STEADY-STATE AND PICOSECOND LASER FLUORESCENCE STUDIES OF NONRADIATIVE PATHWAYS IN TRICARBOCYANINE DYES - IMPLICATIONS TO THE DESIGN OF NEAR-IR FLUOROCHROMES WITH HIGH FLUORESCENCE EFFICIENCIES

In order to rationally design probes appropriate for sensitive near-in frared (NIR) applications, fluorescence studies of two representative tricarbocyanine NIR dyes, IR-125 and IR-132, were undertaken to evalua te solvent-dependent and independent nonradiative relaxation pathways. The fluorescence quantum yields, lifetimes, and the radiative and non radiative rates in aqueous solvents, organic alcohols, and binary mixt ures 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. T he quantum yields were less than 15% with subnanosecond lifetimes in a ll solvent systems investigated with severely reduced quantum yields a nd lifetimes in aqueous solvents when compared to those in the neat or ganic alcohols. Inspection of the absorption spectra indicated extensi ve ground-state aggregation for IR-132 in aqueous solvents, while IR-1 25 showed little evidence of aggregation. The fluorescence lifetimes f or both dyes demonstrated negligible dependence on solution viscosity, indicating that photoisomerization is not a major nonradiative path f or these tricarbocyanine dyes. Linear free energy plots of the nonradi ative rates (k(nr)) and the solvent's E(T)(30) value (parameter indica tive of solvent polarity and hydrogen bond donating ability) showed a linear relationship in the neat alcohols and H2O/methanol binary mixtu res, 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 (anion ic), and tert-octylphenoxy polyethoxyethanol (nonionic) above their cr itical micelle concentrations improved the photophysical properties of these dyes when compared to the pure aqueous solvents. Internal conve rsion resulting from the small electronic energy difference between th e ground and first excited singlet, a nonrigid molecular structure giv ing rise to many vibrational degrees of freedom and distortion of the molecule from planarity, was surmised to be the major nonradiative man ifold of the singlet excited state. A solvent-dependent nonradiative r ate was also discovered, with the efficiency of this process determine d by the hydrogen bond donating strength and/or polarity of the solven t. The photophysical results warrant consideration of the following co nstraints in the design of new fluorochromes requiring high fluorescen ce efficiencies appropriate for the NIR: inclusion of charged function alities on the dye to prevent aggregation; exclusion of heavy atoms wi thin the dye structure; structural reinforcement within the polymethin e chain to reduce the rate of internal conversion; and inclusion of or ganized media in the aqueous environment when appropriate to shield th e dye from the strong hydrogen bond donating strength and/or polarity of the interstitial solution.