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Rotational invariance of the fluorescence quatum yield in S1 pyridine
Volume
15I, number 3
CHEMICAL
14 October
PHYSICS LETTERS
1988
COMMENT ROTATIONAL INVARIANCE OF THE FLUORESCENCE QUANTUM YIELD IN S, PYRIDINE Young-Dong
Shin and E.C. LIM
Department of Chemistry,
Wayne State Unrversity, Detroit, MI 48202, USA
Received 25 August 1988
In a recent paper (Chem. Phys. Letters 148 (1988) 507) the reported rotational dependence of the fluorescence experImental artifact. There is, m fact, no variation m fluorescence quantum yield across the rotational envelope.
In a recent paper [l] from this laboratory, it has been reported that S,(‘B, n7c*) pyridine exhibits a strong angular momentum dependence of fluorescence quantum yield in a supersonic jet that can be explained by invoking the coupling of the S,(nx*) state with nit* singlet states via orbital-rotation interaction. The purpose of this Comment is to point out that the “rotational dependence of the fluorescence” is not genuine, but is caused by the fluorescence saturation of the photomultiplier, which occurred despite the extremely small quantum yield (< 10T4) [2], the very low molecular density of the seeded supersonic beam and what we believed to be reasonable caution. The USC of a high-gain photomultiplier and/or strong attenuation of the fluorescence leads to the fluorescence excitation spectrum which is identical (within experimental errors) to the corresponding absorption spectrum. Thus, there is no variation in fluorescence quantum yield across the rotational envelope. Orbital-rotation interaction may be important in influencing photophysical properties [I], but the S, radiative decay of pyridine does not provide such an example as proposed in the earlier paper. The rotational invariance of the fluo-
308
is due to an
rescence yield in supersonic jet invites a conclusion that singlet-triplet intersystem crossing in pyridine belongs to the statistical limit [3] of electronic relaxation, in sharp contrast to the behavior of other azabenzenes (pyrimidine, pyrazine and s-triazine) (see for a review, ref. [4]). WC wish to thank Dr. Josef Dresner of this laboratory and Professor Aviv Amirav-of Tel Aviv University for pointing out the possibility of photomultiplier saturation and the National Science Foundation for financial support.
References [II A. Meenakshi, W. Chen and E.C. Lim. Chem. Phys. Letters 148(39X8) 507.
[21 1. Yamazaki, T. Murao;K.
Yoshihara, M. Fujita, K. Sushida and H. Baba, Chem. Phys. Letters 92 ( 1982) 42 1. [31 A. Nitzan, J. Jortner and P.M. RentTepis, Proc. Roy. Sot. A 327 (1972) 367. in: Excited states, Vol. 7, [41 K.E. Drabe and J. Kommandeur, eds. E.C. Lim and K.K. Innes (Academic Press, New York, IY88)p. 107.
0 009-2614/88/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
B.V.
15I, number 3
CHEMICAL
14 October
PHYSICS LETTERS
1988
COMMENT ROTATIONAL INVARIANCE OF THE FLUORESCENCE QUANTUM YIELD IN S, PYRIDINE Young-Dong
Shin and E.C. LIM
Department of Chemistry,
Wayne State Unrversity, Detroit, MI 48202, USA
Received 25 August 1988
In a recent paper (Chem. Phys. Letters 148 (1988) 507) the reported rotational dependence of the fluorescence experImental artifact. There is, m fact, no variation m fluorescence quantum yield across the rotational envelope.
In a recent paper [l] from this laboratory, it has been reported that S,(‘B, n7c*) pyridine exhibits a strong angular momentum dependence of fluorescence quantum yield in a supersonic jet that can be explained by invoking the coupling of the S,(nx*) state with nit* singlet states via orbital-rotation interaction. The purpose of this Comment is to point out that the “rotational dependence of the fluorescence” is not genuine, but is caused by the fluorescence saturation of the photomultiplier, which occurred despite the extremely small quantum yield (< 10T4) [2], the very low molecular density of the seeded supersonic beam and what we believed to be reasonable caution. The USC of a high-gain photomultiplier and/or strong attenuation of the fluorescence leads to the fluorescence excitation spectrum which is identical (within experimental errors) to the corresponding absorption spectrum. Thus, there is no variation in fluorescence quantum yield across the rotational envelope. Orbital-rotation interaction may be important in influencing photophysical properties [I], but the S, radiative decay of pyridine does not provide such an example as proposed in the earlier paper. The rotational invariance of the fluo-
308
is due to an
rescence yield in supersonic jet invites a conclusion that singlet-triplet intersystem crossing in pyridine belongs to the statistical limit [3] of electronic relaxation, in sharp contrast to the behavior of other azabenzenes (pyrimidine, pyrazine and s-triazine) (see for a review, ref. [4]). WC wish to thank Dr. Josef Dresner of this laboratory and Professor Aviv Amirav-of Tel Aviv University for pointing out the possibility of photomultiplier saturation and the National Science Foundation for financial support.
References [II A. Meenakshi, W. Chen and E.C. Lim. Chem. Phys. Letters 148(39X8) 507.
[21 1. Yamazaki, T. Murao;K.
Yoshihara, M. Fujita, K. Sushida and H. Baba, Chem. Phys. Letters 92 ( 1982) 42 1. [31 A. Nitzan, J. Jortner and P.M. RentTepis, Proc. Roy. Sot. A 327 (1972) 367. in: Excited states, Vol. 7, [41 K.E. Drabe and J. Kommandeur, eds. E.C. Lim and K.K. Innes (Academic Press, New York, IY88)p. 107.
0 009-2614/88/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
B.V.