(More) About biphenyl first excited triplet state energy

Biphenyl photophysics was extensively studied in the past and much attention was given to the measurement of its first excited triplet state energy. Phosphorescence and the corresponding T1 ← S0 absorption give triplet energies that differ by 10 kcal mol−1. We revisited biphenyl photophysics using photoacoustic calorimetry (PAC), a technique that directly measures the thermochemistry and kinetics of short-lived transients in solution. PAC measurements give the relaxed biphenyl triplet energy of 67.8 ± 0.6 kcal mol−1; 2.3 kcal mol−1 higher than the spectroscopic value. The direct measurement of the biphenyl relaxed triplet energy reported in this work should resolve the continued misuse of its spectroscopic energy, in particular in the growing field of polyphenylene polymers.