Near-infrared absorbing dual-dye integrated azaborondipyrromethenes: Synthesis, characterization, and unveiling the role of naphthalene and phenothiazine as energy and electron donors in light-induced events

Near-infrared absorbing azaborondipyrromethenes (azaBODIPYs), 1, 2, and 3, tethered with phenothiazine (PTZ) and/or naphthalene at 1,7 and/or 3,5 positions, were synthesized and photoinduced energy (PEnT) and electron (PET) events occurring within these compounds were systematically studied. Steady-state fluorescence studies have shown that, when the phenothiazine moiety, in 1 and 2, was selectively excited, the fluorescence of the PTZ and azaBODIPY was quenched, indicating the occurrence of PET from the singlet excited phenothiazine to the azaBODIPY. On the other hand, when the naphthalene moiety in 3 was excited, the emission of the naphthalene was observed to be quenched with the concomitant appearance of the azaBODIPY emission indicating the display of PEnT from 1 naphthalene* to the azaBODIPY. Interestingly, in the case of 1, selective excitation of naphthalene resulted in the quenching of both the naphthalene and azaBODIPY emissions, indicating the possibility of PEnT from 1 naphthalene* to azaBODIPY followed by a tandem PET from the ground state of the phenothiazine moiety to the azaBODIPY. The present work showed the synthetic scope of introducing two different chromophores onto the azaBODIPY platform to produce panchromatic dyes and systematically revealed the significance of the individual constituents, PTZ and naphthalene as electron and energy donors, and azaBODIPY as an excited state energy or electron receiver in the PET and PEnT processes.