The Journal of Physical Chemistry, Vol. 83, No. 26, 1979 3393
Triplet State Line Narrowing of 1-Naphthol
(18) Janis Research Co., 22 Spencer St., Stoneham, Mass. (19) A Lakeshore Cryogenics Model CGR-1-1000 carbonglass resistance thermometer was used. Lakeshore Cryogenics, Columbus, Ohio. (20) G. Donnay and C. 6. Storm, Mol. Cryst., 2, 287 (1967); S. J. Silvers and A. Tulinsky, J. Am. Chem. SOC.,89, 3331 (1967). (21) T.G. Brown, J. L. Petersen, G. P. Lozos, J. R. Anderson, and 6. M. Hoffman, Inorg. Chem., 16, 1563 (1977). (22) E. 6. Fleischer, C. K. Mlller, and L. E. Webb, J. Am. Chem. Soc., 86, 2342 (1964). (23) G. N. LaMar and F. A. Walker, J . Am. Chem. Soc., 95, 1782 (1973). (24) J. M. Assour, J . Chem. Phys., 43, 2477 (1965). (25) J. Subramanian in “Porphyrins and Metalloporphyrlns”, K. M. Smith, Ed., Elsevier, New York, 1975, p 567. (26) J. F. Gibson, D. J. E. Ingram, and D. Schoniand, Discuss. Faraday SOC.,26, 72 (1958); H. Morimoto and M. Kotani, Biochem. Biophys. Acta, 126, 176 (1966). (27) S. Sullivan, P. Hambright, 6. Evans, A. Thorpe, and J. Weaver, Arch. Biochem. Biophys., 137, 51 (1970); C. Maricondi, W. Swift, and D. Straub, J . Am. Chem. SOC.,91, 5205 (1969).
and R. L. Ballard, ibid., 28, 1099 (1972). (8) G. N. LaMar, W. D. Horrocks, and R. H. Holm, Ed., ”NMR of Paramagnetic Molecules”, Academic Press, New York, 1973. (9) A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour, and L. Korsakoff, J . Org. Chem., 32, 476 (1967). (10) E. 6. Fieischer, J. M. Palmer, T. S. Srivastava, and A. Chatterjee, J. Am. Chem. SOC.,93, 3162 (1971). (11) P. Rothemund and A. R. Menotti, J . Am. Chem. SOC.,70, 1808 ( 1948). (12) 6. Loev and M. M. Goodman, Prog. Sep. Purif., 3, 73 (1970). (13) G. D. Dorough, J. R. Miller, and F. M. Huennekens, J . Am. Chem. Soc., 73, 4315 (1951). (14) J. T. Horeczy, 6. N. Hill, A. E. Walters, H. G. Schutze, and W. H. Bonner, Anal. Chem., 27, 1899 (1955). (15) M. 2. Atassi, Biochem. J., 103, 29 (1967). (16) G. F. Hatch, J. W. Neely, and R. W. Kreilick, J. Mag. Reson., 16, 408 (1974). (17) G. F. Hatch, D. Ondercin, T.Sandreczki, and R. W. Kreilick, J . Mag. Reson., 27, 261 (1977).
Narrowing of Triplet State Spectral Lines in Laser-Excited Experiments R. L. Wllllamson and Alvin L. Kwlram” Department of Chemistry, University of Washington, Seattle, Washington 98 195 (Recelved August 27, 1979) Publication costs assisted by the National Institutes of Health
-
-
Although line narrowing in the phosphorescent spectrum is not normally achieved under conditions of S1 So pumping, it is observed under TI SOpumping conditions. We present results for 1-naphthol, a highly polar molecule in a polar solvent, which demonstrate (i) line narrowing of the phosphorescent spectrum, (ii) a blue shift in the phosphorescent spectrum in concert with an increase in the S1 St, excitation energy, and (iii) increased resolution in the magnetic resonance spectrum under T1 Sopumping conditions. In addition, we introduce a phenomenological model which provides a coherent framework within which these observations can be rationalized.
- -
Introduction The zero-field magnetic resonance transitions observed at very low temperatures for the lowest excited triplet state of organic molecules are inhomogeneously b r ~ a d e n e d - l - ~ In dilute mixed crystals or in appropriate n-alkane hosts the zero field magnetic resonance lines are typically of the order of 0.5-5 MHz in width4 (the optical lines are approximately 1cm-I widek7). However, if the guest molecules are not isolated in such unique environments the magnetic resonance transitions2 (as well as the optical transitions8) are one to two order-of-magnitude broader. One attributes this solvent broadening in the condensed phase to an inhomogeneous distribution of environments for the guest molecules in the (solvent) host. The distribution of shifts in the optical transition frequencies results in part from perturbations of both the ground and excited states caused by variations in the local electric field. Such local field effects on the wave function give rise to corresponding changes in the relative energies of the triplet sublevels. In optically detected magnetic resonance (ODMR) e x p e r i m e n t ~where ~ l ~ ~one ~ can monitor selected portions of the phosphorescent band, one might expect that the zero field splitting (zfs) parameters measured for one subset of molecules (monitored at A,) would be different than those measured (at A,,) for another subset.12 Moreover, one can ask whether the ODMR line width for such a subset of “site selected” molecules is narrower than that observed without site selection. The answer to the latter question appears to be negative for conventional ODMR experiments in which SI So excitation is employed to indirectly populate the triplet state.13 In other words, monitoring a narrower and nar-
-
0022-3654/79/2083-3393$01 .OO/O
rower bandwidth (range of sites) in the optical (phosphorescence) domain does not normally lead to any appreciable change in the width of the ODMR transitions. On the other hand, any number of experiments suggest that there exists a linear correlation between the optical and the magnetic resonance line s h i f t ~ . ~ ~This J ~ correJ~ lation can be attributed to spin-orbit coupling contributions to the energy of the triplet state sublevels, and a theoretical treatment of this problem has just appeared.16 Further it has been shown that, whereas narrow band pumping of the S1 So transition (which yields site-selected (narrowed) fluorescence lines) does not yield narrowed phosphorescence lines,17direct T1 So pumping does lead to narrowed phosphorescence lines.I7 We show in this paper that in the latter case some increase in the resolution of the ODMR line(s) can also be achieved. In light of this observation and the arguments above one can ask how the characteristics of the ODMR lines would be expected to change if the triplet state were populated directly (via T1 So pumping) rather than indirectly (via S1 So pumping). In addition we have observed an approximately linear dependence of the triplet emission maximum on the S1 So excitation energy. We present these preliminary results below and introduce a highly simplified model to rationalize the various observations.
-
+-
-
-
-
Experimental Section 1-Naphthol (Sigma, grade 111) was hosted in an ethanol glass at concentrations of approximately 0.1 and M; the S1 So transition was pumped either by a Xe-Hg compact arc lamp (Canrad Hanovia No. L5305) or with the frequency doubled output of a tunable dye laser
-
0 1979 American Chemical Society
3394
The Journal of Physical Chemistty, Vol. 83,No. 26, 1979
Williamson and Kwiram NAPTHOL- E t O H i B r B u
NAPTHOL- EPAiBrBu
8,
S;So a t 3130
A(&
4800
5874
Flgure 1. Phosphorescent spectrum of 1-naphthol under conditions of S1 + So excitation. See text for solvent details.
NAPTHOL-EtOH/ B r B u
T,+So a t 4 8 8 0
D - E TRANS 6
F
z
z
vj
A
RES
r a
9a MA)
5950
Flgure 2. Phosphorescent spectrum as In Flgure 1 but under conditions of T1 So excltatlon. The 0-0 band was not recorded because of scattered exciting Ilght. +
-
(Chromatix, Inc., Model CMX-4). TI So pumping was accomplished through use of the 4880-8, line of an argonion laser at a power level of 700 mW. The sample cell was mounted in a microwave helix at the end of a probe and immersed in liquid He. The space above the liquid helium was evacuated in order to lower the boiling point of the liquid helium to approximately 1.4 K. The detection system consisted primarily of an Jobin-Yvon Model HR1000 high-resolution monochromator and a cooled EM1 Inc. Model 9558 photomultiplier tube. The sample was irradiated for periods of from 1 to 2 s after which the exciting light from the laser was blocked with a shutter and the emission decay monitored alternately with and without microwave energy applied. ODMR signals were obtained by subtracting the emission decay intensity taken without microwave power from that obtained with microwave power applied. The microwave frequency was scanned from 3.00 to 3.60 GHz to encompass the (D-E) transition found a t 3.250 GHz.
Results
A
hex-3130
A
F
4900
5273
Figure 3. The 5234-A band (the most intense band in Figure 2) at 1.4-A resolution.
NAPTHOL- BrBuIEPA
I
A(&
5200
-
Figures 1 and 2 show the effect of T1 So laser excitation on the 1-naphthol system. The 0-0 band is missing from the narrowed spectrum due to interference from scattered laser light. These spectra are of 1-naphthol in a glass composed of 10 parts EPA to one part l-bromobutane by volume. The structural characteristics of these spectra are identical with those obtained in a 15:l mixture of ethanol and 1-bromobutane, the solvent in Figures 3-6. Figure 3 shows the vibronic band located a t 5234 8, at higher resolution. Apparently there are two components contributing to this band, one at 5234 8, and one at 5230 A. Careful inspection of Figure 2 reveals that these two components are more readily observable in the 5000-8,
0
3,60
freq.(GHz)
Flgure 4. The D-E transltion in I-naphthol with S1 + So excltatlon whlle monitorlng the emlssion at 5226 A. NAPTHOL- EtOH i B r B u
hex- 4 8 8 0
A
I-
z
4
3 00
f r e q (GHz)
3 60
Flgure 5. The D-E transition In I-na hthol with T, + So exckation while monitoring the emission at 5226
vibronic band where they are separated by 15 8, instead of 4 8,. All of these spectra exhibit relatively intense phonon wings due to the strong electron-phonon coupling in these polar and (presumably) strongly hydrogen bonded systems. Figure 4 shows the D-E transition observed when pumping indirectly via S1 So and monitoring the phosphorescence at 5226 8, with 6 8, resolution. Figure 5 shows the same transition observed when pumping directly (TI So) and monitoring the phosphorescence under essentially identical conditions. As is evident from the figures, the ODMR line is narrower in the latter case. Note, however, that 5226 8, corresponds to the extreme blue edge of the emission where one is presumably only monitoring the emission from one of the components present. If one monitors at 5236 8, where one picks up contributions from other components the ODMR line
-
-
The Journal of Physical Chemistv, Vol. 83, No. 26, 1979 3395
Triplet State Line Narrowing of 1-Naphthol NAPTHOL- E t O H I B r B u
A
3 00
-4880s
3 60
f r e q . (GHz)
Figure 6. The D-E transition in 1-na hthol with Ti monitoring the emission at 5236
+-
I
So excitation while
I
4733
4979
A
Figure 9, Shift in the phosphorescent spectrum of 1-naphthol as the excitation wavelen th is changed from 3260 A for curve 1, 3270 A for curve 2, 3280 for curve 3, and 3290 A for curve 4.
NAPTHOL-EtOHI KBr
1
3 00
f r e q (GHz)
-
360
Figure 7. The D-E transition in 1-naphthol with Ti So excitation while monitoring the emission at 5234- and 16-A resolution. NAPTHOL- ETOHIKBr
D - E TRANS
A,,-4880 5 A ,2 ,3 ,4
8 8
4 3 00
f r e q (GHz)
360
Figure 8. The D-E transition in 1-naphthol with Ti + So excitation while monitoring the emission at 5234- and 6-A resolution.
broadens and develops a reproducible high-frequency shoulder (Figure 6). Figures 7 and 8 show the same effect obtained by using the resolution of the monitoring system t o partially eliminate contributions from one of the phosphorescent components. These last two spectra are of 1-naphthol hosted in slightly acidified ethanol glass saturated with KBr. Figure 9 shows the shift in the phosphorescent bands as the S1 Soexcitation energy is changed for 1-naphthol. The dependence is approximately linear toward the red edge of the absorption band and corresponds to a ratio of about 0.6 (shift in the wavelength of the phosphorescent band to change in exciting wavelength).
-
Discussion Phosphorescent Results. Phosphorescent line narrowing can be achieved in symmetrical hydrocarbons in nonpolar
solvents as reported earlier by Al'shits et al." Indeed in such cases mixed single crystals can usually be prepared. We chose naphthol for these experiments in order to determine whether line narrowing could be achieved even in the extreme case of polar molecules in polar solvents. That situation prevails for most studies involving biologically important molecules where single crystals are often not obtainable. Moreover, 1-naphthol serves as a substrate for an important class of enzymes. As is evident from Figure 1, naphthol does exhibit characteristics similar to those observed for symmetrical hydrocarbons: indirect population of the triplet state (via SI So pumping) leads to typical broad lines (even in cases of narrow band laser excitation) whereas direct population via narrow band T1 So pumping leads to significantly decreased optical line widths. The width of the phosphorescent lines in the latter case are roughly 0.14 nm (compared to 6 nm with S1 Sopumping). The resolution of the 1-m monochromator was 0.13 nm. Unfortunately,,given the forbidden character of the T, +-- Sotransition and the power limitation of our present laser sources, we cannot a t this time explore the higher resolution cases because of the lack of adequate phosphorescent intensity. I t is reasonable to expect, however, that these lines are not as narrow as those obtained in systems with considerably smaller electronphonon coupling. Phosphorescent lines as narrow as 0.3 cm-l have been observed for iodonaphthalene in EPA.18 At higher resolution (Figure 3) it is seen that the individual bands exhibit structure. In particular, two sharp peaks at 523.0 and 523.4 nm are indicated. These have continued to decrease in width with increasing resolution to the limit of our sensitivity. Two origins are indicated but this question will require further study especially as a function of pH to clarify the role of the acidic proton in this system. In order to explain these observations we assume that there is a distribution, designated by A&,,,in the absolute energy of the molecules in the ground state. This distribution can be divided into arbitrarily small but uniform energy increments 6E. We can designate a particular increment at absolute energy E%@)by the index k. Vertical S1 Soexcitation with quanta of energy hue, leads to a similar distribution AEs, of excited states at Es (k)= E&) + hu,,. Molecules lying within 6E a t E&) ao not have identical environments but are represented by an ensemble of quasi-degenerate sites. Each ensemble of sites at E&), for example, can itself be represented by a Gaussian distribution of width 6E.
-
-
-
+ -
3390
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979
Flgure 10. The distribution of "absolute" energies representing different sites, in So, S , , and TI. The index k, for example, represents an ensemble of sites which are quasi-degenerate in the singlet state but not in the triplet state. Thus, whereas the distribution designated by k in S, can be represented by a narrow Gaussian of width 6E, that sources distributionof sites in TI has a width 615'. All transitions shown So are isoenergetic at A,. See discussion in text. for SI +
Although it is not our purpose to explore the nature of ,the optical line shape in this report, we note that emission from such site selected states is often characterized by very narrow zero-phonon lines together with the broad phonon ~ i n g . 6 9 ~The 1 ~ ~specific behavior for a given system depends on the strength of the electron-phonon coupling and the homogeneous and inhomogeneous line broadening processes. For our limited aim here the important point is that although the transition energies are isoenergetic, a distribution of excited states having different absolute energies is created, reflecting the distribution of (absolute) energies in the ground state. These aspects are depicted very schematically in Figure 10 where for simplicity we show only a few representative cases of ground state molecules having nearly isoenergetic transition energies A1. It should be emphasized again that an ensemble of distinctly different sites, which are accidentally degenerate (or nearly so within 6E),contribute to the population at Es(k)." Molecules in the excited singlet state may in turn undergo intersystem crossing (isc) to the triplet manifold. Molecules within 6E a t E,,&) after isc and nonradiative relaxation will have energies lying between ET,(k) and ET,(k) + 6E'where now the size of 6E'is expected to be significantly different than 6E given the difference, for example, in the dipole moments and the polarizability tensors for the two states. Based on the observed wavelength dependence between S1 Soexcitation and T1 So emission described earlier in the Results section, we assume that a piecewise linear correlation exists such that AE,,O'k) = dAEs,O'k), where d is an arbitrary constant and A E Q k ) = E ( k ) - EG). Thus although the ensemble of (distinct) quasi-degenerate sites lying within 6E at Es,(k) now gives rise to a highly nondegenerate range of energies 6E'at &,(k), nevertheless the energy ordering i, j , k in S1 and T1 is preserved. Consequently emission from the resulting distribution of T1 sites will give rise to broad phosphorescent lines even under narrowband S1 So excitation a t A1. Nevertheless, excitation at another wavelength A2 # AI will give rise to a shift in the center of the phosphorescent emission band because of an underlying correlation between S1 and T1 energies. By contrast, direct T1 So excitation selects an entirely different ensemble of sites. Under these conditions of excitation the phosphorescent line narrowing can be viewed by analogy to the fluorescent line narrowing observed
-
-
-
-
Williamson and Kwiranl
Aem Figure 11. The wavelength dependence of the zero-field transition frequency on the wavelength of optical detectlon. The distribution of absolute energies in the triplet state can be represented by a family of straight lines. This figure suggests why, despite the hear dependence of D-E on A, the ODMR line wldth is not substantially decreased as AA is decreased.
under S1 So excitation. The important point, however, is that the excited states created in all cases cover a range o f absolute T1energies. We can now ask what effect this distribution will have on the character of the ODMR lines. ODMR Results. Despite the observed linear dependence of the ODMR transition frequency on the wavelength of the optical emission,14J6the ODMR line width remains virtually independent of optical resolution. To understand this we return to Figure 10. Under S1 So pumping an ensemble of sites characterized by a broad energy dispersion is populated in T1. Of these, a subset, having different absolute T1energies will have essentially isoenergetic transition energies hv, = Em = E(Tl)-E(S,). We can designate that subset of emission lines detected a t a fixed wavelength by A,. Since shifts in the triplet state energy are linearly related to shifts in the frequency of the zero field ODMR transition, we expect to see a distribution in the ODMR transition frequencies detected a t A, (and a concomitant broadening of the ODMR lines) because sites of different absolute energy contribute to the emission at the same wavelength, A,. Thus rather than representing the wavelength dependence of the ODMR transition frequencies by a single line, a family of lines such as shown in Figure 11 can be introduced. If we represent the wavelength dependence by y = mx + b then the range Ay is given by Ay = m(Ax) + 6b where 6b is equivalent to the range of the vertical shifts of the family of lines. If the slope is relatively small such that 6b >> m(Ax) a moderate change in Ax will not result in a detectable change in Ay. In the case of 1-naphthol, the slope is roughly 0.5 MHz/A whereas the line width (ab) is roughly 80 MHz. Thus the condition suggested above obtains and no substantial change in line width is expected as the optical resolution is altered. The observation that narrower emission windows do not appreciably narrow the ODMR line is a result, therefore, of the distribution of absolute energies in the triplet state created by the SISo excitation process. It is immediately apparent therefore that direct TISo excitation cannot in general totally circumvent this problem. Given the distribution of absolute ground state energies, narrow band T1 Soexcitation though giving rise to narrow T1 Soemission will not, because of the +-
-
- -
The Journal of Pbysical Chemistry, Vol. 83, No. 26, 1979 3397
EPR of EDA Triplet In Mixed Crystals
distribution of absolute triplet state energies created in the excitation process, necessarily give rise to narrow ODMR lines. On the other hand, one can expect that, in some cases depending on the particular properties of a given system, line narrowing might be observed under T, Sopumping when compared to S1 So pumping since different sites are excited in the two cases. The example of 1-naphthol is a case in point. As seen in Figure 5 the resolution of the ODMR transition is clearly improved with T1 So excitation. This can be explained by assuming that two components contribute to the emission as is suggested by the narrowed phosphorescent spectra. Since S1 So excitation gives rise to broad phosphorescent lines, both components contribute to the emission and thus to the “broadened” ODMR signal. However, under T1 So pumping the two components can be resolved optically and the ODMR signals that result when only one component is monitored yield “narrowed” lines. Thus the resolution in the ODMR domain can indeed be improved somewhat by T1 So pumping but the extent of such improved resolution must await further study of this and other systems and the development of a quantitative model. Acknowledgment. We thank Dr. Thijs Aartsma for helpful discussions, James van Zee and Douglas Lantrip for contributions to the computer programs used for experimental control, and Sheldon Danielson and Mark Champion for design and fabrication of various interface modules for this instrumentation. This work was sup-
-
+-
+
-
-
-
ported by Grants GM22603 and CA 19101 awarded by the National Institutes of Health, DHEW.
References and Notes (1) J. Schmktt and J. H. Van der Waals, Cbem. Pbys. Lett., 2, 640 (1968). (2) M. Gouterman, B. S. Yamanashi, and A. L. Kwiram, J. Cbem. Pbys., 56. 4073 (1972). (3) C. A. Hutctbofhr,, J. V. Nlcholas, and G. W. Scott, J. Cbem. Pbys., 53, 1906 (1970). (4) . , A. L. Kwiram, MTP Int. Rev. Sci., Pbys. Cbem., Ser. 1 , 4, 271 (1972). (5) D.S. McClure, “Electronic Spectra of Molecules and Ions in Crystals”, Academic Press, New York, 1959. (6) E. V. Shpol’skli, Sov. Pbys. Usp., 6, 411 (1963). (7) K. K. Rebane, ”Impurity Spectra of Solis”, Plenum Press, New York, 1970. (8) N. S. Bayliss and E. G. McRae, J . Phys. Cbem., 58, 1002 (1954). (9) M. Sharnoff, J. Cbem. Pbys., 46, 3263 (1967). (10) A. L. Kwiram, Cbem. Pbys. Lett., 1, 272 (1967). (11) M. A. El-Sayed, Annu. Rev. Pbys. Cbem., 26, 235 (1975). (12) J. U. von Schutz, J. Zucllch, and A. H. Makl, J. Am. Cbem. SOC., 96, 714 (1974). (13) A. H. Makl and J. Zuclich, Top. Current Cbem., 54, 115 (1975). (14) J. van Egmond, B. E. Kohler, and I. Y. Chan, Cbem. Pbys. Lett., 34. 423 (1975). (15) A. L. Kwiiarn, J: B. A. Ross, and D. A. Deranleau, Chem. phys. Lett., 54, 506 (1978). (16) A. H. Zewail, J. Cbem. Pbys., 70, 5759 (1979). 117) E. I. Al’shits, B. I. Personov, and B. M. Kharlamov, Cbem. Pbys. Lett., 40, 116 (1976). (18) J. Funfschilling,E. Wasmer, and I. Zschokke, J . Cbem. Phys., 60, 2949 (1978). (19) A Szabo, Pbys. Rev. Lett., 25, 924 (1970). (20) It should be emphasized that a given molecule In a site of energy E#) does not have access to states at a Merent energy E&./) unless the local lattice structure is somehow converted from configuration ktoj.
EPR Studies of the Phosphorescent State of EDA Complexes in Mixed Crystal Systems Chong-lao Yu and lien-Sung Lin“ Department of Chemistty, Washington University, St. Louis, Missouri 63 130 (Received August 13, 1979)
The EPR spectra of the phosphorescent state of the following electron donor-acceptor (EDA) complexes imbedded in naphthalene-tetrachlorophthalic anhydride (N-TCPA) crystals have been measured: phenanthrene-TCPA, anthracene-TCPA, and phenazine-TCPA. We have observed triplet signals from both the guest and the host complexes with different spin polarization patterns which indicate the energy transfer from the host to the guest complex is ineffective. The charge-transfer (CT) character, measured from the change in the zero-field splittings and hyperfine splittings upon complexation,for each of the above complexes is 45 f 5% for phenanthreneTCPA, 25 f 10% for anthraceneTCPA, and 14 f 7% for phenazine-TCPA. The orientational studies show that the in-plane axes of the guest donor are distorted with respect to those of the host donor. The distortion is explained in terms of the stacking patterns of donors and acceptors (overlapping principle) in the solid.
Introduction Previously we have reported a completed EPR study of the phosphorescent state of 1:l naphthalene-tetrachlorophthalic anhydride (N-TCPA) complex crystals.’ The study enabled us to obtain the degree of charge-transfer (CT) character, to measure the population and decay rates, to probe the spin polarization, and to determine the structural and dynamical effects in the photoexcitation of the lowest triplet state of N-TCPA crystals. Here we report further EPR studies on the following electron donor-acceptor (EDA) complexes imbedded in N-TCPA crystals: phenanthrene-TCPA, anthracene-TCPA, and phenazine-TCPA. The objectives of the mixed crystal studies are to investigate the energy transfer process between the host complex and the guest complex, and to 0022-365417912083-3397$0 1.OOlO
examine the CT character of the guest complexes (with the same number of 7r electrons) in the same host complex crystal. It has been shown that the hyperfine structure (hfs) data of EDA complex provide a more direct probe than the fine structure (fs) data for estimating the degree of the CT character.2 However, the hfs in neat crystal system is washed out by the exciton motion. Thus one can obtain the hfs data only in mixed crystal systems. This is clearly brought out in our mixed crystal studies.
Experimental Section All of the chemicals used in this experiment were purchased from the Aldrich Chemical Co. and purified extensively by the zone-refining method. The 1:l mixture of N-TCPA was further zone-refined before the final 0 1979 American Chemical Society
