Time-resolved Area Normalized Emission Spectra Research Articles

Excited state dynamics of a double ESIPT unit comprising fluorophore

A comprehensive experimental and computational analysis on the photophysical characteristics of a double ESIPT unit based fluorescent molecule, 2,2′-(1,4-phenylenebis(1H-benzo[d]imidazole-1,2-diyl))diphenol (PBIDP), was carried out. Photoluminescence spectra and lifetime decays have been measured in various polar and nonpolar solvents to explore the fluorescence properties. Furthermore, time-resolved area normalized emission spectra are exploited to identify the different conformers to the unsymmetrical keto emission band correlating with their lifetime values. To corroborate the experimental observations, computations have been enacted by calculating the relative energy, excitation and emission energies of different conformers. Especially, the simulation facilitates to trace the reason behind the triple emissions, uncovering the proton transfer dynamics in the double ESIPT based PBIDP system. Additionally, to understand the ESIPT mechanism in the fluorophore, the potential energy curve and transition state in the ground, and excited state were calculated.

Exploring cationic polyelectrolyte-micelle interaction via excited-state proton transfer. Signatures of probe transfer.

Excited-state proton transfer (ESPT) is a sensitive tool for the delicate monitoring of structural reorganization, hydration level, and confinement within surfactant and polymer assemblies. Here, we investigate the interaction of a cationic polyelectrolyte, poly(diallyl dimethylammonium chloride) (PDADMAC), with micelles of differently charged surfactants using 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) as an ESPT probe. We used three surfactants: anionic sodium dodecyl sulfate (SDS), cationic dodecyl trimethylammonium bromide (DTAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12), possessing the same alkyl (dodecyl) chain but varying headgroup charges. The fluorescence of HPTS residing initially within the micellar medium modulates differently in the presence of PDADMAC. For the anionic SDS and cationic DTAB micelles, the emission spectrum of HPTS does not alter significantly; however, for SB12 micelles, the emission spectrum undergoes a strong modulation upon adding the polyelectrolyte. The emission intensities quench strongly at a low concentration of PDADMAC but recover at a higher concentration. The emission intensity ratio of the two emission bands also changes significantly, implying strong modulation of the ESPT process with varying PDADMAC concentrations. The time-resolved area normalized emission spectra (TRANES) disclose single isoemissive points in the SB12 micelle at low and high concentrations of PDADMAC but two different isoemissive points (one characteristic of the SB12 micelle at 500 nm and another characteristic of the PDADMAC interface at 480 nm) in the mixed assembly at an intermediate concentration. Detailed analysis suggests that the polyelectrolyte can enforce the transfer of the anionic probe HPTS from the zwitterionic micelle to the PDADMAC assembly above a specific PDADMAC concentration. The transfer of the molecular probe between two assemblies resembles a drug sequestration event, and the study reveals necessary emission signatures.

Coexistence of ground and excited state intramolecular proton transfer of the Schiff base 2-((E)-(naphthalene-3-ylimino)-methyl)phenol: A combined experimental and computational study

Prototropic behavior of the Schiff base compound, 2-((E)-(naphthalene-3-ylimino)-methyl)phenol (NMP), has been explored by photophysical/photochemical means and reported in this paper in a comprehensive manner. The study, comprising of absorption, steady state and time resolved fluorescence, 1H NMR, together with quantum chemical calculations, establishes that the probe undergoes intramolecular proton transfer (IPT) process in both ground and lowest excited singlet state. Resolution of the low energy absorption band of NMP in different solvents and observation of two different emission bands upon excitation at two different positions within the low energy absorption band indicates co-existence of two distinct species of the probe in the ground state, implying occurrence of the ground state intramolecular proton transfer (GSIPT) process. Large downshifted 1H NMR peak at δ > 13 ppm suggests a strong intramolecular H-bonding in the ground state and goes in favor of the GSIPT process. Excited state intramolecular proton transfer (ESIPT) of the probe has been confirmed by the existence of the large Stokes shifted emission band of the tautomer along with the normal (enol) emission band, especially in non-polar media. Observation of the single isoemissive point in the time resolved area normalized emission spectra (TRANES) in nonpolar solvents confirms existence of two distinct species (enol (N*) and tautomer (T*)) in the photoexcited state. Quantum chemical calculations using DFT/TDDFT have been performed to provide support to the experimental findings.

Synthesis and Characterization of Far-Red Emissive Boron-Based Triads Showing Large Stokes Shifts: Optical, TRANES, and Electrochemical Studies.

Herein, we report the design and synthesis of far-red emissive boryl-thiophene-BODIPY triads 1-3. The π-conjugation length and electronic communication between borane and BODIPY moieties are tuned by judiciously varying the size of the oligothiophene spacer in these triads (1, terthiophene; 2, quarterthiophene; and 3, pentathiophene). Conjugates 1-3 showed intriguing triple emissions in the blue to far-red regions. Detailed optical, time-resolved decay kinetics, time-resolved area-normalized emission spectra (TRANES), fluoride binding, and computational studies suggest that the multiple emissions in these triads are due to an inefficient transfer of energy from the boryl-oligothiophene to the BODIPY unit. In addition, all of the conjugates showed a ratiometric fluorescence response to fluoride ions.

Exploration of Self-Aggregation of Coumarin 7 and Coumarin 30 in Water: Role of β-Cyclodextrin as a Modulator.

Steady-state and time-resolved spectroscopic studies demonstrate that two members of the coumarin class of dyes, coumarin 7 (C7) and coumarin 30 (C30), undergo self-aggregation in water. The development of hypsochromically shifted new absorption bands in addition to the existing monomer bands with an increase in concentration of the dyes in an aqueous medium suggests that the aggregates are of H-type. An absorption-based kinetic study reveals that the rate of aggregation of C30 is an order of magnitude faster than that of C7. Second-order rate kinetics, as obtained from the half-life (t1/2) data, implies that the aggregates are dimeric in nature. Observations of isosbestic points in area-normalized absorption spectra (ANAS) and isoemissive points in area-normalized fluorescence excitation spectra (ANFES) and time-resolved area-normalized emission spectra (TRANES) establish that ground-state monomer ⇌ dimer equilibria for both of the systems are preserved in the photoexcited state. The present study further establishes that β-cyclodextrin is the most efficient of the three common cyclodextrins in shifting the equilibria toward the monomer by encapsulating the monomers within its cavity, making β-CD a convenient modulator to control the self-aggregation process. Dynamic light scattering (DLS), quantum chemical calculations, and molecular docking studies provide further support to our propositions.

The Role of Hydrogen Bonding in the Preferential Solvation of 5-Aminoquinoline in Binary Solvent Mixtures.

5-Aminoquinoline (5AQ) has been used as a fluorescent probe of preferential solvation (PS) in binary solvent mixtures in which the nonpolar component is diethyl ether and the polar component is protic (methanol) or aprotic (acetonitrile). Hence, the roles of solvent polarity and solute-solvent hydrogen bonding have been delineated. Positive deviations of spectral shifts from a linear dependence on the concentration of the polar component, signifying PS, are markedly more pronounced in case of the protic solvent. Solvation dynamics on a nanosecond time scale mark the formation of the solvation shell around the fluorescent probe. Time-resolved area-normalized emission spectra indicate the occurrence of the continuous solvation of the excited state when the polar component is acetonitrile. In contrast, two distinct states were observed when the polar component was methanol, the second state being the hydrogen bonded one. Translational diffusion is the rate-determining step for formation of the solvation shell. The time constant associated with it has been estimated from rise times observed in fluorescence transients monitored at the red end of the fluorescence spectra and also from the time evolution of the spectral width of time-resolved emission spectra.

Dynamics of Anthracene Excimer Formation within a Water-Soluble Nanocavity at Room Temperature.

Excited anthracene is well-known to photodimerize and not to exhibit excimer emission in isotropic organic solvents. Anthracene (AN) forms two types of supramolecular host-guest complexes (2:1 and 2:2, H:G) with the synthetic host octa acid in aqueous medium. Excitation of the 2:2 complex results in intense excimer emission, as reported previously, while the 2:1 complex, as expected, yields only monomer emission. This study includes confirming of host-guest complexation by NMR, probing the host-guest structure by molecular dynamics simulation, following the dynamics AN molecules in the excited state by ultrafast time-resolved experiments, and mapping of the excited surface through quantum chemical calculations (QM/MM-TDDFT method). Importantly, time-resolved emission experiments revealed the excimer emission maximum to be time dependent. This observation is unique and is not in line with the textbook examples of time-independent monomer-excimer emission maxima of aromatics in solution. The presence of at least one intermediate between the monomer and the excimer is inferred from time-resolved area normalized emission spectra. Potential energy curves calculated for the ground and excited states of two adjacent anthracene molecules via the QM/MM-TDDFT method support the model proposed on the basis of time-resolved experiments. The results presented here on the excited-state behavior of a well-investigated aromatic molecule, namely the parent anthracene, establish that the behavior of a molecule drastically changes under confinement. The results presented here have implications on the behavior of molecules in biological systems.

Lumichrome tautomerism in alcohol-water mixtures: Effect of carbon chain length and mole fraction of alcohols

Alcohol-water mixtures are important solvent systems, widely used in many applications. In this study, the intriguing water induced tautomerism of the fluorescent chromophore, Lumichrome (LC), has been explored to reveal interesting changes in the solvent environments of alcohol-water mixtures of methanol (MeOH), ethanol (EtOH), n-propanol (PrOH) and tert-butanol (t-BuOH), as a function of the carbon chain length/alkyl group size and concentrations of the alcohols. Since LC remains in the alloxazine form in pure aqueous medium, the appearance of emission from the isoalloxazine tautomer in alcohol-water mixtures provides compelling evidence that addition of alcohol disrupts the H-bonded network structure of bulk water to facilitate the availability of isolated water molecules in the solvent medium, which can form the required LC-water precursor complex for conversion of the alloxazine form to isoalloxazine form. Interestingly, two estimated parameters for LC tautomerism, namely, Rlong, which is the relative emission contribution of the isoalloxazine form in the fluorescence decay traces of LC (and hence a measure of the population of isoalloxazine form in the solution), and τlag, which is the lagtime involved in the evolution of the isoalloxazine emission band in the time-resolved area normalized emission spectra of LC (and hence a measure of the ease with which the LC-water precursor complex is formed), show a non-monotonic dependence on the mole fraction of alcohols (χalcohol) in the mixed solvent systems. The unique variation of Rlong and τlag with χalcohol, indicates the occurrence of three composition zones in the alcohol-water mixtures, which we correlate with characteristic transformations in the microscopic structures of the solvent mixtures. Our results reveal that the ability of the alcohols to disrupt the H-bonded network structure of bulk water increases with increasing size of their hydrophobic alkyl groups, following the trend, t-BuOH > PrOH > EtOH > MeOH.

Time Evolution of Local pH Around a Photo-Acid in Water and a Polymer Hydrogel: Time Resolved Fluorescence Spectroscopy of Pyranine.

In this work, we propose a new analysis of the time resolved emission spectra of a photo-acid, HA, pyranine (8-hydroxypyrene-1,3,6-trisulphonic acid, HPTS) based on time resolved area normalized emission spectra (TRANES). Presence of an isoemissive point in TRANES confirms the presence of two emissive species (HA and A- ) inside the system in bulk water and inside a co-polymer hydrogel [F127, (PEO)100 -(PPO)70 -(PEO)100 ]. We show that following electronic excitation, the local pH around HPTS, is much lower than the bulk pH presumably because of ejection of proton from the photo-acid in the excited state. With increase in time, the local pH increases and reaches the bulk value. We further, demonstrate that the excited state pKa of HPTS may be estimated from the emission intensities of HA and A- at long time. The time constant for time evolution of pH is ∼630 ps in water, ∼1300 ps in F127 gel and ∼4700 ps in CTAB micelle. The location and local viscosity sensed by the probe is ascertained using fluorescence correlation spectroscopy (FCS) and fluorescence anisotropy decay. The different values of the local viscosity reported by these two methods are reconciled.

Investigation of Different Prototropic Forms of Biologically Active Flavin Lumichrome in the Presence of Liposome.

Herein, we reported the photophysical behavior of lumichrome (LC), one of the biologically active flavin molecules, in the presence of small unilamellar vesicle of anionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). With the help of different spectroscopic techniques, we have proposed that anionic DMPC liposome interacts with the cationic form LC in ground state and in excited state and modulate the spectral properties of LC. Photophysical study reveals that different prototropic forms of LC are present in DMPC liposome medium. In the presence of DMPC liposome, fluorescence emission properties of LC vary with change in excitation and emission wavelengths. This indicates switch over between different structural forms of LC. From fluorescence lifetime measurements and fluorescence lifetime imaging (FLIM) study, it was revealed that emission decay profile of LC was fitted biexponentially in the presence of liposome. It suggests that in the presence of liposome, more than one form of LC is present. We have constructed the time-resolved area-normalized emission spectra (TRANES) of LC in the liposome and found one isoemissive point. This confirmed that two emissive species of LC are present in liposome. FLIM study and FE-SEM study give an idea about the structural feature of the complex between LC and liposome.

Aggregation of Nile Red in Water: Prevention through Encapsulation in β-Cyclodextrin.

The present work, based on various spectroscopic investigations, vividly demonstrates the self-association of Nile red (NR) in aqueous medium. The rapid decrease in the absorbance as well as emission of NR in water bears the signature of the aggregation process. Appearance of a new blue-shifted absorption band in addition to the original one and a drastic decrease in the emission intensity imply that the aggregation is of H-type. Poor solubility of NR in water, hydrophobic interaction, and the planar structure of the dye are ascribed to favor the formation of the aggregate in the aqueous medium. Absorption-based kinetic studies reveal the aggregation process to be second order, thereby establishing the aggregate to be a dimer. Similar kinetic profiles of the absorbance of NR in the presence and absence of light confirm that the aggregation process is not photoassisted. The presence of an isosbestic point in the absorbance spectra and an isoemissive point in the time-resolved area normalized emission spectra bears the evidence of equilibrium between the dimeric and the monomeric species of NR in the ground state as well as in the photoexcited state. Encapsulation of the monomer of NR within the hydrophobic cavity of β-cyclodextrin is demonstrated to prevent the aggregation process.

A Ratio-Analysis Method for the Dynamics of Excited State Proton Transfer: Pyranine in Water and Micelles.

The emission spectrum of a fluorophore undergoing excited state proton transfer (ESPT) often exhibits two distinct bands each representing emissions from protonated and deprotonated forms. The relative contribution of the two bands, best represented by an emission intensity ratio ( R) (intensity maximum of the protonated band/intensity maximum of the deprotonated band), is an important parameter which usually denotes feasibility or promptness of the ESPT process. However, the use of a ratio is only limited to the interpretation of steady-state fluorescence spectra. Here, for the first time, we exploit the time dependence of the ratio ( R( t)), calculated from time-resolved emission spectra (TRES) at different times, to analyze ESPT dynamics. TRES at different times were fitted with a sum of two log-normal functions representing each peak, and then, the peak intensity ratio, R( t), was calculated and further fitted with an analytical function. Recently, a time-resolved area-normalized emission spectra (TRANES)-based analysis was presented where the decay of protonated emission or the rise of deprotonated emission intensity conveniently accounts for the ESPT dynamics. We show that these two methods are equivalent but the new method provides more insights on the nature of the ESPT process.

An Approach to a Model Free Analysis of Excited-State Proton Transfer Kinetics in a Reverse Micelle

Time resolved area normalized emission spectra (TRANES) of a photoacid dye HPTS, confined within the water-pools of reverse micelles (RMs), are analyzed for a direct and model free observation of excited state proton transfer (ESPT) kinetics. When area normalized emission spectra of HPTS at different times are overlapped in a single window, we find population of RO–* (PTS–*) form of HPTS increases at the cost of a concomitant decreasing of the population of ROH* (HPTS*) form as time progresses. Migration of excited state population from ROH* form to RO–* form causes the emergence of an isoemissive point in TRANES, which retains throughout the entire course of ESPT process. An estimation of ESPT time scale is obtained either from the population depleting rate of ROH* form or from the population increasing rate of RO–* form; both are practically the same here. Emergence of an isoemissive point in TRANES implies that there are only two kinetically and reversibly coupled emitting species (ROH* and RO–*) of HPTS are present within the water pools of RMs. Continuous spectral relaxations of HPTS due to excited state solvation dynamics apparently have no effect on the ESPT kinetics of HPTS within the RMs; otherwise spectral shifting, caused by the excited state solvation, would have destroyed the isoemissive point of TRANES.

Proton Transfer Reaction Dynamics of Pyranine in DMSO/Water Mixtures.

Photoinduced intermolecular excited-state proton transfer (ESPT) reactions are ubiquitous in chemistry and biology. ESPT reactions are extremely sensitive to the nature of water molecules in its microenvironment and thus serve as a sensitive reporter for the water structure and dynamics in a system. Herein, the photoinduced intermolecular ESPT reaction of 8-hydroxypyrene-1,3,5-trisulfonic acid (HPTS, also known as pyranine) has been investigated in various DMSO/water mixtures by using steady-state and time-resolved emission spectroscopy. The DMSO/water binary mixture yields an interesting and anomalous behavior for the proton transfer reaction dynamics of HPTS at a mole fraction of DMSO (XDMSO ) of 0.41-0.51, which has also been previously investigated and projected as an anomalous region by molecular dynamics simulation and other experimental techniques. The extreme slowdown of the proton transfer reaction observed at XDMSO =0.41-0.51 has been attributed to the slow solvation dynamics, as well as the non-availability of free water molecules at this composition, which are required to solvate the newly generated proton. These observations have been also corroborated by time-resolved area-normalized emission spectra. The dimensionality of the proton diffusion process has been investigated by analyzing the geminate recombination process, and is found to be significantly different in DMSO/water mixtures (XDMSO =0.41-0.51) compared with three-dimensional proton diffusion in neat water.

On the Molecular Form of Amyloid Marker, Auramine O, in Human Insulin Fibrils.

Designing extrinsic fluorescence sensors for amyloid fibrils is a very active and important area of research. Recently, an ultrafast molecule rotor dye, Auramine O (AuO), has been projected as a fluorescent amyloid marker. It has been claimed that AuO scores better than the most extensively utilized gold-standard amyloid probe, Thioflavin-T (ThT). This advantage arises from the fact that AuO, in addition to its usual emission band (∼500 nm), also displays a large red-shifted emission band (∼560 nm), exclusively in the presence of human insulin fibril medium and not in the native protein or buffer media. On the contrary, for ThT, the emission maximum (∼490 nm) largely remains unchanged while going from protein to fibril. This otherwise unknown large red-shifted emission band of AuO, observed in the presence of human insulin fibrils, was tentatively attributed to a species formed upon fast proton dissociation from excited AuO. It was proposed that because of the long excited-state lifetime (∼1.8 ns) of AuO upon association with human insulin fibrils, this fast proton dissociation from excited AuO could be observed, which is otherwise not observed in buffer or native protein media, owing to its very short excited-state lifetime (∼1 ps). Herein, we show that despite the long excited-state lifetime of AuO in other fibrillar media (human serum albumin and lysozyme), the new red-shifted emission band at 560 nm is not observed, thus possibly suggesting a different origin of the red-shifted emission band of AuO in human insulin fibril medium. We convincingly show that this red-shifted band of AuO (∼560 nm) could be observed under conditions that promote dye aggregation, such as a premicellar concentration of surfactants and polyelectrolytes. These AuO aggregates display strong emission wavelength dependence of transient decay traces, similar to that for AuO in human insulin fibril medium. Detailed time-resolved emission spectral (TRES) measurements suggest that the AuO/premicellar surfactant and AuO/human insulin fibril system share similar features, such as a dynamic red-shift in TRES and an isoemissive point in the time-resolved area-normalized emission spectra, suggesting that the characteristic red-shifted emission band of AuO in human insulin fibril medium may arise from AuO aggregates.

Unravelling the Multiple Emissive States in Citric-Acid-Derived Carbon Dots

Steady-state and time-resolved fluorescence spectroscopy techniques were used to probe multifluorescence resulting from citric-acid-derived carbon dots (C-dots). Commonly, both carboxyl-/amine-functionalized C-dots exhibit three distinct emissive states corresponding to the carbon-core and surface domain. The shorter-wavelength fluorescence (below 400 nm) originates from the carbon-core absorption band at ∼290 nm, whereas the fluorescence (above 400 nm) is caused by two surface states at ∼350 and 385 nm. In addition to three emissive states, a molecular state was also found in amine-functionalized C-dots. Time-resolved emission spectra (TRES) and time-resolved area normalized emission spectra (TRANES) were analyzed to confirm the origin of excitation wavelength-dependent fluorescence of C-dots. The surface functional groups on the C-dots are capable of regulating the electron transfer to affect the multifluorescence behavior. The electron transfer takes place from the carbon-core to surface domain by the presence of −COOH on the surface and vice versa for the case of −NH2 present on the surface. To the best of our knowledge, this is the first report that the multiemissive states are probed in C-dots systems using TRES and TRANES analyses, and related fluorescence mechanisms are verified clearly.

Excited-state proton transfer dynamics of firefly's chromophore D-luciferin in DMSO-water binary mixture.

In this article we have investigated intermolecular excited-state proton transfer (ESPT) of firefly's chromophore D-luciferin in DMSO-water binary mixtures using steady-state and time-resolved fluorescence spectroscopy. The unusual behavior of DMSO-water binary mixture as reported by Bagchi et al. (J. Phys. Chem. B 2010, 114, 12875-12882) was also found using D-luciferin as intermolecular ESPT probe. The binary mixture has given evidence of its anomalous nature at low mole fractions of DMSO (below XD = 0.4) in our systematic investigation. Upon excitation of neutral D-luciferin molecule, dual fluorescence emissions (protonated and deprotonated form) are observed in DMSO-water binary mixture. A clear isoemissive point in the time-resolved area normalized emission spectra further indicates two emissive species in the excited state of D-luciferin in DMSO-water binary mixture. DMSO-water binary mixtures of different compositions are fascinating hydrogen bonding systems. Therefore, we have observed unusual changes in the fluorescence emission intensity, fluorescence quantum yield, and fluorescence lifetime of more hydrogen bonding sensitive anionic form of D-luciferin in low DMSO content of DMSO-water binary mixture.

Unusual H-Type Aggregation of Coumarin-481 Dye in Polar Organic Solvents

Coumarin-481 (C481) dye shows intriguing time-resolved (TR) fluorescence behavior in polar organic solvents of both protic and aprotic nature, namely, ethanol (EtOH) and acetonitrile (ACN), demonstrating the presence of multiple emitting species in the solution. Following concentration-dependent and wavelength-dependent TR fluorescence measurements and the time-resolved emission spectra (TRES) and time-resolved area-normalized emission spectra (TRANES) subsequently constructed using wavelength dependent decay parameters, we convincingly conclude that in the studied solvents a part of the dissolved dye undergoes H-type of aggregation, even at a very low dye concentration. This is quite an unusual finding because the dye C481 apparently shows reasonably good solubility in these organic solvents. As the TR measurements indicate, major contribution in the fluorescence decays is due to monomeric dye, having reasonably short lifetime (∼0.64-0.68 ns), which is in accordance with the conversion of fluorescent intramolecular charge transfer (ICT) state to nonfluorescent twisted intramolecular charge transfer (TICT) state suggested for the dye in high polarity solvents, causing an efficient nonradiative deexcitation. The minor contributions arising from the aggregated dyes show its clear presence in the decays at the blue edge of the emission spectra and have relatively longer lifetimes (∼1.2-5.2 ns) because the steric hindrance caused by the stacked dyes resists the ICT to TICT conversion. Aggregation of C481 dye as observed in the present study in polar organic solvents is an intriguing finding, as the dye is a widely used fluorescent probe for various photochemical studies, where overlooking such aggregation can mislead the observed results.

Ultrafast excited state dynamics of 1-nitropyrene: Effect of H-bonding

Abstract Ultrafast excited state dynamics of 1-nitropyrene has been carried out using femtosecond fluorescence upconversion technique. Detailed time-resolved emission measurements were performed in protic and aprotic solvents of similar polarity. It is seen that although the singlet–triplet energy gap in polar solvents is quite large, the excited state decay shows intersystem crossing in sub-picosecond time scale as observed for nonpolar solvents with isoenergetic singlet and triplet state. The appearance of the sub-picosecond decay component in polar solvents has been explained on the basis of the dynamical nature of the solvent relaxation process. Further, the contribution of the sub-picosecond decay component is much more for the solvent with slower solvent relaxation time. Time-resolved area normalized emission spectra (TRANES) show the presence of two emissive species for 1-nitropyrene in aprotic solvent while only one emissive species has been observed in protic solvent. The additional emissive species in aprotic solvent has been assigned to the structurally relaxed excited singlet state formed due to the rotation of the nitro group around the pyrene moiety. However, such rotation of the nitro group is prevented in protic solvent due to the formation of H-bonded complex between the solute and the solvent. Formation of H-bonding between protic solvent and nitropyrene is also confirmed by quantum chemical calculations.

Revisit of 4,4′-Diaminodiphenyl Sulfone Photophysics in Different Solvents

This paper aims at getting the origin of dual emission of 4,4′-diaminodiphenyl sulfone (Dapsone) in various solvents using steady state and time-resolved emission spectroscopy in conjunction with ground and excited state optimized geometry calculations in gaseous phase and in solvents. The dual fluorescence in different solvents and a distinctive bathochromic spectral shift of the low energy band in polar solvents indicate the possibility of intramolecular charge transfer (ICT) from the donor site (−NH2) to the acceptor moiety in the excited state. Time-resolved emission spectra (TRES) and more precisely time-resolved area normalized emission spectra (TRANES) depict the evolution of the dynamics of excited state species and the charge transfer. Hindrance of rotational relaxation in restricted environments causes intensified charge transfer emission quantum yield. A plausible model for excited state deactivation through ICT has been proposed on the basis of both theoretical and experimental findings. The o...