Rotational effects on the quantum yield and lifetime of the slow component of fluorescence from s-triazine

CHEMICAL. PHYSICS LETTERS

Volume 84, number 2

ROTATIONAL

EFFECTS

OF THE

COMPONENT

SLOW

ON THE

QUANTUM

YlELD

OF FLUORESCENCE

AND

FROM

Nobuhiro OHTA and Hiroaki BABA Dirisim of Chemistry. Research Institute of Applied Electricity, Sapporo 060. Japun

1 December 1981

LIFETlME

s-TRIAZINE

Hokkaido

Univem’ty.

Received 3 August 1981

The tluorescence quantum yield and lifetime of the slow component of fluorescence obtained along the rotational contour of the 6; and 6; absorption bands of s-triazinc at low pressure show a marked variation. For each band, the quultumyield spectrum shows a sharp peak at the Q-branch edge. the lifetime spectrum exhibits a valley at the same position.

1. introduction

2. Experimental

Rotational effects on electronic relaxation processes in polyatomic molecules have recently received considerable interest. In formaldehyde, both the fluorescence quantum yield and the lifetime vary greatly with the rovibronic level excited. However, the decay rate shows no smooth variation with the rotational quantum numbers .I’ and K’ or the rotational energy I!?,~ in S, , though, on the average, the rate increases with increxing K’ or Erot [I] _With azabenzenes, it has recently been shown that there is a marked variation of the fluorescence quantum yield along the rotational contour of the O-O and IOaA absorption bands of

s-triazine (Nakarai Chemicals, Ltd., Kyoto) was purified by vacuum sublimation. The pressure of the sample vapor was determined by measuring the absorption intensity. All the measurements were carried out

pyrazine

and of the O-O band of pytimidtne,

the yield

value

peaking

near

the band

origin

belonging

over a rotational contour, absorption and fluorescence excitation spectra were measured simultaneously with a high-resolution laser spectrophotometric system, the details of which were described in previous papers [2,5]. A frequency-doubled dye laser, pumped by a nitrogen laser, is used in this system to generate W pukes with a bandwidth

with [2]_

The rotational state dependence of electronic relaxation in pynzine has been confirmed by fluorescence decay measurements that were made for rotationally cooied pyrazine in a supersonic jet [3]. On the other hand, rotational effects are practically absent in such larger molecules as benzene and naphthalene the statistical limit [4-61.

at room temperature. To obtain the relative fluorescence quantum yields

of -0.5 cm-1 and a duration of -3 ns. Fluorescence decays were measured by attaching a single photon counting lifetime apparatus [9], equipped with a time-to-amplitude converter,

to the spectrophotometric system. During the decay measurement, fluctuation of laser frequency was checked by monitoring the absorption intensity.

to

Recently, it has been shown that s-triazine behaves molecule which involves fast and slow components of fluorescence [7,8]. In this

3. Results

and discussion

as an intermediate-case

paper, we report the first observation of significant rotational effects on the fluorescence lifetime, as well as on the fluorescence quantum yield, of s-triazine vapor at low pressure_ 308

Figs. 1 and 2 show absorption, fluorescence excitation and fluorescence quantum-yield spectra of s-triazine vapor at 160 mTorr obtained over the rotational contours of low-lying vibronic bands at 3 16.9 (band I) and 312.0 run (band II). Both bands are as0 009-2614/81/0000-0000/$02.75

0 1981 North-Holland

Volume 84, number 2 WAVENUMBER

i December 1981

CHEMICAL PHYSICS LETI-ERS ( cni’

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Fig. I. A set of spectra for the 6; band at 316.5 nm ofs-Ukzirte vapor at r*om temperaruse. (a) Ftuorescenfe excit%tion spec) monitored at 34Q nm with 12 nm bandwidth; abtrum (sorption spectrum (:..). @) Relative fluorescence quantum-yield spectrum (--- ) obtained as the ratio of the excitation spectrum to the absorptiqn spectrum; absorption spectrum (...)_ (c) Lifetime spectrum of slow ~mponent of fluorescence (-) derived from the observed lifetime values (0); absorption spectrum (...I. The pressures ofrtriazine are 160 mTorr for the fluorescence excitation spectrum and lifetimes, and ~5.0 Torr for the absorption spectrum. See text for estimation of the,position of the band origin denoted by 0. sociated with the S, +- SO electronic transition, where S, is a i E”(n, R*) electronic state [ 101: Fischer and. SmalI [ 101 assigned these two absoiption bands as

4

l

8

”

12

16

( cm-’ 1

F&z.1. A set of spectra for the 62 band nt 311.0 nm ofs-tiabne

vapor at room temperature. (af Fluorescenu2 excitation spec) monitored at 330 nm with 12 nm bandwidth; abtnrm (sorption spectrum (...). (b) Relative fluorescence quantumyield spectrum () obtained as the ratio of the excitation spectrum to the absorption spectrum; absorption spectrum C-.-).(c) Lifetime spectru? of slow component of fluorescence f-) derived from the observed Iifetime values (0); absorption spectrum (...). The pressures of s-triazine are 160 mTorr for the fluorescence excitation spectrum and lifetimes, and 55.0 Torr for the absorption spectrum. The band ori_& de-

noted by 0 is at 32041.75 cm-‘. 6(e’)A and S(e’)A, respectively. The assignment of band I was supported by a study of single vibronic level (WT.+) ffuorescence spectra [ t 11. The assignment 309

Volume 84, number 2

CHEMICAL

PHYSICS

rotational

ofband if was not confirmed by the SVL fluorescence study. and tllis band was reassi~qned as 6; [ 12,13].

s-triazine a three-fold

is an ablate

symmetric

top molecule

with

asis. Both the absorption bands f and II are considered to be parallel bands [ 131. In fact, as is seen in figs. 1 and 2, these bands each have a very sharp Q-branch edge near the center of the band. a series of P-branch lines on the low-frequency side and a peak arising from unresolved R-branch lines on the hi$t-frequency side of the Q edge. According to the rotational annlysis of band II by Udagawa et al. [ 141, each of the P- and R-branch lines consists of many rotational lines with the same J” value and different K” vahtcs. The wavenumbers in cm- f of the P- and R-brancll lines arc espressed by v = 3204 1.75 + 0.426&11

- 0.0076m

,

(1)

where t~z= -J” for the P-branch lines and M =J” + 1 for the R-branch lines. The position of the band origin, determined to be at 3204 I .75 cm-‘, is represented by 0 OE the wavenumber asis in fig. 3. On tile other

1 December 1981

LETTERS

structure

f8]. The experiments

were car-

ried out under such pressure conditions that only the slow-fluorescence component is affected by collision. The results show that the fluorescence obtained by excitation collisions;

at band I is most liable to be quenched by still its quantum yield at 160 mTorr is high

(a4.5 X I O-3) compared with the yields obtained by excitation at the other bands. For excitation at band I, the ratio of the yield of the slow component of the fluorescence to the yield of the fast one is farger than

ten at IGO mTorr. For excitation at band II, the coliisional quenching of the slow fluorescence is not so efficient as in the case of band I; on the other hand, the ratio of the slow- to the fast-fluorescence yield is relatively small, and still it is larger than three at 160

mTorr. Therefore, the variation of the fluorescence quantum yield observed for both bands I and II is to be associated largely tith the slow component. This view is confirmed by the fact that the peaks and maxima in the quantum-yield

spectra disappear in the presence

of a foreign gas (nitrogen)

at such a moderately

high

hand. the rotation31 fine structure of band I is too complicated to be analyzed. The location of the origin of band 1. represented by 0 in fig. I. WIS estimated by

pressure that the slow fluorescence alone is affected

rcferencc to the position of the Q edge of this band and also to the ori@ns of analogous parallel bands of the same and different molecules. 1t is seen in figs. 1 and 2 that. For each of the bands I

conclusions drawn from the foregoing experimental

and 11. the fluorescence quantum yield at a pressure of f 60 mTorr &ows a marked variation along the rota-

tional contour. A sharp peak appears in the quantumyield spectrum

at the Q edge near the band origin, the

peak vaiue of the yield being about four times as large as the average. These observations are similar to what has been found in the case of pyrazine and pyrimidine [?I. In addition, the quantum-yield spectrum in fig. f or 2 shows a broad maximum in the region between the band origin and the R-branch peak. The features of the ffuorescence quantum-yield spectrum may possibly be affected by a continuous

background which is supposed to underlie the absorption band i or II. Nevertheless, it can safely be said that the observed variation of the fluorescence quantum yield reflects the rotational level dependence of electronic relaxation in the s-triazine molecule. The pressure dependence of the fluorescence quantum yields has been studied at low resolution for various vibronic bands of s-triazine without regard to

310

by collisions fluorescence

and is fully quenched. Furthermore. the decay data give evidence in favor of the

data. Fig. 3 shows fluorescence decays which were obtained by excitation at three representative positions in band 11, i.e. the Q-branch peak, the peak of the fiuorescence quantum-yield spectrum and the R-branch peak (at - 1.8, -0 and 12.4 cm- * , respectively). With

the present time resolution, afl the photons corresponding to the fast-fluorescence component, the iifetime of which is estimated to be shorter than 1 ns, fall on the first channel. Each of the fluorescence decays in fig. 3 is thus seen to involve fast and siow components, the decay of the latter component being exponential. An analysis of the observed decays reveals that the values of the ratio of the slow- to the fastfluorescence

quantum

yield are 6.2, 13.7 and 2.5, re-

spectively, for the three excitation positions mentioned above. The relative values of the total fluorescence quantum yields (i.e. the sums of the fastand slow-fluorescence yields), as obtained from the data given in fig. 2, are 2.7, 5.2 and 1.0, respectively, for the same excitation positions. By combining these two sets of values, it can be shown that the fast-fluorescence quantum

yield is constant

within

the 20%

CHEhUCAL PHYSICS LETTERS

Volume 84. number 2

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1 December 198i

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Fig. 3. Fluorescence decays of different rotational states belonging to viironic level 62 of s-triazine vapor at 160 rnTorr, obtained by mo~to~g the SVL fluorescence band at 338.4 nm with a b~d~d~ of =1 nm (6.4 nsfchannei time scale). The positions of the exciting light within 6; absorption band: (a)Qbranch peak; @)peak of the fluorescence quantum-yield spectrum; (c) R-branch peak. The intensity at the first channel, which is indicated by an arrow, is normalized to 1O3 counts/channel. The lifetimes of slow fluorescence. evaluated from the Sropes of the stra@ t lines, are shown in the figure. involved in the experiment and analysis, the observed variaticn of the total quantum yield along the rotational contour is due mainIy to the variation of the slow-fluorescence yield. The decay data for band i also lead to a similar conclusion_ The lifetimes of the slow fluorescence excited at various positions within the absorption band I or II give new ~nFo~atjon as to the rotational effect under study. These lifetimes, which were obtained f&m analysis of such decay data as given in fig. 3, are shown in figs. 1 and 2. In either case, the Lifetime varies significantly with excitation frequency, and the “lifetime spectrum” exhibits a valley at the Q edge, i.e. just at the position where the quantum-yield spectrum gives a peak. The magnitude of the lifetime variation amounts to 20% of the average value of the lifetimes in band I, and to 30% in bmd II. According to a mixed state model for interrnediatecase molecules [15,16], the quantum yield and decay rate of the slow fluorescence increase with decreasing uncertainty and that

number of triplet vibronic levels that are coupled effectively to the initially prepared singlet vibronic fevel. The present observations on the quantum yields and lifetimes, therefore, suggest that in s-trizine the number of effectiveIy coupled triplet levels should decrease as J’ or Erot in the singIet vibronic level decreases, and that the number is especially small at very IowJ’.

This work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education,

Science and Culture.

References (I ] J.C. Weissbaarand C-B. hfoore. 1. C%em. Phys. 72 (1980) 54 15, and references therein.

Volume 84. number 2

CHEMICAL

[ 21 H. Baba, hi. Fujita and K. Uchida, Chem. Phys. Letters 73 fi980) 425. 131 G. tcr Horst, D.W. Pratt and J. Kommandeur. J. Cbem. Phys.

74

(1981)

3616.

141 C.S. Pw%entet and M.D. &huh, Clrem, Phys, Letters 13 !1972) 120. 151 Ii. Baba, hl. Fujita, N. Ohta and Y. Shindo. J. Spectrosc. sot. Japan 29 (1980) 387. [ 61 N. Ohta and H. Baba, to be published.

[7] P.R. Nott and B.K. Selinger. Australian (1978) 1889.

J. Chem. 31.

181 N. Ohta and H. Baba, to be published.

]Y] I. Yamazaki, hf. Fujita, K. Uchida, Y. Shindo and H. Baba, Bull. Rcs. Inst. Appt. Elec. Hokknido 124.

312

Univ.

16 (1974)

1 December

PHYSICS LETTERS

1981

[ 101 G. Fischer and G.J. SmaU. J. C&em. Phys. 56 (1972) 5934. [ 1 I] A.E.W. Knight and C.S. Parmenter, Chem. Phys. 43 (1979) 2.37. [ f2] J. Barnard, Ph. D. Thesis, University College, London (1974).

and

references

therein.

131 J.D. Webb, K.M. Swift and E.R. Bemstcin, J. Chem. Phys. 73 (1980) 4891. [ 141 Y. Udagawa, M. ito and S. Nagakura, J. Mol. Spectry. 39 (2971)400. [15] F. Lahmani, A. Tramer and C. ‘EC, J. Chem. Phys. 60 (1974) 4431. ] 161 R. van der Wcrf and 3. Kommandeur, Cbem. Phys. 16 (1976) 125. [