Shortwave infrared polymethine dyes for bioimaging: ultrafast relaxation dynamics and excited-state decay pathways

Excited-state relaxation in two prototypical shortwave infrared (SWIR) polymethine dyes developed for bioimaging, heptamethine chromenylium Chrom7 and flavylium Flav7, is studied by means of femtosecond transient absorption with broadband ultraviolet-to-SWIR probing complemented by steady-state and time-resolved fluorescence and phosphorescence measurements. The relaxation processes of the dyes in dichloromethane are resolved with sub-100 fs temporal resolution using SWIR, near-IR, and visible photoexcitation. Different population members of the ground-state inhomogeneous ensemble are found to equilibrate via skeletal deformation changes with time constants of 90 fs and either 230 fs (Chrom7) and 350 fs (Flav7) followed by slower evolution matching the 1-ps timescale of diffusive solvation dynamics. Molecules excited into high-lying singlet electronic states (S) by visible excitation repopulate with time constants of 400 fs (Chrom7) and 450 fs (Flav7) the corresponding first excited singlet S₁ states, which decay within several hundreds of picoseconds in dichloromethane and chloroform solvents. Vibrational relaxation in S₁ for both Chrom7 and Flav7 in dichloromethane occurs with time constants of 350 and 800 fs for excess of vibrational energy of ∼1000 and 10 000 cm^-1 deposited by near-IR and visible excitation, respectively. Two competing non-radiative processes are present in S₁: temperature-independent internal conversion, and thermally-activated twisting about a carbon-carbon bond of the conjugated chain, which is substantial at room temperature but essentially nonreactive, producing traces of isomer product. Intersystem crossing in S₁, and thus the triplet quantum yield, is minor. The importance of absorption bands from the excited S₁ state in applications requiring high-intensity excitation conditions is discussed.