Accurate & cheap calculations of the lowest triplet state energy: an experimentalist's guide

Triplet-triplet annihilation and singlet fission are bimolecular processes which can be exploited in a range of technological applications. These processes involve the first excited singlet and triplet states (S1 and T1), and have restrictions on their relative energies. While singlet-singlet energy differences are easily measured using optical spectroscopy, the singlet-triplet energy gap is less amenable to experiment. We report a computationally inexpensive method for the calculation of the energy of the lowest singlet-triplet transition for a range of extended pi chromophores. Excellent correlation (mean absolute displacement less than or similar to 0.05 eV) between experiment and calculation is achieved for a wide range of molecules without requiring zero point energy calculations. This provides the experimental chemist with the necessary tools to accurately predict T1 energies for novel molecules that are candidates for triplet-triplet annihilation or singlet fission. We demonstrate that the lowest triplet energy of many technologically relevant molecules can be inexpensively calculated. This will be particularly useful for triplet-triplet annihilation and singlet fission applications.