Multichromophore Integrated Panchromatic Azaborondipyrromethenes: Synthesis, Characterization, and Exploring the Role of AzaBODIPY as an Energy and Electron Relay in Photoinduced Processes

Panchromatic azaborondipyrromethenes directly connected with N,N-ditolylaniline (TPA) and naphthalene (Naph) at 1,7- and/or 3,5-positions of the azaBODIPY platform, 1, 2, and 3, were synthesized and the roles of the individual chromophore constituents in the photo-induced energy and electron transfer processes have been investigated. Optical absorption studies have indicated that integrating the complementary absorbers, naphthalene and TPA moieties, into the azaBODIPY core yielded broad-band capturing dyes with the absorption ranging from 250-1000 nm. Parallel electrochemical studies revealed that TPA moiety in 1 and 2 is easier to oxidize when compared to azaBODIPY moiety, which are in congruent with the computational studies indicating that TPA moiety would behave as an electron donor and azaBODIPY as an acceptor in PET processes. Steady-state fluorescence studies indicated that the photo-excitation of the TPA moiety in 2 resulted in the occurrence of PET from 1 TPA* to azaBODIPY generating (TPA)2+ ⋅-(azaBODIPY)- ⋅ while that of naphthalene in 3 resulted in PEnT from 1 (naphthalene)* to azaBODIPY forming (Naph)2 -1 (azaBODIPY)*. Interestingly, in 1, excitation of naphthalene moiety resulted in sequential PEnT from 1 (naphthalene)* to azaBODIPY followed by the PET from TPA to 1 (azaBODIPY)* generating a charge-separated state, (TPA)2+ ⋅-(azaBODIPY)- ⋅-(Naph)2 Fluorescence lifetime studies have indicated that the electron and energy transfer processes occurred in nanosecond time scales.