The Structure and Decay Dynamics of Exciplexes Derived from Dibenzoylmethanatoboron Difluoride and Alkylbenzenes in Cyclohexane
✍ Scribed by Yuan L. Chow; Zhong-Li Liu; C. Ingemer Johansson; Jun-ichi Ishiyama
- Publisher
- John Wiley and Sons
- Year
- 2000
- Tongue
- English
- Weight
- 135 KB
- Volume
- 6
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Dibenzomethanatoboron difluoride (DBMBF 2 ) interacted with alkylbenzenes from its singlet excited state to form exciplexes ranging from weak polarity up to contact radical ion pairs (CRIP); this exciplex series shows the characteristics in the Marcus ªnormalº region. In cyclohexane these exciplexes gave intense fluorescence spectra and high quantum yields (F I ex . The dipole moment of these exciplexes calculated from the solvatochromic shift of the fluorescence maximums (n max ) was used to estimate the coefficient (ªaº and ªcº) of the CT and LE terms in the exciplex wavefunction. On the basis of the measured lifetimes and F I ex of these exciplexes, the radiative (k ex f and nonradiative (k ex NR rate constants were calculated. The former k ex f were also computed from a semi-empirical approach based on the assumption that the exci-plex wavefunction could be adequately described by CT and LE states, and that *DBMBF 2 primarily contributes to the probability of exciplex emission. Two results agree with each other with small systematic deviations for those less polar exciplexes. The plots of k ex f and k ex NR (or their logarithmic value) against the LE contribution (c 2 ) and transition energy gaps (hn max ) afford better correlation than those against À DG Àet . This indicates the role played by the LE contribution in generating the stabilization energy (U s ) in these exciplexes through the j A À D i > j *ADi resonance interaction; U s , in turn, modifies À DG Àet to afford the decay driving force hn max . Also, those plots from k ex f values (being determined directly from experiments) show better correlation than those from k ex NR . In contrast to the CRIP type exciplexes in the Marcus ªinvertedº region, these k ex f and k ex NR increase in the common trend with increasing transition energy gaps. The k ex NR plots show less steep slopes and attains more quickly a minimum toward the CRIP region; the latter is identified as the turning point from the ªnormalº to ªinvertedº region. Both the attenuation and reversal of the k ex NR value with increasing polarity are believed to be generated by the emerging contribution of the intersystem crossing process as an additional nonradiative process, which is induced by the increased spin ± orbit coupling in highly polar exciplexes.