A. De

Steady state and time resolved studies on photophysical properties of carbazole and 9-phenyl carbazole molecules and their quenching reactions with suitable electron acceptors in the excited electronic states both at the ambient temperature and at 77 K

Studies at the ambient temperature by employing both steady state and time resolved techniques reveal that the observed fluorescence quenching phenomena of the present electron donor molecules, carbazole (C) and 9-phenyl carbazole (9PC) in the presence of the well-known electron acceptors 9-fluorenone (9FL) and 2-nitro-9-fluorenone (2N9FL) in acetonitrile (ACN) fluid solution are due to the combined effect of the static and dynamic processes involved. To dissect the donor fluorescence quenching data into its dynamic and static components, a model in the form of a modified Stern-Volmer (SV) relation has been proposed. By treating the data, obtained from the present investigation when the donor chromophores are excited, by nonlinear least squares curve fitting procedure, static (V) and dynamic (K SV ) contributions in overall quenching mechanisms were evaluated separately. The contribution of the static component (V) is observed to be so large that it overwhelms the dynamic process and plays major role in overall quenching mechanisms. In dynamic quenching, photoinduced electron transfer (PET) process, whose occurrence being confirmed by measuring redox potentials of the reacting systems (C and 9PC as electron donors and 9FL and 2N9FL as electron acceptors) in ACN solvent, is found to be operative concurrently with the Forster energy transfer process. However. when the electron acceptor molecules are excited in presence of a ground state donor, the quenching of the acceptor fluorescence appears to be mainly of dynamic nature. In this dynamic process, electron transfer (ET) seems to play the major role in nonradiative deactivation of the lowest excited singlet state (S 1 ) of the acceptor species. From the observed results at 77K, it is inferred that donor fluorescence quenching is primarily due to the combined effect of the concurrent occurrences of ET in the excited singlet state and the Forster long range energy transfer (S I D → S I A ). Low temperature studies further demonstrate that the triplet donors are not involved in energy transfer as well as in ET reactions with the acceptors. From the observed room temperature oxidation potential values of the donors, it is apparent that the electron donating capability decreases when substitution is made along the short molecular axis of symmetry of C, i.e., by replacing > NH hydrogen atom of C molecule by a phenyl ring (in case of 9PC).

Effects of protic and aprotic solvents on quenching mechanisms involving dimethyl-substituted donors and tetracyanoquinodimethane (TCNQ)

Steady-state and time-resolved spectroscopic techniques have been used to study photoinduced quenching reactions e.g. electron transfer (ET), energy transfer processes etc. between the electron donors 3,5-dimethylphenol (3,5-DMP) and 3,5-dimethylanisole (3,5-DMA) and the electron acceptor TCNQ in polar aprotic acetonitrile (ACN) and polar protic ethanol (EtOH) at ambient temperature. In both solvents photoinduced ET reactions are found to be highly exothermic (ΔG0 λ (where λ is the nuclear reorganization energy parameter) and because the electron transfer rate kET decreases with increasing exothermicity, to occur in the Marcus inverted region (MIR). However, relatively larger kET values are observed in ACN than in EtOH. This has been explained in terms of the ordered structure of EtOH due to H-bonding. In ACN, the primary process responsible for quenching of the excited singlet (S1) of the donors in the presence of TCNQ seems to be ET whereas, in EtOH, several other non-radiative processes can occur together with photoinduced ET. ACN would appear to be a better solvent in which to investigate the mechanism of the ET reactions. Reaction schemes showing the possible non-radiative deactivation routes within the donor–acceptor systems in both ACN and EtOH have been proposed.

Photophysical properties of 5-hydroxyindole (5HI): Laser flash photolysis study

Steady state fluorescence emission and transient absorption spectra of 9-fluorenone (9FL) were measured in the presence of 5-hydroxyindole (5HI) in highly polar acetonitrile (ACN) environment at ambient temperature. Cyclic voltammetry measurements demonstrate that ground state 5HI as a donor could take part in highly exothermic electron transfer (ET) reactions with excited 9FL, which should serve as electron acceptor. From the transient absorption measurements it is inferred that in geminate ion-pair (GIP) (or contact ion pair), formed initially due to photoinduced ET, the decay of this contact ion-pair occurs not only through ion recombination (back electron transfer to ground state of reactants), but through the other processes also such as proton-transfer (hydrogen abstraction) from radical cation to anion and separation of ion-pair producing the free ions. From the computed reorganisation energy parameter (λ) and experimentally observed -‡ET0 values it is hinted that there is a possibility that highly exothermic forward electron transfer reactions in the singlet stateS1 occur, within present reacting systems, in Marcus inverted region. Back transfer seems to follow the same path. Investigations with similar other reacting systems are underway.

Studies on unimolecular and bimolecular photoprocesses of a newly synthesized selenium compound, 7-chloro-2-phenyl-9H-[1]-benzopyrano[3,2-b]-selenophene-9-one (SeP)

Using steady state/time resolved spectroscopic and electrochemical techniques the spectroscopic and photophysical studies were made on a novel synthesized selenophene compound SeP in nonpolar methylcyclohexane (MCH), polar aprotic acetonitrile (ACN) and polar protic ethanol (EtOH) solvents at the ambient temperature as well as at 77 K. Both from the studies on unimolecular and bimolecular photoprocesses this selenophene compound was found to possess several electronic levels, 1Bb, 1La, 1Lb (all are of pi pi* nature and 1Lb is hidden within 1La band envelop like the characteristics of most of the acenes) and 1(nO pi*) state arising due to carbonyl oxygen atom. In polar ACN environment this nO pi* state disappears because it moves within the envelop of intense 1La band due to large destabilization. Large overlapping of different band systems within the 1La band of SeP was confirmed from the observed depolarization effect. The lack of phosphorescence of SeP both in MCH and EtOH rigid glassy matrix at 77 K has been inferred due to large vibronic interactions between closely lying triplets of the corresponding 1nO pi* and 1Lb states. From the bimolecular investigations, it reveals that SeP acts as a good electron donor in presence of the well known electron acceptor 9 cyanoanthracene (9CNA). Transient absorption spectra measured by laser flash photolysis technique demonstrate the formation of ion-pair when the acceptor is excited. From the analysis of the fluorescence quenching data it seemingly indicates that the major contribution in the diminution of the fluorescence intensity of the acceptor 9CNA in presence of SeP is not only due to the photoinduced electron transfer (ET) but also originates from static type (instantaneous) quenching processes along with external heavy atom effect. The possibility of occurrence of photoinduced ET reaction in Marcus inverted region is hinted.

Electron transfer compatible molecular design of benzothiophene-acetophenone bichromophores: a theoretical approach.

Computational work has been done for a bichromophore (4MBA) comprising a donor 4-methoxy-benzo[b]thiophene (4MBT) and an acceptor molecule p-chloro-acetophenone (pclA) linked together by a HC=CH bond which shows large hyperpolarizability. The charge transfer in this bichromophoric system is computed by semiemperical theoretical calculation. Ground state and excited state dipole moment difference of the bichromophore 4MBA indicates a large electron transfer probability.