T1–T2 inversion in aromatic thiones

T1–T2 inversion in aromatic thiones

Volume 143, number 6 CHEMICAL PHYSICS LETTERS 5 February 1988 T,-T2 INVERSION IN AROMATIC THLONES A. MACIEJEWSKI Faculty of Chemistry, Adam Mickiew...

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Volume 143, number 6

CHEMICAL PHYSICS LETTERS

5 February 1988

T,-T2 INVERSION IN AROMATIC THLONES A. MACIEJEWSKI Faculty of Chemistry, Adam Mickiewicz University,Grunwaldzka 6, 60-780 Poznan. Poland

M. SZYMANSIU Institute of Physics, Adam Mickiewicz University,Grunwaldzka 6, 60- 780 Poznan, Poland

and R.P. STEER ’ Depariment of Chemistry, Universityof Saskatchewan, Saskaloon, Saskatchewan, Canada S7N 0 WO Received 8 October I987

The emission spectra and phosphorescence lifetimes and quantum yields, of xanthione (XT), 4H-pyran-4-thione (PT) and 4H- I-benzopyran-4-thione (BPT) have been measured in five solvents of varying polarities at room temperature. A dramatic change in the T, +SO phosphorescence spectrum is observed, and this is interpreted in terms of inversion of the 3(n,x*) and ‘(RJ*) states with increasing solvent polarity. Only modest variations in the radiative and non-radiative decay constants for the triplet are observed, and no evidence of bi-exponential decay is found. Strong coupling between T,-T2 and between the singlet and triplet manifolds is indicated. Comparisons are drawn with ketones of similar structure.

1. Introduction

Reordering of the energies of the n,ff* and A,IC* triplet states of polyatomic molecules induced by changes in the polarity or temperature of the host medium is well established for several classes of compounds, including the aromatic ketones [l-6] and N-heterocyclics [ 1,7,8]. Variations in hE( T,-T, ) can be detected by characteristic changes in the steady-state phosphorescence emission spectra, by the appearance of bi-exponential phosphorescence decays, by quite dramatic changes in the phosphorescence quantum yields in some cases, and by changes in the polarization of the emission as a function of wavelength. The aromatic thiones appear to offer excellent opportunities for the observation of T,-T;? inversion. In non-polar solvents their lowest triplet and lowest excited singlet states are of n,A* configuration ( ‘A2 ’ To whom correspondence should be addressed.

and ‘Al in CZvsymmetry) and are separated by only ~400-1500 cm-’ [9-121, For some compounds, thermally activated delayed fluorescence has been observed in fluid solutions at room temperature [ 131. The second excited singlet states are most often of IC,K*configuration (‘A,) and are well separated from S, [ 14-l 61. Large singlet-triplet separations are characteristic of ~,n* states, so it is anticipated that T2 ( 3A,) might be found at energies similar to those of S, and T,. This qualitative expectation has been borne out by semi-empirical calculations [ 121 and by measurements of the zero-field splitting parameters of several structurally related thiones, including xanthione, in both low-temperature Shpolskii matrices and xanthione crystalline hosts [9-l 2,171. Unusually large values of 1D* I (up to 24 cm- ’) were obtained in the latter experiments, and these have been interpreted in terms of strong spin-orbit interactions resulting from the close proximity of T, and Tz [9-111. Despite these clear indications of the possibility of

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T,-T2 reordering with changing solvent polarity, to our knowledge no experimental investigation of this matter in thiones has been reported. In an earlier report we hypothesized that the changes in the shapes of the phosphorescence spectra of several thiones (exhibiting thermally activated delayed fluorescence) with increasing solvent polarity might be a consequence of T,-T2 reordering [ 131. The present paper describes our further investigation of this possibility.

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also corrected to infinite dilution for S,+T, excitation in order to get values of &( T, ) , as previously described [ 201. Absorption spectra were measured with Cary 118 or Specord M-40 (Carl Zeiss, Jena) instruments, and corrected emission spectra were obtained using a Spex Fluorolog 222 spectrofluorometer controlled by a Datamate computer. Some uncorrected spectra were obtained in early work using a modified Perkin-Elmer MPF-3 instrument. Lifetimes were obtained by exciting with the 5 14.5 nm line of a cavitydumped cw argon ion laser of Spectra Physics origin.

2. Experimental 3. Results and discussion The structures of the thiones studied in the present work are shown in fig. 1. The thiones were synthesized from the corresponding ketones by sulfuration with P4S10using published methods [ 18,191. Both thiones and solvents were purified by standard techniques. Perfluoro-1,3-dimethylcyclohexane (PF-1,3DMCH, PCR Chemicals) was purified by distillation and column chromatography and was checked for H-containing impurities by NMR and gas chromatography. Samples were rigorously deoxygenated by repeated freeze-pump-thaw cycles or by flushing with 02-free He. Phosphorescence quantum yields, $,,, and T, lifetimes, rr, were measured as previously described [ 20,2 11. In PF-1 ,fDMCH and 3-methylpentane (3MP) solutions, rT, was measured at several concentrations and the results were cast in Stem-Volmer form to obtain values at infinite dilution, rf,, by short extrapolation [20]. In other solvents, measurements of TV, were made at a single finite concentration, and the corresponding values of 79, were calculated by correcting for self-quenching using accurately known values of the bimolecular selfquenching rate constant [ 221. Quantum yields were

PT

BPT

XT

Fig. 1. Structures of the thiones examined in the present work. PT = 4H-pyran4thione; BPT = 4H- I -benzopyran4thione; XT = xanthione.

560

Copious data concerning the structure and decay dynamics of thione excited states which are relevant to the present studies have previously been published [ 9-17,23-261. These include the absorption and emission spectra of many thiones in a variety of media at temperatures from = 1 to 298 K [9-l 3,231, studies of the rates and mechanism of S, *T, decay [25], ODMR studies of T, [9-111, and our previous work on thermally activated delayed fluorescence [ 131, triplet and singlet decay dynamics [ 14,24,26] and photochemistry [24]. The accumulated data have permitted the construction of detailed Jablonski diagrams for a number of thiones, as depicted in fig. 2 for xanthione embedded in a nonpolar medium. Accumulated spectroscopic data show that the S,-T, energy gap ranges from = 400 to 1500 cm- ’ for aromatic thiones in non-polar media [ 13,201. Calculations and the results of ODMR experiments suggest that the T,-T2 gap may range from %400 to w 2500 cm-’ in isolated molecules [ 9- 111. Based on the well-known behavior of the ketones, it is therefore apparent that the energy difference between the T, and T2 states should diminish as the polarity of the solvent is increased, and may become inverted in media of high polarity [ 1,3,5,27-301. Fig. 3 shows the corrected emission spectra obtained on exciting XT in five different solvents of widely differing polarity (PF-1,3,-DMCH, 3-MP, benzene, methanol and acetonitrile) at room temperature. In all cases excitation was in the strong So-tSz absorption system at 365 nm using spectral bandwidths of 1.8 nm for both excitation and emis-

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Volume 143, number 6

s2

5 February 1988

2333Etcm-’

-----

T2 C3A,l

s2-40

SI

t

)

1514E T,

(3A2)

Fig. 2. Jablonski diagram for XT in non-polar media. The rate constants are those quoted and obtained in refs. [ 13123,241for 3-MP solvent. S,, S2 and T, energies are from ref. [ 121.

sion. The concentration of XT were from 0.9X lo-’ to 1.5 x 10-j M where self-quenching was negligible for Sr, but accounted for roughly half of the T, decay events. The phosphorescence observed in PF-1,3DMCH and 3-MP solution is typical of that ohserved from the 3(n,lc*) states of rigid thiones in nonpolar solvents at room temperature, and consists of a number of broad vibrational features diminishing monotonically in intensity to longer wavelengths. (The weak feature near 630 nm, located immediately to the blue of the first strong phosphorescence band at 668 nm is thermally activated delayed fluorescence from S, [ 131.) By comparison, the phosphorescence spectrum in the most polar solvent, acetonitrile, consists of only one broad feature with A,,,,, at 720 nm. In solvents characterized by intermediate values of the solvatochromic parameter, A*, the spectra exhibit some residual structure but are much broader than in 3-MP or PF-1,3-DMCH. Similar effects are observed for 4H-pyran-4-thione (PT) and benzopyranthione (BPT) in the same five solvents. Based upon these initial observations, it appeared possible that T,-TZ inversion in more polar

WAVELENGTH

(nm)

Fig. 3. Corrected emission spectra of XT in (a) PF-l,EDMCH, (b) 3-MP, (c) benzene, (d) methanol, (e) acetonitrile, at room temperature. Concentrations are from 0.9X 10e5 to 1.5X 10m5 M.

solvents could be responsible for these changes in the phosphorescence spectra. This possibility seemed all the more likely when the effects of solvent polarity on the S2+S0 fluorescence and T,dS,, phosphorescence spectra were compared. Using ketones as a guide, both dispersive and dipole-dipole (or dipole-induced-dipole) interactions are expected [ 3 l-341 to be greater in the Sz, ’(n ,x*) states owing to the much larger oscillator strengths of their radiative transitions to the ground state and their larger dipole moments [ 35-371. As expected, a large bathochromic shift in more strongly interacting solvents is observed in the S2+S0 fluorescence spectra but, although some resolution is lost and the overall intensities vary [ 141, the shape of the spectrum remains approximately the same (cf. fig. 3; accurate spectral shift data are available [ 141). The phosphorescence spectra, whose band frequencies are expected to be less strongly influenced by solvent polarity on the grounds of the lower T,-S, 561

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oscillator strengths and triplet dipole moments, change shape drastically. The shift in the frequencies of resolved bands, however, is minor between PF- 1,3DMCH and 3-MP, the two solvents for which accurate band frequencies can be determined, and amounts to only 5 30 cm- ‘. This is to be compared with x 200 cm- ’for the corresponding shifts in the fluorescence bands [ 141. In addition, one may note that the T,-& phosphorescence spectra of compounds, whose n,lc*, T, states are well separated from the upper triplets, exhibit only minor changes in ‘shape with increasing solvent polarity [ 32,38,39]. Visual inspection of the spectra (fig. 3) might suggest that in benzene the 3(n,rr*) state is lower than the ‘(Ic,R*) state and that the majority of the emission intensity is of ‘( n,rc*) origin, On the other hand in methanol the 3(rc,~*) state would seem to be almost equiergic with the 3(n,z*) state and contributes about the same emission intensity. The spectra in methanol and ethanol are identical at room temperature, but the spectrum in ethanol at 77 K (not shown) resembles that in 3-methylpentane at 298 K. These observations suggest that the diminution in the solvent-solute interaction which accompanies cooling of the ethanolic solution to 77 K is sufficient to reverse the order of T, and T,, and that T, is of 3(n,a*) character is the glass. The appearance of bi-exponential phosphorescence decay of compounds dissolved in solvents chosen so that T, and Tz are nearly equiergic constitutes almost conclusive evidence for the T,-T2 inversion process [ 2-4,28,30,40,4 11. We therefore measured the phosphorescence lifetimes of XT at various

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wavelengths spanning the emission spectra in all five solvents. Single-exponential decay was observed in all cases. The lifetimes depended mildly on solvent but were completely independent of &,,, for a given solvent. Phosphorescence quantum yields were also measured, and were corrected [24,26] for &,(S&$,(T,), which is < 1. Together with the lifetimes, the resulting values were then corrected for self-quenching using l/r -,.,= UT:, + ,& [ thione] and 4: =q&t~,/~~, to give the values of rt, and & at infinite dilution given in table 1. The values of the radiative rate constant, k,, and the apparent nonradiative rate constant, k,, were then calculated from k,=&(T,)/r$ and k,,= [ 1-@(T,)]/T?,. These results are also given in table 1. The values of k, exhibit a modest increase with increase solvent IC*.Except for methanol the values of k,, do the same, and it is possible that the factor of 3 increase in methanol may be due to reversible photochemistry since triplet XT is photochemically active in ethanol but not methanol [ 23,441, even though the triplet lifetimes are similar in the two alcohols but shorter than in 3-MP or acetonitrile [ 441. The remarkable feature of these data is that the radiative and non-radiative rate constants remain relatively constant while the spectra change so drastically. We rationalize these observations on the basis of strong coupling between the T, and Tz states and between the singlet and triplet manifolds. Large spin-orbit interactions between the 3(n,n*) and the ‘( ~c,Ic*) states have previously been observed by ODMR, Zeeman effect, and high-resolution optical

Table 1 Properties of XT decay in live solvents and relevant solvent properties Solvent

lo-‘$;(sz)

=’

0 b,

7TI

T(SZ,T,)

(PI

PF-1.3-DMCH 3-MP C,H, MeOH H,CCN

3.8 3.5 2.9 8’ 0.9 8’ 2.6 8’

8.2 7.1 5.6 g’ 1.7 r) 4.9 8’

0.65 0.59 0.48 h’ 0.48 h.‘r 0.48 h.il

1k.r (10-S s-1)

k c’

1.2 1.4 1.8 5.8 2.0

7.1 7.6 10 11 11

31For excitation to S1. b, Efficiency of T, formation. ‘) k,=@~(&)lq(&, T,)T?,. e, For perfluoro-n-heptane. n For n-hexane. r’ Corrected for selfquenching using data of ref. [ 221. h, Calculated assuming &,( S, )/$,(T,) =0.75 as for XT in 3-MP. ‘I Calculated assuming r)(S1, S,) =0.64 as for XT in C6H+

dJFrom refs. [42,43].

562

.z

P

l.765e’ 1.895 2.28 32.62 35.95

0 0 0 1.7 3.7

(ks-‘)

fad,

(D) -0.39 =’ - 0.08 ” 0.588 0.586 0.71

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CHEMICALPHYSICSLETTERS

spectroscopy of triplet XT in low-temperature matrices [ 9,11,17 1. On the basis of the observed values of 1D* I= 11 and 20 cm-‘, Burland [ 111 estimated that the T,-T2 energy gap was 1600 and 2400 cm- ’

in two sites in a xanthone host. Taherian and Maki came to essentially the same conclusion on the basis of spectroscopic measurements in n-alkane matrices [ 9, lo], but estimated that the gap could be as low as 600 cm-‘. It is therefore anticipated that in more polar media where AE( TZ-T, ) is much smaller, T2-T, spin-orbit interactions will produce even greater triplet spin sublevel displacements. Strong spin-orbit coupling could also occur between the ‘(n,x*) and ‘(x,x*) states, which must be in close proximity in these molecules and between the ‘(n,n*) and ‘(n,K*) states, which are separated by only x 8200 cm-’ in XT. The ’(n,n*) state is expected to increase in energy with increasing solvent polarity relativetothe3(7r,rc*) state [31,33].IfE(T,)>E(S,) in the isolated molecule, the degree of singlet character admixed into the triplet states (and vice versa) will increase in more polar solvents. The observed increase in k, with increasing solvent polarity may be a consequence of this effect. Vibronic coupling between the two triplets, induced by out-of-plane bending modes, is also expected [ 11, and these could be particularly important given the fact that xanthione is likely slightly bent in its equilibrium conformation even in wholly non-interacting environments [ 231. Additional evidence for strong coupling between T, and Tz may be found in measurements of the rates of photochemical consumption and self-quenching of triplet XT in a variety of aprotic solvents [ 22-24,441. Bruhlmann and Huber [ 22,441 have shown that the rate constants for triplet self-quenching are diffusion limited and that both these and the rate constants for net photochemical consumption of XT in aprotic solvents (including 3-MP, benzene and acetonitrile) vary inversely with the viscosity over a range of polarities. As with the values of k, and k,,, obtained in the present work, no exceptional change in reactivity or self-quenching rate constant is found for large changes in solvent polarity. The implication is that T, and T, are mixed to the extent of indistinguishability with respect to their chemical selfquenching and photoreactivity in all solvents. This behavior is to be contrasted with that of ketones such as acetophenone [ 291, indanone [ 6,401,

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benzophenone and its derivatives [ 30,4 11, and especially xanthone [ 2-51, In the latter molecule, the structural analog of XT, T, and TZ are very closely spaced and are present in proportions determined by the Boltzmann factor, exp[ -AQT,-T,)IkT] [2,5]. However, large differences in photochemical reactivity and self-quenching rate constants are found as the equilibrium proportions of T, and T2 change with solvent polarity [ 5 ] . The lowest triplet of xanthone, is of X,R* configuration [ 2-51, and interacts with Tz, of n,tt* configuration by strong, environment-sensitive, spinorbit interaction [2-41 just as in xanthione. Spin-orbit mixing with singlet states is much weaker than in the thione, however, as indicated by the difference in T,-S, oscillator strengths of more than two orders of magnitude. The vibrational structure in the ketone phosphorescence spectra remains similar in a variety of hosts, and that identified with the 3(n,x*) state consists mainly of a totally symmetric progression in the C=O stretching mode. In xanthione, however, the spectral shapes are very sensitive to the polarity of the environment, and the 3(n,n*) emission in low-temperature matrices involves very little of the C=S stretching mode. Such differences suggest that a strong vibronic coupling mechanism, such as the pseudo Jahn-Teller case described by Lim and co-workers [ 1,27,45] leading to large distortions of the potential energy surfaces, may also be operative in xanthione. The available evidence therefore suggests that strong coupling, perhaps involving both spin-orbit and vibronic mechanisms, exists between T, and T2 and between the singlet and triplet manifolds in these molecules. One therefore observes phosphorescence from highly mixed states, leading to single exponential phosphorescence decay and only small variations in k, and k,, as the solvent polarity is changed and T, and Tz invert in energy, Experimental and theoretical studies of the T,-T2 interaction in xanthione and other thiones are continuing in our laboratories.

Acknowledgement The authors gratefully acknowledge the continuing financial support of the Natural Sciences and Engi563

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neering Research Council of Canada. Part of this work was also supported under Polish Research Project CPBP 1.19.

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