Exciton dynamics and binding energy relation in crystal-cocrystal system

This study presents a theoretical exploration of light-matter interaction in crystal (CuCl₂4-aminoacetophenone) and cocrystal (CuCl₂4-aminoacetophenone 1,4-diiodotetraflourobenzene) systems using the density functional theory approach. The findings not only identify realistic charge transfer (CT) channels within these systems but also reveal their implications for the field of materials science. The natural bond orbital analysis on the molecules shows that the copper atoms produce lone pair interactions and construct<i>π</i>-conjugated pathways with ligands. Density of states analysis reveals that compared to the parent crystal, the cocrystal accumulates more electronic states. The exciton descriptors like<i>D</i>_index,<i>H</i>_index, Sr,<i>t</i>_index, hole delocalization index, and electron delocalization index were used to study the exciton dynamics. Both compounds have CT and local excitation (LE) character, and CT is dominant over LE. The descriptor values revealed how strong or weak the exciton pair is regarding binding energy. The binding energy strength is illustrated with exciton descriptor indexes and a graphical overlap integral. It is evident that as the excitation states increase, the binding energy of the exciton gradually decreases. After the excitation process, the e-h delocalization spreads within the molecular unit, validating the theoretical coexistence of Frenkel-CT excitons with a radius of a few angstroms (Å) and an immense binding energy.