J. Phys. Chem. 1986, 90, 6314-6318
6314
Thermally Activated Delayed S, Fluorescence of Aromatic Thrones Andrzej Maciejewski,* Marian Szymanski: Department of Chemistry and Institute of Physics, A. Mickiewicz University, 60-780 Poznan, Poland
and Ronald P. Steer Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0WO (Received: June 16, 1986) Absorption, emission, and emission excitation spectra, emission lifetimes, and quantum yields have been measured for four aromatic thiones, 4H-pyran-4-thione (PT),4H-1-benzopyran- 1-thione (BPT), xanthione (XT), and 2,2,3,3-tetramethylindanethione (TMIT), in several solvents. Emission to the blue of TI phosphorescence has been identified as thermally activated delayed fluorescence and its temperature dependence in a perfluoroalkane and a hydrocarbon solvent investigated. The effect of quenching by O2has also been studied. For XT in 3-methylpentane at 293 K the quantum yield of delayed fluorescence and the rate constant of T1 SI intersystem crossing is 5.6 X lo9 s-I. is estimated to be 1.1 X
--
Introduction Although the mechanisms leading to its appearance have been known for many years,'J the phenomenon of delayed fluorescence (DF) has been observed only relatively rarely. P-type D F can be. observed following triplet-triplet annihilati~n,~ i.e. TI + TI So SI followed by SI So hvf,as exemplified by the recent work of Nickel et aLM on aromatic hydrocarbons. E-type3 or thermally activated delayed fluorescenc;r(TDF) results from SI followed by SI thermal activation processes such as T1 So hvf,and was originally described in detail by Parker and Hatchard for eosin3v7and for other dyess The intensity of TDF is proportional's2 to exA-AE/kT) and is therefore only observed in compounds with relatively small AE = Es, - ET, and then only at sufficiently high temperatures. Thus T D F has been observed in room temperature solutions of a number of d~es,'"*~3~ benzil,"' benzophen~ne,'~-'~ anthraquinone,16 and several other aromatic ketones,13 all of which are characterized by AE values from 1400 to 2500 cm-I. The aliphatic and aromatic thioketones exhibit even smaller values of AE'7-20and many have relatively long-lived triplet states. Englebrecht et al." identified them as potential T D F candidates over 10 years ago, although their interpretation of the results of Emeis and OosterhofP' would now appear to be incorrect. Despite this, no reports of T D F in the thiones have yet appeared in the literature. The thiones exhibit spectroscopic, photochemical, and photophysical properties which would seem to be well-suited for the observation of TDF.20v22-24 The So TI absorption system is relatively intense and the associated 0-0 band is often clearly identifiable and well-separated from the 0-0 band of the So S, absorption ~ y s t e m . ' ~Direct -~ So TI excitation is thus readily achievable. TI So phosphorescence can be relatively intense even in solution at room temperature and TI lifetimes in the microsecond range have been m e a s ~ r e d . ' ~ Most * ~ ~ thiones - ~ ~ are also photochemically stable in inert solvents.23 In this paper we report the observation of T D F from several thiones in nonpolar solvents at room temperature. We also discuss the reasons why the T D F of thiones is of relatively low intensity and is difficult to identify unambiguously. Experimental Section Materials. 4H-Pyran-4-thione (PT),4H- 1-benzopyran-4-thione (BPT), xanthione (XT), and 2,2,3,3-tetramethylindanethione (TMIT) were all prepared by reaction of the corresponding ketone
-
+
-
--
+
-
PT
-
+
BPT
-
XT
-
TMIT
*To whom correspondence should be addressed at the Department of Chemistry. 7 Institute of Physics.
0022-3654/86/2090-63 14$01.50/0
with P2Ss (P4S10) following the methods of Arndt and lor en^,^^ Pedersen et a1.:6 and Scheermer et alez7 The crude thiones were purified by repeated fractional crystallization from ethanol and toluene. Purity (>99%) was monitored by IR and UV spectrophotometry and by gas chromatography using an H P Model 5880A chromatograph with a 50-m SE-54 capillary column and a Pye 104 chromatograph with 3-m OV-17 column.
(1) Parker, C. A. Photoluminescence of Solutions; Elsevier: Amsterdam, 1968. (2) McGlynn, S . P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice-Hall: New York, 1969. (3) Parker, C. A.; Hatchard, C. G. Trans. Faraday SOC.1963, 59, 284. Parker, C. A. Adu. Photochem. 1964, 2, 305. (4) Nickel, B. Helv. Chim. Acta 1978, 61, 198, and references therein. (5) Kray, H. J.; Nickel, B. Chem. Phys. 1980, 53, 235. (6) Nickel, B.; Roden, G. Chem. Phys. 1982, 66, 365, and references therein. (7) Parker, C. A.; Hatchard, C. G. Trans. Faraday Soc. 1961,57, 1894. (8) Parker, C. A.; Hatchard, C. G. J. Phys. Chem. 1962,66,2506. Parker, C . A.; Hatchard, C. G.; Joyce, T. A. J . Mol. Spectrosc. 1964, 14, 311. (9) Parker, C. A.; Joyce, T. A. J. Chem. SOC.,Chem. Commun. 1968, 1421. (10) Fang, T. S.; Brown, R.E.; Kwan, Ch. L.; Singer, L. A. J. Chem. Phys. 1978.82, 2489. (1 1) Flamigni, L.; Barigelletti, F.; Dellonte, S.; Orlandi, G. J . Photochem. 1983, 21, 237. (12) Parker, C. A,; Joyce, T. A. J. Chem. SOC.,Chem. Commun. 1968, 749. (13) Saltiel, J.; Curtis, H. C.; Metts, L.; Miley, J. W.; Winterle, J.; Wrighton, M. J . Am. Chem. SOC.1970, 92, 410. (14) Jonps, P. F.; Calloway, A. R. J. Am. Chem. SOC.1970, 92, 4997. Jones, P. F.; Calloway, A. R. Chem. Phys. Lett. 1971, 10, 438. (15) Wolf, M. W.; Legg, K. D.; Brown, R.E.; Singer, L. A,; Parks, J. H. J . Am. Chem. SOC.1974, 97, 4490. (16) Carlson, S.E.; Hercules, D. M. J. Am. Chem. SOC.1971, 93, 561 1. (17) Engelbrecht, J. P.; Anderson, G. D.; Linder, R. E.; Barth, G.; Bunnenberg, E.; Djerassi, C.; Seamans, L.; Moscowitz, A. Spectrochim. Acta 1975, 31A, 507. (18) Blackwell, D. S . L.; Liao, C. C.; Loutfy, R.0.;deMayo, P.; Paszyc, S . Mol. Photochem. 1972, 4, 171. (19) Safarzadeh-Amiri, A.; Verrall, R. E.; Steer, R. P. Can. J . Chem. 1983, 61, 894. (20) Steer, R.P. Rev. Chem. Intermed. 1981, 4, 1. (21) Emeis, C. A.; Oosterhoff, L. J. J. Chem. Phys. 1971,54,4809. TDF cannot occur at 77 K in this system. (22) deMayo, P. Acc. Chem. Res. 1976, 9, 52. (23) Maciejewski, A.; Steer, R.P. J . Am. Chem. SOC.1983, 105, 6738. Mackejewski, A.; Demmer, D. R.;James, D. R.;Safarzadeh-Amiri, A,; Verrall, R. E.; Steer, R.P. J. Am. Chem. Soc. 1985, 107, 2831. (24) Mahaney, M.; Huber, J. R.Chem. Phys. 1975,9,371. J. Photochem. 1976, 5, 333. (25) Arndt, F.; Lorenz, L. Berichre 1930, 63, 3121. (26) Pcdersen, B. S.; Scheibye, S . ; Nilsson. N. H.; Lawesson, S . 0. Bull. SOC.Chim. Belg. 1978, 87, 223. (27) Scheermer, J. W.; Ooms, P. H. J.; Nivard, R. J. F. Synthesis 1973, 149.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6315
SI Fluorescence of Aromatic Thiones
1
12
I\
0 05
I
I
I
XT in 3-MP
'.\ I
,
I
\.
500
600
550
650
700
750
800
WAVELENC-TH InmJ
WAVELENGTH l n m l
0 0018 '
Figure 1. Phosphorescence spectrum of T M I T and PT. The vertical arrows indicate the thermally activated emission bands.
5 00016-
All solvents, perfluoro- 1,3-dimethylcyclohexane (PF- 1,3DMCH), 3-methylpentane (3-MP), cyclohexane (CH), methylcyclohexane (MeCH), and acetonitrile, were purified by fractional distillation followed by column chromatography, and contained BPT > X T > TMIT, Le. with decreasing singlet-triplet energy gap. (See Table
--
t
620
6LO
660
680 WAVELENGTH ( n m j
700
Figure 3. A part of the phosphorescence spectrum of XT. Arrows indicate the wavelengths selected for measurements of excitation spectra and decay dynamics.
1.)
3. Corrected excitation spectra were measured a t seveial different emission wavelengths, Aem: 627,638, 648, 668, and 700 nm for XT in PF-1,3-DMCH and 3-MP (cf. Figure 3), and 598, 603, 609,627, and 675 nm for BTP in the same solvents. These emission wavelengths encompass both the phosphorescence region and the blue-shifted emission band. The corrected excitation spectra are independent of emission wavelength and are identical with the absorption spectra in the spectral regions encompassing the strong So S2 and So S3 electronic transitions of both thiones. 4. Without exception, the decay of emission from BPT and XT in PF-1,3-DMCH and 3-MP excited in the So SIabsorption system is well-described by a single-exponential decay function. The lifetimes measured at several different &,, encompassing both the phosphorescence and blue-shifted emission bands, are identical and are the same as those obtained when So S2 excitation is employed. The data are summarized in Table 11. 5 . The blue-shifted emission gradually disappears with decreasing temperature, while vibrational bands in the phosphorescence spectrum become narrower and better resolved. This is illustrated in Figure 4 for X T in PF-1,3-DMCH; the data are summarized in Table 111. Similar results are obtained for the other thiones and, in addition, for BFT and PT the emission peaks of low intensity to the red of the phosphorescence origin (which are probably S I So + hvf vibronic bands) vanish (cf. Figures 1 and 2b).
-
-
-
-
WAVELENGTH
I nml
Figure 4. Changes in the uncorrected emission spectrum of XT as a function of temperature. Peak intensities are normalized to unity.
6. Quenching caused by the addition of oxygen to M) does not change the shape of the emission spectrum, i.e. it diminishes the blue-shifted emission and the phosphorescence by
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6317
SI Fluorescence of Aromatic Thiones TABLE III: Temperature Dependence of Intensities at the Maximum of the (0,O) Bands of Blue-Shifted Emission, IE,and Phosphorescence, Ip,for XT in PF-1J-DMCH
T,K
PIP
213 223 230 235 25 1 26 1 27 1 276 280 297 307 31 1 312
0 0.009 0.014 0.020 0.036 0.056 0.063 0.070 0.070 0.078 0.080 0.092 0.086
anm x:
Ai?( 1/2):
cm-l
672.6 672.4 671.8
312 317 316 320 330 348 358 367
372.2 672.5 pTDF
673.5 674.0 673.1 673.5
376 383 385 385
kF
kSqSO
‘The half-width of the (0,O)band of phosphorescence and the position of the maximum of this band are denoted by AFp(1/2) and ,A,,: respectively. TABLE I V Molar Absorption Coefficients and Ratios of Intensities of the (0,O) Band of Thermally Activated Emission and Phosphorescence of BPT and XT in Different Solvents
XT CH
3-MP
BPT PF‘
CH
3-MP
PF“
16.9 16.3 11.9 10.4 10.1 10.4 1.1 1.6 1.6 12.2 20 8.3 18.7 12.2 7.0 PIP 0.051 0.044 0.027 0.075 0.063 0.036 Ado*o)(Sl-Tl),cm-’ 850 840 b 680 695 690 AD$(E-TJ, cm-I 590 580 600 470 430 440 “PF-1,3-DMCH. bNot measured owing to the low solubility of XT
in PF-1.3-DMCH.
the same relative amounts. Oxygen quenching experiments were performed for both XT in C H and BPT in 3-MP. 7. The ratio of the intensities of blue-shifted emission to phosphorescence, PIP, (for the 0-0 band) increases when the solvent is changed, in the order PF-1,3-DMCH < 3-MP < C H < MeCH. This corresponds to the order of the changes in the ratios of the molar extinction coefficents, @‘)/4?),of the 0-0 bands in the So SI and So TI absorption spectra in the same solvents, as shown in Table IV for X T and BPT.
-
-
Discussion It is not easy to prove conclusively that the observed blue-shifted emission is TDF. We begin by considering several other possible interpretations. First, one can exclude the possibility that the emission is due to trace impurities in the thiones or the solvents because the excitation spectra measured for several different A,, covering the TDF and phosphorescence regions are identical with the corresponding thione absorption spectra. In addition, the emission decay time is independent of both A,, (cf. Table 11) and A,, the latter encompassing excitation to the thione T1, S1, and S2 states. Emission from a triplet excimer can be excluded because of the spectral distribution of the temperature-sensitive emission and because the ratio of the intensity of this emission to that of phosphorescence is independent of thione concentration over a wide concentration range (lW3 to 10” M). Emission from higher vibrational levels of the triplet state cannot be the source of the blue-shifted emission because, for a given thione, the ratio of its intensity to that of phosphorescence changes significant1 when the solvent changes, and parallels the changes in c&030),/4~oy.(See Table IV.) Moreover, one would not expect the hequency of a hot vibrational band which carries a large fraction of the oscillator strength to vary in frequency by over 700 cm-l for the four thiones studied. Prompt Sl fluorescence can be eliminated because the measured microsecond decay time of the emission is many orders of magnitude too long (vide infra) and because the emission totally
Figure 5. Typical energy level diagram of aromatic thiones.
disappears at temperatures lower than about 220 K. (See Figure 4, Table 111.) Prompt fluorescence has never been observed from the larger thiones excited to their S1 states either at room temperature or at 77 K because kf is small whereas intersystem crossing to Tl is both rapid and efficient. In XT, for example, $,sc approaches unity24,34,35 and is, is as short as 20 ps36even at 1.7 K. We now consider the possibility, albeit remote, that the thermally activated emission might arise from T2(?r,a*). Figure 5 is a typical energy level diagram of an aromatic thione. On the basis of the spectral data presented above (Figure 2 and Table I), the energy gap between the vibrationless Franck-Condon levels of the S1 and T, states varies between ca. 600 and 1400 cm-’ for the four thiones studied. Thus, at room temperature SI can be populated significantly when in thermal equilibrium with T I , fulfilling the main condition for observing TDF from S1. However, ODMR studies have revealed that the energy gap between the Tl(n,?r*) and T2(?r,7r*)states can be of the same magnitude as the +TI gap. Values of hE(T2-T1) of between 600 and 2400 cm-I have been obtained for XT,37*38 while for PT the gap may be even smaller.39 Thus, no definitive conclusion regarding the nature of the state which is the source of the thermally activated emission can be drawn from any of the spectral, temporal decay, oxygen quenching, or temperature dependence data. The SI and T, states of the thiones are of n,?r* character whereas T2is of ?r,?r* character. It is well-known that n,?r* excited states are less strongly solvated than the corresponding ground states, leading to a hypsochromic shift in an electronic spectrum associated with an n ?r* transition with increasing solvent polarity. Conversely, ?r,?r* states become slightly better solvated than the corresponding ground states with increasing solvent polarity, leading to small but distinct bathochromic spectra shifts for ?r ?r* transitions. Thus, for a given thione AE(S,-T,) should remain approximately constant whereas AE(T,-T,) should decrease as the polarity of the solvent is increased. Thus, in order to identify the state which gives rise to the thermally activated emission, we sought to determine whether the activation energy -+
-
(34) Serafimov, 0.;Bruhlmann, U.; Huber, J. R. Ber. Bunsen-ges. Phys. Chem. 1975, 79, 202. Serafimov, 0.;Haink, H. J.; Huber, J. R. Ber. Bunsen-ges.Phys. Chem. 1976, 80, 536. (35) Kumar, Ch. V.;Qin, L.; Das, P.K. J . Chem. SOC.,Faraday Trans. 2 1984, 80, 783. (36) Molenkamp, L. W.; Weitekamp, D. P.;Wiersma, D. A. Chem. Phys. Lett. 1983, 99, 382. (37) Maki, A. H.; Svejda, P.;Huber, J. R. Chem. Phys. 1978, 32, 369. Taherian, M. R.; Maki, A. H. Chem. Phys. 1982, 68, 179. (38) Burland, D. M. Chem. Phys. Lett. 1980, 70, 508. Burland, D. M. J . Chem. Phys. 1981, 75, 2635. (39) Taherian, M. R.; Maki, A. H. Chem. Phys. Lett. 1983, 96, 541.
Maciejewski et al.
6318 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 -08
M
3 -1
-
0
o XT in PF-1.3-DMCH
\
X T i n 3-MP
m
-
-1 2
-1 i
-1 6
-1
e
-2
c
35
40
L.5 1/T [xlO'K.'I
Figure 6. log
(P/P) vs.
1 / T for XT in PF-1,3-DMCH and 3-MP.
associated with the thermal activation process is a function of the nature of the solvent. If it is assumed that the emission in question is TDF (as is shown in Figure 5), then the temperature dependance of its quantum yield, $TDF, relative to the quantum yield of phosphorescence, $p, may be written as follows1~10~15~16~40
spectra vary in parallel as the solvent is varied for both XT and BPT. PT behaves similarly, but because AE(SI-TI) is relatively small both for BPT and especially for PT, estimation of the positions and intensities of their T D F and 0-0 phosphorescence bands and their corresponding 0-0 absorption bands are subject to considerable uncertainty. We therefore ascribe the weak blue-shifted features in the room temperature emission spectra of TMIT, XT, BPT, and PT in perfluoroalkanes and relatively nonpolar solvents such as 3-MP and C H to T D F from the SI state. In these same solvents at temperatures below about 220 K, the T D F component of the emission has completely vanished leaving only phosphorescence from TI in the same spectral region. In more polar solvents, such as acetonitrile, the room temperature emission spectrum of the same thiones consists of a single broad unstructured band as a consequence of a solvent-induced reordering of the TI and TZ states. The latter observation will be the subject of a separate paper. We now examine the kinetics of the SI 2% TI process. For XT in 3-MP at 293 K, the value of kTI--.S, can be calculated from eq 1. A value of ksI--TI = 5 X 1Olos-l has been obtained p r e v i o ~ s l y .Phosphorescence ~~ quantum yield and TI lifetime data give kp = 9.8 X lo3 s-1,43 while kF calculated from the Strickler and Berg equation44is ca. 5 X lo4 s-l. Values of $TDF/$P 0.04 and AE = 600 cm-' are obtained from our emission measurements. The resulting value of kT,---sI is equal to 5.6 X lo9 SI. This is a factor of ten smaller than ksl--T, for XT, but is higher than that for benz~phenone,'~.~~ anthraquinone,I6 and c o ~ o n e n e . From ~ ~ independently measured values of $p and the ratios of $TDF/$P we can calculate $TDF. For XT in 3-MP at 293 K the value of $p extrapolated to zero concentration is 4.5 X when excited in the So SI absorption system, leading to ~ T D F= 1.1 x 10-3. This value of $TDF is large enough in XT to be readily measurable, a situation which is in contrast to its immeasurably small It may prompt SI So fluorescence quantum yield of thus seem strange that TDF has not previously been reported in thione systems. We note in this regard that, although a small value of AE(SI-Tl) favors obtaining a relatively large $TDF, the greater overlap of the phosphorescence and T D F spectra make the identification of the T D F more difficult. This difficulty is exacerbated in the thiones by the fact that kp for T I is very large whereas kF for SI is relatively small, compared with other classes of compounds in which TDF has been observed. Efficient selfquenching which, even at concentrations of 1 X M constitutes a significant TI quenching process, also contributes to the difficulty in characterizing the long wavelength emission of these compounds. Despite these problems, we believe that we have observed T D F in four thiones and have unambiguously identified SI as the emitting state. To our knowledge these are the first measurements of the photophysical properties of the SI states of aromatic thiones that have been obtained directly from emission measurements.
-
-
The notation is given in Figure 5. A plot of log (#TDF/$P) vs. 1/ T (Figure 6) is linear and yields the same value of AE = 600 cm-' for XT in both PF-1,3-DMCH and 3-MP. This activation energy corresponds precisely to the energy difference between the relaxed vibrationless SI and TI states obtained from the emission spectrum. This excellent agreement notwithstanding, because the value of AE(Tz-TI) for XT37938is not accurately known, we did not wish to exclude the possibility that its T2-T1 energy gap might also be ca. 600 cm-'. Note that temperature dependences of the same form would be observed for thermally activated emission from either SI or TZ. More meaningful, however, is the fact that AE is the same for two solvents (PF-1,3-DMCH and 3-MP) which have previously been shown to exhibit very different interaction energies with electronically excited thiones.z3*30*41,42 Moreover, although the 600-cm-I separation between the blue-shifted thermally activated emission band and the 0-0 phosphorescence band remains constant for X T in PF-1,3-DMCH, 3-MP, and MeCH, the relative intensities of these bands (at the same temperature) change significantly (cf. Table IV). Importantly, p / p from the emission spectra and t&@/e$y) from the absorption (40) Stockburger, M. In Organic Molecular Photophysics, Birks, J. B., Ed.; Vol. 1; Wiley: New York, 1973, p 91. Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970, p 373. (41) Fuchs, R.; Peacock, L. A,; Das, K. Can. J . Chem. 1980, 58, 2301. (42) Hamza, M. A.; Serratrice, G.; Stebe, W. J.; Delpuech, J. J. J . A m . Chem. SOC.1981, 103, 3733. Maciejewski, A. Proc. IUPACSymp. Photochem. 10th 1984, 143.
-
Acknowledgment. The authors acknowledge financial support through Polish Research Project CPBP 01 -19 and the Natural Sciences and Engineering Research Council of Canada. Registry No. PT, 1120-93-0; BPT, 6005-15-8; XT,492-21-7; TMIT, 74768-62-0; 02,7782-44-7.
(43) Maciejewski, A.; Szymanski, M.; Steer, R. P. unpublished results. (44) Strickler, S. J.; Berg, R. A. J . Chem. Phys. 1962, 37, 814. (45) Kropp, J. L.; Dawson, W. R. J . Phys. Chem. 1967, 71, 4499.
