Intramolecular charge transfer in 2-methyl-1,3-dihydrobenz[d,e]isoquinoline:: Calculation of the electronic coupling matrix element

2-Methyl-1,3-dihydrobenz[d, e] isoquinoline (DHBIQ) undergoes rapid intramolecular charge transfer from the amino donor to a locally-excited singlet state of the naphthalene acceptor. In order to better understand the mechanism of charge transfer in this class of molecules, we have performed a series of quantum chemical calculations intended to compute the relevant electronic coupling matrix elements. The reactants are separated by sigma-bonds and it appears that both through-space and through-bond electronic coupling must be taken into account. Furthermore, the ground state of DHBIQ exists as a mixture of two interconverting conformers and, surprisingly, the extent of through-space interactions is significantly greater than the corresponding through-bond interactions for both conformers. Comparison of electronic coupling within the two conformers shows that an axial conformation more strongly favours charge transfer than does the complementary equatorial geometry. An additional calculation made on a twisted conformation of DHBIQ further illustrates the effect of molecular geometry on the extent of electronic coupling, especially for through-bond interactions. Overall, the calculations agree well with experimental observations and it is stressed that the configuration interaction single and double excitation method, used with 12 molecular orbitals in the active space, has distinct advantages relative to other methods available for calculating the extent of electronic coupling in donor - connector acceptor systems.