The rate of photoinduced ET in molecular systems is controlled by electronic coupling of the locally excited and charge transfer states. We generalize the Bixon–Jortner–Verhoeven expression for electronic coupling to systems with a small energy gap and derive the transfer integral for charge separation in two model heterojunctions using the excitation energies and oscillator strengths computed with TD DFT. The estimated couplings are in good agreement with the reference values

The authors acknowledge the support of the Ministerio de Economía y Competitividad of Spain (project CTQ2015-69363-P), Generalitat de Catalunya (project number 2014SGR931), and Xarxa de Referència en Química Teòrica i Computacional