Optical detection of the lowest triplet state of azulene

Journal of Luminescence 24/25 (1981) 487—490 North-Holland Publishing Company

487

OPTICAL OETECTION OF THE LOWEST TRIPLET OF AZULENE Hans-Joachim Kray and Bernhard Nickel Ahteilung Spektroskopio Nax—Planck—Institut fuer Biophysikaliache 0-3400 Goettingen, FRG

STATE

Chemiu

The lowest triplet state of azulene, Ti (AZ) , is populated by triplet energy transfer from fluoranthene (Fl) and is detected by the delayed fluorescence S2(Az) -.Sp(Az), which is caused by hetero triplettriplet annihilation T 1 (Az) + Ti (Fl) oio-* ~2 (Az) + Sp(Fl) From the time dependence of this delayed fluorescence the triplet lifetime of azulene is obtained. The temperature dependence of the triplet lifetime is explained by thermally activated intersystem crossing T1w~Si, followed by very fast internal conversion Si ~JO+ Azulene is an exception to Kasha’s rule in every respect: First, the only essily observable fluorescence is the fluorescence ~2~So [1) (lifetime 1.6 ns, guantum yield 0.03). Second, the lifetime of Si is very short (2 ps, [2]) and the guantum yield of the fluorescence Si -. SQ is very low (2 x 10—6). Third, it has not yet been possible to detect the lowest triplet stste Ti by the phosphorescence Ti • S~ or by the E-type or P-type delayed fluorescence Si • S~ or by Tn ÷ Ti absorption. Kim succeeded in the first convincing detection of T1 of azuleno in sn EPR experiment [3]; from his data follows that the 1) 0.5 is ms. known from triplet value guenching exlifetime of Ti is shorter than An spproximate of the periments [4,5]. (mhcn i3650cm triplet energy Recently we have shown that hetero triplet—triplet annihilation (TTA) opens the wsy to the indirect optical detection [6] and the determination of the lifetime [7] of Ti of azulene: In a liguid solution of azulene (AZ) and s suitable triplet sensitizer like fluorsnthene (Fl) (cf. fig. i), the following processes, So(Fl) Ti (Fl) T 1 (Az)to lesd

ho ÷ S~(Az) +

• nJO+

Si (Fl) ÷o-* Ti (Fl), S~j(Fl) + Ti (Az) ,

5o (Fl) + aT1 (Fl) °‘°fluorescence ~2 (Az) + delayed

(1), (2) (3) (4)

,

(BF)

S

2(Az) —‘S0(Az). In fig. 2 the spectrum of the OF of s solution of fluorsnthene and szulene is shown. Between 350 and 390 nm this OF spectrum is prsctically identical with the spectrum of the prompt fluorescence 52(Az) •S~(Az), excited at 313 nm. In principle, the BFs Sn(Fl) •S~(Fl), n=4, 3,2, sl— so contribute to the total BF in this spectral rsnge (cf. fig. i) but, under the chosen experimentsl conditions, they are much weaker than the OF ~2 (Az) ‘ ~o(Az). The rise of the OF intensity at 390 nm is due to hot bsnds of the OF Si (Fl) • ~o(Fl).

0 022—2313/81/0000—0000/502.75 © North-Holland

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Azulene

Fig. i. Photophysical processes leading to a delayed fluorescence S2 -s S0 of azulene (i—4) and to a delayed fluorescence of fluoran— thene (a, b, c) ; double lines mark the energies of triplet pairs.

330

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350

360

370

380

390

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8 / nm 6 M) and fluoranthene (i x io—4 in K Fig. 2. Delayed—fluorescence spectrum (—) N) of isopentane a solution atof 152 azu— (excitation lens (i i0 at 363.8 nm) ; absorption spectra of azulene in isopen— tans at i50 K C ) and of fluoranthene in methylcyclohexane at 193 K (—.—.—)

H. -i Kraj-, B. ~~\~~ckel / Lowest triplet state of azulene

489

The kinetic analysis of the time dependence of the DF S

2(Az) -.S0(Az) allows to determine the lifetime of T1 (Az). Let a solution of azulene and fluoranthene be excited by a nanosecond laser flash at time t=0. Then, on a time scale of microseconds and with neglect of TTA as a decay process (first—order kinetics with respect to triplet concentrations), the time dependence of the total DF is given by

‘DF = A1 exp(—2k t) — A2 exp(—(k+k’)t), (5) where A1 A2 and k and k are the first—order decay constants of T1 (AZ) and T1 (Fl) , respectively, [7] . A1 = A2 if, first, the population of Ti (AZ) by light absorption S2 (AZ) + S0 (AZ) and intersystem crossing (ISC) S2(AZ)ft!~T1 (Az) is negligible, and, second, the DF in the spectral range, where the time dependence is measured, results only from hetero TTA (processes (4), (b) and (c) in fig. 1) Solutions of azulene and fluoranthene were excited at 337 nm (where both compounds absorb) or at 383 nm (where practically only fluoranthene absorbs) , and the time—dependence of the DF was measured at 353 nm (0,0 transition of S2(AZ) -.S0(AZ)) or 374 nm. With excitation at 337 nm, the results were less accurate, because a mechanical chopper had to be used for suppression of the strong prompt fluorescence S2(AZ) -.S0(AZ), and because the photochemical decomposition or isomerisation of aZulene was no longer negligible. Nevertheless excitation in the S2(AZ) +S0(AZ) absorption band at 337 nm is of interest of ISC Sat least an upper bound of the quanbecause it allows to2)determine tum efficiency qisc( 2(AZ)-W~T1(AZ). Here we only mention the result, qisc(2) 5 0.04 [7]. The results obtained with excitation at 383 nm are shown in fig. 3. The values of k and k’ were obtained by fitting (5) to the measured

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150

200 T/K

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3.

Temperature dependence of the decay constants k of T 4 M, isopentane, 1(Az) excitation at 383 nm. and k of T1(Fl); [Az] = 1.0 x 10—6 M, [Fl] = 1.0 x i0

II-.! JCrav, /3 V/eke! / Lulls// Ill/I/It state

490

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time dependence of the OF. The temperature dependence constant k of T 1 (Az) can be represented by k

=

k9

+

of the decay

k1 exp(-AE/kpT),

(6)

where k0 and k1 do not depend on temperature, AS is an activation energy, k3 is Holtzmann’s constant, and T is the absolute temperature. The fit of (6) to the experimental values of 695 k in fig. 3 yields 6 5~i, AS = hex cm1. (7) k0 = curve 2.1 io~ ~ in fig. ki 3= was 2.0 xcalculated io The with the parameters (7) The temperature dependence of the decay constant k of T 1of azulene may have different causes: temperature-dependent quenching of T1 by an impurity, temperature—dependent ISO T1rr.,-*SQ, and thermally activated ISO T1 iR=Si, followed by very fast internal conversion ~i ~ (this is the radiationless analogue of the F—type riP). In ref. [7] it has been shown, that of these three possible causes of the temperature dependence of k, the first one can be excluded with certainty and only the third one is likely. This interpretation is strongly 1, [4,5]). A consequence this supported by the fact that AR approximately equals the energy ofdifferinterpretation the (7) is that for the rate constant k ence between S~ofand T1 data )~hc x 700cm 150(1( of ISO S1rc’=T1 the relationship k150 (1) 3 x k1 should hold (the factor of 3 takes into account the threefold degeneracy of T1) . With the lifetime 2 ps of ~i, a quantum efficiency q15g(l) ~1 x 10~ results for the ISO S1ftru-~T1. This very low value of q1gg(i( is in agreement with the fact that, within experimental accuracy, depopulation of ~i and repopulation of S~ are equally fast processes [8]. It also explains why T1 of azulene in a phenazine host crystal was detected by EPR only when the host was excited, but not when azulene was excited to S~ [3). The value of the low—temperature decay constant k0 of Ti of azulene is about 400 times greater than one would expect on the basis of Siebrand’s theory for the radiationless ISO ~1~r~>50 of aromatic hydrocarbons [9]. Hence, both lowest excited states of azulene, T1 and are exceptionally short—lived, and one may suspect that, on account of the proximity of T1 and S~, the cause for the short lifetime is the same for both states.

References: [1) [2) [3) [4) [5) [6) [7) [8) [9)

Beer, H. and Longuet—Higgins, B.C., J. Chem. Phys. 23 (1955) 1390. Ippen, E.P., Shank, g~y~ and Woerner, R.L., Chem. Phys. Letters 46 (1977) 20. Kim, S.S., J. Chem. Phys. 68 (1978) 333. Glandien, H. and Kroening, P., Z. Physik. Chem. HF 71 (1970) 149. Herkstroeter, B.C., J. Am. Chars. Soc. 97 (1975) 4161. Hickel, B., Chem. Phys. Lett. 68 (1979) 17. Fray, H.—J. and Hickel, B., Chem. Phys. 53 (1980) 235. Shank, ~ Ippen, F.P., Teschke, 0. and Fork, R.L., ghem. Phys. Letters 57 (1978) 433. Henry, B.R. and Siebrand, W., Radiationless transitions, in: Birks, J.B. (ed.), Organic molecular photophysics 1, p. 206 (/1. Wiley, London, 1973).