Thermodynamic energies of donor and acceptor triplet states

Thermodynamic energies (ET) of lowest triplet states (T1) have been measured for 60 triplet energy donors and acceptors. Laser flash photolysis procedures were used to monitor the equilibration between the triplet–triplet absorptions of reference and unknown compounds in ethyl acetate at 295 K. These measurements provide triplet free energies that reflect both enthalpy and entropy differences between T1 and the ground state (S0). These thermodynamic energies are the basis of quantitative models for triplet energy transfer kinetics in solution. For comparison, T1 energies were also measured via phosphorescence at 295 K for most of the donors and many of the acceptors of this study. The phosphorescence of naphthalene at 295 K served as a reference for relating the thermodynamic and phosphorescent triplet energies (EP). For most rigid molecules the ET and EP values are nearly identical. However, for molecules with considerable rotational or conformational freedom, such as ketocoumarin and α-diketone donors and polyphenyl acceptors, ET values tend to be substantially greater than EP values. This difference partially reflects the tendency for reduced rotational freedom in T1 relative to S0, which leads to a reduction in entropy and a corresponding increase in free energy (from −TΔS) in T1. For nonrigid chromophores triplet energies obtained from low-temperature measurements tend to approximate ET values in some cases, because the energy increases associated with emission from unrelaxed triplets tend to parallel the positive contributions of decreased T1 entropy to ET. Triplet–triplet absorption maxima and extinction coefficients are provided for all of the donors and acceptors. Intersystem crossing quantum yields, also measured by laser flash photolysis, are provided for the triplet energy donors.