Excited state dynamics of 9,9'-bianthryl in room temperature ionic liquids as revealed by picosecond time-resolved fluorescence study

Picosecond time-resolved fluorescence measurements have been carried out on 9,9'-bianthryl in three imidazolium ionic liquids to probe the excited state dynamics. In the early time-scale, the fluorescence spectra of bianthryl have been found to consist of emission from both locally excited (LE) and charge transfer (CT) states. The LE → CT relaxation time, as estimated from the decay of the fluorescence intensity of the LE emission, is found to vary between 230 and 390 ps, while the average solvent relaxation time, as estimated from the analysis of time-dependent fluorescence Stokes shift, is found to vary between 620 ps and 1840 ps, depending on the viscosity of the ionic liquids. The results confirm that while in conventional less viscous solvents the CT formation kinetics of bianthryl occurs simultaneously with the solvation dynamics, in ionic liquids the two processes mostly occur in different time scales

What determines the rate of excited-state intramolecular electron-transfer reaction of 4-(N,N'-dimethylamino)benzonitrile in room temperature ionic liquids? A study in [bmim][PF<SUB>6</SUB>]

The kinetics of excited-state intramolecular electron-transfer reaction and dynamics of solvation of the intramolecular charge transfer (ICT) state of 4-(N,N'-dimethylamino)benzonitrile (DMABN) was studied in 1-butyl-3-methylimidazloium hexafluorophosphate, [bmim][PF6], by monitoring the dual fluorescence of the system. The picosecond time-resolved emission spectra (TRES) of DMABN exhibit decay of the locally excited (LE) emission intensity and shift of the ICT emission peak position with time, thus capturing the kinetics of evolution of the ICT state from the LE state and solvent relaxation of the ICT state. These results show that the LE→ICT transformation rate is determined not by the slow dynamics of solvation in ionic liquid, but is controlled mainly by the rate of structural reorganization of the molecule, which accompanies the electron-transfer process in this polar viscous medium. Even though both solvent reorganization around photo-excited DMABN and structural rearrangement of the molecule are dependent on the viscosity of the medium, it is the latter process that contributes to the viscosity dependence of the LE→ICT transformation

Modulation of the excited state intramolecular electron transfer reaction and dual fluorescence of crystal violet lactone in room temperature ionic liquids

The influence of polarity, viscosity, and hydrogen bond donating ability of the medium on the fluorescence behavior of crystal violet lactone (CVL), which undergoes excited state electron transfer reaction and exhibits dual fluorescence from two different electronic states, termed as CT<SUB>A</SUB> and CT<SUB>B</SUB>, has been studied in six different room temperature ionic liquids (ILs) using steady state and time-resolved emission techniques. It is shown that the excited state CT<SUB>A</SUB> &#8594; CT<SUB>B</SUB> transformation and dual fluorescence of CVL can be controlled by appropriate choice of the ILs. While dual fluorescence of CVL is clearly observed in pyrrolidinium IL, the molecule exhibits a single fluorescence band in ammonium IL. While the second emission from the CT<SUB>B</SUB> state can barely be seen in 1,3-dialkylimidazolium ILs, dual fluorescence is quite prominent in 1-butyl-2,3-dimethylimidazolium IL, [bmMim][Tf<SUB>2</SUB>N]. These contrasting results have been explained taking into account the hydrogen bonding interactions of the 1,3-dialkylimidazolium ions (mediated through the C(2)-hydrogen) with CVL and the viscosity of the ILs. The excited state CT<SUB>A</SUB> &#8594; CT<SUB>B</SUB> reaction kinetics has been studied in IL by monitoring the time-evolution of the CT<SUB>B</SUB> emission in [bmMim][Tf<SUB>2</SUB>N]. The solvation dynamics in this IL has been studied by following the dynamic fluorescence Stokes shift of C153, which is used as a probe molecule. A comparison of the excited state reaction time and solvation time suggests that the rate of the CT<SUB>A</SUB> &#8594; CT<SUB>B</SUB> reaction in moderately viscous ILs is primarily dictated by the rate of solvation. Very little or negligible excitation wavelength dependence of the emission behavior of CVL can be observed in these ILs

Fluorescence response of 4-(N,N'-dimethylamino)benzonitrile in room temperature ionic liquids: observation of photobleaching under mild excitation condition and multiphoton confocal microscopic study of the fluorescence recovery dynamics

The fluorescence behavior of 4-(N,N'-dimethylamino) benzonitrile has been studied in room temperature ionic liquids (ILs) as a function of temperature, excitation wavelength, and exposure time. Dual emission from the locally excited (LE) and intramolecular charge transfer (ICT) states of the molecule has been observed and the relative intensities of the two emission bands and the peak position of the ICT emission are found consistent with the viscosity and polarity of the ILs. Temperature dependence study reveals a blue shift of the ICT emission peak with lowering of temperature indicating that under this condition the emission occurs from incompletely solvated state of the molecule. The observed excitation wavelength dependence of the emission behavior has been attributed to the microheterogeneity of the media. Exposure of the solution to the exciting radiation under very mild condition is found to influence the relative intensities of the two emission bands; an enhancement of the LE emission accompanied by a slight decrease of the ICT emission is observed. The emission intensities, however, return almost to their original values when the exposed solution is kept in the dark. The observation has been attributed to photoreaction of the exposed molecules and the recovery to replenishment of phototransformed molecules by the surrounding unexposed molecules. Fluorescence recovery after photobleaching has been studied by multiphoton confocal fluorescence microscopic technique to obtain insight into the recovery dynamics. The diffusion coefficient estimated from this study is found to be lower than that predicted by the Stokes-Einstein equation by a factor of nearly 7 indicating the microheterogeneous nature of the ILs

Photophysical and Theoretical Insights on Fullerene/Zincphthalocyanine Supramolecular Interaction in Solution

The present article reports photophysical studies on supramolecular interaction of a zinc phthalocyanine derivative, namely, zinc-2,9,16,23-tetra-<i>tert</i>-butyl phthalocyanine (<b>1</b>) with C₆₀ and C₇₀ in solvents having varying polarity, i.e., toluene and 1,2-dichlorobenzene (DCB). The interesting feature of the present work is the observation of charge transfer (CT) absorption bands of the fullerene/<b>1</b> complexes in DCB. Utilizing the CT transition energy, many important physicochemical parameters like vertical ionization potential of <b>1</b>, degrees of CT, oscillator strength, transition dipole moment, and resonance energy of interaction have been determined in the present case. The influences of <b>1</b> on the UV–vis spectral characteristics of C₆₀ and C₇₀ have been explained using a theoretical model that takes into account the interaction between electronic subsystems of <b>1</b> with fullerenes. Steady state fluorescence experiment elicits efficient quenching of the fluorescence intensity of <b>1</b> in the presence of both C₆₀ and C₇₀. The average binding constants of the C₆₀ and C₇₀ complexes of <b>1</b> (estimated by UV–vis and steady state fluorescence measurements) are determined to be 18 330 dm³·mol^–1 (12 595 dm³·mol^–1) and 19 160 dm³·mol^–1 (15 292 dm³·mol^–1) in toluene (DCB), respectively. Lifetime experiment yields a larger magnitude of charge separated rate constant for the C₇₀/<b>1</b> species. The faster charge recombination of the fullerene/<b>1</b> systems observed in more polar solvent results from solvent reorganization energies. Quantum chemical calculations by the <i>ab initio</i> method explore the geometry and electronic structure of the supramolecules and testify the significant redistribution of charge between fullerenes and <b>1</b> during fullerene/<b>1</b> interaction. A variable temperature ¹³C NMR study nicely demonstrates that the end-on orientation of C₇₀ is very much responsible for the low selectivity in binding between C₆₀/<b>1</b> and C₇₀/<b>1</b> systems. Free energy of charge recombination and free energy of radical ion-pair formation signify that electron transfer from the excited <b>1</b> to C₆₀ and C₇₀ in the C₆₀/<b>1</b> and C₇₀/<b>1</b> complexes, respectively, is an unlikely process. Finally, transient absorption measurements in the visible region establish that energy transfer from ^TC_60* (and ^TC_70*) to <b>1</b> occurs predominantly in both toluene and DCB, which is subsequently confirmed by the consecutive appearance of the triplet state of <b>1</b>

Fluorescence quenching of CdS quantum dots by 4-azetidinyl-7-nitrobenz-2-Oxa- 1,3-diazole: a mechanistic study

Fluorescence quenching of CdS quantum dots (QDs) by 4-azetidinyl-7-nitrobenz-2-oxa-1,3-diazole (NBD), where the two quenching partners satisfy the spectral overlap criterion necessary for Forster resonance energy transfer (FRET), is studied by steady-state and time-resolved fluorescence techniques. The fluorescence quenching of the QDs is accompanied by an enhancement of the acceptor fluorescence and a reduction of the average fluorescence lifetime of the donor. Even though these observations are suggestive of a dynamic energy transfer process, it is shown that the quenching actually proceeds through a static interaction between the quenching partners and is probably mediated by charge-transfer interactions. The bimolecular quenching rate constant estimated from the Stern-Volmer plot of the fluorescence intensities, is found to be exceptionally high and unrealistic for the dynamic quenching process. Hence, a kinetic model is employed for the estimation of actual quencher/QD ratio dependent exciton quenching rate constants of the fluorescence quenching of CdS by NBD. The present results point to the need for a deeper analysis of the experimental quenching data to avoid erroneous conclusions

CdTe Quantum Dots in Ionic Liquid: Stability and Hole Scavenging in the Presence of a Sulfide Salt

The light-harvesting properties of both CdSe and CdTe nanocrystals are ideally suited for their use in quantum dot (QD)-sensitized solar cells. However, corrosion of the CdTe QD in an aqueous environment in the presence of sulfide/polysulfide electrolyte renders it unsuitable despite its better electron injection ability (compared to CdSe QD) to a large band-gap semiconductor like TiO₂. In this work, we explore the stability of a CdTe QD, which we have developed exclusively for its use in ionic liquids, in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid in the presence of S^2– and investigate the hole transfer process from this photoexcited QD to S^2–. We not only demonstrate that an appropriate capping of the CdTe QD and use of an ionic liquid in place of the aqueous medium enhances the stability of the QD significantly in the presence of S^2– but also provide evidence of hole transfer from a photoexcited QD to the sulfide salt using steady-state and time-resolved emission and ultrafast transient absorption measurements

Surface plasmon enhanced spectroscopies and time and space resolved methods : general discussion

F. Javier Garcıa de Abajo opened a general discussion of the two papers by Jeremy Baumberg and Javier Aizpurua