Phenoxazine and Naphthalene Tethered Dual-Dye Azadipyrromethenes: Synthesis and Photophysical Study

Panchromatic azaborondipyrromethenes, 1, 2, and 3, containing phenoxazine (PHO) and naphthalene tethered at 1,7 and/or 3,5-positions of the azaBODIPY platform were synthesized and photo-induced energy (PEnT) and electron transfer (PET) events were investigated. Optical absorption spectra have displayed broad absorption, from 250 to 1000 nm, in 1 and 2. Parallel electrochemical and computational studies revealed that PHO moiety in 1 and 2 was easier to oxidize compared to azaBODIPY suggesting that PHO moiety would behave as an electron donor and azaBODIPY as electron acceptor in PET reactions in these compounds. Steady-state fluorescence studies have revealed that, upon excitation of PHO moiety in 1 and 2, the emission of PHO and azaBODIPY moieties were quenched indicating the occurrence of PET from the singlet excited phenoxazine to the azaBODIPY. In contrast, excitation of naphthalene moiety in 3 revealed the quenched emission of naphthalene with concomitant appearance of the azaBODIPY emission suggesting the occurence of PEnT from (1)naphthalene* to the azaBODIPY. Fascinatingly, in 1, excitation of naphthalene moiety yielded quenched emissions of naphthalene and azaBODIPY moieties indicating the occurence of sequential PEnT from (1)(naphthalene)* to azaBODIPY followed by the PET from PHO to (1)(azaBODIPY)* leading to the formation of a charge-separated state (PHO)(2)(+center dot)-azaBODIPY(center dot)-(Naph)(2). Fluorescence decay studies revealed that the PEnT and PET in these compounds occur with rate constants of similar to 10(9) s(-1) - 10(10) s(-1).