bichromophoric model compounds. Table I contains the abbreviations for compounds.

320 340 Mo 380 400 420 *(o 460 uy) 5oo

h (nm)

Figure 3. Emission spectra in ethylene glycol at 25", normalized at 370 nm, for cycN and the four bichromophoric model compounds.

where I A B d A , 4 W denotes the normalized intensity at 400 nm for the bichromophoric model compound (400 nm was selected as the wavelength for moni- toring the excimer, in analogy with previous work), 1,4,7*8 and IcycN,400 is the normalized intensity obtained with cycN a t 400 nm. I,,,,, denotes the intensity used for normalization a t the maximum of the emission band for cycN, which appears in the range 368-370 nm. The values of I D / I M depend on the structure of the glycol. In both solvents the largest values of I D / I M are obtained with T2 and CTT4, and the smallest values are obtained with C2 and CTT3.

The viscosity of the medium was changed iso- thermally by the use of mixtures of methanol and ethylene glycol, and it was changed a t constant composition by the use of ethylene glycol in the range 240°C. The dependence of I D / I M on the viscosity is depicted in Figure 4 for the isothermal series, with the data restricted to the region where q > 1.5 cp. In this range, I D / I M is nearly independent of q, which is in contrast to results obtained previously with others systems with acyclic spacers, 1,4,7,8

where I D / I M was found to depend on q in this range. Figure 5 shows that I D / I M is also nearly independent of q in the studies using ethylene glycol at various temperatures, where q is in the range 2.5-42 cp. The very weak dependence of I D / I M on q, at high q, is


A A A

I-Pr A - A A

Figure 4. and ethylene glycol at 25°C for (A) C2, (B) T2, ( 0 ) CTT3, and (+) CTT4.

Dependence of lD/lM on the reciprocal of the viscosity of mixtures of methanol

attributed to the strongly hindered internal dynamics of the bichromophoric model compounds, arising from the large naphthalene groups in A and the cyclohexane ring in B.

Extrapolation of the data in Figures 4 and 5 to 1 /T,J + 0 yields the limiting values that are collected in Table 11. The maximum occurs with T2, and the minimum is seen with CTT3.


4 -

l h (CP -1)

Figure 5. (A) C2, (D) T2, ( 0 ) CTT3, and (+) CTT4.

Dependence of ZD/IM on the reciprocal of the viscosity of ethylene glycol for

CONFORMATIONAL ANALYSIS

The methodology employed in the conformational analysis was similar to that used previously,6 using

Sybyl (Tripos Associates, Inc., St. Louis, MO) with the default force field.’ When applied to sixteen model compounds studied by Hirayama, lo it cleanly separates the nine compounds that exhibit excimer


Table 11. Values of ID/IM as 1/q + 0

ID/IM Mixed Ethylene

Sample Solvent Glycol Average

T2 0.47 f 0.05 0.41 2 0.05 0.44 k 0.05 c 2 0.36 f 0.05 0.32 f 0.03 0.34 k 0.04 CTT3 0.32 k 0.07 0.25 f 0.03 0.29 f 0.05 CTT4 0.45 f 0.06 0.38 f 0.05 0.42 _+ 0.06

emission from the seven compounds that do not." The molecules studied in the present work were the bichromophoric model compounds with exclusively cis or trans configurations of the attachments to the cyclohexane ring. The cyclohexane group was initially placed in the boat and chair configurations in separate calculations. The chromophores were studied with attachments in the axial and equatorial positions. For each conformer obtained by rotation at the Car-C' (Car denotes a carbon atom in an aromatic ring and C' denotes the carbonyl carbon atom) and 0 - C bonds, four situations were studied. These torsions were initially placed in the c and tstates (forC"'-C') andinthet,g+,andg-states (for 0 - C) . Each conformation was then subjected to an energy minimization, during which bond lengths, bond angles, and torsion angles were vari- able. For each cis or trans configuration of the at- tachment to the cyclohexane ring, Z 2 X 32 conformations were studied. Here Z 2 counts the conformations at the two Car-C' bonds, and 32 counts the conformations at the two C - 0 bonds. If more than one initial conformation produced similar torsional angles (and hence a similar conformation) after minimization of the conformational energy, only one member of the redundant set was retained for subsequent analysis. The computations employed Syby15.5 from Tripos Associates, as was done in the previous study in which A contained a benzene ring instead of a naphthalene ring.6

As in our previous work, the finite width of the minimum in the torsion potential for the C - 0 bonds was taken into account by assigning three discrete torsion angles to each of the three rotational isomers. These torsion angles were located at the optimal position, and at displacements of kAC#J from that position. No distinction was made in the energy of the conformations that correspond to the three torsion angles. Hence the finite width of the minimum in the torsional potential is approximated using a square well, which in turn is approximated with three discrete torsion angles spaced by A4.

A conformational partition function, 2, was as- signed to each unit as

Z = 32 2 exp(-E/RT) ( 2 )

where the summation extends over all the indistin- guishable rotational isomers obtained upon minimization of the conformational energy. The factor of 32 is included because the assessment of the population of the conformations conducive to the formation an excimer evaluates

si is an integer in the range 0 I si I 9, which counts the number of conformers that satisfy the criteria for the formation of an excimer, and that are produced by use of AC#J at each of the two C - 0 bonds.

Three geometric properties were evaluated for each conformation: d,, which denotes the shortest distance between the center of mass of a six-membered ring in one naphthalene unit and the mean plane of the other naphthalene unit, dry, which denotes the lateral offset of the six-membered rings that define d,, and \k, which denotes the angle between the normals to the mean planes of the naphthalene units. The tolerances used for the complexes are the same as those used previously,6 namely 3.35 A < dz < 3.9 A, 0 I dry < 1.35 A, and 0 I < 40".

The computed probabilities depend strongly on the type of aromatic ring system present. In the present series where A contains a naphthalene unit, the maximum probability is computed for T2, but in a previous series where A contained a single six- membered ring, the computed probability for T2 was very small.6 The distribution of conformations for these two versions of T2, the one containing naphthalene, the other benzene, is depicted in Figure 6. A much larger range of conformations is accessible in the bichromophoric molecule with the smaller ring system. The conformations of the bichromophoric molecule containing naphthalene are found in a restricted region of conformational space. Further- more, this region is close to the region that contains the excimer (\k < 40" and a separation of the centers of the ring systems of about 3.5 A ) . The comparison of the simulations for T2 containing naphthalene and benzene in A is in harmony with the experi- mental results, because it predicts the molecule containing naphthalene can more easily form an excimer.

’8

(b)

Figure 6. systems in T2, using A 4 = 20°, when A contains (a) naphthalene or (b) benzene.

Probability profile for \k and the distance between the centers of the ring


Figure 7. Conformation of T 2 with the cyclohexane ring in a chair conformation, torsion angles (C"'-C', 0 -C, C-0 , C'- C a r ) of 179.3", -99.9", -79.8", and 179.6", and d, = 3.86 A, d,, = 0.76 A, and 9 = 20".

When A contains naphthalene, probabilities of zero are obtained for the bichromophoric molecules in which the spacer is based on 1,3-trans-cyclohexanediol or 1,4-trans-cyclohexanediol. The four other molecules have probabilities larger than zero. The conformation that makes the largest contribution to the probability for each of these four molecules is depicted in Figures 7-10.

The probabilities are collected in Table 111, for the same stereochemical composition as in the bichromophoric model compounds used in the exper- iments. Most of the probability comes from complexes of a six-membered ring in one naphthalene with a six-membered ring in the other naphthalene. There is a minor contribution from the complete

Figure 9. Conformation of the bichromophoric model compound with 1,3-cis-cyclohexanediol in the spacer. The cyclohexane ring is in a chair conformation, torsion angles

-88.8", and 7.9", and d, = 3.36 A, d *,,, = 1.16 A, and 9 = 21.3".

(Car-C', 0 -C, C - 0 , C'-CC"') of -7.9", 68.8",

overlap of the two naphthalenes in T2 and C2. Mol- ecules with chair conformations for the cyclohexane rings contribute most of the probability for the bichromophoric model compounds with 1 3 and 1,3- cyclohexane in their spacers, but all of the very small probability for the bichromophoric model compound with the 1,4-spacer comes from conformations with boat conformations for the cyclohexane ring.

The probabilities in Table I11 correctly identify T2 as being the bichromophoric compound with the highest probability for the intramolecular formation of an excimer, but the agreement between the order of the remaining compounds in the calculations and in the experiment is disappointing. There is a sug- gestion in the Table that the problem might lie in part in an inability of a single A$ to serve as a useful approximation for the finite width of the range of 4 for each rotational isomer. The computed probability for C2, for example, changes by well over an order of magnitude when A$ is changed from 10 to 20". In the real system, of course, the range of 6 accessible

Figure 10. Conformation of the bichromophoric model compound with 1,4-cis-cyclohexanediol in the spacer. The cyclohexane ring is in a boat conformation, torsion angles (Car-C', 0 - C , C-0 , C'- C a r ) of -148.5", -178.4", 154.8", and 157.6", and d, = 3.62 A, d,, = 0.57 8, and 9 = 25.7".

Figure 8. Conformation of C2 with the cyclohexane ring in a chair conformation, torsion angles (C"'-C', 0 -C, C-0 , C'- Car ) of 0.7", -83.3", -41.8", and 1.6", and d, = 3.47 A, dx.\ = 0.58 A, and 9 = 37.5'.


Table 111. Values of p Using Two Different Estimates for A+

Spacer Stereochemistry p (A+ = loo) p (A+ = 20')

1,2-Cyclohexanediol trans 0.128 0.401

1,3-Cycloheanediol cistrans 1 : 2 0.074 0.074 1,4-Cyclohexanediol cis:trans 1 : 2 0.006 0.004

1,2-Cyclohexanediol C i S 0.052 0.002

to each rotational isomer is not described by a square well, but instead requires a continuously variably function for the torsion potential energy function.

This research was supported by National Science Foun- dation grant DMR 92-20369, DGICYT PB91-0166 and a fellowship from the Comunidad de Madrid.

REFERENCES AND NOTES

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J. Gallego, F. Mendicuti, E. Saiz, and W. L. Mattice, Polymer, 34,2475 (1993). J. M. F. Gunn and M. Warner, Phys. Rev. Lett., 58, 393 ( 1987). F. Mendicuti and W. L. Mattice, Polymer, 33, 4180 (1992). F. Mendicuti, B. Patel, and W. L. Mattice, Polymer, 31,1877 (1990). F. Mendicuti, E. Saiz, and W. L. Mattice, J. Polym. Sci: Part B: Polym. Phys., 31, 213 (1993). M. Clark, R. D. Cramer 111, and N. Van Opdenbosh, J. Comput. Chem., 10,892 (1989). F. Hirayama, J. Chem. Phys., 42,3163 (1963). F. Mendicuti and W. L. Mattice, Comput. Polym. Sci. (in press).

Received October 5, 1993 Revised December 7, 1993 Accepted December 17, 1993

intramolecular formation of excimers in model compounds for polyesters obtained from 2,6-naphthalene...

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