Existence of a New Emitting Singlet State of Proflavine: Femtosecond Dynamics of the Excited State Processes and Quantum Chemical Studies in Different Solvents

Abstract

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.

Nature of Linear Spectral Properties and Fast Electronic Relaxations in Green Fluorescent Pyrrolo[3,4-c]Pyridine Derivative

“…All of the kinetic curves start with negative signals ( < 0), which gradually decrease to the positive values, remain constant, or further increase depending on the spectral range (460–480 nm, 500–520 nm, and 530–620 nm, respectively). These relatively long changes are described by the characteristic times in the range of 20 to 30 ps and can be attributed to solvent reorganization phenomena, frequently observed for different organic molecules in protic solvents [ 25 , 26 , 27 , 28 ]. The largest values of the first emerged subpicosecond negative signals were observed in the short wavelength range ( ≤ 550 nm, Figure 4 a,b) and are, presumably, associated with the dominant role of optical gain from the unrelaxed first excited state S 1 along with some possible input from the SA of the ground state S 0 .…”

Electronic and spatial control over the formation of transient ion pairs during photoinduced electron transfer between proflavine–amine systems in a subpicosecond time regime

“…The second time constant s 2 varies from 50 to 215 ps depending on the solvent. 27 We studied proavine-TEA and proavine-DMA as donoracceptor couples in different solvents of varying dielectric constants in a femtosecond-picosecond time-resolved uorescence set-up. Our previous studies showed that the addition of TEA to proavine increases its lifetime from 4.45 ns to 5.00 ns in ethanol.…”

“…The observed decay times are presumably due to the emission from the close lying singlet excited states ( 1 A 1 and 1 B 2 ) of proavine in homogeneous media and the variation in s 2 is due to the variation in Stokes shi and uorescence quantum yield of the dye with solvent polarity. 27 By considering the preferential residence of DMA and PF + molecules in different regions of the CTAB micelle, it could be inferred that the PF + molecules are repelled by its positively charged head groups and go into the water phase and thus cannot access the DMA molecules located in the hydrophobic tail region. 23 Due to the preferential residence of DMA molecules in the micellar core environment, probably the lone pair of nitrogen of this tertiary amine becomes protonated by the nearby NH 4 + group of the CTAB micelle, which is impossible in the case of TEA since it easily diffuses into the water phase owing to its small size and hydrophilicity ( Fig.…”

Host–guest composite materials of dyes loaded zeolite LTL for antenna applications

“…Dye molecules were measured either in various solvents in quartz cuvette, or incorporated in zeolite LTL by suspending dyes/zeolite LTL in chloroform ($ 1 mg in 100 mL). [20][21][22]. The absorption and emission bands of AF at different concentrations in K-LTL are shown in Figs.…”