Investigations on nonradiative transitions in different environments usingexcited (or ground state) 1,2,3,4-tetrahydroquinoline (THQ) as donor and ground state (or excited) 9-fluorenone (9FL) or 2-nitro-9-fluorenone (2N9FL)as acceptors

By using steady state and time resolved spectroscopic techniques, investiga tions were made on the nature and mechanisms of different nonradiative proc esses involved within the excited donor (or acceptor) and ground state acce ptor (or donor) in nonpolar n-heptane (NH), polar protic ethanol (EtOH), an d polar aprotic acetonitrile (ACN) solvents at the ambient temperature as w ell as in EtOH rigid glassy matrix at 77 K. The bicyclic molecule 1,2,3,4-t etrahydroquinoline (THQ) was used as a donor (D) in the present investigati on whereas 9-fluorenone (9FL) and 2-nitro-9-fluorenone (2N9FL) were chosen as electron accepters (A). When the donor chromophore was excited in presen ce of an acceptor, highly exothermic electron transfer GET) reactions seeme d to occur from observed large negative driving energy (Delta G(o)) values, measured by electrochemical techniques, Forster's type energy transfer, st atic quenching, etc. An attempt was made to estimate separately the contrib utions of static and dynamic quenching modes in the overall quenching mecha nism of donor fluorescence by using a model of modified Stern-Volmer (SV) r elation. From this relation it seemingly indicated that the major contribut ion in quenching originated from the static mode. When excited acceptors re act with the ground state donor THQ it is primarily ET reactions that seem to occur. Observations of the transient absorption spectra, by laser flash photolysis techniques, of contact ion-pairs of the present D-A systems alon g with the triplet absorption spectra of the monomeric acceptor (9FL and 2N 9Fl) corroborate our views regarding the occurrence of photoinduced ET reac tions within the present reacting systems. At 77 K the combined effect of F orster type energy transfer and static quenching seems to be responsible fo r observed lowering of donor fluorescence emission intensity in presence of accepters (9FL or 2N9FL). However, the phosphorescence Lifetime measuremen ts reveal that the triplet donor might be involved in photoinduced ET react ion with the unexcited acceptor in rigid glassy matrix at 77 K and this pos sibly causes the reduction in the phosphorescence band intensity of the don or THQ in presence of the latter one.