Photophysical investigations on some synthesized electron donor aryl-bridged compounds

The measurements of electronic absorption, steady-state fluorescence emission and time resolved transient spectra are made for two novel synthesized aryl-bridged compounds: 2,6-bis(4,5-dihydro-7,8-dimethoxy)-isoquinolino benzene(DIB) and 2,6-bis(4,5-dihydro-7,8-dimethoxy)-isoquinolino pyridine(DIP) at the ambient temperature in presence of the well-known electron acceptor 9-fluorenone (9FL). The energy positions of the absorption bands of both DIB and DIP are found to be similar to those of the corresponding bands of naphthalene and isoquinoline. Solvent effect shows that the two closely lying bands (260 and around 310 nm) of both DIB and DIP are of ππ* type. Steady-state polarization experiments demonstrate that these two close-lying bands are responsible for two different type of transitions, 260 nm is responsible for 1La←1A and 310 nm is due to 1Lb←1A. This polarization study also shows that 1La and 1Lb bands are largely overlapping. Large bimolecular fluorescence quenching rates (kq∼1011 M−1 s−1) observed from both intensity reduction and fluorescence lifetime quenching of the acceptor 9-fluorenone(9FL) in presence of the aryl-bridged donors indicate that the electron transfer (ET) radius is slightly greater than the hydrodynamic radius. The possibility of occurrence of some other fast non-radiative process along with photoinduced electron transfer whose possibility was tested by cyclic voltammetry measurements and presence was confirmed by laser flash photolysis study has been discussed. Transient absorption spectral measurements reveal that in the excited singlet (S1) of 9FL photoinduced ET reaction is present but in its triplet (T1) only energy transfer process is operative. Laser flash photolysis experiments provide the direct evidence for ion recombination mechanisms for production of monomeric triplets of both 9FL and the donors DIB and DIP.