Deciphering the Role of Azaborondipyrromethene as an Electron Relay in Photoinduced Events Operating in Near-Infrared Absorbing Heteroaromatics Based Dual-Dye Substituted AzaBODIPYs

Panchromatic dyes extending the absorption up to the near-infrared region stand out as excellent candidates for light harvesting and biological applications. One of the viable ways to construct panchromatic dyes involves the strategic selection of a molecular platform that can accommodate multiple chromophores absorbing at varied wavelength ranges. Even though azadipyrromethene (azaBODIPY) offers such a molecular skeleton, reports on broadband absorbing azaBODIPYs and related photoinduced interchromophore energy/electron transfer events intending to provide desirable functions such as electron migration and charge separation (CS) are still inadequate. In this context, multiheteroaromatic tethered azaBODIPY, (PTZ)2-AB-(TPA)2, containing phenothiazine (PTZ) and triphenylamine (TPA) integrated into azaBODIPY core has been synthesized and light-induced electron transfer events were explored. Parallely, control compounds involving azaBODIPYs with either TPA or PTZ moieties, (Ph)2-AB-(TPA)2 and (PTZ)2-AB-(Ph)2, and pristine Et-PTZ and TPA were synthesized, and the roles of the individual constituents in the photoinduced events are investigated. Optical absorption studies have revealed that the substitution of azaBODIPY skeleton with PTZ and TPA moieties at 1,7- and 3,5-positions enhanced the extended π-conjugation and resulted in broader absorption extending beyond 1000 nm. Electrochemical studies have displayed first oxidation from either TPA or PTZ, and first reduction from the azaBODIPY moieties indicating that TPA or PTZ would behave as electron donors and azaBODIPY as the electron acceptor, and computational studies have corroborated the results. Steady-state fluorescence studies in solvents of varied polarity, involving selective excitation of PTZ at 265 nm and TPA at 300 nm resulted in quenching of the PTZ or TPA emission indicating the occurrence of photoinduced electron transfer (PET) from 1PTZ* or 1TPA* to azaBODIPY. Time-correlated single photon counting studies confirmed the quenching of overall lifetimes of the azaBODIPYs indicating the presence of PET within these systems. Systematic femtosecond transient absorption studies revealed the optical signatures of TPA+• or PTZ+• displayed at 550 and 650 nm, respectively, authenticating the occurrence of PET from excited TPA or PTZ to azaBODIPY with a very short formation time of CS states (14, 61, and 7 ps for (PTZ)2-AB-(Ph)2, (Ph)2-AB-(TPA)2 and (PTZ)2-AB-(TPA)2, respectively), and a long charge recombination in the nanosecond time domain, and highlighted the versatility of azaBODIPY as an electron relay in light-induced events.