685
J. Phys. Chem. 1994,98, 685-691
Energetics of Organic Solid-state Reactions: Electronic Structure and Photoreaction Mechanism of the 2,5-Distyrylpyrazine Oligomer Molecular Crystal N. M. Peachey and C . J. Eckhardt’ Department of Chemistry. University of Nebraska, Lincoln, Nebraska 68588-0304 Received: July 27, 1993; In Final Form: October 19, 1993’
An investigation of the 2,5-distyrylpyrazine (DSP) oligomer crystal electronic structure is presented in order to more completely characterize the oligomer photoreaction leading to the formation of the crystal of the macromolecule. The electronic transitions observed in the oligomer crystal include an A* n transition at 30 250 cm-* and a a* a transition at 34 000 cm-l which are both due to the styrylpyrazine moiety in the oligomer. Higher energy transitions include those from the pyrazine groups in the oligomer backbone and the phenyl side groups. In solution these transitions occur at lower energies, indicating a change in molecular conformation of the oligomer from that in the crystal. The extensive excitonic interaction of the monomer crystal is absent in the oligomer, causing it to behave as an oriented gas. While the ?r* n transition that was crucial to understanding the monomer reactivity is still active in the oligomer crystal, its role in the photophysics of the oligomer crystal is changed. In the oligomer crystal, the A* n transition no longer serves as an exciton-phonon trap for the A* A exciton but rather leads directly to the formation of product.
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I. Introduction Obtaining single-crystal polymers remains an important goal of synthetic solid-state chemistry. That such are desirable is evident when one considers the advantages of combining the inherent characteristics of polymers with the collective properties of crystallinesolids. Crystallographicallyordered macromolecules are expected to display novel mechanical, electronic,and nonlinear optical properties and thus hold promise for future technological developments. Unfortunately, the difficulty of obtaining desirable single-crystal polymers has tempered the development of such applications. An important route for obtaining crystalline polymers available to the solid-state chemist is to crystallize the monomer and polymerize it in situ by such methods as heat or electromagnetic radiation. This poses a considerable challenge since predicting apriori the crystal lattice into which a monomer will pack is beyond the present capabilities of computational science. It is thus impossible to predict or calculate with any certitude whether a given monomer will pack into a geometry which allows reaction and, furthermore, whether such a reaction would result in a single-crystal polymer. While it is not yet possible to “engineer”specific, single-crystal macromolecules, further investigation of those examples which do result in ordered polymers can provide clues to the future development of this field. Previous studies have delineated the geometric requirements that determine the possibility of reaction in organic crystals.1.2 While it is clear that solid-state reactions arecontrolled by more than geometricconsiderations,the reactive moieties in the crystal must assume an arrangement that allows the possibility of product formation. Furthermore, in order to synthesize single-crystal polymers, the reactants and products must remain a homogeneous solid solution throughout the bulk ofthecrystalduring thecourseofthereaction. Any heterogeneous separation of the reactant and product phases usually results in thedisintegration of thecrystal. Certain symmetry requirements must also be met in order for homogeneity to occur.3 To obtain single-crystal polymers, the reaction process must result in retention of the structural symmetry elements present in the monomer crystal. There must also be minimal variations of the lattice parameters during polymerization;otherwise internal strain forces phase separation. ~~
~~~
Abstract publiahcd in Advance ACT Absrracrs, December 15, 1993.
0022-3654/94/2098-0685S04.50/0
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On occasion, solid-state reactions which were initially not thought to result in single-crystal products were found to do so upon closer e~amination.~.~ Such variations in the reaction pathway are achieved by careful control of reaction parameters. This has been the case for the oligomerization of DSP. This classical four-centered polymerization reaction was first studied by Hasegawa and co-workers.5 Here, two neighboring carboncarbon double bonds undergo a 2+2 cycloaddition to form a cyclobutane ring. It is a two-stage reaction in which the monomer was reported to form an oligomer of three to five monomer units upon irradiation with 400480-nm light. Additional irradiation at wavelengths shorter than 400 nm yields a high polymer (Figure 1). Alternatively, the high polymer will form directly if the monomer is irradiated with light of shorter wavelength than 400 nm throughout the reaction. Initially, this reaction was reported to be heterogeneous, proceeding from the surface of the crystal.6 Later investigations by Wegner and co-workers revealed that, by careful selection of the wavelength of the incident illumination, the crystal could be irradiated in a uniform manner throughout the bulk.4 Under these conditions, they showed that macroscopic single crystals of the oligomer could be obtained. Research in our laboratory has shown that extreme care must be taken to remove all impurities if better quality single-crystal oligomers are to be obtained.’ By investigation of the evolving lattice vibrational modes, and under the proper experimental conditions, the oligomerization reaction has been shown to remain homogeneous on the scale of the wavelength of light.* The unusual reactivity of the DSP monomer has been explained by a biexcitonicmechani~m.~ Thelargesplittingof thedelocalized ?r* A monomer crystal exciton lowers the energy of one of its branches to such an extent that it overlaps the lower energy n exciton couples a* n transition. Furthermore, the u* with the lattice phonons to form a local dynamic distortion. The reactivity of DSP to form oligomers results from this distortion that serves as a trap for the delocalized a* a exciton causing the reaction.7 In this manner, both excitations are excited simultaneously and cooperate to make the DSP crystal a highly reactive crystal. The local trapping of the delocalized exciton is expected to ensure that the reaction remains short-range and thus rationalizes the formation of short oligomers. Although the DSP monomer crystal and the concomitant reaction have been studied extensively, detailed characterization of the oligomer crystal photoreaction has not been pursued. The
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686 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994
Peachey and Eckhardt
TABLE 1: Experimental Lattice Parameters for DSP Monomer, Oligomer, and Polymer Crystplsz1 DSP a (A) b (4 c (4 monomer 20.638 9.599 7.655 oligomer 20.34 10.43 7.31 10.89 7.45 18.40 polymer such restricted motion does not exist in solution and the molecule is expected to take the conformation of lowest internal energy. This report presents the results of a polarized, single-crystal, low-temperature reflection study of the DSP oligomer. The electronic transitions present in this crystal are investigated and are used to formulate a photopolymerization reaction mechanism. By analyzing the factor group (Davydov) splitting observed in the spectra and comparing these results tocalculations, the nature of the excitonic interactions is established. Since excitons may have implications for any p r o p e d reaction mechanism, the results of the exciton analysisare related to the photophysics. Thesinglecrystal spectra are compared to the spectrumobtained in solution to investigate thediffering conformations assumed by the oligomer in each phase. Section I1 outlines the experimental methods and procedures used in this investigation. This is followed in section I11by an analysisofthesolution spectrumand thelow-temperature single-crystal spectra. This section concludes with a presentation of the results of the excitonic analysis and the assignment of the polarization of the transitions. The discussion in section IV begins with an investigation of the differing conformations assumed by the oligomer. Following this, the nature of the electronic excitations in the oligomer is compared with those observed in the monomer crystal. Finally, a discussion of the mechanism of the polymerization reaction asit relates to theelectronic transitions is presented. Figure 1. (a) DSP monomer molecule. (b) Repeat unit for the DSP oligomer and polymer. (c) Two-stage DSP reaction in the crystal.
nature of the electronic excited states of the oligomer, albeit important to the understanding of the polymerization reaction, remains to be assigned. Clarifying the nature of the electronic transitions should provide additional insight into the wavelength dependence of this reaction mechanism. Such an investigation would shed further light on both the oligomerization and polymerization mechanisms. The DSP monomer, which is a highly conjugated system, assumes a nearly planar conformation in thecrystal. In theoligomer, thisextended conjugation is broken by the formation of the cyclobutane rings. Such a modification should radically affect the r* r transition and resulting exciton formation. Loss of thedelocalized exciton would necessarily alter the reaction mechanism for polymerization from that for oligomerization. A significantly reduced intensity would lead to diminished coupling such that the oriented gas model can be employed. Indeed, the most extensive conjugation remaining is that of the unreacted styrylpyrazine terminal groups of the oligomer. Thus, analysis of the 2-styrylpyrazine spectrum is expected to provide additional insights into the oligomer electronic properties. The breaking of the extended conjugation in the oligomer molecule would normally lead to a greater degree of rotational freedom about the macromolecular backbone. The solid state, however, does not allow such movement. Rather, the oligomer is forced to reside in a metastable configuration within a modified lattice which originally held monomers. The photoreaction results inconsiderablestrain within thelattice as thebondsof theoligomer are formed. Indeed, it is the evolution of such strain fields which produces a chemical pressure described by Luty and Fouret9 as driving the reaction.8 Comparison of the oligomer crystal spectra to that of the oligomer in solution may yield information about the conformation of the free molecule. Although the oligomer is constrained to a particular geometric conformation in the crystal,
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11. Experimental Section
DSP was synthesizedand recrystallized from xylene as reported by H a ~ e g a w a .The ~ product was further purified by column chromatography as described elsewhere.' DSP monomer crystals were obtained by slow evaporation from tetrahydrofuran (THF) or acetone solutions. Crystals used for the investigation were typically 2 X 2 X 1.5 mm. The DSP monomer crystal packs in the orthorhombic Pbca space group and has four molecules in the unit cell.'O The oligomer and polymer crystals are isomorphous with the monomer crystal.$ The unit cell dimensions of the monomer, oligomer, and polymer are given in Table 1. Monomer DSP crystals were irradiated under controlled conditions in order to obtain high-quality oligomer crystals in which virtually all monomers are converted to oligomers. Light from a 1000-W quartz halogen source was rendered monochromatic using a JarrelAsh 82-410 monochromator. The oligomerization was carried out on a rotating microscopestage which was turned continuously about an axis perpendicular to the a crystallographic axis of the crystal. This was done to offset inhomogeneities in the light source and polarization effects due to the monochromator and crystal optics. The monomer crystals were oligomerized using light of 4 8 8 4 5 0 nm with the shorter wavelengths used at higher conversions. The extent of oligomerization was verified by observing changes in the color of the crystal and in the frequencies of the Raman vibrational spectrum. Yellow monomer crystals, which display an ethylene vibrational mode at 865 cm-*, become clear upon oligomerization and the ethylene vibrational mode disappears. The Raman molecular-vibrational spectra of both the monomer and the oligomer are displayed in Figure 2. The procedure for the synthesis of 2-styrylpyrazine was a modification of that used to synthesize DSP4 where 2-methylpyrazine was substituted for 2,5-dimethylpyrazine and the quantities of benzoic anhydride and benzaldehyde were reduced by half. The product was dissolved in 20% ethyl acetate/hexane and then washed with a 5% NaOH solution to remove the benzoic
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 687
2,S-Distyrylpyrazine Oligomer Molecular Crystal
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ea
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4m
OM
lo00
1200
1400
1800
ENERGY (em-;)
(b)
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F i p c 2. Raman spectra of the DSP molecular vibrational modes in the
Figure 3. Solution spectra of (a) DSP oligomer in THF and (b) 2-styrylpyrazinein acetonitrile.
side product. The 2-styrylpyrazine product was then extracted by column chromatography and verified by proton and 13CNMR. The solution spectra of both the DSP oligomer and 2-styrylpyrazine were obtained using a Cary 210 spectrophotometer. The oligomer crystals used in the low-temperature single-crystal reflection experiments were subsequently used in forming solutions for spectroscopy. This wasdone to avoid introduction of anomalies due to slight differencesin the extent of oligomerizationor crystal composition. The oligomer and 2-styrylpyrazinesolution spectra were obtained at ambient temperature and are shown in Figure 3. The low-temperature, polarized, single-crystal spectra of the DSP oligomer crystals were obtained at 8 K from the (100) and (010) faces of the crystal by specular reflection. Spectra were obtained from 20 000-44 OOO cm-1 for light polarized along each of the crystallographic axes. A single-channel microspectroreflectometer utilizing a THR 1500 Jobin Yvon monochromator anda 1OOO-WXearclamp(Orie1) wereusedtoobtain thespectra. A helium refrigerator (CTI-Cryogenics Model 2 1) cooled the crystal to 8 K. Thecrystal reflection data for each crystallographic axis were correlated to absolute reflectivities and then transformed to absorption spectra using a fast Fourier transform.11 Both the reflection and absorption spectra for the region of interest are shown in Figure 4. As an aid to interpret the spectra, the crystal packing of the oligomer is shown in Figure 5. Since the X-ray structure of the oligomer crystal has not yet been established, the crystal packing shown in Figure 5 has been constructed from the coordinates obtained from a crystal in which about 14% of the monomers had been converted to oligomers.12
HI. Results A. Free Molecule Spectraand INDO/SCaIculatioas. A curvefitting routine was used to deconvolute the bands in the solution spectrum of the DSP oligomer. Vibrons were found at 27 850, 29 450, 31 150,and 35 300 cm-1. To compliment the analysis of the observed electronic structure, an INDO/S calculation was performed following the approach of Ridley and Zerner.I3 This is an LCAO-MO-SCF-CI calculational model designed to accommodate u* n transitions such as those encountered in nitrogen heterocycles. Due to the loss of conjugation resulting from the formation of the cyclobutane moiety in the oligomerization, a molecules-in-molecules approach was used. The observed electronictransitions are expected to be essentially those of 2-styrylpyrazine, 2,5-dimethylpyrazine, and toluene. The results of these calculations performed for 2-styrylpyrazine, 2 3 dimethylpyrazine, and toluene are listed in Tables 2 4 , respectively. The lowest energy transition that appears at 27 850 cm-I as a leading shoulder in the oligomer spectrum is characteristic of an u* n transition. Such low intensity u* n transitions are typical of nitrogen-containing heterocycles.14 This is supported by the comparison of the spectra taken in polar and nonpolar solvents (Figure 6). The characteristic blue shift of the leading edge of the spectrum is observed upon dissolution in a polar solvent. This also agrees with the INDO/S calculationfor 2-styrylpyridine that indicates the u* n transition should be the lowest energy transition. Both the u* n transition of 2,s-dimethylpyrazine and the r* + u transition of 2-styrylpyrazine should occur at approximately 30 000cm-1. The 2-styrylpyrazine transition is centered around 31 400 cm-1 while the 2,5-dimethylpyrazine, r* n transition is at 30 912 cm-l.I5 In the DSP oligomer spectrum,
(a) monomer crystal and (b) oligomer crystal.
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688 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 .11
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'.w O7I
c-axis
ENERGY (em.',
Bf :
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% :
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r
I b-axis
0
ENERGY (em") Figure 4. (a, Top) Reflection spectra and (b, bottom) absorption spectra of the DSP oligomer at 8 K obtained by transform of reflection spectra. theintensedoublepeakataround30 0 0 0 ~ m -(29 ~ 450and31 150 cm-1 peaks) results from a combination of these two transitions. The peak at 29 450 cm-1 is the low-intensity a* n transition of the isolated pyrazine moiety in the oligomer backbone being raised by the much more intense ?r* a transition of the styrylpyrazineterminal groups. Several observations support this. First, the leading peak of this doublet does display a blue shift in a polar solvent (see Figure 6). Second, since the a* n transition in 2,s-dimethylpyrazineis at slightly lower energy than the 2-styrylpyrazine a* -a transition, it is reasonable to expect the DSP oligomer spectrum to be consistent with this. The highest energy transition observed in the oligomer solution spectrum appears to be another combination of two peaks. In this case, the second 2-styrylpyrazine a* a transition (Figure 3) is approximately at the same energy that the INDO/S calculation predicts for the first a* a transition in 2,sdimethylpyrazine. The relative intensity of this peak in the DSP oligomer spectrum is greater than the corresponding band in 2-styrylpyrazine,indicating that it is indeed a combination of the second a* a styrylpyrazine transition and the first ?r* ?r dimethylpyrazine transition. B. Single-Crystal Spectra. Solution spectra do not provide information about transition polarizations, and thus assignments cannot be certain. This difficulty can be overcome by solid-state studies since molecules are confined to known, fixed orientations. Consequently, much more detailed information can be expected from the low-temperature, polarized, single-crystal absorption spectra. Comparison of the solution and crystal spectra (Figures 3 and 4, respectively) shows the latter has much richer structure. This results from both the selectivity of the polarized measurements and the increased resolution afforded by reduced temperature. The four transitions observed in the solution spectrum are clearly evident in the crystal spectra, albeit displaying energy shifts characteristic of spectra obtained at lower temperatures. No apparent factor group splitting is observed in these spectra,
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Figure 5. Proposed crystal structure of the DSP oligomer following the crystal structure of a partially oligomerized crysta1:ll Projection of (a) the (010) face and (b) the (100) face. The molecules shown by dashed lines are translated into the plane by half a unit cell.
TABLE 2 INDO/S Calculational Results for 2-Styrylpyrazine. Axis System Shown Below energy (cm-1) PX YY PI Planar Molecule 29 914 33 143 3 1 984 38 326
0.0 6.9394 1.1861 4.8923
29 918 33 299 38 128 38 503
4,1564 6.1172 0.9410 5.0238
0.0 0.2097 0.2987 2.6161
osc. str.
1.0625 0.0 0.0 0.0
0.01 59 0.751 1 0.0267 0.5546
1.0618 0.0561 0.1136 0.0767
0.0162 0.7195 0.0166 0.5840
Molecule Twisted 1So 4.0155 0.1497 0.1619 2.6483
Molecule Twisted an Additional 15' 30 007 33 512 38 165 38 661
0.1666 6.6498 0.7420 -5.0501
0.0325 0.0632 0.0996 -2.6218
1.0489 4.3802 0.1526 0.2865
TABLE 3 INDO/S Calculational Results for 23-Mmethylpyrazine. Axis System Shown Below energy (cm-I) PX PY Pz 30 161 36 719 48 912 51 671
0.0 2.5810 -2.6804 2.4541
0.0 2.1747 1.4256 -2.8579
1.0983 0.0 0.0 0.0
0.0159 0.7012 0.0105 0.5900
0%
str.
0.0171 0.1966 0.2120 0.3447
so they can be analyzed satisfactorily by utilizing the oriented
gas model. Nevertheless, to test this assertion and, more importantly, to compare the exciton interactions of the monomer and oligomer crystals, exciton calculations were performed for this crystal. These calculations were carried out by consideration of only single-exciton states. Furthermore, since most molecular crystals exhibit a large energy gap between the ground and excited states, the theory assumes no mixing of these states. These calculations are described in greater detail elsewhere.7J6 The
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 689
2,5-Distyrylpyrazine Oligomer Molecular Crystal
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ENERGY (cm.7 Figure 6. DSP oligomer solution spectrum in (-) benzene and (- - -) THF.
TABLE 4 INDO/S Calculational Results for Toluene. Axis System Shown Below energy (cm-I) PX MY pz osc. str. 40 373 51 079 55 303 55 044
-0.0154 2.2783 -0.0769 6.1220
0.6022 0.2348 -6.1065 -0.2556
0.0 0.0 0.0 0.0
0.0069 0.1260 0.9697 0.9893
results of the calculations which include only the two lowest energy transitions are presented in Table 5. The Franck-Condon factors ([) included in the exciton calculations were obtained from the crystal spectra since the oligomer may assume a different conformation in solution than in the solid state. Since the factor group splittings calculated for the observed spectra were all less than 85 cm-1, the use of the oriented gas model in analyzing the spectra is reasonable, considering the band-passes used for the photochemical reactions. The lowest energy A* n transition is most prominent in the a crystallographic axis (100) spectrum. This transition is less intense in the b-axis (010) spectrum and completely absent in the c-axis (001) spectrum. The?r*-n transitionoriginatesat 30 250 cm-1 and includes several vibrational progressions which are not resolved in the solution spectrum, the most prominent of which is the totally symmetric pyrazine ring vibration at 600 f 25 cm-I. The oligomer Raman vibrational spectrum shows that this molecular mode has a frequency of 617 cm-1. This corresponds well with the vibrational structure of the A* n transition observed in the spectrum of pyrazine. In that spectrum, the A* n transition is strongly coupled to the vibrational modes, the most conspicuous being the totally symmetric v6a(ag) 597 cm-1 ~ i b r a t i o n . ' ~AJ ~second vibronic progression in the oligomer spectrum which corresponds to this transition in pyrazine is the
out-of-plane, hydrogen-bending vibration.'* In pyrazine, this is the v ~ ( b 2 ~vibration ) at 383 cm-1, while in the oligomer the vibrational frequency is 350 f 50 cm-1. At least one other vibrational mode is associated with this transition. A low-frequency vibron at 225 f 25 cm-1 is present in the oligomer spectrum. This vibron is absent in the pyrazine spectrum, indicating that it is unique to the oligomer structure. Since the 2-styrylpyrazine vibrational spectrum was not available, it was not possible to see whether it displayed this vibron. It does appear in the Raman spectrum of the oligomer at 206 cm-1 at a frequency which corresponds to that of a low-frequencycarboncarbon torsion or bending mode.19 The dichroic (polarization) ratio indicates that the transition moment vector for this transition is polarized out of the plane of the pyrazine ring. Since complete structural information on the oligomer crystal is not available, the geometry of the partially oligomerized crystal is utilized in the analysis of the transition moment polarizations.12 Using the oriented gas model, the calculated dichroic ratiofo.axis/ for the partially oligomerized crystal of a vector polarized perpendicular to the pyrazine ring is 11.4 while the c-axis spectrum should show no intensity. The experimental results do indeed show that this transition has no c-axis intensity. However, the dichroic ratio fa-axii/fb-axis was measured to be 2.0. This corresponds to a transition moment polarized 19O off the normal to the pyrazine ring, assuming the partially oligomerized crystal. The second, more intense, transition is most pronounced in the c-axis spectrum where it is centered at 34 000 cm-1. It also contains a vibrational progression, although it is much more congested than that in thelowest energy transition. This transition is absent in the a-axis spectrum and is merely a shoulder on the lowest energy transition in the 6-axis spectrum. This is the lowest energy A* A transition of the styrylpyrazine terminal group that is polarized in the pyrazine plane. Since the oligomer backbone forms along the c crystallographic axis (Figure 5), any transition in the plane of the pyrazine ring will have little or no intensity projected upon the a-axis. The crystal spectra of this transition are consistent with this description (Figure 4). The INDO/S calculations of the 2-styrylpyrazine indicate that its lowest energy A* A transition is polarized essentially along the long axis of the molecule (Figure 7a). The experimental dichroic ratio fc-axis/h.axis for this transition is 7.3. This corresponds to either a vector lying along the long axis of the styrylpyrazine moiety or a vector nearly perpendicular to the nitrogen-nitrogen vector in the pyrazine plane (Figure 7a and 7b, respectively). Since the a-axis intensity of this transition is essentially zero, little insight is gained from other dichroic ratios. The INDO/S calculations agree that the polarization of this transition is along the long axis of the molecule (Figure 7a), and thus, this transition is assigned such a polarization. The only vibronic progression observed associated with this transition is the pyrazine ring
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TABLE 5: Calculated Values of the Exciton Energies of the K na Transition and the A* d Transition of the DSP Oligomer Crystal transition calculated values (cm-I) (100)face calculated values (cm-I) vibron type b-axis c-axis splitting a-axis 6-axis splitting 0 1 2 3 4 5 6 0 1 2 3
r* A*
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n n n
+
r* r*+n r* n +
+
r*+n u*
n
r*+u **-u **+u **-if
34 094 35 122 36 125 37 121
34 012 35 072 36 079 37 075
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30 250 30 853 31 453
30 226 30 822 31 423
24 31 30
32 052
32 024
28
32 650 33 248 33 839
32 626 33 228 33 829
24 20 10
82 50 46 46
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a Free molecule transition energies for the r* n transitions were 30 228 + n600 cm-' for n = 0-6, E 0.361,0.411, 0.402,0.375,0.348, 0.335, and p 0.329A. b Free molecule transition energiesfor the r* u transitions were 34 219 + nlOOO cm-1 for n &3, = 0.634,0.485,0.471,0.375, and p 0.504A. +
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should be polarized directly along the normal to the pyrazine plane if the system is planar. That the polarization in the crystal diverges from the normal by 19' suggests that this end of the molecule may not be planar. The 16' angle between the pyrazine and end phenyl rings in the partially oligomerized crystal12 is an increase of 4 O from that observed in the monomer crysta1,lOSince latticevibrational spectroscopy has shown that most of the changes in the lattice occur at approximately the midpoint of the reaction: it is conceivable that in the completely oligomerized crystal, the end styrylpyrazine moiety is twisted even further. Calculating the electronic transitions for a twisted conformation indicates that such perturbations to the conjugation would result in the A* n transition polarization diverging from the pyrazine plane normal with all the transition shifting to higher energy (see Table 2). Alternately, if the A* +n transition is assumed to be normal to the pyrazine ring plane, the entire styrylpyrazine terminal group must be rotated 19' from its orientation in the partially reacted crystal. The positive assignment of the oligomer structure in the crystal will need to await definite confirmation by X-ray crystallography. A comparison of the oscillator strengths of the transitions of 2-styrylpyrazine and the DSP oligomer in solution should also provide information about the average oligomer length. Caution must be exercised since the oligomer spectrum in solution includes more than purestyrylpyrazine transitionsin the region from 25 000 to 33 000 cm-l as discussed above. Such errors could lead to higher apparent oscillator strengths for the styrylpyrazine transitions in the oligomer which would result in an estimation of oligomer lengths which is too short. Nevertheless, a comparison of these transitions suggests that the average oligomer length is approximately three monomer units. This corresponds well with the average oligomer length of 3.2 monomer units which Wegner found.22 Of greater significance to the photochemistry is the exact nature of the electronic transitions in the oligomer crystal. Further insight is gained by comparing the excited state of the monomer crystal with those of the oligomer. The high reactivity of crystalline DSP has been attributed to a dynamic lattice distortion caused by a local exciton-phonon interaction from the A* n transition trapping the delocalized exciton from the A* A transition.' Although the A* A transition remains an important feature in the electronic excitations of the unreacted styrylpyrazine terminal groups of the oligomer, its excitation energy is higher than the illumination generally used to initiate the photopolymerization. Typically, the oligomer is polymerized using illumination by wavelengths of 350-300 nm (28 570-33 333 cm-1).17 The A* n pyrazine-like transition, albeit shifted to higher energy from that in the monomer crystal, also continues to be a feature in the oligomer spectrum. Since its energy coincides with that required for the photopolymerization, it appears to be responsible for the photochemistry. Such A* n transitions have been shown to be active in the photochemistry of nitrogen-containing polyatomic molecules.23 The above analysis and discussion allows formulation of a mechanism for the photopolymerization of the oligomer crystal. This reaction is activated by a single-photon absorption process since there is no overlapping of exciton branches as was observed in the monomer crystal. At the frequencies required to initiate the photoreaction, only the A* n styrylpyrazine transition is excited. Instead of forming a lattice distortion as seen in the monomer crystal, the A* n transition leads to the formation of polymer directly. The difference between the monomer and the oligomer reaction mechanisms is emphasized by the shorter distance between thereactivedoublebondsin theoligomer crystal. In the monomer, these bonds are 3.939 A apart9 while, in the oligomer, this distance is reduced to 3.27 A.34 This closer proximity should lower the activation energy over that in the monomer crystal since it is topochemically more favorable. + -
Figure 7. Polarization of the ?r*
r transition moment vector of the pyrazine end group of the styrylpyrazine oligomer: (a) Polarization of this transition in 2-styrylpyrazineand (b) the other possibility allowed by the vector polarization calculations from the crystal spectra. +
-- -
breathing vibration at 965 cm-l. This vibration typically dominates A* T transitions in azabenzenes.12 The first A* n and A* A transitions are the only ones of interest to the photochemistry of the crystal since the others lie at energies higher than those employed for the reaction. Consequently, the analysis of the crystal spectra will be limited to these lower electronic transitions.
IV. Discussion Obtaining single-crystal polymers by methods other than in situ polymerization of a monomer crystal has proven problematic. One reason for this difficulty is that polymers, especially those permitting rotation about bonds, assume a randomly oriented conformation which is not conducive to the ordering required for crystallization. Such ordering is not thermodynamically favorable due to entropic constraints which are clearly evident when an attempt is made to recrystallize polymers from solution. The DSP oligomer and polymer exhibit such behavior."J The highly conjugated system of the DSP monomer molecule is broken by the formation of the cyclobutane ring in the oligomer. The single carbon4arbon bonds resulting from the photoreaction allow free rotation; however, in the solid state, such movement is constrained. The crystalline oligomer remains highly ordered, but these rotational restrictions are removed upon dissolving the crystal, and theoligomer can assume the most thermodynamically favored conformation. These conformational changes in the DSPoligomer may include more than mere rotations about single bonds. Since the oligomer is forced to reside in a lattice not initially designed to accommodate it, the styrylpyrazine terminal groups may be twisted or bent in such a manner as to lower the effective conjugation. Furthermore, residual strain from the reaction process is almost certain to be present in the product crystal. The contrasting environments and conformations of the solution and crystalline phases are evidenced in a comparison of the electronic spectra. While the lowest transition in solution is at 27 850 cm-1, the lowest observed in the crystal spectrum is at 30 250 cm-1. Indeed, the solution spectrum transitions are approximately 2 500-5 000 cm-l lower in energy than those in the crystal. Normally, crystallization lowers the electronic transition energies due to dielectric interactions in the solid state. For the DSP monomer, this stabilization lowers the transitions in thecrystal by approximately 1 OOOcm-1 from those in solution.7 That such is not the case for the oligomer supports the assertion that the free oligomer molecule assumes a more energetically favored, planar conformation which, in turn, lowers theelectronic transition energies. Additional evidence for this is obtained by examining the polarization vector for the A* n transition along with calculations of twisted conformations of 2-styrylpyrazine. The INDO/S calculation shows that this transition in styrylpyrazine
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2,5-Distyrylpyrazine Oligomer Molecular Crystal Increased reactivity was also observed at low temperature where, unlike the monomer, the oligomer reacted even at 8 K. This indicatesthat there is no trapping or phonon-coupling mechanism active in this reaction as there was in the monomer crystal but that electronicenergy from a singleexcitation alone is responsible for the oligomer reactivity.
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The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 691
further development of highly ordered macromolecular crystals. Although much remains to be learned about synthetic solid-state chemistry before rational design of crystalline polymers is a reality, thorough investigation of known systems can provide the necessary physical understanding such a goal requires. More complete characterizationof thesesystemswill neceaarily precedea rational attack upon the engineering of novel crystalline polymers.
conclusions
The DSP photoreaction is a crystal-to-crystal reaction which has provided considerableinsight into solid-state polymerizations. By proper control of crystal purity and irradiation conditions, single-crystal oligomers can be formed. Examination of the resulting crystals allows characterization of the electronic excited states and yields information about the mechanism of the second photopolymerization stage of this two-stage reaction. With the loss of the extended conjugation present in the monomer, the oligomer transitions are shifted to higher energy and become essentially those of styrylpyrazine and disubstituted pyrazine superimposed upon each other. The weak ‘K* n and stronger ‘K* T transitions of both styrylpyrazine and disubstituted pyrazine are present in the spectra along with their characteristic vibronic progressions. The oligomer in the crystal environment apparently differs from the conformationassumed by the free molecule. This results from the metastable lattice in which the oligomer is forced to reside following the photoreaction. Clearly, the residual strain is not too great to be accommodated by the lattice, or else the crystal would disintegrate. Besides the entropic barriers from the random rotations of the oligomer backbone, this distortion helps to explain why the DSP oligomer cannot be recrystallized from solution and why solid-state reactions remain the premier method of obtaining most crystalline polymers. The investigation of the electronic transitions also serves to reveal the photopolymerization mechanism of the oligomer crystal. Unlike the monomer, the oligomer reaction is not biexcitonic. In the oligomer crystal, the reaction is initiated by a single 350-nm photon excitation of the n* ‘K styrylpyrazine transition. The reactivity of the oligomer is increased by the close proximity of the reactive double bonds in the lattice. The activation energy for this reaction is supplied purely by the electronic excitation since it is not highly temperature dependent. In contrast to the nontopochemical behavior of the DSP monomer crystal oligomerization, the oligomer crystal reaction is topochemically controlled. The continued study of such archetypical systems as the DSP solid-state reaction is expected to yield insights which will enable
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Acknowledgment. N.M.P. gratefully acknowledges the University of Nebraska and the University of Nebraska Foundation for the award of graduate fellowships. Prof. Gordon Gallup provided the INDO/S program used for the calculations of the spectra in this research. References and Notes (1) Schmidt,G. M. J . InSolidState Chemistry;Ginsberg,D.,Ed.;Verlag Chemie: Weinheim, Germany, 1976. (2) Bloor, D.In Developments in Crystalline Polymers-l; Basset, D. C., Ed.; Applied Science Publishers: London, 1982; p 151. (3) Baughman, R. H. J. Polym. Sci. 1974, 12, 1511. (4) Braun, H.-G.; Wegner, G. Makromol. Chem. 1983, 184, 1103. (5) Hasegawa, M.; Suzuki,Y.; Suzuki,F.;Nakanishi, H. J. Polym. Sci. Part A-1 1969, 7 , 743. (6) Nakanishi, H.; Hasegawa, M. J . Polym. Sci. Part A-2 1972, 10, 1537. (7) Peachey,N. M.;Eckhardt,C. J.J.Am. Chem.Soc. 1993,115,3519. (8) Peachey, N. M.; Eckhardt, C. J. J . Phys. Chem. 1993, 97, 10849. (9) Luty, T.; Fouret, R.J . Chem. Phys. 1989, 90,5696. (10) Sasada, Y.; Shimanouchi, H.; Nakanishi, H.; Hasegawa, M. Bull. Chem. Soc. Jpn. 1971,44, 1262. (11) Peterson, C. W.; Knight, B. W. J . Opt. Soc. Am. 1973, 63, 1238. (12) Stezowski, J. J.; Peachey, N. M.; Goebel, P.; Eckhardt, C. J . J. Am. Chem. Soc. 1993, 115, 6499. (13) Ridley, J.; Zemer, M. Theor. Chim. Acta 1973, 32, 115. (14) Innes, K. K.; Byrne, J . P.; Ross, I. G. J. Mol. Spectrosc. 1967, 22, 15. (15) Narasimha, H. A.; Shashidhar, M. A. Indian J. Phys. 1982,568, 124. (16) Philpott, M. R. J . Chem. Phys. 1969,50,5117. Clark, L. B.; Philpott, M. R. J. Chem. Phys. 1970, 53, 3790. Philpott, M. R. Adv. Chem. Phys. 1973, 23, 227. (17) Lewis, T. P.; Ragin, H. R. J . Am. Chem. SOC.1972, 94, 5566. (18) Suzuki, I.; Mikami, N.; Ito, M. J. Mol. Spectrosc. 1974, 52, 21. (19) Tobin, M. C. Laser Raman Spectroscopy, Chemical Analysis; Wiley: New York, 1971; Vol. 35. (20) Tamaki, T.; Suzuki,Y.; Hasegawa, M. Bull. Chem. Soc. Jpn. 1972, 45, 1988. (21) Nakanishi, H.; Suzuki,Y.; Suzuki,F.; Hasegawa, M. J . Polym. Sci. Part A-1 1969, 7 , 753. (22) Braun, H.-G.; Wegner, G. Makromol. Chem. 1983,186, 1103. (23) Calvert, J. G.;Pitts, J . N. Photochemistry;Wiley: New York, 1966. (24) Braun, H.-G.; Wegner. G. Mol. Cryst. Liq. Cryst. 1983, 96, 121.
