ICG revisited: Excited-state dynamics as a function of dye concentration and solvent environment

Indocyanine green (ICG) is a clinically approved tricarbocyanine dye widely used in medical imaging and photodynamic therapy. Its incorporation into inorganic hybrid nanoparticles (IOH-NPs) offers a highly promising strategy for the targeted delivery of therapeutic agents, particularly in photothermal applications. Despite extensive use of ICG, the influence of solvent and concentration on its excited-state behaviour remains incompletely understood, but an in-depth understanding of these photophysical properties is essential for elucidating its functional role within the IOH-NPs. Therefore, this study combines steady-state and time-resolved spectroscopic methods to examine the dependence of the excited-state dynamics of the first excited singlet state of ICG on both solvent environments and dye concentration. The photophysical behaviour of ICG was characterised in ethanol (EtOH), dimethyl sulfoxide (DMSO) and demineralised water across a systematically varied concentration range from 0.08 to 100 μM. The steady-state absorption behaviour of ICG in EtOH and DMSO largely showed a concentration independence, whereas in water, concentration-dependent H-aggregation was observed. The fluorescence quantum yield (fQY) decreased with increasing dye concentration above approximately 0.2 μM, beginning from approximately 22% in EtOH, 42% in DMSO and 5% in water. The time-resolved studies were conducted by time-correlated-single-photon-counting (TCSPC) at λ_ex = 366 nm and transient-absorption spectroscopy using femtosecond laser pulses at λ_ex = 800 nm. Relaxation from the first excited singlet state of ICG occurs on timescales of 500-600 ps in EtOH, 700-900 ps in DMSO and 120-160 ps in water, reflecting increased nonradiative decay in aqueous solution. In EtOH and DMSO, excited-state dynamics remained largely concentration-independent, while in water aggregation effects became more pronounced at higher concentration. A clear correlation between excited-state lifetime and fQY was observed across all solvents.