J. Phys. Chem. 1990, 94, 1185-1 189
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Molecular Phosphorescence Enhancement via Tunneling through Proton-Transfer Potentialst David Gormin, Jozef Heldt,* and Michael Kasha* Institute of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee, Florida 32306-301 5 (Received: August 25, 1989)
A set of four substituted aminosalicylates, whose respective structures permit normalfluorescence, twisted intramolecular charge-transferfluorescence, and proton-transfer fluorescence, in various combinations, were studied at I1 K to observe the effect on intersystem crossing. It is shown that the molecular structures which are capable of intramolecular proton transfer exhibit greatly enhanced normal molecule phosphorescence. The excitation mechanism involves proton-tunneling S1 S,’(PT) after primary excitation, enhanced intersystem crossing from the tautomer excited state Sl’(PT) T,’(PT), TI. The competitive singlet-state excitation followed by reverse proton-transfer tunneling via the triplet potential Tl’(PT) processes are delineated by picosecond transient absorption spectroscopy. The mechanism described requires the specfic electronic state energy ordering So < So’ C T,< T1’< SI’< SI... where the primes represent the states of the tautomer form of the molecule.
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Molecular electronic mechanisms for the excitation of lowest triplet states have occupied spectroscopists and photochemists for many years. In this paper we report a new mechanism of triple state excitaton enhancement via proton-transfer potentials. When state-ordering permits the mechanism to be active, it proves to be extraordinarily efficient. We believe this to be the first new mode of triplet state excitation enhancement to be reported in the past several decades. The assignment of molecular phosphorescence as the intrinsic lowest TI So transitiodl in molecules has been greatly extended by subsequent researches on the factors influencing spin-orbital perturbation and the dynamics of excitation mechanisms. The recognition of intersystem crossing’ as a spin-orbital-restricted radiationless transition in molecules permitted the correlation of the intersystem crossing with bipartition of excitation between S1 Soand TI So luminescences for A A* excitation. The case of n A* excitation required not only consideration of relative rates of fluorescence emission and intersystem c r ~ s s i n g but ~ . ~also the influence of the special role of one-center spin-orbital matrix element^.^^^
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1. Enhancement Mechanisms for Intersystem Crossing The atomic-number effect on spin-orbital perturbation (Z effect) offered the first tool for enhancement (and diagnosis) of triplet excitation modes in organic molecules containing C, N, 0,and H atoms. Both internal and external “heavy atom” perturbations proved to be effective in enhancing singlet-triplet transitions, radiationle~s~q~*~ and radiative.*s9 A second enhancement mechanism for lowest triplet state excitation in molecules came through studies of molecular exciton effects.I0-l2 In this case, the formation of dimers (and higher aggregates), in appropriate geometries, led to the generation of metastable lowest excited singlet states, with a kinetic enhancement of intersystem crossing by a dramatic change in kF/kIsc (fluorescence/intersystem crossing rate) ratios, instead of through changes in electronic matrix elements for spin-orbital coupling. This mechanism proves to be important in “dye sensitization” of triplet state formation because of the propensity of aggregation of ionic dyes in water. A third possible mechanism for lowest triplet state population enhancement has presented itself in the field of proton-transfer spectroscopy. If we consider the proton-transfer potentials (Figure 1) and the dynamics of excitation, a new mechanism of triplet state population can be predicted. It is recognized that the ex-
* Author to whom correspondence should be sent. ‘This paper is dedicated to Professor Harry G. Drickamer as a salute to his illustrious research career. *Visiting Research Scientist from the Institute of Experimental Physics, University of Gdansk, Poland.
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cited-state intramolecular proton transfer, SI SI’(PT), is in the rapid picosecond domain. Taking a typical measuredI3 rise time of 2.8 ps, it is evident that with a rise time of 10 ns or more normal intersystem crossing SI TI cannot compete with the much more rapid proton transfer SI SI’(PT). The pathway for enhanced TI state excitation via protontransfer potential tunneling is depicted in Figure 1 . The SI T1intersystem crossing is bypassed by the dominance of the SI SI’(PT) proton-transfer tunneling, if this is present in a molecule. The S,(PT) state has a normal nanosecond range decay constant for the Sl’(PT) Sd(PT) fluorescence, but S,’(PT) T,’(PT) intersystem crossing is expected to be considerably enhanced, over SI T, intersystem crossing, because of the smaller A E s ~ , gap. - ~ ~ Subsequently, ~ a reverse proton tunneling can lead to enhanced TIpopulation over what would have been achievable by direct SI TI intersystem crossing (cf. section 4). The intersystem crossing from the Sl’(PT)state competes with the proton-transfer fluorescence SI’(PT) S,’(PT) and may be a factor in ASE and lasing ~ompetition.’~J~ The current discussion suggests strongly that a proton-transfer phosphorescence (T,’(PT)-Sd(PT)) for the tautomer would be unobservable for the energetics illustrated for Figure 1. None has been reported from among the many molecules for which proton-transfer spectroscopy has been investigated. KlopffeP
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(1) Lewis, G. N.; Kasha, M. J . Am. Chem. Soc. 1944,66,2100; 1945,67, 994. (2) For accounts of the belated evolution of this topic cf.: Kasha, M. The
Triplet State-An Example of G. N. Lewis Research Style. J. Chem. Educ. 1984,61,204-215. Kasha, M. Fifty Years of the Jablonski Diagram. Acra Phys. Polon. 1987, A71, 661-670 (in English). (3) Kasha, M. Discuss. Faraday SOC.1950, No. 9, 14-19. (4) Kasha, M. Radial. Res. 1960, Suppl. 2, 243-275. (5) El-Sayed, M. F. J . Chem. Phys. 1962, 36, 573. (6) Clementi, E.; Kasha, M. J . Mol. Specrrosc. 1958, 2, 297. (7) Clementi, E.; Kasha, M. J . Chem. Phys. 1957, 26, 967. (8) McClure, D. S . J. Chem. Phys. 1949, 17, 905. (9) Kasha, M.J. Chem. Phys. 1952, 20, 71-74. (IO) Levinson, G. L.; Simpson, W. T.; Curtis, W. J. Am. Chem. Soc. 1957, 79, 4314. (11) McRae, E. G.; Kasha, M. J . Chem. Phys. 1958, 28, 721. (12) Kasha, M.Molecular Excitons in Small Aggregates. In Specrroscopy of rhe Excited Stare; Dibartolo, B.,Ed.; Plenum Press: New York, 1976; pp 337-363. (13) Dick, B.;Ernsting, N. P. J . Phys. Chem. 1987, 91, 4261. (14) Chou, P.; McMorrow, D.; Aartsma, T. J.; Kasha, M. J . Phys. Chem. 1984,88,4596. (1 5 ) Kasha, M. In Mo/ecular Elecrronic Deuices; Carter, F.L., Siatkowski, R. E., Wohltjen, H., Eds.; Elsevier Science: Amsterdam, 1988; pp 107-121. (16) Kbpffer, W. Intramolecular Proton Transfer in Electronicaly Excited
Molecules. In Aduances in Photochemistry; Pitts, Jr., J. N., Hammond, G. S., Gollnick, K., Eds.; Wiley Interscience: New York, 1977; Vol. 10, pp 31 1-358.
0022-3654/90/2094-1185$02.50/00 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 3, 1990
Gormin et al.
I
II
lu
m
MABAE
MDABAE
PASE
PDASE
Figure 2. Structures of some selected aminosalicylates.
NORMAL
TAUTOMER
Q (PT) Figure 1 . Schematic singlet and triplet potential curves for aminosalicylates capable of tautomer formation by excited-state proton transfer. SI’(PT) and T,’(PT) The dual proton-tunneling pathway SI T I dominates, with intermediate enhanced intersystem crossing S,’(PT) Tl’(PT).
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TABLE I: Selected Aminosalicylates and their Fluorescence Behavior I
MABAE
2-methoxy-4-aminobenzoic acid methyl ester
I1
MDABAE 2-methoxy-4-dimethylaminobenzoic acid methyl ester Ill PASE p-aminosalicylic acid methyl ester IV PDASE p-dimethylaminosalicylic acid methyl ester
F, F,, F F
F,,F,’ F,,Fi, F F
has commented on the absence of phosphorescence in his early review of proton-transfer systems. Transient absorption spectroscopy following proton-transfer excitation SI S,’(PT) has been investigated widely, and the transients of delayed rise time have been attributed to a triplettriplet absorption. In particular, the excitation pathway for TI via double-proton tunneling has been assumed in the treatment of triplet-triplet absorption in a proton-transfer system by Mordzinski and Grellmann,I7 but their study did not focus on a TI excitation enhancement mechanism. In the next section we shall compare molecules whose structures permit the study of intersystem crossing to the Ti state, with and without the intervention of the proton-transfer tautomer state S,’(PT) and its accompanying excitation pathways.
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2. Phosphorescence Enhancement via Proton Tunneling A special series of aminosalicylates has been under study in our laboratory, permitting the observationI8 of three discrete fluorescences, singly, or in combination: S I ’ (PT)
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Sl”(CT)
SI
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So
So’(PT)
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So(FC)
F,
normal fluorescence
Fi
proton-transfer fluorescence
F2” twisted intramolecular charge-transfer state (TICT) fluorescence
The molecules selected for this study are shown in Figure 2. Table I names the molecules and their fluorescence behavior. Molecules I and I1 cannot exhibit intramolecular proton transfer because of the presence of the methoxy group ortho to the ester group containing the carbonyl. In contrast, molecules I11 and IV contain an o-hydroxy group which can H bond to the carbonyl of the ester substituent, and which upon excitation is capable of an intra( I 7) Mordzinski, A.; Grellmann, H. K. J . Phys. Chem. 1986, 90, 5503. (18) Heldt, J.; Gormin, D.; Kasha, M. Chem. Phys., 1989, 136, 321-344 (Special Issue on Proton Transfer, Trommsdorf, P., Barbara, P., Eds.).
molecular proton transfer to the carbonyl group. The absorption and luminescence spectra of the four aminosalicylates (Figure 2 and Table I) in EPA (5 parts ethyl ether, 5 parts isopentane, 2 parts ethyl alcohol, parts by volume) solution are given in Figure 3. The onset of the UV absorption is at -320-330 nm for each of the molecules, and the normal fluorescence emission FI occurs at 298 K, correspondingly, with onsets at -320 nm, with the usual overlap of absorption and fluorescence. The second fluorescence (Fz) emission at 298 K (cf. Table I) may be single (in the case of molecules I1 and III), but is establishedlE to be double in the case of molecule IV. At 77 K, molecule I exhibits a narrower F, fluorescence band with A,, 330 nm, and a weak phosphorescence band (Ti So), exhibiting some unresolved vibrational structure (first peak at -405 nm). Molecule I1 behaves analogously. In both of these cases the phosphorescence arises from direct SI TI intersystem crossing, no proton-transfer potential being available. Molecules I11 and IV exhibit a t 298 K, in addition to F1, also a pronounced F2/ proton-transfer fluorescence band at A, -450 nm (with additional FF emissionla for molecule IV at hx -440 nm). In a six-membered H-bonded cycle, the H bond is stable against solvent H-bonding competition, well-known from experience in methyl salicylate proton-transfer studies.19 This is in marked contrast to the H-bonding solvent perturbation sensitivityB which is found for a five-membered H-bonded cycle such as occurs in 3-hydroxyflavone. The ratio of normal fluorescence F1to normal phosphorescence PI for the four aminosalicylates at 77 K in the right-hand column of Figure 3 contrasts the behavior of the molecules of Table I. At 77 K molecules 111 and IV exhibit a dramatic enhancement of phosphorescence as shown in Figure 3, right. These latter molecules possess the additional capability of excited-state intramolecular proton transfer (cf. Table I); this is confirmed by picosecond transient absorption spectroscopy as shown in section 3 (cf. ref 18 and 21). Thus, it would appear that the excitation pathways described at the beginning of this section and in Figure 1 could prevail. In section 4 a discussion of the relative probabilities of the fluorescences Fl(normal molecule), F2/(proton transfer), F F (twisted intramolecular charge transfer), and P,(normal molecule) phosphorescence is presented.
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3. Picosecond Transient Absorption Spectroscopy The complex competition in the excitation mechanisms of molecules I to IV of Figure 2 should reveal itself in electronic absorption spectroscopy of transients after picosecond laser excitation (cf. section 5, Experimental Section). Molecule I1 (MDABAE (Figure 2, Table I) is capable of exhibiting F,(normal) and F,”(TICT) fluorescences. The transient absorption spectra for this molecule (Figure 4) exhibits a principal broad absorption centering on 480 nm, appearing in less than 25 ps and maximizing in intensity at about 100 ps. We assign this S,,”(CT) transition from the TICT transient as a S1”(CT) potential minimum. Molecule 111 (PASE) (Figure 2, Table I) is capable of exhibiting F, (normal) and F2’(proton transfer) fluorescence. The transient absorption spectra for this molecule (Figure 5 ) exhibits a principal broad absorption centering on -560 nm, appearing in less than
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(19) Weller, A. Z . Elektrochem. 1956.60, 1144. Klopffer, W.; Naundorf, G. J . Lumin. 1974, 475, and observations in this laboratory. (20) McMorrow, D.; Kasha, M. J . Phys. Chem. 1984, 88, 2235-2243.
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1187
Molecular Phosphorescence Enhancement
77 K
6.1, Y j.,JL 1 !
I
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400 500 300 400 500 WAVELENGTH, nm Figure 3. Absorption and luminescence spectra of selected aminosalicylates (cf. Figure 2). Numbering corresponds to molecules I-IV of Figure 2 and Table 1. First column, absorptions (A); second and third columns luminescences (L). The second fluorescences (cf. Table I) at 298 K (second column) are as follows: in LIl, TICT-state emissions; in LI11, proton-transfer fluorescence; in Llv, both TICT-state and proton-transfer fluorescence. At 77 K (third column) the luminescences with onset -400 nm are TI So phosphorescence, greatly enhanced for molecules I11 and IV. Cf. Table I1 for excitation parameters. All spectra for EPA dilute solution (ethyl ether, 5 ; isopentane, 5 ; ethyl alcohol, 2; all parts by volume). 200
250
300
350 300
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25 ps, and developing its maximum intensity in 50 ps. We assign this transient as a SI’(PT) S,’(PT) transition from the excited tautmer SI’(PT) potential minimum. A slowly developing secondary transient appears at about 450 nm, maximizing in intensity at about 5 ps. This latter transient we would assign to a TI T, absorption. Molecule 1V (PDASE) (Figure 2, Table I) is capable of exhibiting Fl(normal), F,’(proton transfer) and F,”(TICT) fluorescences, and thus transient absorption SI’(PT) S,,’(PT) and SI”(CT) S,,”(CT) should be observable simultaneously, in comparison with the behaviors of molecules I1 and 111. Figure 6 presents the transient absorption spectroscopy of molecule IV. It is clear that by 25 ps a very general absorption covering the visible region has developed and reaches a maximum intensity at 100 ps. At 50 ps the A, is at 520 nm, emphasizing the S1’(PT) S,,’(PT) transient (A, 560, cf. Figure 5), and at 100 ps the A, is at 500 nm, emphasizing the SI”(CT) S,,”(CT) transient (A,, 480 nm, cf. Figure 4). The overall character of the fast transient for molecule IV reflects the expected mixture of the transients of molecules I1 and Ill (Figure 2, Table I). The slowly developing triplet-triplet transients of molecules 111 and IV are of principal interest to the theme of this paper. Between 5 and 20 ns, maximizing intensity at 10 ns, is a powerful and distinctive absorption at 475 nm for molecule IV which we assign to TI T, of the normal molecule. In this time range all the singlet state transients have decayed. Molecule IV and molecule 111 are just the structures (Figure 2, Table I) which are capable of proton-transfer excitation and which exhibit greatly enhanced phosphorescence at 77 K (Figure 3). However, there
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is a striking difference in intensity of the triplet-triplet transients when Figures 5 and 6 are compared. Molecules 111 and IV differ in their behavior at room temperature in that the latter is capable of SI”(CT) state excitation as well as S,’(PT) state excitation. But at 77 K, the SI”(CT) state is frozen out in the 77 K rigid glass solvent, so this state cannot be a factor in the marked difference in TI T, intensity observed. However, the dimethylamino group of molecule IV has a marked effect on the electronic structure and electron distribution in the excited states (cf. absorptions of Figure 3). We attribute the high TI T, intensity in molecule IV to the strong electron-donating character of the dimethylamino group as the primary factor causing an absolute intensity enhancement, rather than to a triplet population difference for molecules Ill and IV.
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4. Competitive Excitation Pathways in Aminosalicylate In a previous study1*we have compared the excitation behavior of the four aminosalicylates (Figure 2, Table I) in various solvents and excitation conditions at 298 K. Here a discussion of the differential behavior at 77 K vs 298 K is essential for understanding of the phosphorescence enhancement mechanism which we are describing. The TICT-state potentialsI8 for twisted intramolecular charge-transfer states of the aminosalicylates leading to F2/1 fluorescence, S1”(CT) S,(FC) (which ends on a FranckCondon level of the ground state) do not influence intersystem crossing in these molecules in the perturbation range (cf. section 6) studied in Figure 3. This is clear in comparing molecules I and 11, where the S,”(CT) state excitation has no great effect on
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Gormin et al.
1188 The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 06r
w
0
z
r
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a
r
X
V W
V
3a
0 v)
m
a
10 nr
100 ps
n I
I
I
r
.06r .04
20 ns
200 ps
0 I
400 WAVELENGTH, nm
Figure 4. Time-resolved picosecond transient absorption spectra Of MDABAE, molecde 11 (Figure 2, Table I), in methylene chloride at different time delays, 298 K.
.04
Baseline
500 ps
2s pr
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n
+
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:
I
6CO 4CO 500 WAVELENGTH, nm
I
6CO
Figure 6. Time-resolved picosecond transient absorption spectra of PDASE, molecule IV (Figure 2, Table I), in methylene chloride at different time delays, 298 K. Dashed curve corresponds to runs in which the continuum filter system admitted more blue light (cf. section 5). TABLE II: Excitation Parameters (Baird Spectrofluorimeter) for Aminosalicylate Luminescences 298 K I1 K h.. nm slits. nm gain X.. nm slits, nm gain L, 298 515 30X 296 515 1x LII 285 2.515 30X 285 2.512.5 1OX Llll 280 515 lOOX 280 1015 3x Lrv 314 515 30X 310 212 3x
06
,041
500
L
Figure 5. Time-resolved picosecond transient absorption spectra of PASE, molecule 111 (Figure 2, Table I), in methylene chloride at different time delays, 298 K.
the intensity of phophorescence observed. Thus, molecules I1 and IV which exhibit F2/1 fluorescence have contrasting behavior with regard to intersystem crossing enhancement. However, in section 6 the effect of stronger dielectric relaxation perturbations is discussed. In Table I I we present semiquantitative data on excitation parameters which can be used in lieu of precise quantum yield determinations. Comparing molecule I at 298 and 77 K, it is clear that in the rigid-glass low-temperature solvent about a 30-fold increase in
fluorescence intensity is observed, indicating considerable quenching in the 298 K solution. Comparing molecule I and I1 a t 298 K, it is seen that the SI”(CT) excitation channel enhances the total fluorescence yield. Comparing molecule I1 at 298 and 77 K demonstrates the freezing out of the SI”(CT) or TICT-state excitation in the rigid glass solvent at 77 K. We can anticipate that molecule IV will experience analogous quenching of its SI”(CT) state excitation mode at 77 K. Comparing molecule 111 at 298 and 77 K we first make the observation that if a TI enhancement mechanism is operative in this proton-transfer-capable molecule, then this mechanism should be present under all solvent conditions and temperatures. Thus we should see considerable quenching of total fluorescence at 298 K compared with molecules incapable of S,’(PT) excitation. The excitation parameters for molecules, I, 11, and I11 indicate that there is substantial quenching of room-temperature fluorescences. Comparing molecule I11 at 298 and 77 K shows the phosphorescence enhancement described in section 2. We anticipate that F, and F,’(PT) fluorescences are both present at 77 K, but the latter is masked by the strengthened phosphorescence at the excitation parameters used. Comparing molecule IV with I1 at 298 K we see a comparable behavior in which quenching of F, is strongly compensated by the rapid excitation of the SI”(CT) state, enhancing total fluorescence. Comparing molecule IV at 298 and 77 K, the S1”(CT) state playing no role a t 77 K, we see a behavior paralleling that of molecule 111 in going from 298 to 77 K. The results for molecules 111 and IV suggest a particular distinction which we have not probed. It is clear that these two aminosalicylates, both exhibiting S,’(PT) pathways, show en-
Molecular Phosphorescence Enhancement
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hanced triplet excitation manifested as phosphorescence enhancement, and an enhanced TI T, transient absorption, although with intensity differences referred to in section 3. However, the TI enhancement at 298 K in fluid solvents could be markedly different: the total fluorescence quenching for molecule I11 suggests a strong TI excitation pathway at room temperature, whereas the presence of SI”(CT) excitation in molecule IV (as in molecule 11) may diminish the TI excitation probability at 298 K for this species. This suggested difference could be probed by flash spectroscopy.
5. Experimental Section Low-temperature (77 K) luminescence studies were done on a Baird-Atomic recording spectrofluorimeter, Model SFR 100. The spectrofluorimeter sample compartment was modified to allow for the introduction of a vacuum-sealed Suprasil quartz cold finger Dewar. The sample was contained in a 1 mm diameter quartz tube, which was then placed in the Dewar containing liquid nitrogen. Upon equilibration of the sample with the liquid nitrogen, the 77 K luminescence was studied. Phosphorescence spectra were recorded by the same apparatus employing a chopper as a phosphoroscope. A specially developed21v22picosecond absorption and emission spectrometer was used for the transient excited-state spectroscopy studies. The equipment included the use of an optical multichannel analyzer (OMA), EG &G Princeton Applied Research Model 1216, detector controlled; a Model 1254E silicon intensified target detector; and a Hamamatsu Model C979-01 streak camera. Light pulses (1064 nm), for both OMA and streak camera resolved spectra were generated by an optically pumped, 7 mm diameter Nd:YAG rod. Picosecond pulses are both actively and passively mode locked. For transient absorption or OMA resolved spectra the laser pulse was split into two parts: one part was used to generate a light continuum, and the other part was passed through a second-harmonic and fourth-harmonic generating crystal to produce 266-nm light. The 266-nm light prepared the sample molecule in the excited state. The continuum light was passed through the excited system at delay times that could be adjusted from 20 ps to 20 ns from the initial molecular excitation, being the agency by which the excited-state absorption spectrum is observed. Distinguishing selected emission bands (the F1 or the F2)for the streak camera observations was accomplished by using different filter combinations. The F,was monitored by using a filter pack that transmitted light between 300 and 410 nm (Corning Glass filter 7-54 and 7-60). The F2 was monitored by using a 18922,23
(21) Gormin, D.; Kasha, M . Chem. Phys. Lett. 1988, 153, 574. (22) Schmidt, J. A,; Hilinski, E. F.; Bouchard, D. A.; Hill, C. L. Chem. Phys. Lett. 1987, 138, 346-351. (23) Schmidt, J . A.; Hilinski, E. F. Rev. Sci. Instrum., in press.
The Journal of Physical Chemistry, Vol. 94, No. 3, 1990 1189
filter that cut off light with a wavelength lower than 440 nm (Corning Glass filter 3-72). 6. Conclusion: Variations in Energy Schemes and Excitation Mechanisms The four aminosalicylates studied in this paper (Figure 2, Table I) present the case for which the lowest triplet state potential minimum for the normal form of the molecule lies below the lowest triplet state potential minimum for the tautomeric structure, where accessible by intramolecular proton transfer. Thus, in this case the pattern of energies for the potential minima of TI and TI’ states parallels the relative pattern of energies of the So and S,,’ states. Photochemists familiar with pK studies for proton dissociation for So, TI, and SIstates believe that this will be a general pattern, the pKs for So and TI generally being rather similar. However, the electronic orbital configurations for the SI and TI states are generally analogous, and depending on overall electronic structure of a molecule and excited state interactions, we can anticipate that various other electronic state energy orderings (primes represent tautomer molecule) can occur, such as
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in which case tautomer phosphorescence T,’(PT) Sd(PT)would be expected, as compared with the present case of aminosalicylates I11 and IV (Figure 2, Table I), where the state order is So C S,,’ C TI C TI’ C SI’C SI ...
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and enhanced T, So normal molecule phosphorescence is observed. Further research over a wide variety of molecular electronic structural types in which intramolecular proton-transfer tautomerism is possible should reveal a series of different energy level progressons, with distinctive excitation mechanisms for observed luminescences. Finally, the S1”(TICT) state (twisted intramolecular charge transfer) potentials could occur so as to offer an avoided-crossing path to the T1state excitation of a molecule (cf. ref 18). Using dielectric medium perturbations on the SI”(TICT) potential is one of the parameters worth exploring, as yet another lowest triplet state T I excitation enhancement mechanism for the normal molecule structure. Acknowledgment. This work was done under Contract No. DE-FG-5-87ER605 17 between the Division of Biomedical and Environmental Research, US.Department of Energy and the Florida State University; and National Science Foundation, Chemical Instrumentation Program (CHEO-8602678), Purchase of Laser Instrumentation for Time-Resolved Spectroscopy, and the Florida State University. Registry No. I, 27492-84-8; 11, 42832-22-4; 111, 4136-97-4; IV, 27559-59-7.
