The Structure and Decay Dynamics of Exciplexes Derived from Dibenzoylmethanatoboron Difluoride and Alkylbenzenes in Cyclohexane

Abstract

Dibenzomethanatoboron difluoride (DBMBF2) 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 "normal" region. In cyclohexane these exciplexes gave intense fluorescence spectra and high quantum yields (phi(infinity)ex). The dipole moment of these exciplexes calculated from the solvatochromic shift of the fluorescence maximums (vmax) was used to estimate the coefficient ("a" and "c") of the CT and LE terms in the exciplex wavefunction. On the basis of the measured lifetimes and phi(infinity)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 exciplex wavefunction could be adequately described by CT and LE states, and that *DBMBF2 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)r and k(ex)NR (or their logarithmic value) against the LE contribution (c2) and transition energy gaps (hvmax) afford better correlation than those against -deltaG(-et). This indicates the role played by the LE contribution in generating the stabilization energy (U(s)) in these exciplexes through the A-D+)<==> *AD) resonance interaction; U(s), in turn, modifies -deltaG(-et) to afford the decay driving force hvmax. 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 "inverted" 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 "normal" to "inverted" 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.

Vibronic bandshape of the absorption spectra of dibenzoylmethanatoboron difluoride derivatives: analysis based on ab initio calculations

“…Recently, both time-dependent and time-independent approaches to calculations of vibronic absorption and emission band shapes have been developed. [37][38][39][40][41][42][43] A typical representative of this class of compounds, dibenzoylmethanatoboron difluoride (DBMBF 2 ), its derivatives, and complexes with some aromatic compounds were studied experimentally in detail 35,37,42 with the aim of using DBMBF 2 in optical chemical sensors for various aromatic compounds. In optical chemical sensors, the absorption or emission bandshape can serve as an analytical signal responding to the interaction of the chromophore with analytes or environment.…”

Synthesis and Structure Study of Asymmetric Binuclear Complex of Boron Difluoride

“…Исследование фотофизических свойств дикетонатных комплексов бора, растворенных в органических растворителях, показало значительную зависимость спектральных свойств от молекулярной архитектуры и в первую очередь от наличия заместителей, сопряженных с хелатным циклом. Для кристаллических комплексов бора спектральные свойства зависят от межмолекулярных взаимодействий даже таких слабых, как водородные связи, пи-стекинг взаимодействия, силы Ван-дер-Ваальса, приводящих к межмолекулярной самоорганизации, образованию эксимеров и эксиплексов [11][12][13][14][15][16]. Поэтому мы попытались получить и исследовать комплекс, в котором к борнохелатному циклу присоединены два заместителя, содержащих  (p) электроны: это фенильная группа и атом серы, соединенный с борноацетилацетонатным циклом, что должно привести к увеличению цепи сопряжения в комплексе.…”

Synthesis and Photophysical Properties of Alkoxysilyl Derivatives of Dibenzoylmethanatoboron Difluoride

“…A distinctive feature of BF 2 bdks is their ability to form exciplexes in the first excited singlet states with aromatic hydrocarbons that have strong fluorescence with a fluorescence maximum shifted to longer wavelengths . The magnitude of the shift depends on the electron‐donating ability of the hydrocarbon.…”