Thermodynamic equilibrium between locally excited and charge-transfer states through thermally activated charge transfer in 1-(pyren-2′-yl)-o-carborane

Reversible conversion between excited-states plays an important role in many photophysical phenomena. Using 1-(pyren-2 '-yl)-o-carborane as a model, we studied the photoinduced reversible charge-transfer (CT) process and the thermodynamic equilibrium between the locally-excited (LE) state and CT state, by combining steady state, time-resolved, and temperature-dependent fluorescence spectroscopy, fs- and ns-transient absorption, and DFT and LR-TDDFT calculations. Our results show that the energy gaps and energy barriers between the LE, CT, and a non-emissive 'mixed' state of 1-(pyren-2 '-yl)-o-carborane are very small, and all three excited states are accessible at room temperature. The internal-conversion and reverse internal-conversion between LE and CT states are significantly faster than the radiative decay, and the two states have the same lifetimes and are in thermodynamic equilibrium.

Automated summary

Reversible conversion between excited states is crucial in various photophysical phenomena. By conducting experiments and calculations, we discovered that the energy gaps and barriers among the locally-excited and charge-transfer states in 1-(pyren-2'-yl)-o-carborane are negligible, making all three states accessible at room temperature. Furthermore, the internal-conversion and reverse internal-conversion processes occur much faster than radiative decay, resulting in the same lifetimes and thermodynamic equilibrium of the two states.