Theoretical studies on molecular and structures of mono- and binuclear chromium carbazole derivatives for optoelectronics

Studies on the molecular geometries, electronic properties and second-order nonlinearities of a series of mono- and binuclear chromium carbazole complexes: (N-vinyl-carbazole)Cr(CO) ₃ (M1), (N-vinylcarbazole)Cr(CO) ₂PPh ₃ (M2), (CO) ₃Cr(N-vinylcarbazole)-Cr(CO) ₃ (B1), and (CO) ₃Cr(N-vinylcarbazole)Cr(CO) ₂PPh ₃ (B2) were carried out, using the density functional theory (DFT) at the B3LYP//LanL2DZ/6-31G(d) level. The experimental singlet metal-to-ligand charge transfer ( ¹MLCT) spectra of these complexes can also be well simulated and discussed by the time-dependent DFT (TDDFT) at the B3LYP//LanL2DZ/6-311+G(d) level associated with the polarizable continuum model (PCM). The computational results show that an unusual characteristic of chromium carbazole structures is explained in terms of interaction between frontier molecular orbitals of the metal and its ligands. The highest occupied molecular orbitals (HOMOs) of these complexes are composed of a set of distorted degenerated Cr 3d orbitals, whereas the lowest unoccupied molecular orbitals (LUMOs) are predominantly the N-vinylcarbazole ligand π ^* orbitals. The HOMO - LUMO energy gaps decrease in the order NVC > M1 > B1 > M2 > B2. The considerable coupling between the carbazole and (CO)3 in M1 creates an asymmetric environment about the chromium atom, leading to modest second-order responses. The PPh ₃ ligand is acting as a donor which increases the donating strength of the d _π orbitals in chromium carbazole species, resulting in the large electronic asymmetry in M2. As for the binuclear chromium carbazole chromophores, a wide-range ¹MLCT band and large oscillator strength are found, allowing for the electronic interactions between two metal centers which can be modified by altering the ligand bound to the metals to induce peculiar asymmetry. Essentially, Cr(CO) ₃ acceptor and Cr(CO) ₂PPh ₃ donor units in B2 make significant contribution to the charge-transfer process or NLO responses via conventional push - pull chromophoric architecture. (Figure presented)