摘要:
8 CHEMICAL COMMUNICATIONS A Photoionisation Mechanism for the Delayed Fluorescence of Perylene in a Rigid Glass By B. STEVENS and M. S. WALKER (Department of Chemistry, The University, Shefield 10) DELAYED fluorescence has been observed for rigid singlet is the emitting state, and the absence of the homogeneous solutions of perylene in liquid green, structured- and orange, structureless- paraffin at 7 7 " ~ over the concentration range emission bands of the p and a crystalline modifica- 10-6-10-3~ both in the presence and absence of tions of perylenel indicates that solute aggregation, dissolved oxygen; the spectral identity of the de- which could be responsible for emission of the layed and normal (unsectored) components shown delayed component,2 does not take place under the in the Figure establishes that the lowest exciting conditions cited. J.Tanaka, Bull. Chem. Soc. Japan, 1963, 36, 1237. B. Stevens and E. Hutton, Proc. Phys. Soc., 1963, 81, 893.NUMBER 1, 1965 As required by the triplet-triplet annihilation mechanism3 under conditions such that the triplet relaxation rate is annihilation-~ontrolled,~ the intensity I,, of delayed fluorescence was found to be a linear function of the incident light intensity I, and to decay non-exponentially over an observa- tion period of at least 10 seconds after excitation cut-off. However, although the normal fluores- cence excitation spectrum recorded for a lo-% solution closely resembles the molecular absorption spectrum, delayed fluorescence could not be excited in the first absorption band and its excita- tion spectrum, also shown in the Figure, coincides with the triplet-state absorption continuum below 280 m ~ .~ Wavelength (my) Corrected excitation ( I ) and emission (11) spectra of lO-'%-perylene an liquid parafin at 7 7 " ~ , recorded on Aminco-Keirs spectrophotophosphorimeter; - delayed fluorescence; - - - normal puorescence ; shaded area corresponds to continuous triplet state absorption.6 This behaviour is consistent with a mechanism in which photoionisation of the triplet state during the excitation period precedes slow electron capture by the perylene cation A+ which indirectly populates the lowest excited singlet and triplet states A* and 3A in the expected statistical ratio of 1 : 3. The overall scheme and the correspond sions for a solute concentration marised as follows.Process A + h v + A * Is N IoEg A* +3A P I S 9 ng rate expres- [A] are sum- Rate ~ 4 1 - C~AIP 3A + hv +A+ + e IT N I o E ~ [ ~ A ] ~ 3A + A k i PA1 A + + e + A * kr [A+] [el A+ + e -t3A A * + A + h v kP [A+] 3kr [A+] [el where e denotes a solvated electron or solvent anion, /3 is the intersystem crossing yield to the lowest triplet state, and the optical densities eS( [A] - [3A])d and cTrA]d associated with the transitions indicated are assumed to be sufficiently low that second and higher order terms in the expanded Beer-Lambert exponential terms can be neglected. Since the duration of delayed fluorescence is at least three orders of magnitude greater than the interval between successive periods of illumination under the conditions of observation, the justifiable assumption of a photostationary state leads to the expressions I, = %b[A+l [el - - d O E T [ 3 A l d/4 where yF is the quantum yield of molecular fluorescence; or since If the rate of unimolecular triplet-state relaxation is slow compared with its rate of photoionisation, or more specifically if equation (1) reduces to which accounts for the linear dependence of ID on I, for the biphotonic process.According to equation (2) the excitation spec- trum of the delayed component is a function of both E~ and E~ in the photoionisation region; however if 4 8 ~ ~ >> E ~ , I, varies linearly with eT as suggested for solutions of perylene. On the a C. A. Parker and C . G. Hatchard, Proc. Roy. Soc. 1962, A , 269, 574.T. Azumi and S . P. McGlynn, J . Chem. Phys., 1963,39, 1186. G. Porter and M. W. Windsor, Discuss. Faraday Soc., 1954, 17, 178.10 CHEMICAL COMMUNICATIONS other hand if this inequality is reversed equation (2) becomes and the excitation spectrum should coincide with the molecular absorption spectrum in the triplet photoionisation region. Finally, it is suggested that (a) the non-exponen- tial decay of delayed fluorescence indicates extensive electron migration through the glass, otherwise the rate of doublet-doublet annihilation at ion-pair sites should be proportional to the number of sites; (b) if the continuous regions of I D = YFrBIOE8 [Aid triplet absorption reported6 for other aromatic hydrocarbons corresponds to photoionisation, this may be a general phenomenon which could lead to a non-exponential decay of phosphorescence emis- sion. (c) the lowest triplet state of perylene should lie at - 20,000 cm.-l or - 1.0 ev above the reported values of 1.5 ev unless the molecular ionisation potential' of 7.15 ev is considerably reduced in the glassy environment.8 The phosphorescence of this molecule has yet t o be reported. (Received, December 2nd, 1964.) J. B. Birks and hf. A. Slifkin, Nature, 1961, 191, 761. G. Briegleb and J. Czekalla, 2. Elektrochewz., 1959, 63, 6. * e.g., P. Bennema, G. J. Hoijtink, J. H. Lupinski, L. J. Oosterhoff, P. Selier, and J. D. W. van Voorst, Mol. Phys., 1959, 2, 431.
ISSN:0009-241X
DOI:10.1039/C19650000008
出版商:RSC
年代:1965
数据来源: RSC