DFT AND TDDFT calculations of linear and nonlinear optical properties of Ruthenium Alkynyls with extended π-Bridges

Abstract

Materials with large Nonlinear Optical (NLO) coefficients are great of interest in a wide range of technological applications. Amongst these NLO materials, organometallic and coordination complexes have attracted significant attention in recent years owing to their record values of NLO coefficients and greater design flexibility. The present investigation reports Density Functional Theory (DFT) and Time-Dependent (TD) DFT calculations of linear and nonlinear optical properties of a series of ruthenium complexes with different extended π-bridges: trans-[Ru(C≡C(B)-4-C6H4-1-NO2)Cl(κ2-dppm)2, B = [–C6H4–]n (OPS); [–C≡CC6H4–]n (OPE); [–(E)-HC=CHC6H4–]n (OPV); [–C4H2S–]n (OTS); [–(E)-HC=CHC4H2S–]n (OTV); [–(E)-N=NC6H4–]n (OPA) with n = 1-5. For OPS, OPE, OPV and OTS, the total static first hyperpolarizability (βtot) calculated with CAM-B3LYP increases considerably upon extending the bridge length up to three repeating units. After that it seems to saturate. For OPA and OTV, βtot increases up to four and five units, respectively. For any n, βtot decreases in the order of OTV > OPA > OPV > OPE > OTS > OPS. The TD- CAM-B3LYP calculations showed that for OPE, OPV and OTV, the main optical band is red-shifted until three linker units and further extending the conjugation has a minor effect on the peak position. This trend in excitation energies is consistent with the calculated HOMO-LUMO energy gaps. The βtot saturation predicted at greater lengths may thus be linked to the excitation energy of the main optical band. The energy barrier to the internal rotation of phenyl units in OPS, OPE and OPV is markedly low, permitting planar and different twisted conformers to coexist under the laboratory conditions. The βtot values were found to decrease significantly for twisted structures, with the communication between the electron donor and acceptor hampered by the non-planarity. The number of conformers increase with increasing the length of the bridge. Therefore, the predicted β saturation at greater bridge length may be attributed to the conformational flexibility associated with these molecules.