The structure of C2 was then optimised, and TD-DFT calculations carried out in the same manner as used for A1, A2, Please do not adjust margins Please do not adjust margins and B1 to determine the nature of the excitations from the ground state. Results of these calculations are presented in Table . A comparison of B1 and C2 excitations show that the 418.5 nm absorption for the chromophore is split into two excitations at 420.8 nm and 411.1 nm for the dimer. The oscillator strengths per chromophore for these excitations decrease slightly from 0.8680 for B1 to 0.0214 and 0.8360 in C2. The 322.3 nm HOMO-2 → LUMO absorption for the chromophore corresponds to two excitations (Exc 3,4 in Table ) in the dimer, with wavelengths of 324.9 nm and 323.2 nm. Oscillator strengths decrease significantly from 0.1361 for the chromophore to 0.0059 and 0.0060 for the dimer. The NTOs of these excitations show their charge transfer nature and thus can partially explain the lower absorbance observed experimentally. Please do not adjust margins Please do not adjust margins