Experimental Investigation by Laser Flash Photolysis To Reveal the Optical and Electron-Donating Properties of Benzothiazole Derivatives and a Theoretical Approach by Using Time-Dependent Density Functional Theory

Synthesis, materials characterization and opto(electrical) properties of unsymmetrical azomethines with benzothiazole core

Optical (UV-vis and photoluminescence) properties of two soluble organic molecules based on azomethines with benzothiazole core (BTA1 and BTA2) were reported. The structures of both compounds are characterized by means FTIR, (1)H NMR, and (13)C NMR spectroscopy and elemental analysis; the results show an agreement with the proposed structure. The investigated compounds emitted blue light. Influence of excitation wavelength and concentration on maximum and intensity of emission of BTA1 and BTA2 was found. Electrochemical properties of the compounds were studied by differential pulse voltammetry. Introduction of fluorine moieties (BTA1) resulted in lower energy band gap (E(g)) of approximately ∼0.5 eV, whereas BTA2 showed E(g) of ∼2.8 eV. The devices comprised of BTA1 with P3HT:PCBM (1:1:1) showed an open circuit voltage (V(OC)) of 0.40 V, a short circuit current (J(SC)) of 1.19 mA/cm(2), and a fill factor (FF) of 0.23, giving a power-conversion efficiency (PCE) of 0.10% under AM1.5 G irradiation (100 mW/cm(2)).

Role of upper triplet states on the photophysics of nitrated polyaromatic compounds: S(1) lifetimes of singly nitrated pyrenes

The photophysics of most nitrated polycyclic aromatic compounds is dominated by an ultrafast intersystem crossing channel, which makes their first singlet excited states decay with rates on the order of 10(12) to 10(13) s(-1). Some questions, however, remain about the nature of the receiver triplet states, which have been in principle assigned to specific triplets of a different electronic configuration from T(1). In particular, it could be suggested that even a small degree of n-π* character of the T(1) state may be enough to allow the S(1) state to couple to upper vibronic states of the lowest energy triplet, without the requirement for specific upper triplet states. In this report, we show that there are, in fact, nitroaromatic compounds that do not show the ultrafast intersystem crossing channel but instead have S(1) states that are two to three orders of magnitude longer lived. Our studies focused on the time resolution of the emission from singly nitrated pyrenes, which show a strong photophysical dependence on the position of the NO(2) group: Whereas S(1) in 1-nitropyrene is short-lived (up to 3 ps), in 4-nitropyrene and 2-nitropyrene this state has 0.41 and 1.2 ns lifetimes, respectively, in acetonitrile solution. Computational work at the TD-DFT level of theory indicates that such remarkable increase in the first excited singlet lifetime can indeed be explained by a loss of the energy coincidence between the S(1) state with specific upper triplet states formed from transitions that involve the nonbonding orbitals at the oxygen atoms. These results are in strong support of the previous descriptions about the requirement for intermediacy of specific triplet states in the ultrafast decay of the fluorescent state present in most nitroaromatics. The implications for the photochemistry of this group of toxic atmospheric pollutants, including the channel that redounds in the dissociation of the NO· fragment, are discussed in view of the present results.

N-hetero atomic effect on the photophysics of 2,2'-dipyridylketone

Emission characteristics of 2,2'-dipyridylketone (DPK) have been studied in the rigid glass matrices of ethanol (EtOH) and methyl cyclohexane (MCH) at 77 K. As in the case of the aromatic analogue benzophenone (BOP), DPK is found to exhibit only phosphorescence emission. Although both BOP and DPK have the lowest triplet states chiefly of n pi* nature, the presence of other triplet states in the neighbourhood of the lowest triplet one is found to play some significant roles in determining the photophysical properties of DPK, specially in the non-polar solvent (MCH). All the photophysical properties including the polarization characteristics have been explored in both the molecules. Experimental observations are corroborated with necessary quantum chemical calculations.

Photophysical processes involved within the bichromophoric system 9-benzotriazole-1-ylmethyl-9H-carbazole and its role as an artificial photosynthetic device

From both steady state and fluorescence lifetime measurements it reveals that due to photoexcitation of benzotriazole (BZ) part of the bichromophore, 9(1-H-benzotriazole-lylmethyl)-9H-carbazole (BHC), singlet-singlet energy transfer takes place to populate the lowest excited singlet of carbazole (CZ). CZ, thus being excited indirectly via energy transfer process, undergoes strong charge transfer (CT) reaction with the surrounding polar medium acetonitrile (ACN). On the other hand, very weak CT band was apparent when CZ part, within BHC, was directly excited. In less polar tetrahydrofuran (THF) and polar benzonitrile (BN) environment, lack of formation of CT band strongly suggests in favor of the electron-accepting behavior of ACN. Moreover, by measuring the emission spectra of BHC in microcrystals and of 30 bilayers mixed LB film at high mole fraction of BHC molecules, the possibility of excimer formation or aggregation has been ruled out. Thus, BHC, when dissolved in ACN, acts as a triad system of BZ-CZ-ACN where BZ acts as an antenna molecule and CZ as a reaction center. The possible role of the bichromophoric system BHC as an artificial photosynthetic or solar energy conversion device has been hinted.