J. Phys. Chem. 19%5,89, 1217-1220
1217
Photoisomerization and Fluorescence of Surfactant and Hydrophobic 4- and 4,4’-Substituted Stilbenes in Homogeneous Solution. Observation of Strong Fluorescence Enhancement by Dialkyi Substitution Patti EUer Brown and David G. Whitten* Departments of Chemistry, University of Rochester, Rochester, New York 14627, and University of North Carolina, Chapel Hill, North Carolina 27514 (Received: August 20, 1984; In Final Form: November 6, 1984)
Several stilbenes substituted at one (4-) or both (4,4’-) para positions with linear alkyl chains terminating in hydrogen or carboxyl groups have been prepared. These surfactant or hydrophobic stilbenes have been studied primarily in organized assemblies; this paper reports on their behavior in homogeneous (benzene and methylcyclohexane) solution. The behavior of these stilbenes on direct irradiation is characterized by strong increases (4-5-fold) in both fluorescence and singlet lifetimes for all of the disubstituted or “intrachain” derivatives compared to trans-stilbene; in contrast, relatively little effect is observed for the monosubstitutedderivatives. Although the quantum yield for direct trans cis isomerization is reduced as a consequence of the enhanced fluorescence, the cis/trans excited singlet decay ratio is unaffected. Studies of the sensitized isomerization indicate that triplets of the substituted stilbenes have similar energies and cis/trans decay ratios to trans-stilbene; however, a somewhat longer triplet lifetime is indicated. The primary changes noted can be accounted for in terms of an enhanced sensitivity of the intrachain stilbenes to viscosity-torsional interactions between solute and solvent.
-
Introduction Stilbene and a variety of diarylethylenes have been the subject of intense study insofar as their solution photochemistry and photophysics are concerned.’-’* Although quite characteristic patterns of reactivity have emerged, it is often found that strong structural effects on both photophysical and photochemical behavior exist. We have recently synthesized several surfactant and hydrophobic tram-stilbene derivatives having the general structures 1 and 2 for use as probe-solubilizates in a variety of organized
2; A, A’. H , COOH
assemblies including micelles, vesicles, various types of microemulsions, monolayer films at the air-water interface, and supported m ~ 1 t i l a y e r s . l ~These ~ ~ molecules offer several advantages as probes for different organized media: first of all they are relatively compatible with surfactant molecules consisting of a single paraffm chain attached to a polar or charged head group-in fact, pure or mixed films of water-insoluble derivatives of 1 or 2 terminating with a single carboxyl group exhibit surface pressure-area isotherms closely resembling those of fatty acids having similar effective chain 1e11gths.l~ A second advantage is the well-characterized photochemical reactivity which in dilute solution consists mainly of fluorescence and trans -. cis isomerization for simple tram-stilbenes; the viscosity dependence3JlJ6 of this behavior suggests that surfactant molecules such as 1 and 2 can be useful indicators of microviscosity for different sites in various organized assemblies. A third attractive aspect of stilbene chromophores is their potential ground- and excited-state reactivity with a variety of reagents; here one anticipates that reactivity can be modified strongly by the precise microenvironment experienced by the chromophore in different assemblies. Prerequisite to a study of molecules such as those in the series 1 and 2 is a knowledge of how closely their behavior in homogeneous solution resembles that of the “parent” trans-stilbene. Previously we reported preliminary studies of several stilbenes having a single chain at the *Address correspondenceto this author at the University of Rochester.
0022-3654/85/2089- 1217$01.50/0
4-position of one of the aromatic rings (1; n = 5 , 9 , 11, 15, A = COOH). The photochemical and photophysical behavior of these molecules was found to be nearly identical with that of transtilb bene.'^*'^ More recently we have prepared several intrachain stilbenes, 2, which also can be incorporated into assemblies. We were somewhat surprised to find that these molecules generally exhibit much higher fluorescence quantum efficiencies (by factors of 2-4) than trans-stilbene or the single-chain derivatives of 1. In the present paper we report a study of the solution photochemistry of some of the intrachain stilbenes 2; these studies indicate that while there are some significant differences in the photochemistry of the “intrachain” stilbenes, their overall behavior is similar enough to that of trans-stilbene to permit them to be useful surfactant “probes”.
Experimental Section Materials. The synthesis of several of the surfactant stilbenes has been described in detail e1se~here.I~ The same procedures were used to synthesize 6S4A (mp 154 “C) which has been the compound most used in this study. Anal. Calcd for 6S4A: C,
(1) Hammond, G. S.;Saltiel, J.; Lamola, A. A.; Turro, N. J.; Bradshaw, J. S.;Cowan, D. 0.;Counsell, R. C.; Vogt, V.; Dalton, C. J . Am. Chem. Soc. 1964,86, 3197. (2) Gegiou, D.; Muszkat, K. A,; Fischer, E. J . Am. Chem. SOC.1968,90, 3907. (3) Gegiou, D.; Muszkat, K. A.; Fischer, E. J. Am. Chem. Soc. 1968,90, 12.
(4) Gegiou, D.; Muszkat, K. A.; Fischer, E. J . Am. Chem. SOC.1967,89, 4814. (5) Saltiel, J.; Megarity, E. D.; Kneipp, K. G. J . Am. Chem. SOC.1966, 88, 2336. (6) Saltiel, J. J. Am. Chem. SOC.1967, 89, 1036. (7) Saltiel, J. J. Am. Chem. SOC.1968, 90, 6394. (8) Saltiel, J.; Zafiriou, 0. C.; Megarity, E. D.; Lamola, A. A. J . Am. Chem. SOC.1968, 90,4769. (9) Mulliken, R. S. J. Chem. Phys. 1977, 66, 2448 and references therein. (10) Whitten, D. G.; McCall, M. T. J . A m . Chem. SOC.1968, 90, 5097. (1 1) Saltiel, J.; D’Agostino, J. T.; Megarity, E. D.; Metts, L.; Neuberger, K. R.; Wrighton, M.; Zafiriou, 0. C. Org. Phofochem. 1973, 3, 1. (12) Herkstroeter, W. G.; Lamola, A. A.; Hammond, G. S. J. Am. Chem. SOC.1964.86. 4537. (13) Russeil, J. C.; Costa, S. B.; Seiders, R. P.; Whitten, D. G. J . Am. Chem. SOC.1980, 102, 5678. (14) Russell, J. C.: Whitten, D. G. J . Am. Chem. SOC.1981, 104, 5937. (15) Mooney, W. M.; Brown, P. E.; Russell, J. C.; Pedersen, L. G.; Whitten, D. G. J. Am. Chem. Soc. 1984, 106, 5659. (16) Saltiel, J.; DAgostino, J. T. J . Am. Chem. SOC.1972, 94, 6445.
0 1985 American Chemical Society
1218 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
82.24; H, 8.63. Found: C, 81.88; H, 8.69. The nonpolar stilbene derivative 4S4 (mp 107 "C) was synthesized from p-n-butylbenzaldehyde by converting a portion to the corresponding benzyl bromide and thence to a Wittig reagent which could be condensed with the remaining aldehyde. Anal. Calcd for 4S4: C, 90.35; H, 9.65. Found: C, 90.26; H, 9.73. Azulene (Baker) was twice vacuum sublimed prior to use. Benzil (Aldrich) was recrystallized twice from 95% ethanol. Benzophenone (James Hinton, zone refined) was used as received. Benzene (Aldrich, Gold Label) and methylcyclohexane (Matheson Coleman and Bell, Spectral Quality) were used as received. cis-6S4A was prepared by benzil-sensitized photoisomerization of trans-6S4A. A solution of 50 mg of trans-6S4A and 15 mg of benzil in 50 mL of benzene was irradiated with a high-pressure 1000-W mercury lamp through a monochromator set at 350 nm and through an 0-51 (340 nm) cutoff filter. After ca. 2-h irradiation analytical HPLC indicated >85% cis formation. The irradiated solution was reduced in volume to ca. 1 mL; 1OO-wL samples of this solution were separated by preparative HPLC using a Prep PAC normal-bonded phase column. The cis-6S4A was obtained as a colorless viscous oil. Measurements. Fluorescence spectra were obtained on an SLM Instruments Fluorometer; quantum yields were obtained with trans-stilbene in methylcyclohexane as a reference.I6 Isomerization quantum yields were measured after irradiating 5-mL samples (stilbene derivative concentration 1 X 104,M) with a high-pressure mercury lamp using the 31 3-nm line; both argon-deaerated and freeze-pumpthaw vacuum degassed samples gave similar results. Potassium ferrioxalte actinometry was used, and the amounts of isomeric stilbene formed were determined by analytical HPLC in both direct and sensitized irradiations. Ultraviolet and visible spectra were recorded on a Perkin-Elmer Model 576 S T or a Bausch and Lomb Spectronic 2000 spectrophotometer. Highperformance liquid chromatography was performed with a Perkin-Elmer Series 1 pump, a Varian Vari-chrom UV-visible detector, and a Whatman Partisil PAC 10/25 normal-bonded phase column. The solvent system consisted of 90.1% hexanes, 9% chloroform, and 0.9% acetic acid for both preparative and analytical work. The flow rate was maintained at 2.0 mL/min for the analytical work and 4.0 mL/min in preparative studies. The sensitized isomerizations were carried out at 366 nm by using 1,Zdiphenylpropene isomerization as a secondary actinometer. The 1,2-diphenyIpropene isomers were separated by gas chromatography using a 20%SF-96 on Chromosorb W column with temperature programming from 50 to 200 O C .
Results and Discussion Photochemical Behavior of Surfactant Stilbenes on Direct Irradiation. As noted above, initial examination of several surfactant stilbenes having the general structure 1 or 2 led to the finding that intrachain compounds 2 such as 6S4A or 4S4 have considerably higher quantum yields for fluorescence than transstilbene (TS) itself or the single-chain derivatives 1. Table I compares fluorescence quantum yields for several of these compounds measured in a nonviscous hydrocarbon solvent at 25 OC. Thus, it appears that 4-monosubstitution increases $f by about 60%, regardless of chain length, while 4,4'-disubstitution results in a 4-5-fold increase. If it is assumed that the generally accepted mechanism for direct isomerization of trans-stilbene (eq 1-5)
hu
transo trans*'
ki
trans*'
trans*'
"p"
kd
trans*'
transo
k t
+ hv
trans*3
k a
atranso
,p"
+ (1 - a)ciso
(1)
(2) (3)
Brown and Whitten TABLE I: Quantum EMciencies for Fluorescenceo of Trans Isomers of SnA and mSoA Compounds
compd TS S4A S6A S7A SlOA S12A
@f
compd
9r
0.05* 0.08 f 0.011 0.09 f 0.009 0.09 f 0.004 0.08 f 0 . 0 0 3 0.08 f 0.006
S16A 6S4A 4S6A 2S8A 4S4
0.08 f 0.007 0.21 f 0.002 0.25 f 0.01 1 0.21 f 0.012 0.20 f 0.013
Methylcyclohexane solutions; olefin concentration, 1 X M; excitation X = 310 nm; emission X = 320-470 nm. bData from ref 4. TABLE 11: Rate Constants ( k M ) and Activation Energies ( E ' ) for Twisting within the Singlet Manifold of SnA and mSnA" compd 1 0 9 ~ . , . s-1 E', kcal/mol stilbene 11.19 3.48 S4A 6.8 3.77 S6A 6.0 3.85 6.0 3.85 S7A SlOA 6.8 3.77 S12A 6.8 3.77 3.77 S16A 6.8 6S4A 2.2 4.44 1.8 4S6A 4.60 4.44 2.2 2S8A 2.4 4s4 4.39
Methylcyclohexane solution, 25
O C
tersystem crossing, twisting, and decay are denoted by kf,kk, kobsd, and kd,respectively, the fluorescence quantum yield is given by eq 6. Because reaction 4 is temperature dependent,"^'^ the
Arrhenius equation can be used to describe the rate constant for twisting (eq 7). If we assume that the rate of intersystem crossing (7)
is much slower than the other rates, eq 6 may be used to calculate kobsd,given ki = 5.89 X lo8 s-'.I8 The frequency factor, A, has been found to be 4.0 X 10l2s-' by Saltiel.I8 Table I1 contains kohd values for SnA and mSnA at 25 OC, along with activation energies obtained from eq 7. An independent determination of E* and A from a study of the temperature dependence of & for several of these stilbenes leads to very similar results, indicating that the attribution of enhanced fluorescence to increased E* is justified.20 This treatment predicts that singlet lifetimes should vary with the ratio of fluorescence quantum yields. Although we have not measured lifetimes for all of the intrachain stilbenes, this has generally been found true.19 For example, the lifetime of 4S4 in methylcyclohexane is 0.344 f 0.004 ns by single-photon counting; the lifetime ratio for this compound to TS is thus 4.119 while the quantum yield ratio is 4.0, in good agreement. The increase in fluorescence observed for the intrachain stilbenes is expected to come at the expense of a decreased efficiency for the trans cis direct photoisomerization. Previous studies of viscosity effects on fluorescence and cis-trans photoisomerization for TS have shown that 6,- is reduced as $f increases with increasing solvent v i s ~ o s i t y . ~ * "Since ~ ' ~ the present results with fluorescence suggest that 4,4'-disubstitution on TS significantly retards twisting to the "p" geometry, it is interesting to determine whether the isomerization process itself is much affected once the fluorescent state has been passed. Table I11 lists ,$, and
-
(4)
(5)
applies for these compounds, where "p" represents the twisted singlet geometry, while the rate constants for fluorescence, in-
(17) Dyck, R. H.; McClure, D. S.J . Chem. Phys. 1962, 36, 2326. (18) Saltiel, J.; Charlton, J. L. "Rearrangements on Ground and Excited States"; deMayo, P.,Ed.; Academic Press: New York, 1980 Vol. 3, p 25. (19) Suddaby, B. R., unpublished results. (20) Russell, J. C. Ph. D. Dissertation, University of North Carolina, Chapel Hill, NC, 1982.
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1219
4- and 4,4'-Substituted Stilbenes TABLE III: Initial Quantum Yields for Direct Isomerization' of SnA and mSaA
compd stilbeneb S16AC S4A 6S4A 4S6A 4s4
photostationary state, % cis 90
41-
0.50 0.50 0.49 0.37 0.36 0.34
TABLE V Benzophenone-SensitizedPhotoisomerization" of Stilbene Surfactants mSnA
4tT compd stilbeneb 0.55 1,2-diphenylpr~pene~ 0.54 6S4A 0.58 4S6A 4s4
91 91
"Benzene solutions; olefin concentration, 0.001 M; X = 313 nm. bData from ref 2 and 18. CDatafrom ref 20; methylene chloride solution.
stationarv state, % cis
b,=+ 0.42
60 55
0.44
0.39
60 58 56
' A = 366 nm; olefin concentration, 0.001 M; benzophenone concentration, 0.05 M; solvent, benzene; temperature, 25 O C . bData from ref 11. eData from ref 12.
sufficient to react with all sensitizer triplets, the following relationships should be observed:
TABLE I V Extinction Coefficient Ratios, Decay Ratios, and Fluorescence Quantum Yields for Stilbene and S I A " Ctrdccis
compd stilbene 6S4A
(313 nm) 6.80b 8.66c
a / ( l - a)
df
O.7lc
0.05d 0.21
0.68
dt-c
kC
= h,iskc
' A = 313 nm; methylcyclohexane solution, 25 O C . bData from ref = 11 and 21. c a / ( l - a) = 0.69; ref 11. dData from ref 4.
24150; tcis = 2790.
+ kt kc
stationary state, fraction ciso = kc
photostationary states measured for irradiation at 3 13 nm for several substituted stilbenes. The monosubstituted TS derivatives have &, values nearly the same as that for TS while the disubstituted (intrachain) stilbenes give substantially lower values. Interestingly, however, all of the substituted stilbenes examined give similar cis-rich photostationary-state compositions. For the substituted stilbenes it was found that, as with TS, prolonged irradiation leads to the corresponding dihydrophenanthrene (or phenanthrene, if oxygen is present) in very low yield. The effect of 4,4'-disubstitution past the fluorescent state can be evaluated quantitatively by using the calculated stationary-state equation (neglecting any dihydrophenanthrene formation) [transIp,
-=..(g_)(L) 4f [cis],,
1
€trans
a
1-
(8)
where ek/etrana is the extinction coefficient ratio at the excitation wavelength (313 nm) and a / ( l - a) is the decay ratio from the "p" singlet state. (ais the fraction which decays to trans0). Table IV contains photostationary state ratios, extinction coefficient ratios, and & values for stilbene and 6S4A obtained in methylcyclohexane. The finding that "decay ratios" for stilbene and 6S4A are identical, within experimental error, supports a tenet that the disubstitution has a net effect of increasing the effective viscosity "experienced" by the stilbene chromophore without greatly modifying decay characteristics of the twisted state, "p", once it is formed. (Viscosity changes have been found not to affect the decay ratio of stilbene itself.)" Sensitized Photoisomerization of Surfactant Stilbenes in Solution. The simplest mechanism for sensitized photoisomerization of an olefin is given by the following equations'
so
- + + + - + hv
S*3 S*3
intersystem
S*'
s*3
crossing
cis0 4 3p*
trans0
3p*
3p*
ko
kt
3p*
SO
So
(9)
(10) (1 1)
ciso
(12)
transo
(13)
where 3p* denotes a twisted state having geometry analogous to the state "p" discussed above. When the energy of the sensitizer is high enough, energy transfer to both cis and trans isomers is diffusion controlled. Assuming that the olefin concentration is
4-t
+ kt
+ 4 t - c = h.is
where &+t is the initial quantum yield of cis to trans and +s,iSc is the sensitizer intersystem crossing efficiency. The triplet energies of cis- and trans-stilbene are 57 and 49 kcal/mol, respectively;1s it is reasonable to assume that the SnA and mSnA triplets are not significantly different from these values. Benzil has a triplet energy of 53.7 kcal/mol,I2 intermediate between cis- and transstilbene triplets. Therefore, benzil should transfer triplet energy efficiently only to trans-stilbene; consequently, the benzil-sensitized photostationary state for stilbene is ca. 95% cis.' The benzilsensitized photostationary states of several surfactant stilbenes are also cis rich, 87% cis for 6S4A, 94% cis for S16A,20and greater than 90% cis for 4S4 (a nonpolar double-chain stilbene derivative). Because of these very high yields, benzil was used as the sensitizer in the quantitative preparation of cis-6S4A. These results strongly suggest that the triplet energies for cis and trans isomers of SnA and mSnA are quite similar to those of stilbene. When benzophenone (ET = 69 kcal/mol)12 is used as a sensitizer (intersystem crossing efficiency equals 1; #s,isc = l), the sum of the initial quantum yields (eq 9) should be close to unity and thus the isomerization quantum yields should be predicted by the photostationary states (eq 7 and 8).2' Quantum yields and photostationary states for benzophenone-sensitized photoisomerization of mSnA are given in Table V. For 6S4A the sum of the quantum yields is 1, within experimental error. Since photostationary states for 4S6A and 4S4 are also similar to that of stilbene, it seems reasonable to assume that they also should undergo benzophenone-sensitized isomerization with similar efficiency to that of stilbene. The values are uncorrected for back-reaction; however, since the experiments were carried out to approximately 5% conversion, none of the yields are significantly changed by the back-reeaction correction.22 An interesting feature of TS which has been widely investigated by Saltiel and co-workersM-8.18 is the selective quenching of stilbene triplets by triplet energy acceptors to produce, in most cases, selectively ground-state TS. This has been explained in terms of a triplet potential surface on which there are either two minima or a broad shallow minimum extending from transoid to twisted or 3p* geometry.1*",23 Addition of azulene results in enrichment of the trans isomer for 6S4A, and the effect observed is somewhat more pronounced than with stilbene. In previous studies it has ~ been demonstrated that azulene (ET = 39 k ~ a l / m o l ) *quenches (21) Valentine, D. H.; Hammond, G. S . J. Am. Chem. Soc. 1972,94,3449. (22) Lamola, A. A,; Hammond, G. S. J. Chem. Phys. 1965, 43, 2129. (23) Saltiel, J.; Marchand, G. R.;Kirkor-Kaminska, E.; Smothers, W. K.; Mueller, W. B.; Charlton, J. L. J . Am. Chem. SOC.1984, 206, 3144.
J . Phys. Chem. 1985,89, 1220-1227
1220
stilbene triplets a t rates near diffusion controlled to selectively produce ground-state trans. If it is assumed that there are two triplets, 3p* and 3trans*, in equilibrium
S 3p*
+ 3trans*
k, + k, k,
3p*
3trans*
+ transo trans3 + Az*
ciso
k4
Az
(18) (19) (20)
and azulene quenches only %rams*at the diffusion-controlled rate, the values of KAzfrom a quantum yield study of the azulene effect according to the Stern-Volmer e q u a t i ~ n ’ ~ ~ ~ ’ ~ ~ ~ 4Ot-Jcbt-C
= 1 + K*z[AzI
(21)
can be related to these constants by KAz
=
kq/Kkp
(22)
The effect of azulene was investigated by using trans-1,2-diphenylpropene as an external actinometer as previously described. The slope/intercept ratio (Kh)obtained for the quantum yield plot for 6S4A is 220; this compares with a value of 120 for stilbene under the same conditi0ns.l If it is assumed either (a) that a single minimum in the triplet potential energy surface exists or (b) that (24) Herhtroeter, W. G. J . Am. Chem. Soc. 1975, 97,4161. (25) Whitten, D. G.; Lee, Y. J. J . Am. Chem. Soc. 1972,949142. (26) G6mer, H.; Schulte-Frohlinde, D. J . Phys. Chem. 1981, 85, 1835.
K is similar for stilbene and 6S4A (which seems reasonable to view of the remote nature of the substituents) and that kg is the same for both compounds, the triplet lifetime of 6S4A is estimated to be ca. 1.85 times that of stilbene or approximately 1.9 X s. The increase in triplet lifetimes observed here is substantial but not dramatic when compared to other modifications of the stilbene chromophore such as substitution of a ring carbon by n i t r ~ g e n . ’ ~ *Overall ~’ the net results of the photosensitization experiments is to demonstrate relatively little effect on the triplet behavior by 4- and 4,4’-disubstitution. In summary, we find that the hydrophobic and surfactant 4and 4,4’-substituted stilbenes how generally similar photochemical and photophysical behavior in homogeneous, nonviscous solutions. The most striking difference is the increased fluorescence yields in all of the 4,4’-disubstituted stilbenes studied which appear more or less constant, regardless of the chain length of the para substituent. Interestingly, there appears to be no effect of the substituents on either singlet or triplet decay ratios and the major effects on both paths can probably be ascribed to kinetics; the strong increases in fluorescence (and corresponding increase in singlet lifetimes) can be attributed to effective increases in viscosity-torsional barriers with substitution. The more modest increase in triplet lifetime upon substitution is less readily interpreted but could be due to a slowing down of geometric changes requisite in nonradiative decay. Acknowledgment. We are grateful to the National Science Foundation (Grand CHE8315303) for support of this research. We also thank Professor J. Saltiel for helpful suggestion.
A Time-Resolved EPR Investigation of Very Short-Lived Triplet States: 3n1r* and 9,lO-Diazaphenanthrene
311r*
Masahide Terazima, Seigo Yamauchi, and Noboru Hirota* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received: September 4, 1984)
The nature of the lowest excited triplet (TI) state of 9,lO-diazaphenanthrene (DAP) was investigated by the time-resolved EPR (TREPR) technique in a variety of environments. The spectra obtained in single crystals of biphenyl, fluorene, and dibenzofuran give large zero field splitting (zfs) (X= 0.0892 cm-I, Y = -0.1333 an-’, Z = 0.0446 cm-I) and hyperfine coupling constants due to nitrogen nuclei (A, = 19 G, A, = 24 G). These values together with the result of an INDO calculation unambiguously show that the TI state of DAP is %1r* in character in these mixed crystals. The TI state was also found to be %a* in character in many solid solutions including ethanol and in the neat crystal. The 3na* assignment of the TI state in the neat crystal contradicts the previous 31r7r* assignment of the same system, but reexamination of the spectroscopic data justifies the present assignment. Only in very polar ptotic solvents such as 2,2,2-trifluoroethanoIis DAP phosphorescent and the TI state was found to possess a 31ra*character. The temperature and solvent dependences of the zfs are interpreted in terms of the vibronic coupling between the Tl(nr*) and Tz(alr*) states. The magnitude of this coupling is estimated to be -300 cm-’ in the biphenyl host. The radiationless decay rate constant of the Ti state of DAP is estimated, and the value is discussed on the basis of a direct spin-orbit coupling mechanism.
1. Introduction
Most aromatic molecules exhibit strong phosphorescence and their lowest excited triplet (Ti) states have been studied extensively over the past four decades. The properties of most phosphorescent triplet states are now reasonably well understood. On the other hand, there are a number of interesting molecules which are nonphosphorescent or very weakly phosphorescent. The properties of these triplet states are often far less understood. Among such molecules are ortho diazaaromatics such as pyridazine, phthalazine, and 9,lO-diazaphenanthrene. Though considerable efforts especially have been made to understand their triplet (1) M. A. El-Sayed, J. Chem. Phys., 36,573 (1962); J . Chem. Phys., 38, 2834 (1963). (2) B. J. Cohen and L. Goodman, J . Chem. Phys., 46,7 13 ( 1 967).
the cause for the lack of phosphorescence, there remain uncertain aspects about their triplet states. For instance, the exact natures of the Ti states are mostly unknown. Even whether the TI state is 3 n ~ or * 31r1r* in character is often ambiguous. In view of large quantum yields for intersystem crossing found for pyridazine and phthalazine6 an anomalously fast T, So radiationless decay is considered to be responsible for the lack of phosphorescence.
-
(3) R. M. Hochstrasscr and C. Marzzacco, J . Chem. Phys., 45, 4681 (1966). (4) R. M. Hochstrasser and C. Marzzacco, J . Chem. Phys., 49, 971 i1968).
(5) H. Baba, I. Yamazaki, and T. Takcmura, Spectrochim. Acta Purr A , 27, 1271 (1971). (6) T. Takemura, I. Yamazaki, and H. Baba, Bull. Chem. Soc. Jpn., 45, 1936 (1972). (7) C. T. Lin and J. A. Stikeleather, Chem. Phys. Leu.,38, 561 (1976).
0022-3654/85/2089-1220$01.50/00 1985 American Chemical Society
