Early Events in the Photoinduced Electron Transfer between Carbazole and Divinylbenzene in a Silylene-Bridged Donor-Acceptor Compound

Abstract

Meeting the challenge of designing new light-emitting materials, we synthesized the compound N-isopropylcarbazole (CBL)-SiMe2-divinylbenzene (DVB), which represents the general idea of building attractive emitters by stitching together simple, well-known block elements. Following this strategy, an extraordinary emission emerges from photoinduced intramolecular processes between silylene-bridged adjacent chromophores, e.g., intramolecular energy and/or electron transfer (PET). The reported compound displays an attractive blue emission that occurs no matter which of the linked chromophores is excited (i.e., regardless of the excitation wavelength in the range 240-360 nm). Excitation of CBL leads directly to intramolecular charge transfer (ICT) state formation within 35 ps, whereas excitation of DVB results in "pumping"the CBL excited state via 300 fs energy transfer. In the latter case, DVB acts as an intramolecular photosensitizer of the ICT precursor. Both mechanisms, proceeding via ultrafast processes, are confirmed by femtosecond transient absorption experiments performed on the investigated bichromophoric compound and its individual Si-containing chromophores in acetonitrile solution. Analysis of the transient absorption bands allowed us to characterize the ICT excited state as a radical ion pair of carbazole radical cation and divinylbenzene radical anion linked by a silylene bridge.