Absorption and emission spectra of 9-N,N-dimethylaniline decahydroacridinedione (DMAADD) have been studied in different solvents. The fluorescence spectra of DMAADD are found to exhibit dual emission in aprotic solvents and single emission in protic solvents. The effect of solvent polarity and viscosity on the absorption and emission spectra has also been studied. The fluorescence excitation spectra of DMAADD monitored at both the emission bands are different. The presence of two different conformation of the same molecule in the ground state has lead to two close lying excited states, local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the dye. A CTstate involving the N,N-dimethylaniline group and the decahy droacridinedione chromophore as donor and acceptor, respectively, has been identified as the source of the long wavelength anomalous fluorescence. The experimental studies were supported by ab initio time dependent-density functional theory (TDDFT) calculations performed at the B3LYP/6-31G* level. The molecule possesses photoinduced electron transfer (PET) quenching in the LE state, which is confirmed by the fluorescence lifetime and fluorescent intensity enhancement in the presence of transition metal ions.

Photoionization, fluorescence time-dependent Stokes shift (TDSS), and rotational dynamics of coumarin 307 (C307) have been investigated in soft matter system such as micelles using time-resolved transient absorption and fluorescence spectroscopy. Photoionization of C307 leads to the formation of coumarin radical cation and hydrated electron, which were characterized by their respective transient absorption. The photoionization yields are significantly higher in anionic sodium dodecyl sulfate (SDS) micelle than in cationic cetyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) micelles, indicating the influence of micellar surface charge on the efficient separation of radical cation-hydrated electron pair. The CTAB micelle favors the recombination of radical cation and hydrated electron leading to the formation of triplet state of C307, which causes a decrease in the photoionization yield. C307 exhibits TDSS in all micelles; the time evolution and the magnitude of the TDSS depend on nature of the micelle. In TX-100 micelles, the decay of the TDSS exhibits ultraslow component (165 ns) and is affected by the presence of electron scavengers. The ultraslow component in TX-100 micelle originates from the recombination of radical cation-hydrated electron, which results in the formation of twisted intramolecular charge transfer (TICT) state; such formation of TICT state was not observed in SDS and CTAB micelles. To the best of our knowledge, this is the first report where the radical-ion pair recombination dynamics is probed using TDSS in combination with time-resolved transient absorption studies. The activation energy for the solvent relaxation and radical-ion pair (solvent separated) recombination process was found to be 6.1 and 3.0 kcal mol(-1), respectively. Temperature effect on TDSS in TX-100 micelles confirmed the increase in the water hydration, and size of the micelle influences the relative contribution of the solvation and radical-ion pair recombination dynamics toward the total TDSS. We propose that TDSS observed in neutral micelles and reported in other biomolecules such as proteins by the 7-amino coumarin probe is not only due to the solvation dynamics alone but also due to the radical-ion pair recombination dynamics.

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.

The dynamics and mechanism of the photoinduced electron transfer (PET) reaction between coumarin 307 (C307) and aromatic amines in micelles have been studied by using steady-state (S-S) and time-resolved (T-R) absorption and fluorescence spectroscopy. Based on the fluorescence quenching time scale, PET in micelles is grouped into two types: (i) ultrafast electron transfer (ET) due to the close contact of the donor and acceptor in micelles and (ii) diffusion averaged dynamic electron transfer (DADET) which is controlled by the diffusion of the reactants in micellar Stern layer and diffusion of the micelles. The DADET does not affect the photoionization and solvation processes whereas ultrafast ET competes with the photoionization and faster than the solvation process. Both ultrafast and DADET shows Marcus inversion in the ET rates at the similar exergonicity and indicates that the role of diffusion and solvent reorganization is negligible toward the activation barrier for the ET reaction in micelles. The activation barrier for the ET reactions in micelles is mainly due to intramolecular reorganization energy. The intramolecular reorganization energy must be higher in CTAB due to the photoionization and subsequent recombination and also involvement of triplet state in the PET. The ET reaction between coumarin radical cation and amine is reported for the first time in the C307-amine systems in micelles which are confirmed by the effect on amine concentration of the decay of coumarin radical cation and the dynamics of the ground-state recovery of C307. A mechanism for the PET reaction between C307-amine systems is proposed in micelles including photoionization, ultrafast and dynamic ET, and solvation dynamics.

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