J. Phys. Chem. 1982, 86, 1764-1767
1764
Phonon Spectroscopy of Photochemical Reactions in Organic SolMs: Photodknerizatlon of 2-BenzyC5-benzylMenecyclopentanone and Photopolymeritation of 2,5-Distyryipyrazine J. Swiatklewkz,+0. Elsenhardt, P. N. Prasad,' D e p e H m t of Chemlsby, State Unhmky of New York at Buffalo, Buffalo. New York 14214
J. M. Thomas,' W. Jones, and C. R. Theocharb BPa-nt
of phvsical Chemism, C e m b r w Universky, Cam-,
united Kingdom (Received: N o v e " r 10, 1981)
The nature of the photoreactions in 2-benzyl-5-benzylidenecyclopentanone (abbreviated as BBCP) and 2,5distyrylpyrazine (abbreviated as DSP) is investigated by a combination of Raman spectroscopy and electronic spectroscopy. The Raman phonon spectra, obtained as a function of reaction progress, reveal that photodimerization of BBCP in the solid state proceeds by a homogeneous mechanism. But a similar study of photopolymerization of the DSP cryataJ indicates a heterogeneous behavior for this reaction. The 4.2 K electronic spectra for both systems show a strong electron-phonon coupling in the reactive excited electronic state. The observation of a strong electron-phonon coupling in the reactive electronic state supports a model of phonon-assisted photochemical aggregation (dimerization, polymerization) reaction. In this model, a strong electron-phonon coupling localizes the excitation energy as well as provides a local lattice configuration,which is a precursor of the product lattice.
Introduction Phonon spectroscopy has recently been introduced12 as a suitable method to investigate the reaction mechanism and to examine the role of the low-frequency motions in determining reactivity in condensed phase. Owing to the short-range nature of intermolecular interactions in molecular solids, the phonon spectra, sampled as a function of reaction progress, can conveniently be used to determine whether a reaction is truly homogeneous at the molecular level. Also, the electron-phonon interaction can play an important role in determining the photochemical aggregation (dimerization and polymerization) processes in solids. With the application of electron microscopy3 for the study of solid-state reactions, it has been apparent that there are examples of photochemical reactions in solids that are not sensitive to structural imperfections in the lattice. In such cases, the electron-phonon coupling4 may play an important role in effectively trapping the excitation energy. Furthermore, a strong electron-phonon coupling in the reactive electronic state creates a local lattice deformation. The resulting local lattice configuration can, in favorable cases, be a precursor of the photodimer (or the photopolymer) lattice. In other words, a strong electron-phonon coupling can lead to a preformation of the product lattice for a photochemical dimerization or polymerization. Thus, a photochemical reaction can be phonon assisted. The present paper uses phonon spectroscopy to study two systems that are prototypes of two broad classes of compounds. The first reaction is the photodimerization of 2-benzyl-5-benzylidenecyclopentanone(hereinafter abbreviated as BBCPI5. BBCP belongs to a class of photoreactive compounds which consist of a benzylidenecyclopentanone or a benzylidenecyclohexanone nucleus. This particular reaction is a single-crystal-to-single-crystal transformation which proceeds with a minimum rearrangement of the lattice. An extensive amount of work On leave from the Institute of Organic and Physical Chemistry, Technical University of Wroclaw, Poland.
has been done on this type of reaction by Thomas et The second photochemical reaction is the photopolymerization of 2,5-distyrylpyrazine (DSP). It represents a class of compounds of the general formula AR-CC-AR'-C=C-AR. This reaction has been studied in the past by Hasegawa and co-workersg12 and, more reThere are cently, by Jones and Thomas and co-worker~.~~ two crystallographicmodifications of DSP. However, only the a form grown from solution is photoreactive.12 The product is a well-oriented crystalline polymer. Although the reaction proceeds by a s m d molecular rearrangement, but owing to the buildup of mechanical stress, the process ultimately leads to the fracture of the parent crystal. These reactions are investigated by a combination of two spectroscopictechniques: (i) the laser Raman spectroscopy which probes the phonon spectra as a function of reaction progress and (ii) the electronic spectroscopy by which the electron-phonon interaction is studied. The laser Raman spectroscopy is also used to obtain intramolecular vibrations of the reactants and the products as well as of the partially converted samples. These intramolecular via L 6 9 '
(1)K.Dwarkanath and P. N. Prasad, J . Am. Chem. Soc., 102,4254 (1980). (2)P. N. Prasad, Mol. Cryst. Lip. Cryst., 62,63(1979). (3)J. M. Thomas, S. E. Morsi, and J. P. Desvergne, Adu. Phys. Org. Chem., 16, 63 (1977). (4)R.M. Hochatrasaer and P. N. Prasad, "Excited States",Vol. 1, E. C.Lim, Ed.,Academic Press, New York, 1977,p 79.
(5)W.Jones, H. Nakanishi, C. R. Theocharis, and J. M. Thomas, J.
Chem. SOC.,Chem. Commun., 610 (1980). (6)H. Nakaniehi, W.Jones, J. M. Thomas, M. B. Hurathowe, and M. Motevalli, J. Chem. SOC.,Chem. Commun.,611 (1980). (7) H. Nakaniehi, W. Jonea, and J. M. Thomas, Chem. Phys.Lett., 71, 44 (1980);J. M. Thomas,Nature (London),289,633 (1981). (8) M. Hasegawa and Y. Suzuki, J. Polymn. Sci., Part B, 6,813 (1967). (9)M.Iguchi, H. Nakanishi, and M. Hasegawa, J. Polym. Sci., Part A-1, 6,1055 (1968). (10)M. Hasegawa, Y. Suzuki, F. Suzuki, and M. Nakanishi, J. Polym. Sci.. Part A-1. 7 . 743 (1969). (il)H. Nal&hshi, Y . SAuki, F. Suzuki, and M. Hasegawa, J. Polym. Sci., Part A-1, 7 , 753 (1969). (12)M. Hasegawa, Y. Suzuki,H. Nakanishi, and F. Nakanishi, Prog. Polym. Sci., Jpn., 6, 143 (1973). (13)W.Jones, J. Chem. Res. 142 (1978);W.Jones and J. M. Thomas, h o g . Solid State Chem., 12,101(1979);H. Nakanishi, W.Jones, J. M. Thomas, M. Hasegawa, and W. L. Rees, Roc.R. SOC.London, Ser. A, 369, 307 (1980).
0022-3654/82/2086-1764%01.25/0 0 1982 American Chemical Society
Photochemical Reactions In Organlc Solkls
The Journal of phvsical Chemistry, Vol. 86, No. 10, 1982 1785
TABLE I: Intramolecular Vibrations (cm-l) of the BBCP Monomer and the Dimer After Photodimerization, Recorded at 4.2 K monomer dimer assignment
A
878 w, deformation of cyclobutane 979 w ring 997 vs out-of-plane C-H bending of ethylene 1001 s cyclobutane ring “breathing” 1002 s 1002 s benzene ring “breathing” in-plane C-X bending and 1158 m, 1180 m, stretching deformation of 1190 m, 1575 m, ethylene 1593 s 1601 s 1604 m benzene ring C-C stretching 1625 vs ethylene C=C stretching 1713 w 1720 w cyclopentanone C=o stretching
-
0.00
-
0.10
-
0 46
0 65
ki
brations are used to characterize the materials and to study the mechanism of the reaction.
Experimental Section Single crystals of the BBCP monomer were obtained by slow evaporation of the solvent either from a methanolchloroform solution or from a cyclohexane-chloroform solution. Both methods gave the same type of platelike crystals. The platelike crystals of DSP were obtained from a toluene solution. The photodimerization was carried out by using either a 75-W xenon lamp (for BBCP) or a 150-W mercury-xenon lamp (for DSP). For those cases where selective excitation was needed, either a cutoff filter or a monochromator (from S. A. Instxuments Inc., Model H-20) was used to filter out the output of the lamp. The same arrangement was also used to provide excitations for electronic emission studies. The electronic absorption on a single crystal was conducted by using a xenon lamp or a halogen lamp. The absorption spectra of BBCP in solution were recorded on a Cary 17 spectrophotometer. The Raman spectra were recorded on a Spex double monochromator with holographic gratings (Model 14018) by using a 90° scattering geometry. For the Raman investigation of BBCP, the excitation source used was the 5145-A line of an argon ion laser (Coherent Radiation Model CR-8). In order to reduce the fluorescence interference, we used the output of a CW-dye laser to investigate the Raman spectra of DSP during polymerization. To obtain the Raman spectra at 100 K, we cooled the crystal by a flow of cold nitrogen vapor. The Raman spectra at liquid-helium temperatures were obtained by mounting the crystal in a Janis research cryostat. All low-temperature electronic spectra were obtained by placing the crystal in this cryostat. Results and Discussion BBCP. The progress of the photoreaction in the BBCP crystal was monitored by using laser Raman spectroscopy in the regions of both phonons and intramolecular vibrations. It was found that most of the changes in the intramolecular vibration spectra are due to the opening of the double bond when the cyclobutane ring is formed. The cyclobutane ring formation causes an interruption of the conjugated bond system present in the benzylidenecyclopentanone fragment of the monomer molecule. After dimerization, numerous bands of the monomer disappear and new bands of the dimer appear, the latter being less prominent. Some prominent vibrational modes of the monomer and the dimer are listed in Table I. The amount of conversion was calculated by taking the ratio of the monomer peaks at 997 and 1625 cm-’ with respect to the
.__ 1 00
,
,v+-NJ,, . 50
100
150
Figure 1. Laser Raman phonon spectra of the BBCP monomer, partially converted samples, and the dlmer crystal. The conversion yleld Is marked at the right side of each spectrum: (A) Six single crystals separately dlmerlzed at room temperature by exposure to the UV llght from a xenon lamp. The phonon spectra were recorded at 4.2 K. (B) The same single crystal, successhrely exposed to light of X = 345 nm at room temperature. Spectra were recorded at 96 K after each successive exposure.
peak at 1002 cm-’, the latter being unaffected by dimerization. In Figures 1,A and B, two sets of Raman phonon spectra of BBCP crystals are shown. Figure 1A represents phonon spectra obtained for crystals which were exposed to UV light for different times. For improved resolution, these spectra were recorded at 4.2 K. The estimated conversion is indicated on the right-hand side of each spectrum. Figure 1B presents data collected on the same single cryatd which was dimerized stepwise by using monochromatic light of X 345 nm. The spectra shown in Figure 1B were recorded at 90 K. The low-temperature Raman phonon spectra of BBCP monomer consist of sharp structural features which indicate that the monomer lattice is ordered. Except for some anharmonic spectral broadening, observed in the 90 K phonon spectra and shown in Figure 1B compared to the widths of spectral features at 4.2 K shown in Figure l A , there is an excellent correlation of peaks in the two spectra. A careful temperature-dependence study between room temperature and 4.2 K reveals that, except for reduction in line widths and shift of phonon frequencies with the lowering of temperature, no change in spectral features is found. This is taken as evidence that the BBCP monomer crystal does not undergo phase transition between room temperature and liquid-helium temperature. Thus, the low-temperature spectra can provide meaningful information on the crystal that has been photoreacted at room temperature. Both seta of phonon spectra (Figure 1, A and B) reveal amalgamation behavior, i.e., shift in frequency of some phonon bands accompanied by broadening as the reaction pr~gresses.’~The amalgamation behavior is more distinct
-
(14) J. C. Bellows, P. N. Praaad, E. M. Monberg, and R. Kopelman, Chem. Phys. Lett., 54, 439 (1978).
1766
IO, 1982
The Journal of physical Chemistry, Vol. 86, No. nm 280
E
320
300
340
1 0
0.6
360 380 400 A)
Swiatkiewlcz et ai.
TABLE 11: Intramolecular Vibrations of the 2,B-Distyrylpyrazine Monomer and Its Crystalline Polymer
l
~
peak, cm“
I
\
493 642 860 910
I
i
i I
”
I
CI
34
32
30
28
26
(WAVENUMBER) x 10-3em-‘
Figure 2. Absorption spectra of the BBCP In soluHon and in the solkl state: (A) 1.2 X IOd M BBCP h cyclohexane;WBB at frequencies over 30000 cm-‘ were recorded at a hundredfold concentration Increase In sokrtkn; dashed line refers to the BBCP SOkRkn In ethenol. (B) The BBCP monomer single crystal at 4.2 K. (C) The BBCP single crystal after 2 h of exposure to the xenon lamp llght of A > 340 nm when a total conversion to the dimer occurs.
in the spectra in Figure 1B than in Figure lA, even though the former were recorded at higher temperatures and, consequently, showed more anharmonic broadening. The spectra in Figure 1A were obtained for crystals exposed to UV light, which corresponds to the strong absorption band. In this case, the light is expected to be totally absorbed in a thin layer near the surface. Consequently, the surface zone shows more manifestation of the Raman spectra of the monomer or the dimer than that of the intermediate conversion zone. On the other hand, when photodimerization is induced by a longer-wavelength light (A = 345 nm), which refers to the region of weak absorption, the product is formed more uniformly within the crystal, since this light has a longer penetration depth. Hence, the Raman spectra recorded for all layers throughout the crystal are specific for so” solid solutions of the monomeldimer binary system. Figure 1B reflects this behavior. Nonetheless, the phonon amalgamation which shows a homogeneous reaction mechanism for the dimerization of BBCP, is derived from both sets of data (Figure 1, A and B), at least, at the start of the reaction. At a later stage, the interpretation becomes difficult because of tremendous broadening of the spectral features. When the reaction reaches approximately 2/3 of completion, the bands specific for dimer become predominant. The room-temperature electronic absorption spectrum of the BBCP monomer in cyclohexane is shown in Figure 2A. The spectrum consists of two distinct bands a strong absorption band above 30000 cm-’ and a following absorption band below 30 OOO cm-’. This weak absorption band is found to shift toward higher frequencies and merges with the strong band when a polar solvent is used. A portion of the electronic absorption below 30 OOO cm-’ (where the weak band is found in nonpolar solvent), which was obtained for BBCP in ethanol (a polar solvent), is marked by a dashed line in Figure 2A. Because of the shift toward higher frequencies, this weak band cannot be seen
980 1001 1192 1498 1560,1580 1635 325 790 930 1001 1171 1225 1360 1515 1565 1590
assignment Monomer C-H out-of-plane deformation of RC=CR C-H out-of-plane deformation in phenyl group trans-RC=CR C=C deformation benzene ring “breathing” C-Ph stretching, ethylenes pyrazine ring “breathing” in-plane C=C deformation in-plane C=C stretching Polymer C-H out-of-plane deformation of cyclobutane ring cyclobutane skeletal vibration benzene ring “breathing” C-Ph stretching pyrazine ring stretching
-
in this region. This behavior is specific for an n ?r* transition, and this transition is found to be capable of photodimerizing the monomer crystal. The electronic absorption spectrum of the BBCP monomer single crystal at 4.2 K is shown in Figure 2B. The spectrum is broad and again consista of two bands similar to those seen in a nonpolar solvent. This lack of any fine structures (usually found in the electronic spectra of organic cryatah at 4.2 K)can arise from two possible sources (i) disorder in the lattice or (ii) a strong electron-phonon coupling leading to an extremely distorted excited state. As discussed above, our Raman phonon spectra indicate an ordered lattice for BBCP. Therefore, the broad electronic spectra can be taken as evidence of a strong electron-phonon coupling in the reactive electronic state. No detectable emission from the monomer single crystal is observed, which suggesta that the nonradiative energydissipation processes are highly efficient. This behavior is also consistent with a strong electron-phonon coupling in the BBCP monomer crystal. The absorption spectrum of the photodimer single crystal, obtained after an exposure to the light of h > 340 nm for a few hours, is shown in Figure 2C. The weak band disappears; at the same time, a decrease in the absorbance of the long-wave portion of the strong band is observed. The photoreactivity of BBCP monomer depends on the temperature. Even a prolonged exposure to UV at 77 K light does not convert BBCP to the dimer in any measurable quantity. This feature allows one to conveniently obtain the absorption spectra of the crystal at low temperatures. This strong temperature dependence of the reactivity also supports the model of a phonon-assisted reaction. DSP. The laser Raman spectrum of a-DSP monomer in the intramolecular vibrations region is in good agreement with the previously reported spectra by JaneckaStyrcz and Williams.I6 When studied as a function of polymerization, the peaks attributed to the alkene group diminish and peaks characteristic of a cyclobutane group (15) K.Janecka-Styrczand J. Williams, Chem. Phys. Lett., 69, 83
(1980).
The Journal of phvsical Chemistry, Vol. 86, No. IO, 1982 1767
Photochemical Reactions in Organic SoiMs
q, 2,5-0SP Fluoresence
2,5- Distyrylpyrozine (DSP)
d\
(Monomer)
DSP Monomer (pure) Phonon Scon 93'K
h
U I
16000
18000
20000 22000
n
2,5-DSP Photoexcitotion Spectra Monitored at 19850 cm-'
OSP Monomer Partiol Polymerizotion Phonon Scon 94'K
PU 11 22222
24390
27027
cm" Flguro 4. Fluorescence and photoexcitation spectra of the a-DSP monomer crystal at 4.2 K.
"
I
I
I
I
200
150
100
50
lil,L IJ
0
Frequency (cm-'1
Flgure 3. Phonon spectra of a D S P at ~ 9 K.4 Shown are (top) monomer, (middle) partial Polymeriration (41%0),and bottom) polymer. Spectra are shown from 20 to 200 cm-'.
are observed. Table I1 lists the observed intramolecular vibrational frequencies of both the monomer and the polymer. The phonon spectra of DSP obtained as a function of temperature between room temperature and 4.2 K reveal no phase transition in the monomer or polymer DSP crystal. Figure 3 shows the phonon spectra at three points during polymerization: (i) initially when pure monomer is present; (ii) at 41% conversion; and (iii) at completion where essentially pure polymer is present. Again, the sharp phonon spectral features indicate an ordered lattice for the DSP monomer crystal. In the 41% spectra, a direct superposition of monomer and polymer phonon bands is observed with no significant frequency shifts. As the reaction proceeds, the monomer peaks diminish in intensity with a concurrent rise in the intensity of the polymer peaks. This result shows that, within the range of sensitivity of our method (11% of the product), photopolymerization of a-DSP proceeds by a heterogeneous-type mechanism. This inference is borne out by the formation
and subsequent loss of small flakes of the crystalline polymer at the crystal surface. The electronic emission and the photoexcitation spectra of a-DSP monomer, taken at 4.2 K, are shown in Figure 4. Both spectra are devoid of fine structures. A large Stokes shift (1200 cm") is seen between the photoexcitation maximum and the emission maximum. Both the lack of the fine structure and the large Stokes shift provide clear evidence of a strong lattice distortion in the excited state and, thus, a strong electron-phonon coupling. The emission spectrum is of excimer nature. The excimer is formed as a result of the strong local lattice deformation which arises from a strong electron-phonon interaction in the excited electronic state (dynamic mode of excimer formation).
Summary Raman phonon spectroscopy is used to find that photodimerization of the BBCP crystal proceeds by a homogeneous mechanism while the photopolymerization of the a-DSP crystal appears to be heterogeneous in the range of sensitivity of our techniques. Both reactive systems show strong electron-phonon interactions in the reactive excited electronic state and, thus, provide support for the model of phonon-assisted reactivity in the solid state. Acknowledgment. The work at Buffalo was supported in part by NIH grant no. lROl GM 2735702 and in part by AFOSR grant no. 800287; the work at Cambridge was supported by the University of Cambridge and the Science Research Council.
