6550
J . Phys. Chem. 1990. 94, 6550-6555
Excimer Formation Kinetics in Liquid-Crystalline Alkylcyanobiphenyls Tomiki Ikeda,* Seiji Kurihara, and Shigeo Tazuket Photochemical Process Division, Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan (Receioed: September 12, 1989: I n Final Form: April I I , 19901
Excimer formation kinetics of three 4-cyano-4’-alkylbiphenyls(nCB) (alkyl = pentyl (5CB), hexyl (6CB), and octyl (8CB)) were studied by steady-state and time-resolved fluorescence spectroscopy. Excimer formation was significantly enhanced in nematic (N) state compared with isotropic (I) state but depressed in the smectic A (SA) state, although orientational ordering of mesogens in the SA state is much higher than in the N state. Time-resolved fluorescencespectra demonstrated the existence of the ground-state association of mesogens in the N phase, which produced excimer directly on excitation. Kinetical analysis of the excimer formation on the basis of a model that included the ground-state association of the mesogenic chromophores revealed that the rate constants of excimer formation and dissociation in nCB showed discrete change at temperatures close to the N-I phase transition temperatures.
Introduction Excimer formation in liquid-crystalline systems is a topic of interest in view of how morphology of chromophores in the ground state affects the excimer formation. Kinetics of the excimer formation in such anisotropic solvents as liquid crystals (LC) have been extensively investigated by Weiss et al.’.2 However, in these systems the excimer-forming molecules were not mesogenic, and thus orientational ordering of these molecules in the LCs was lower than expected. I n other words, the excimer-forming molecules tend to be aligned in the LC media by the influence of the phase of the LC; however, addition of such nonmesogenic molecules to the LC phase explicitly perturbs the liquid-crystalline order in the vicinity of the location site. For the analysis of the effect of the ground-state morphology on the kinetics of the excimer formation in the LC systems, it is preferred to employ mesogenic compounds with excimer-forming chromophores. However, because of the limited number of such mesogenic compounds available, very few works have been reported so far on the excimer formation behaviors in the LC systems where excimer is formed between mesogens in the excited state and in the ground This type of excimer in the LC systems may be termed an “intrinsic” excimer. The first example of the “intrinsic” excimer in the LC system was reported by Subramanian et aL3 They studied the emission properties of dodecylcyanobiphenyl by the steady-state fluorescence measurements and found that excimer formation was strongly dependent on the phase of the LC. Time-resolved measurements of the ”intrinsic” excimer in the LC systems was first conducted by Yamazaki et al., who showed by the time-resolved fluorescence spectra that excimer is formed by bimolecular process in 4cyan0-4’-octyloxybiphenyl.~ I n this paper, we report the excimer formation behavior of the “intrinsic” excimer of 4-cyano-4’-alkylbiphenyls,studied by steady-state and time-resolved fluorescence spectroscopy, and discuss the kinetics of the excimer formation with special references to the morphology of the systems. Experimental Sect ion Materials. 4-Cyano-4’-pentylbiphenyl (5CB), 4-cyano-4’hexylbiphenyl (6CB), and 4-cyano-4’-octylbiphenyl(8CB) were purchased from Merck Inc. and were used after purification by column chromatography. 2-Propanol used for fluorescence measurements was purified by the conventional method.* Measurements. LC and phase-transition behaviors were examined with an Olympus Model BHSP polarizing microscope equipped with a Mettler hot-stage Model FP-80 and FP-82. Thermodynamic data were obtained with a differential scanning
* Author to whom correspondence should be addressed. ‘Deceased July 1 1, I989 0022-3654/90/2094-6550$02.50/0
calorimeter (SEIKO I&E SSC-5000) operated at a heating rate of 2 “C/min. Steady-state fluorescence spectra (corrected) were measured on a Hitachi F-4000 fluorescence spectrometer. Time-resolved fluorescence measurements were performed with a picosecond time-correlated single-photon-counting system, detail of which has been reported el~ewhere.~ Briefly, a synchronously pumped, cavity-dumped dye laser (Spectra Physics 375B and 3448) operated with a mode-locked Nd:YAG laser (Spectra Physics 3460 and 3240) was an excitation pulse source with a pulse width of 4 ps (fwhm). We obtained a frequency-doubled pulse for the excitation of the samples through a KDP crystal (Inrad 531). Fluorescence from the samples was detected at right angles to the excitation pulse through a monochromator (JASCO CT-25C) with a microchannel-plate photomultiplier (Hamamatsu R 156411-01). Signals from the photomultiplier were amplified (HP 8447D), discriminated (Ortec 583), and used as a stop pulse for a time-to-amplitude converter (Ortec 457). A start pulse was provided from a fast photodiode (Antel AR-S2) monitoring a laser pulse through a discriminator (Ortec 436). Data were stored in a multichannel analyzer (Canberra 35 Plus) and then transferred to a microcomputer (NEC 9801) where decay analysis was performed by an iterative nonlinear least-squares method. The instrument response function of the whole system was 60 ps fwhm. For the measurements of the time-resolved fluorescence spectra, the photon-counting signals were accumulated for 60 s at each wavelength while the monitoring wavelength was driven at 2-nm intervals from 330 to 480 nm. The fluorescence spectra were not corrected for the wavelength dependence of the photomultiplier sensitivity. Fluorescence spectra of nCB in the neat or in the concentrated solution were measured under front-face excitation and front-face ( 1 ) (a) Anderson, V . C.; Craig, B. 8.;Weiss, R. G. J . Am. Chem. SOC. 1982, 103, 1169. (b) Anderson, V. C.; Craig, B. B.; Weiss, R. G. J . Phys. Chem. 1982, 86, 4642. (2) Anderson, V. C.; Craig, B. B.; Weiss, R. G. J . Am. Chem. Soc. 1982, 104, 2972. (3) Subramanian, R.; Patterson, L. K.; Levanon, H. Chem. Phys. Lett. 1982, 93, 578. (4) Tamai, N.; Yamazaki, 1.; Masuhara, H.; Mataga, N. Chem. Phys. L e t t . 1984, 104, 485. ( 5 ) Hatta, I.; Nagai, Y.; Tamai, N.; Yamazaki, I . Mol. Cryst. Liq. Crysr. 1985, 123, 295. (6) Sisido, M.; Kawaguchi, K.; Takeuchi, K.; Imanishi, Y. Mol. Cryst. Liq. Cryst. 1988, 162B, 263. (7) Sisido, M.; Wang, X.-F.; Kawaguchi, K.; Imanishi, Y. J . Phys. Chem. 1988, 92, 4797. (8) Riddick, J . A.; Bunger, W. B. Organic Solvents; Wiley-Interscience:
New York, 1970. (9) Ikeda, T.; Lee, B.; Kurihara, S.; Tazuke, S.; Itoh, S.; Yamamoto, M . J . Ani. Chem. SOC.1988, 110,8299. Ikeda, T.; Lee,B.; Tazuke, S.; Takenaka, A . J . Am. Chem. SOC.1990, 112, 4650.. Tazuke, S.; Guo, R. K.; Ikeda. T . J . Phys Chem. 1990. 94. 1408.
C 1990 American Chemical Society
.Liquid-Lrystaiiine . . ... AiKyicyanooipnenyis . *
.
. I .
1
a .
The Journal of Physical Chemistry, Vol. 94, No. 17, I990 6551
4
10001T
Wavelength ( n m )
Figure 1. Fluorescence spectra of 5CB in 2-propanol: (a) emission measured at 1.7 X lo" M; (b) emission measured at 0.5 M; (c) excitation measured at 1.7 X lod M.
2 00
250 300 Wavelength ( n m )
350
Wavelength ( n m )
Figure 2. Absorption (A) and fluorescence (B) spectra of neat 5CB: (a) nematic state; (b) isotropic state.
measurement of emission with I-mm quarts cells in the presence of air. Temperature of the samples was controlled by means of a JASCO HTV cell coupled with a temperature-controlling unit.
Results and Discussion Steady-State Measurements. Figure 1 shows fluorescence emission (a and b) and excitation (c) spectra of 5CB in 2-propanol. The choice of 2-propanol was based on the fact that 2-propanol has a dielectric constant ( t = 19.9) similar to that of 5CB (ell = 18.5).10 In dilute solution (1.7 X IO" M), 5CB showed a fluorescence spectrum with a maximum at 335 nm (a), and the excitation spectrum (c) was very similar to the absorption spectrum. On the other hand, in concentrated solution ( O S M, b), 5CB exhibited a new peak at 380 nm with a shoulder at 335 nm. By analogy to the fluorescence spectra of 4-cyano-4'-(octyloxy)biphenyl, the peak at 335 nm can be ascribed to a monomer fluorescence and the peak at 380 nm to an excimer e m i ~ s i o n . ~ Figure 2 shows absorption (A) and fluorescence (B) spectra of neat 5CB. The absorption spectra shown by (Aa) and (Ab) are those of nematic ( N ) and of isotropic (I) state, respectively. It can be seen that the absorption spectrum of the I state is somewhat broader than that of the N state; however, the origin of this difference is not clear at the present stage. As shown in Figure 2B, 5CB exhibited similar fluorescence spectra in the N state (Ba) and in the I state (Bb), although the maximum of the excimer fluorescence was slightly shifted to longer wavelength in the N state. As a measure of efficiency for the excimer formation, the ratio of the fluorescence intensity at 450 (IO) Cummins, P. G.; Dunmur, D. A.; Laidler, D. A. Mol. Crysf. Li9. Crysf. 1975, 30, 109.
(K" )
Figure 3. Arrhenius plots for fluorescence intensity ratio of l ~ / l(a) ~ : 5CB; (b) 6CB; (c) 8CB. 1, and I" are fluorescence intensity of excimer (450 nm) and monomer (330 nm), respectively.
nm (excimer fluorescence, ID) to that of 330 nm (monomer fluorescence, IM),ID/IM, was determined. In Figure 3 are plotted the ratios of ID/IM for 5CB (a), 6CB (b), and 8CB (c) as a function of the reciprocal of the absolute temperature. These Arrhenius plots were constructed in the course of cooling. It is clearly seen that in all samples the excimer is formed with high efficiency in the N state, and an abrupt change in the ratio of ID/IM is observed at a temperature close to the N-I phase transition temperature ( T N I indicated ) by arrows in the figure. Of note is the low values of the ID/IM ratio observed in the smectic A (SA) state of 8CB. Fluorescence Decay Profiles. Figure 4 shows typical examples of the decay profiles of the monomer fluorescence (A) monitored at 330 nm and of the excimer fluorescence (B) monitored at 450 nm for 6CB. In Figure 4 two decay curves are shown in the monomer and the excimer emissions, representative of the N state (18 "C, a) and the I state (45 "C, b). In the decay curves of the monomer emissions, we observed fast-decaying component and slowly-decaying component regardless of the phase. In the excimer emissions, we observed rise components as well as the slowly decaying component. Results of the fluorescence decay analysis have shown that both the monomer and excimer emissions were satisfactorily analyzed by the triple-exponential function in the form of A I exp(-t/rl) Az exp(-t/rz) A3 exp(-t/r,) as judged by reduced x2 and Durbin-Watson (DW) parameters (0.9 Ixz I1.2; DW >- 1.85). In fact, the double-exponential function was examined; however, it gave a worse fitting. Temperature dependence of the lifetimes of the monomer and excimer emissions for nCB is shown in Figures 5 and 6, respectively. In Figure 5 are also included the lifetimes of nCB measured in dilute solution (2 X 10" M) of 2-propanol (rM). For 5CB, the monomer emissions were associated with fast-decaying components ( T ~= 50-100 ps), components with a lifetime T~ of 200-800 ps and slowly decaying components ( r 3 = 16-20 ns, Figure 5). Contribution of the second component to the total fluorescence was small (2-7%) as judged by A z ~ 2 / x A i ~ i . In the excimer emissions of 5CB (Figure 6), we obtained rise components with T~ = 100-200 ps (negative values of A I ) , which approximately correspond to the fast-decaying components in the monomer emissions. Furthermore, it is evident that the slowly decaying components in the excimer emissions ( T ~= 16-21 ns) correspond to those of the monomer emissions. Again in the excimer emissions, the contribution of the second component with T~ = 3-4 ns to the total emissions was small. For 6CB, the same tendency was observed as 5CB. Although there is some scattering, agreement in T ] between the monomer and excimer emissions seems to be better than 5CB. Time-Resolved Fluorescence Spectra. Three-dimensional time-resolved fluorescence spectra in the wavelength range 330-480 nm, the third axis being the delay time after pulse excitation, are shown in Figure 7 for 5CB in the different phases N (30.0 "C) and I (45.0 "C). A distinct precursor-successor relationship has been recognized between the monomer emission and the excimer emission. Namely, immediately after pulse excitation, the monomer emission was predominant in all phases as evidenced by the clear maxima around 350 nm, and the excimer emission grew with time, and in the spectra taken at the longest time region the emission maxima were clearly shifted to a longer
+
+
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I keda et al.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
0
A
Figure 4. Decay profile of monomer fluorescence (A) and excimer fluorescence (B) in 6CB: (a) nematic state ( I 8 " C ) ;(b) isotropic state (45 "C). Monomer and excimer fluorescences were monitored at 330 nm and 450 nm, respectively.
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wavelength region (-410 nm). This means that the excimer forms by bimolecular process between the excited 5CB and 5CB in the ground state. It is of particular interest to note that the time-resolved spectra obtained for f = 0 were varied depending on the phase (Figure
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Figure 7. Three-dimensional time-resolved fluorescence spectra of SCB measured at different temperatures: ( A ) at 30.0 "C (N): (B) at 45.0 "C
( I ) . A,, = 310 nm.
Liquid-Crystalline Alkylcyanobiphenyls
The Journal of Physical Chemistry. Vol. 94, No. 17, 1990 6553 SCHEME I M
t
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According to this model, the decays of the monomer emission, IM(t), and of the excimer emission, lD(t), can be expressed by eqs
Figure 8. Time-resolved normalized fluorescence spectra of 5CB: (-) at 30.0 “C (N); at 45.0 O C (I).The number in the figure denote the delay times after pulse excitation. A,, = 310 nm. (e-),
8). At 30.0 “ C where 5CB showed the N phase, the emission intensity at -400 nm was significantly higher than that measured at 45.0 OC, where 5CB exhibited the I phase. This result strongly suggests that although the main portion of the excimer is produced by the bimolecular process, some fraction of the excimer is formed by the excitation of paired chromophores in the ground state. It seems reasonable to assume that the time-resolved spectra at t = 0 reflect the number of such preformed sites for the excimer formation, where two relevant chromophores are already in the face-to-face arrangement in the ground state, so that once excited the pair forms the excimer directly without reorientation process. In the N state, the 5CB mesogens are aligned parallel with each other, and thus the number of such preformed sites is expectedly high. On the other hand, in the I state, orientation of the mesogens is rather random, and thereby the number of the preformed sites is assumed low. The time-resolved fluorescence spectra at r = 0 support this view. Kinetics of Excimer Formation in 4-Cyano-4‘-alkylbiphenyls. I n the early stage of studies on excimer formation, the Birks’ kinetics was successfully applied to solutions of small molecules.ll The Birks’ kinetics, however, was found to be very difficult to apply to molecular aggregate systems such as polymers.12 For example, the Birks’ kinetics predicts that the time variation in monomer (IM(t)) and excimer (ID(t)) emission intensity can be described by the double-exponential functions and ID(t) can be expressed by two-exponential terms with the same but opposite sign of the preexponential factors. Furthermore, the fast-decay component in the monomer emission ( T ~ )should be the same as the rise component in the excimer emission, and both components should have a common slow decay component ( 7 2 ) . However, the more precise the analysis of the decay behaviors in the molecular aggregate systems due to improved instrumentation, the more deviation from the Birks’ model became apparent. A variety of models have been proposed as alternatives for the Birks’ model.12 I n the present system, inapplicability of the Birks’ kinetics is evident. The time-resolved fluorescence spectra (Figure 8) demonstrated the existence of the ground-state association of mesogens in the N phase, which produced excimer directly on excitation. The kinetic model for the excimer formation in the present system, therefore, should include a process of the ground-state association of the mesogenic chromophore^,'^ which is not considered in the simple Birks’ kinetics. The model we considered is shown in Scheme I . ( I I ) Birks. J . B. Photophysics ofAromafic Molecules; Wiley-Interscience: New York, 1971; Chapter 7. ( I 2) (a) Phillips, D.; Roberts, A. J.; Soutar, I. Polymer, 1981, 22,427. (b) Phillips, D.; Roberts, A . J.; Soutar, I. J . Polym. Sci., Polym. Lett. Ed. 1980, 18, 123. (c) Phillips, D.; Roberts, A. J.; Soutar, I . J . Polym. Sci., Polym. Phys. Ed. 1980, 18, 2401. (13) Reynders, P.; Dreeskamp, H.; Kuhnle, W.; Zachariasse, K. A. J . Phys. Chem. 1987, 91, 3982.
On the basis of scheme I, the rate constants can be calculated by the following equations:
X=
-
AM - AD
where AM (=AMI/AM2), AD (=AD,/AD2), XI, and X2 are observables experimentally obtainable from the time-resolved measurements and [D*(O)]/[M*(O)] is the ratio of the concentration of excimer to that of monomer formed at t = 0. We are aware of inadequacy of applying Scheme I to the present system since the time courses of IM and IDderived from Scheme I are still double-exponential while the experimentally observed decay curves were best fitted by the triple-exponential functions. However, as a usefulfirst approximation, we analyzed the excimer formation behaviors in the present system on the basis of Scheme I . This may be rationalized at least partly by the fact that the second components with medium lifetimes in both monomer and excimer emissions (72 in Figures 5 and 6) were only minor components, and their contribution to the total emission was very small. The results obtained in the time-resolved measurements have revealed that the fast-decaying component in the monomer emissions approximately correspond to the rise components observed in the excimer emissions ( T ~in Figures 5 and 6 ) , and both the monomer and excimer emissions have the longest lifetimes with similar values (qin Figures 5 and 6). These are fundamental requirements for the present model as expected by eqs 1 and 2. As described above, the contribution of the second components in both the monomer and the excimer emissions ( 7 2 in Figures
6554
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
Ikeda et al. 22
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5 and 6) to the total emission was very small, we neglected the second component in the analysis of the excimer formation. A3 exp(-t/r,), we calculated Assuming ID(r) = A , exp(-r/r,) the rate constants of the elementary processes described in Scheme I. The values of k , can be calculated if we know the value of K M ( = I / T ~ = kN kiM). This value can be estimated from the lifetimes of nCB in dilute solution. Decay of nCB fluorescence in 2-propanol was found to be reproduced with a single-exponential function (Figure 5). The rate constants of excimer formation (k,) for SCB and 6CB are shown in Figure 9 as a function of temperature. It is clearly seen that in both compounds the values of k , are strongly dependent on the phase. Although the In k , vs l / T plots gave a negative slope in all phases, a clear discrete change was observed at a temperature close to TNI indicated by arrows in the figure. The slopes for the different phases seemed to be nearly the same. It must be mentioned here that the values of k , below T K N seemed not reliable. The fluorescence measurements were performed on cooling; however, the phase of nCB below T K Nseemed to be a supercooled state. Thus, the values of k, below TKN were expected to be those of the supercooled state but not of the crystalline phase. Our concern is the liquid-crystalline state and the I state; thereby, the discussion on the excimer formation behavior below TKN is excluded throughout the paper. The rate constants of excimer dissociation ( k d )for 5CB and 6CB are shown in Figure IO as a function of temperature. As in the case of k,, a discrete change in kd was observed at a temperature near TNI of both compounds. I n Figure I I is shown the ratio of [D*(O)]/[M*(O)] as a function of temperature. Both plots for 5CB and 6CB showed some scatter, which may result from neglecting the second components in the analysis. A general trend, however, seems to be seen that the ratio of [D*(O)]/[M*(O)] is higher in the temperature range where nCB forms the LC phase. Effect of Morphology on Excimer Formation. The high efficiency of the excimer formation in the N state (Figure 3) can be interpreted in terms of favored spatial orientation adopted by the chromophores in the N state. In the N state, the chromophores are aligned parallel with each other in the ground state, which enables the chromophores to attain face-to-face configuration, essential to the excimer formation, quite easily. On the other hand, in the I state, the chromophores are oriented randomly, and thus the efficiency of the excimer formation is reasonably lower than that of the N state. I n other words, excimer formation is more
+
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1000/ T ( K - l ) Figure 10. Arrhenius plots of the rate constants of excimer dissociation
(kd): (A) 5CB; (B) 6CB. Phase-transition temperatures are indicated by arrows in the figures.
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favored entropically in the LC phase than in the I phase. Chromophores in the LC phase are essentially in a low-entropy state, since orientational ordering of the chromophores in the LC state is much higher than in the 1 phase. Thus, entropy loss accompanied by the face-to-face arrangement of the chromophores in excimer formation is much reduced. Entropically favored excimer formation has already been reported for intramolecular excimer f o r m a t i ~ n . ' ~Furthermore, ~'~ the ground-state association of the mesogenic chromophores enhances the excimer formation as evidenced by the time-resolved fluorescence spectra as well as the ratio of [D*(O)]/[M*(O)]. This may be the characteristic
(14) Zachariasse, K. A,; Kuhnle, W.; Weller, A. Chem. f h y s . Lett. 1978, 59, 315. ( I 5) Tazuke, S.;Ooki, H.; Sato. K. Macromolecules 1982, IS, 400.
J . Phys. Chem. 1990, 94, 6555-6564 feature of the excimer formation in LC systems. The discrete points observed in the In k, vs l / T plots and in the In kd vs I / T plots may reflect the ground-state morphology of the LC system. Very few examples are available to compare with this result. Excimer formation kinetics of pyrene in liquid-crystalline media were investigated. I n the case of pyrene dispersed in cholesteric LC, the Arrhenius plot of k, gave a straight line with negative slope in the temperature range where the matrix mesogens showed cholesteric and I phases2 However, no discrete points were observed. Activation energy for excimer formation ( E , ) was found to be higher in the cholesteric phase than in the I phase.2 A mesogenic pyrene derivative with a cholesteryl moiety also showed negative slope of the Arrhenius plot fw k, without discreteness when mixed with other cholesteryl derivatives in the temperature range where the mixture exhibited the cholesteric and 1 phases.’ In this case, however, E , of the cholesteric phase was smaller than that of the 1 phase.7 X-ray diffraction studies on molecular packing in the N and SA phases of cyanobiphenyls revealed that they have a preferred local structure based on head-to-tail overlapping core packing in which the cyanobiphenyl moieties are oriented antiparallel and alkyl tails extrude to an opposite direction.16J7 This local structure is more tightly held in the SA phase than in the N phase since orientational ordering in the SA phase is much higher than that of the N phase. In the SA phase, the orientational ordering of mesogens is high enough to be little affected by temperature. (16) Leadbetter, A. J.; Frost, J. C.; Gaughan, J. P. J . Phys. (France) 1979, 40. 375. (17) Leadbetter, A . J.; Mehta. A. 1. Mol. Cryst. Lig. Cryst. 1981, 72, 51.
6555
Depression of the excimer formation in the SA phase (Figure 3) suggests that crucial factor for the excimer formation in the SA phase is not the ground-state geometry but lateral diffusion of the mesogens (reorientation of chromophores). Viscosity of the SA phase is well-known to be very high, and thus the diffusion process is associated with a high energy barrier, leading to a low efficiency of excimer formation. Conclusion
The present study has clearly demonstrated the important role of the ground-state morphology of the relevant chromophores for excimer formation in the liquid-crystalline systems. A liquidcrystalline matrix may be one of the best matrices to investigate the effect of the ground-state morphology on excimer formation kinetics since the orientational ordering of mesogens is uniquely defined. Crystals, of course, are similarly preferred in view of the orientational ordering of chromophores; however, they lack mobility of the lattice. Unlike, crystals, liquid crystals are essentially liquid and are fluid enough for the mesogens to reorient within the lifetime of excited chromophores. We have shown that excimer formation in cyanobiphenyls is closely related to the ground-state morphology, and some fraction of the excimer arises from the preformed sites where two mesogenic chromophores are aligned in such a way that once excited they form excimer directly. Investigation of the “intrinsic” excimer formation in liquid crystals may enable us to explore microscopic ordering of mesogens, which is difficult to study by other methods. Supplementary Material Available: Decay curves and results of the decay analysis (6 pages). Ordering information is given on any current masthead page.
Matrix Isolation and ab Initio Theoretical Studies of the If? Spectrum of 5-Methylcytosine Leszek Lapinski, Maciej J. Nowak, Jan Fulara, Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Al. Lotnikow 32/46, Poland
Andrzej Lei,+ and Ludwik Adamowicz* Department of Chemistry, The University of Arizona, Tucson, Arizona 85721 (Received: September I I, 1989; In Final Form: February 20, 1990)
Infrared spectra of 5-methylcytosine in an argon matrix are reported. The results indicate that three tautomeric forms, amino-hydroxy, aminc-oxo, and iminc-oxo, exist simultaneously in the matrix. A UV-induced transformation of the amino-oxo form to the amino-hydroxy form was observed and utilized to separate the spectra of the tautomers. Ab initio quantum chemical calculations of the normal modes performed at the SCF/3-21G level of the theory for the three tautomers enabled positive assignment of most of the observed bands.
Introduction 5-Methylcytosine is a minor base of DNA. Its percentage with respect to the total content of cytosine varies over a wide range, from 0.03%in insects, to 2 4 % in mammals, to 50% in the higher plants.’ DNA is modified after synthesis by the enzymatic conversion of many cytosine residues to S-methylcytosine,* and the pattern of DNA methylation is then maintained in the consecutive cell divisions. In actively expressed genes the CCGG sites were found to be undermethylated or unmethylated. However, in other organs, the same genes, but not expressed, were completely Permanent address: Quantum Chemistry Laboratory, University of Warsaw, Pasteura I . 02-093 Warsaw, Poland.
0022-3654/90/2094-6555$02.50/0
methylated in the CCGG sites.l Methylation of a certain sequence of the DNA nucleobases may modulate DNA-protein interaction and hence regulate the gene function and the cell differentiation. It should be clearly stated that the biologically significant compound is not 5-methylcytosine but its N1 derivatives. Nevertheless, the studies of isolated 5-methylcytosine seem to be worthwhile, considering that its various physical properties are not significantly altered upon the formation of chemical bonds in the N 1 position. A similar remark applies to all nucleobases present in the genetic material, which have been intensively studied ( 1 ) Doerfler, W. Annu. Rev. Biochem. 1983, 52, 93. (2) Razin, A.; Riggs, A . D.Science 1980, 210. 604.
0 1990 American Chemical Society
