(Invited) Excited State Dynamics and Ultrafast Electron Transfer in β-Iodo Substituted Bdp and ADP Donor-Acceptor Systems Featuring Fullerene.

Abstract

Novel donor-acceptor systems featuring, as electron donors, triplet sensitizers, viz., β-iodo substituted BDP and ADP, and fullerene as an electron acceptor are designed and synthesized. Iodination of meso-phenyl-tetramethyl-BDP and tetraphenyl-ADP bearing catechol aldehyde groups at the boron center is achieved by treating the precursors with N-iodosuccinimide (NIS) in dry DCM and dry chloroform, respectively, and stirring them for 24 hours under nitrogen at room temperature. Following successful synthesis of pure diiodo-BDP-CHO and diiodo-ADP-CHO, the final donor-acceptor dyads I2BDP-C60 (1) and I2ADP-C60 (2) are synthesized by following Prato reaction using sarcosine in dry toluene and refluxing under nitrogen for 24 hours. The final products which are purified over silica column are achieved as pink-reddish and bluish solid, respectively. Their structural integrity is arrived by both 1H and 13C-NMR, and HR-Mass spectral analysis. The steady-state and time-resolved excited state study of the dyads and control compounds using UV-vis absorption, fluorescence, phosphorescence, TCSPC and transient absorption both in the femtosecond and nanosecond time domain, in both toluene and benzonitrile, reveals very interesting photochemistry. The interplay of the thermodynamics and the kinetics of the singlet to triplet state intersystem crossing in the donor compounds is found to be decisive to the formation and quantum yield of the charge separated state in the dyads 1 and 2. In the case of dyad 1, with a slower rate of intersystem crossing, kISC= 5.18 x 10 9 S-1 in benzonitrile, the electron transfer seems to out compete the intersystem crossing and an ultrafast electron transfer is observed in benzonitrile with kCS= 1.9 x 1011 S-1. In the case of dyad 2 on the other hand, the fast intersystem crossing in diiodo-ADP seems to aggressively compete with the electron transfer especially for excitation initiated from S1 state while a better electron transfer dynamic is observed for electron transfer originating from the higher excited state. The charge separated states are found to persist up to ca. 1 ns, especially in the case of dyad 1 with a life time of 992.5 ps and a kCR= 1.0 x 109 S-1. Figure 1