In order to rationally design probes appropriate for sensitive near-infrared (NIR) applications, fluorescence studies of two representative tricarbocyanine NIR dyes, IR-125 and IR-132, were undertaken to evaluate solvent-dependent and independent nonradiative relaxation pathways. The fluorescence quantum yields, lifetimes, and the radiative and nonradiative rates in aqueous solvents, organic alcohols, and binary mixtures of water/methanol were measured using steady-state and picosecond laser techniques. In addition, organized media and solvent viscosity effects on the NIR dyes' photophysical properties were investigated. The quantum yields were less than 15% with subnanosecond lifetimes in all solvent systems investigated with severely reduced quantum yields and lifetimes in aqueous solvents when compared to those in the neat organic alcohols. Inspection of the absorption spectra indicated extensive ground-state aggregation for IR-132 in aqueous solvents, while IR-125 showed little evidence of aggregation. The fluorescence lifetimes for both dyes demonstrated negligible dependence on solution viscosity, indicating that photoisomerization is not a major nonradiative path for these tricarbocyanine dyes. Linear free energy plots of the nonradiative rates (k(nr)) and the solvent's E(T)(30) value (parameter indicative of solvent polarity and hydrogen bond donating ability) showed a linear relationship in the neat alcohols and H2O/methanol binary mixtures, with larger solvent E(T)(30) values yielding larger nonradiative rates. Inverse linear relationships with poor correlations were found between the solvent's nucleophilicity and the nonradiative rates. The addition of certain surfactants, such as sodium dodecyl sulfate (anionic), and tert-octylphenoxy polyethoxyethanol (nonionic) above their critical micelle concentrations improved the photophysical properties of these dyes when compared to the pure aqueous solvents. Internal conversion resulting from the small electronic energy difference between the ground and first excited singlet, a nonrigid molecular structure giving rise to many vibrational degrees of freedom and distortion of the molecule from planarity, was surmised to be the major nonradiative manifold of the singlet excited state. A solvent-dependent nonradiative rate was also discovered, with the efficiency of this process determined by the hydrogen bond donating strength and/or polarity of the solvent. The photophysical results warrant consideration of the following constraints in the design of new fluorochromes requiring high fluorescence efficiencies appropriate for the NIR: inclusion of charged functionalities on the dye to prevent aggregation; exclusion of heavy atoms within the dye structure; structural reinforcement within the polymethine chain to reduce the rate of internal conversion; and inclusion of organized media in the aqueous environment when appropriate to shield the dye from the strong hydrogen bond donating strength and/or polarity of the interstitial solution.
