Luminescence and triplet decay in quinoxaline vapors

Cberrdcal Physics 7 (1975) 52-61 0 North-Holland Publishing Company

LUMINESCENCE AND TRIPLET DECAY IN QlJlNOXALlNE

VAPORS

B. SOEP and A. TRAMER hbomtoire Received

de Photophysique

hlol~culaire

C N.R.S..

Univenitr! Paris&d,

91405

Orsay. Fmnce

19 July 1974

The effects of the gas pressure on the quinoxaline triplet state kinetics and hunincsamce yields were studied in the 10” -10 ton pressure range. The non-exponential character of the fluorescence decay and long fluorescence lifetimes, previously reported, are contirmed. The collision-free lifetime of the “hot” triplet state measured by triplet-triplet absorption kinetics and by phcsphorescence cbllisional induction is of the order of 100~s. much longer than the long-fluorescence decay rimes (OS-70 14. The results are discussed on the basis of a model involving anharmonic coupling between triplet levels strongly and weakly coupled to the dnglet.

1. Introduction

In a recent paper [ 1) we have demonstrated that the non-exponential decay of the fluorescence of isolated molecules is a necessary consequence of the strong coupling between the radiative and non-radiative states in an intermediate size molecule. We indicated also a class of molecules where such a decay may be observed. In fact, it has been shown that at a sufficiently low gas pressure the fluorescence of pyrazinc [2,3], quinoxaline [4], and biacetyl [S] decays non=exponentially. In the simplest model, described in ref. [l], where the coupling matrix elements Vd between the radia. rive (s) and non-radiative (r) levels are equal (or are randomly distributed around a mean value I-Q), the evolution of the e_xcited system, characterized by its $nglet character S(r) = (sl$(f)) and triplet character T(t) = (Il$~(r)) (monitored respe+ively by the intensity of the fluorescence: I&) = r&f) and of the T-T absorption OD, = emT(r) or the ‘hot’ phosphotescence) may be approximated by a biexponential behavior. The decay of the prompt fluorescence and the growth of the triplet are characterized by rr , while the long fluorescence and the T-T absorption decay

with the same decay time t2. It may be expected that this model is not an adequate description of a big molecule, where some of

the l-levels (t-levels) ate strongly coupled to Is). while other ones (u-levels) are practically uncoupled. In this case. the long fluorescence originates from f-levels - while the evolution of the triplet character depends on the overall properties of t-levels and u-levels. The decay times of the long fluorescence and of the ‘hot’ phosphorescence (and of the T-T absorption) would thus be different, as was observed in the case of benzophenone [7,8] and benzoquinone [9]. This problem was treated in [lo], a somewhat different model taking into account a coupling between t- and u-levels was briefly discussed in [ 1 l] and will be developed in 1121. For those reasons it seems interesting to carry-out a parallel study of the evolution of the S(r) and T(t)

character of an isolated molecule by the fluorescence decay and the triplet-triplet absorption kinetics. Such a study was impossible in the case of pyrazine because of the weakness of the triplet-triplet absorption in the easily accessible spectral range. On the contrary, quinoxaline shows a strong T-T absorption in the 350-450 nm range [ 131. This molecule is fluorescent and phosphorescent in the gas phase [ 141, the non-exponential form of the fluorescence decay was shown by Brus and McDonald [4] who determined the collision-free lifetimes of the long fluorescence for different exciting wavelengths. Its absorption spectrum is known and partially elucidated [ 151 .The radiative and

6. Seep. A. TmmerfLuminescence and niplet

non-radiative rates of the triplet decay have been studied in condensed phases [ 161. In this work, we determined the kinetics of the triplet build-up and decay as well as the fluorescence quenching and phosphorescence induction effects in the gas-pressure range of 1OS3 - 10 torr at different exciting wavelengths in the 260-370 nm spectral region corresponding to the Se + S, , So + S, and So + S, transitions.

2. Experimental C.p. Koch and Light quinoxahne was purified by crjstallisation from cyclohexane and sublimated in vacua. Its purity was controlled by gas chromatography, 99.998% argon and c.p. SF, were used as buffer gas, SF6 was dried and repeatedly distilled under vacuum. The pressure of quinoxahne was controlled by optical-density measurements, that of SF6 - by means of Pirani gauge - calibrated with a McLeod vacumeter. For the measurements of the fluorescence and phosphorescence spectra and relative quantum yields a multi-reflection cell similar to that described in ref. [3] was used. Jobin-Yvon’s prism rnonochromator ‘Georgie’ and grating monochromator M25 were used respectively for excitation and analysis. For the phosphorescence measurements at low gas pressures the system was completed by a two-disk 1 kHz phosphoroscope. Because of the weakness of the quinoxaline lumiwe were obliged to use relatively big spectral (except for the study of finer details in the emis-

~COXIC~

slits

sion Spectnlm). In most measurements of the relative yields and of the excitation spectra, the emission was recorded using a monochromator with 15 run spectral slits centered at 415 nm (fluorescence) and 490 MI (phosphorescence); while the absorption spectrum was scanned with about 5 nm slits. All fme details of the So - St transition are, of course. completely lost in these conditions. The accuracy of the measurements of relative luminescence intensity does not exceed 15%. The main source of the error is a stray-light background and the instability of the quinoxahne pressure

due to long absorption and desorption processes on the surface of the multi-reflection cell.

decay in quinoxahe

vapors

53

Excitation spectra were partially corrected for the energy distribution by recording the excitation spectra of quinine sulfate solutions in a wide concentration range. To this aim, the liquid sample contained in a square silica cell was placed in the center of the (open) multireflection cell. The effects due to the spectral distribution of the light source, to the monochromator and the focusing lens system were thus taken into account, but not the wavelength dependence of the reflectivity of the cell mirrors. The excitation spectra reproduced in the following must thus be taken with caution. The triplet-triplet absorption was studied by conventional spectrokinetic techniques using a laser excitation at 353 or 265 nm obtained from the third and fourth harmonic of a ND3’ glass laser. A CGE laser (model VD) with an about 30 ns FWHM pulse length was used. Quinoxaline was sublimated under vacuum into a thoroughly degassed 1 meter long silica ceil. Its vapor pressure was maintained by piacing the cold finger of the cell into a constant-temperature bath. The pressure could be continuously varied in this way between 15 and 80 mtorr. Similar experiments were performed in presence of buffer gas (SF,& the pressure of which was varied in the 0- 100 torr range. The laser light was focused astigmatically in the center of the cell by means of a F = 5 m concave mirror tilted at 45” providing a constant excitation cross section of 0.19 cm2. The monitoring beam was provided either by a flash lamp (with a constant intensity over about 100 ps) or by a dc 60 W xenon lamp (Osram XBO60). A parallel monitoring beam (a cross section of 0.40 cm*) intercepts the excitation beam over the entire cell length. It was then focxed across a fdter absorbing the laser light (Pyrex glass or saturated sodium nitrite solution) cn the slit of a 0.25 m JaKellAsh monochromator with 3-18 A spectral slits. The signal monitored by a Radiotechnique 150 UVP photomultiplier was displayed on a Fairchild 777 or Tektronix 545 oscilloscope. The same system was used for measurements of decay times of the long fluorescence in a slightly differ-

ent geometry. In order to determine the lifetime of the prompt fluorescence we used the pulsed synchrotron radiation of the electron storage ring in the Orsay Linear Accelerator Laboratory (ACO) and the detection system described in [ 171.

54

El. Seep, A. Tramer/Lumfnescence

and triplet decay in quinoxaline

I 1

3. Results 3. I. Luminescence

vapors

speclm

The emission spectrum at the high-pressure limit (8 X lOBE torr of quinoxaline + 10 torr SF,) is composed of two well separated systems in the 350-460 nm and 460~54@ nm regions, as first shown by Dewey acd Hadley [ 141. The latter one corresponds clearly to the phosphorescence (as may be shown by recording the emission with a rapid phosphoroscope) and its vibrational

structure

is identical with the one pre-

viously reported [ 141. On the other hand, the emission spectrum in the fluorescence region is completely dif-

. I50

300

Xnm

c

300

Xnrn

350

-. 1

Fig. 2. Excitation spectra (normalized) of quinoxaline luminescence: (a) Fluorescence: vapor pressures: (1) 2 mtorr of quinoxaline, (2) 20 mton of quinoxaline, (3) 80 mtorr of quinoxaline, (4) the same + 0.45 torr SF6, (5) + 4.2 torr SF6. (b) Phosphorescence: vapor pressures: (1) 40 mtorr of quinoxaline, (2) 80 mton of quinoxaline. (3) the same + 1.1 torr SF6.

ferent from that reproduced in ref. [ 141 but similar to that obtained by McDonald and Bras [I 8 1: we ob-

Fig. 1. Luminescence spectra (uncorrected) of quinoxaline vapors: (a)~~,=310nm;quinoxalinepressures: (l)p= 12mton. (2) p = 40 mton, (3) p = 70 mton; @) pressure p = 60 mtori; excitation wavelengths: (I) 270 nm. (2) 305 nm. (3) 340 nm.

serve a completely diffuse band with a maximum centered at about 415 run. Both components belong to the same excited species: their excitation spectra are in these conditions practically the same and reproduce the main features of the quinoxaline absorption spectrum. When the gas pressure is decreased, the fluorescence and phosphorescence spectra remain unchanged

8. Soep, A. Tramer/Luminescence and triplet decay in quinoxaline vapors (no new bands corresponding to a ‘hot’ phosphorescence could be detected) but their intensity ratio depends on the pressure and on the exciting wavelength. The relative intensity of phosphorescence is weaker at lower pressures (below low2 torr no phosphorescence emission could be observed) and at higher excitation frequencies (fig. 1). On the other hand, the fluorescence spectrum in the low pressure limit (about 2 X 10M3 torr) is practically~identical for all exciting wavelengths in the 340-260 nm spectral range. The fluorescence excitation spectrum varies in a curious way with the gas pressure: when the pressure of quinoxaline is increased from a few millitorr to about 0.1 torr, its maximum and center of gravity are displaced towards higher frequencies; at a further pressure increase (with SF6 added), the band shifts back to the red (fig. 2a). The variation of tire phosphorescence excitation spectrum is less pronounced: the relative intensity at the high-frequency side increases monotonically with increasing gas pressure. The spectral behavior is essentially the same when SF, is added to about 10d2 torr of quinoxaline as in the case of increasing quinoxaline pressure. This behavior may be easily understood on the basis of further data relative to the fluorescence quenching and phosphorescence induction processes. 3.2. fluorescence

lifetimes

Decay of the fluorescence excited by the third (353 run) and fourth (265 nm) harmonic of the Nd laser are similar to those described in [4] _In both cases, the fluorescence contains a short component, following the exciting pulse. At the 265 nm excitation we observed the long decay with a pressure-dependent decay time yielding - by extrapolation to the zero pressure - the collision free lifetime of 500 -C150 ns. The spectra of the prompt and slow component are, within the error limit, identical. At the 3.53 hm excitation, the amplitude of the long decay is too low for lifetime estimation: its relative intensity with respect to the prompt fluorescence is at least 10 times lower than in the case of the 265 nm excitation. The measurements carried out by the single-photon-counting technique and using the synchrotron radiation (about 1 ns gaussian pulses) (X,x, = 280-310 MI) dlowed to determine only the upper limit of the short decay time: TV < 0.3 ns.

5.5

3.3. Triplet spectrum arld behavior The triplet-triplet absorption spectrum recorded at a high SF6 pressure (50 torr) is not very different from those of liquid solutions [ 131. However, the maximum extinction coefficient given in [ 131 e_ = Its 8000 mole -I P cm-t is certainly overestimated. value obtained by messuring the depopulation of the ground state in presence of the buffer gas seems to be closer to the value of E = 2500 mole-t P cm-* given by Astier et al. [ 191. If this estimation is correct, the ratio of the initial concentration of triplet molecules to the total concentration in the excitation volume varies between l/20 and l/200. At low pressures (p < 8 X tom2 torr and the 265 nm excitation the initial T-T absorption spectrum (recorded with a 100 ns delay with respect to the exciting pulse) is identical, within the error limit, with that observed at high pressures of the buffer gas. The . difference between the ‘hot and relaxed triplet absorption, clearly seen in the case of anthracene 1201 and naphthalene 12 11, is masked by broadness of the absorption band (half-width of the order of 80 nm); the vibrational relaxation within the triplet manifold thus cannot be monitored by T-T absorption measurements. Experimental data obtained at buffer-gas pressures p = 0 and p = 50 - 100 torr and a constant quinoxaline concentration show clearly that: (a) Maximum values of the apparent optical density are almost the same in the high-pressure and low-pressure limits at the same excitation conditions (wavelength and energy of exciting pulse). The triplet yield is thus practically pressure-independent. (b) The transient absorption grows with a characteristic time much shorter than the pulse length, i.e., 7<30ns. (c) In the T-T absorption decay no component shorter than 12i(scould be detected forbothpressu~~ limits and both exciting wavelengths. On the other hand, the form of the triplet decay is different at the high-pressure limit and in the low-pressure case. This effect does not result from the quenching of the quinoxaline triplet by the buffer gas (no quenching by SF6 or argon could be detected) but from the pressure effect on the efficiency of the triplet-triplet annihilation: (a) At the high-pressure limit the decay is essentially

B. Seep. A. Tramer~Luminescenceand rripler decay in quinoxaline vapors

56 IO

er our data only as an estimation of the lower limit of the collision-free ‘hot’ triplet lifetime: T < 100 ~.ls. The triplet decay at times longer than 70 ps can be fitted with the k2 value obtained from high-pressure measurements.

‘IT inillal.1055-1 -al

3.4. Fluorescence quenching and phosphorescence duction process .

0

0

I

8 .

.

’ 50

I

’

’

’

in-

m Torr

m ’

b

100

2

I 3

AO.IO-2

L

Fig. 3. Initial decay rate of the triplet absorption I/r plotted: (a) versus the quinoxslinc pressure, (b) versus the measured triplet OD : AD.

a second-order process; it may be evidenced by linear plots of the inverse optical density l/OD plotted against the time in the 2-500 s range and by the initial decay times proportional to the energy of the exciting pulse. The second-order rate constant can be only roughly estimated (because of the spatial inhomogeneity of the excitation density) as k, = 0.13 X 10” mole-l ~1. The role of the first-order process is vanishingly small, this is consistent with the decay rate of the order of 50 s-1 for the thermally equilibrated phosphorescence [ 181. (b) At the low pressure limit, the inital decay rate is, at both exciting wavelengths a linear function of the quinoxaline pressure yielding a non-zero decay rate at p + 0 (fig. 3). The least-squares treatment gives the first-order rate constant k, = 1.4 X IO4 s-l and the second-order one (rough estimation) k2 = 0.15 X 10” mole-l P s-l. The same order of magnitude of k, (= 1 .l X l& s-l) is found by plotting the initial decay against the initial optical density. Secondary processes resulting from the diffusion of excited molecules from the observation volume were taken into account by monitoring the decrease of the fundamental-singlet absorption. Nevertheless, in view of a large dispersion of experimental results, we prefer to consid-

The dependence of the fluorescence intensity, In, on the gas pressure at fied excitation wavelengths was determined by monitoring the light emitted at 415 nm (after subtraction of the stray-light background) and the absorption A = (IO - I)/lo at A,,,. The relative fluorescence yield, Qf(p), is defined’to be equal to I”(p)/Ah(p)_ The relative phosphorescence yield, Qph(,u). was determined in the same way by monitoring the light signal at 490 nm.. If the phosphoroscope was not used, this signal was corrected also for the fluorescence background estimated as 0.4 of its maximum intensity (see fig. 1). In the lowest pressure range, the excitation monochromator was replaced by a glass Schott UC 11 filter and the absorption

A

3LOnm

D 310 nm 0 280nm

2

h P. 10-2Torr

Fig. 4. Relative fluorescence yield
B. Seep. A. Tramer/Luminescence

and mpler decay in quinoxaiine

57

vapors

Table I

&xc

‘2

(IrS)

this work 265 280 290 300 310 320 340 353

-

100 200 400 100 1200 > 2000

0.5 -

ref. 141 -1 -3 8.2 14 28 - 45 -

was assumed to be proportional to the gas pressure. The relative fluorescence yield’ shows a rapid decrease in the 10S3 - 5 X 10-Z torr pressure range and attains at higher pressure on almost constant value Qr(-) (fig. 4). We consider @r(m) as the relative yield of the prompt fluorescence, not affected by collisions in the pressure range of few torr and Qr(p) Qf(-) as that of the slow one, which is very efficiently quenched. If [Q&J) - Qr(=)]-’ is plotted against p one obtains linear plots conform the Stern-Volmer law. The values of (lit) deduced from the line slopes divided by their origins and ratios [Qf(O) - Qr(=)] /Qr(=) obtained by extrapolation top = 0 are given in table 1 for different exciting wavelengths. The difference between the (/CT) values determined in the case of quinoxaline self-quenching and in that of the quenching of 10d2 torr of quinoxahne by SF6 does not exceed the error limit of about 10 - 20%. (kr) decreases with increasing excitation energy in the 320-280 nm spectral range in the same way as the zero-pressure lifetimes 141. By dividing (kr) by r for a given A,, one finds an almost constant value of the quenching constant k - slightly higher than that determined from the pressure dependence of the long fluorescence lifetime and of the order of the (total) collision rate in the hard-sphere approximation. On the other hand, a very efficient fluorescence quenching in the 340 nm region shows that the long decay takes place also in the case of the So--St excitation. Its relative intensity is however lower and its lifetime * We did not try to determine luminescence yields. A very rough estimalion yields at the high pressure limit the orders of magnitude of Qf(-) = IO4 and Qph(-) = 10m2.

lw212X107

[eftobQff”wC?r<~)

wiplet

bs)

this work

ref. [41

(rrs)

12.5

100 c -

s 10 20 35 60 > LOO

5 8.5 20 27 4s - 75

14 20 20 -

rc400

100cr

Seems to be very long - the fluorescencequenching technique seems to bc more sensitive in this case than the direct determination of decay curves. The values

[Q,(O) - Q,(-)IIQr(-) ratios are in surprisingly good agreement with the data of ref. [4] obtained

of the

by a quite different technique. The pressure dependence of the relative phosphorescence yield Qph(P) may be deduced by combining the high pressure data obtained with a narrow-band excitation and the low-pressure measurements, where we were obliged to use the phosphoroscope and a broadband (glass fiiter) excitation_ As can be seen in fig. 5,a major part of the phosphorescence intensity is collisionally induced, this process taking place in a much wider pressure range than that of the fluorescence quenching. The phosphorescence yield in collision-free conditions, if different from zero, is certainly lower than I/ 100 of its limiting high-pressure vdue. The form of the phosphorescence induction curves suggests that many collisions are necessary for the stabilization of the triplet. If Qph(-)/Qph(p) is plotted against l/p strong positive deviations from the linear dependence are found for any exciting wavelength, as well for quinoxaline-quinoxaline as for quinoxaline -SF6 collisions. Since a linear relationship is expected in the model, where one efficient collision is sufficient for the triplet stabilization, the observed curves correspond to a step-by-step relaxation with a non-neghgible probability of radiationless deactivation from intermediary levels. On the other hand. the dependence of the phosphorescence induction curves on the exciting wavelength is much less pronounced than in the case of the fluorescence quenching. In the preceding, the effects of the triplet self-

I-

B. Seep.

58

0

0.1

A. Tramer/Luminescence

0.L

and triplet decay in @taralike

p Torr

Fig. 5. Relative pbosphorescencc yield plotted agtinrt the toti gx pressure for different exciting wnvelengtbs. (3) (0) 280 nm. (0) 310 nm md (0) 350 nm. (b) Low pressure data in the cz~se of broad-bond excitation.

quenching (estimated from the dependence of rph on the quinoxaline pressure [ 181) have been neglected. If this effect is taken into account, the deviations from the linear dependence Qph(m)/Qph(P) =f( l/p) will be still more pronounced. The pressuredependence of the fluorescence and phosphorescence excitation spectra is a simple consequence of the variation of the fluorescence-quenching and phosphorescence-induction rate constants on the excitation energy. When the gas pressure is increased from a few millitorr to about 100 millitorr, the longlived fluorescence of lower levels is efficiently quenched while that originating from higher ones is relatively less affected - the ma?timum of the excitation spectrum shifts to shorter wavelengths. A further increase of the pressure reduces the fluorescence yield from higher levels and shifts the spectrum back towards the red. In a similar way, higher pressures are necessary to induce the phosphorescence in the case of the shortwavelength excitation.

4.

Discussion

The principal result of this work is the observation of a tbreetomponcnt decay composed of: (i) a prompt fluorescence with a lifetime TV < 1 ns

vapors

as expected in the statistical limit, (ii) a slow fluorescence (r2 = 0.5 to about 50 /_ts), and (iii) a much longer decay of the non-relaxed triplet levels isoenergetic with directly excited singlet ones (73 = 100~s). The (kr) values, deduced from the Stern-Volmer plots are in reasonably good agreement with those of direct lifetime measurements if the quenching constant /i is assumed as equal to about (2-3) X 10’ s-t torr-* corresponding to the collision diameter of 1214 A. By combining both sets of data, we obtain the collision-free lifetimes of the slow fluorescence decreasing with increasing excitation energy from about 100 /JS at the 340 nm excitation to 0.5 ps at 265 run. We were unable to determine directly the lifetime of the ‘hot’ triplet levels. From the kinetics of the TT absorption we may only estimate the lower limit of the collision-free triplet lifetime as TT > 100 ps at both exciting wavelengths of 265 and 353 run. On the other hand, its upper limit may be roughly estimated from the upper limit of the ratio of phosphorescence yields at the low and high-pressure limits Q,,(O)/ Qph(=) < 0.01. Since the phosphorescence lifetime of saturated quinoxaline vapor is of the order of 20 ms, and independent of the buffer gas pressure, this ratio points to a ‘hot’ phosphorescence lifetime at the low-pressure limit shorter than about 400 J.S (the corrections for the efficiency of a high-speed phosphoroscope in the case of a short-living emission being taken into account). The threshold pressure for phosphorescence induction (corresponding to an average between-collision time of about 30 ~.ls) is also consistent with this estimation. At last, a slight dependence of the phosphorescence-induction efficiency on the exciting wavelength as well as the transient absorption kinetics suggest that na drastic changes of the ‘hot’ triplet lifetime may occur between 350 and 265 nm. The lifetime of the long-lived fluorescence is then much shorter than the triplet lifetime for all exciting wavelengths (except, maybe, lowest levels of the St state where the extrapolated value of rn may have the same order of magnitude as rl) and much more sensitive to the vibrational energy excess. One can conclude that the case of quinoxaline is similar to those of benzophenone [7,8] and benzoquinone (91, i.e., that the slow fluorescence originates from specific molecular levels resulting from a strong coupling between singlet

B. Seep. A. TramerfLuminesc~nce

and a limited number of triplet levels, while the triplet decay time is determined by an average lifetime of all levels resulting from the s-1 coupling. Let us make a rough estimation based on the experimenla! data for the 310 nm excitation: 72 = 2.5 X 10m5 s, Qz/Q, = 25 and a tentative assumption of r1 = lo-lo s (consistent with a rough estimation of the singlet radiative lifetime and of the prompt-fluorescence yield). This value corresponds to the total width of the band of coupled states: A = 1/2m1c

= 5 X IO-’

IV=Ap=5510’. On the other hand, the number of effectively coupled levels involved in the long fluorescence at 3 IO nm excitation may be estimated from the ratio of initial amplitudes of the prompt and slow fluorescence[l]:

=Qf(m)~~/iQf(O>-Q~f-)l’l

= 104.

The difference between N and NeK will be still more pronounced for higher excitation energies, because the C,/C, increase is certainly slower than that of the level densities ( 10tl/cm-’ at 280 nm). If the estimation of the St radiative lifetime [4] r, = lo6 s-l is correct, the radiative decay rates of strongly coupled levels would be of the order of I/T,d

= r,&,

=

10’

IS=-

vapors

59

-

cm-‘.

The density of the triplet levels at 310 nm (calculated by the Haarhoff formula 1221 using the set of groundstate frequencies [23] and naphthalene frequencies when the data for quinoxaline are lacking) is about 10q/cm-t for the lowest 3n,n* state and about 7 X lo6 for the second (3n,rr’) state. The total number of states included in the band A would thus be of the order

iv eTT=C,/C2

and triplet decay in quinoxaline

S-I,

negligible with respect to the observed triplet decay lifetimes constant (I/r3 = lOa s-I)_ The microsecond of the long fluorescence cannot be thus explained by the radiative properties of strongly coupled levels. On the other hand, in a model involving a radiative decay of strongly coupled levels, ~2 is proportional to N,, and the ratio of initial amplitudes of the slow-toprompt fluorescence to l/Nen. The relative yields would be thus fairly constant when Nerr varies with &.,,c. In our case, the relative yield of the slow fluorescence is roughly proportional to 72. i.e., to New At

Fig. 6. Scheme of ze!o-order lcvcls and linal disfribution rhe singletcharacter bt*tween molecular levels. last, no drastic

shonening

of 72 with

increasing

of

excess

of the singlet vibrational energy AE, would be expected. If the number of I-levels coupled. to s is approximated by N = ui $and since A = 02 p, Increases with AE,, in polyatomic molecules (see, e.g., (25.261 and also [27]) N would increase still more rapidly. On the other hand, if N is approximated (as it may be done in a kinetic treatment (51) by p,/ps, its value depends slightly on AEvib. All these effects may be easily accounted in the framework of a model extensively described in [I21 and developing the treatment applied to the interpretation of the radiative decay in isolated benzophenone molecules [7,8], its outlines will be briefly summarized here* : The concept of triplet levels strongly coupled to the singlet (because of the presence of active modes, symmetry properties and Franck-Condon factors) are justified as far as the triplet levels may be correctly * An alternative interpretation was proposed by McDonald and BNS 141. Individual sinaet levels would be coupled to different sets of triplet levels giving either the’stalistical’ or the ‘small-molecule’ behavior. The dewy abserved at 3 r&itively broad-band excitation would be a superposition of parallel. independent decays. No such alternation of eoupling schema was observed in the molecules. where singlevibronic levels may be selectively excited [3,X. 261. Such independent decays are very improbable in the case of excitation of higher vibronic levels of Ihc S2 s*%te.where the singlet levels overlap and are strongly mixed.

described in the harmonic

approximation. The anharmonicity (and vibronic coupling if the triplet levels belong to different electronic states) will mix the strongly and weakly coupled levels of the zero-order approximation. If we take as a starting point the f-manifold divided into two groups: t-levels characterized by nonzero value of the s-l coupling matrix element usr and u-levels (LJ_ = 0) and suppose a random distribution oft - u anharmonic coupling constants, urr would be redistributed over the bands 6 of I-levels surrounding a t-level. The decay of an excited state prepared by a coherent excitation of the whole band A takes the form of a sequence: (i) decay of theaon-stationary s-state with a lifetime dependent on the total width A of the band: prompt fluorescence; (ii) decay of non-stationary t-states with a lifetime determined by the width 6 of the u,, distribution: long fluorescence: (iii) incoherent decay of individual molecular states - mainly ‘hot’ phosphorescence or non-radiative decay because of negligible singlet character of these levels. Since the anharmonic coupling increases with the excess of vibrational energy with respect to the lowest levels, l/r2 Z=6 will be higher for higher excitation energies. It should be pointed out that the relative l.ields of the slow fluorescence excited at different wavelengths are roughly proportional to their lifetimes, i.e., that the initial amplitudes are similar at least for X < 320 nm. This would be expected if the s + t + u process may be considered as the main deactivation path. Since the levels resulting from a strong coupling have already a largely predominant triplet character, we can expect that the t-u evolution would not influence f(t) which reaches a value of the order of N’/ (N’+l) after the time corresponding to the short fluerescence lifetime TV, practically identical with its final value ofN/(N+l). In fact, we observe an extremely rapid building up of the T-T absorption and its fur-

ther evolution does not contain any component on the time scale corresponding to the slow fluorescence_ A difference between the efficiency of collisions of the fluorescence quenching and phosphorescence b&action is a natural consequence of the applied model. A very weak interaction, necessary for a transfer of a molecule to a neighbour, weakly coupled level is suf-

ficient to quench its fluorescence. On the other hand,

the low yield of the ‘hot’ phosphorescence .&ems to be due to a more efficient non-radiative deactivation of vibronically excited triplet molecules_ The triplet is ‘stabilized’ by a vibrational relaxation to its lowest vibronic levels which necessitates a number of colhsions. Such a supplementary deactivation channel is not a general property of molecules; it seems to be absent in the case of naphthalene, which shows a pressure-independent phosphorescence yield in the 10s3 - 10-L torr pressure range [24] and of biacetyl (at least for low excitation energies) [S] but was already observed for pyrazine [3]*. In the case of quinoxaline the direct measurements as well as the pressure effects do not suggest any important change in the triplet lifetime between 265 and 350 nm. The hypothesis

that the second n,n* triplet located at 24 600 cm-’ introduces a new deactivation path may be taken in consideration. The difference between the phosphorescence induction curves for lower and higher excitation energies may correspond to the number of collisions necessary to relax the molecule below this threshold_

Acknowledgements The authors are highJy indebted to Dr. J.R. McDonald for his communication of experimental results prior to publication. Thanks are also due to Dr. R. Lopez-Delgado for collaboration in the short-fluorescence lifetime measurements carried out in the LURE (synchrotron radiation) laboratory. *New data for biacetyl (A.Z. Moss. Y.T. Yardley, J. Chem. Phys. (submitted for publication) show a strong decrease of the triplet lifetime with vibrational energy CXCCSS.

References

I1 I F. Labmti,

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and niplet decay in quinoxaiine

vapors

61

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