Symmetry-breaking photoinduced charge transfer state in a near-IR absorbing meso-linked BODIPY dimer

The synthesis and characterization of a conformationally restrained near-IR absorbing homoleptic meso-linked BODIPY dye is reported. The photophysical properties have been investigated using steady-state and time-resolved femtosecond transient absorption spectroscopy. A combination of spectroscopies (steady-state absorption/emission, ultrafast transient absorption spectroscopy measurements), singlet oxygen generation quantum yields, and computational studies were conducted to evaluate the nature of the excited state of the dimer 3D compared to its monomer 3M as a control. The results are consistent with a symmetry-breaking intramolecular charge transfer (ICT) state for 3D being formed in polar solvents, but the formation of the ICT states competes with other photophysical channels (ISC, fluorescence) even in the most polar solvents tested, so the formation efficiency is less than the related "green" light absorbing BODIPY dimer 2 previously reported. The ICT state formation is attributed to the direct meso coupling of the BODIPY dimer, which stymies nonradiative deactivation through both a conformational restraint of the individual BODIPY subunits and a blocking the rotation of the bond connecting the two BODIPY multimers, preventing subunit electronic communication in the excited state and subsequent charge recombination. Consistent with some fraction of intramolecular charge transfer (ICT) states being formed upon photoexcitation (∼1 ps in MeOH and 500 fs in DMF), the lifetime, emission intensity, and absorbance are attenuated in polar solvents for the dimer than the monomer. Further, for 3D diminished ISC is observed in more polar solvents, which is attributed to an increase in the fraction of excited states undergoing the ICT pathway in polar solvents, while for the monomer 3M there is no effect of solvent polarity on ISC. Overall, this study provides insights into the delicate balance between different photophysical channels in symmetric dimers. The exceptional optical properties of this dimer make this chromophore a promising scaffold for initiating charge separation using wavelengths in the challenging-to-access far-red/near-IR region of the optical spectrum, while suggesting further improvements could be made to increase the fraction of excited states undergoing ICT.