Intriguing H-aggregate and H-dimer formation of coumarin-481 dye in aqueous solution as evidenced from photophysical studies

Photophysical properties of coumarin-481 (C481) dye in aqueous solution show intriguing presence of multiple emitting species. Concentration and wavelength dependent fluorescence decays and time-resolved emission spectra and area-normalized emission spectra suggest the coexistence of dye monomers, dimers, and higher aggregates (mostly trimers) in the solution. Because of the efficient intramolecular charge transfer (ICT) state to twisted intramolecular charge transfer (TICT) state conversion, the dye monomers show very short fluorescence lifetime of ~0.2 ns. Fluorescence lifetimes of dimers (~4.1 ns) and higher aggregates (~1.4 ns) are relatively longer due to steric constrain toward ICT to TICT conversion. Observed results indicate that the emission spectra of the aggregates are substantially blue-shifted compared to monomers, suggesting H-aggregation of the dye in the solution. Temperature-dependent fluorescence decays in water and time-resolved fluorescence results in water-acetonitrile solvent mixtures are also in support of the dye aggregation in the solution. Though dynamic light scattering studies could not recognize the dye aggregates in the solution due to their small size and low concentration, fluorescence up-conversion measurements show a relatively higher decay tail in water than in water-acetonitrile solvent mixture, in agreement with higher dye aggregation in aqueous solution. Time-resolved fluorescence results with structurally related non-TICT dyes, especially those of coumarin-153 dye, are also in accordance with the aggregation behavior of these dyes in aqueous solution. To the best of our knowledge, this is the first report on the aggregation of coumarin dyes in aqueous solution. Present results are important because coumarin dyes are widely used as fluorescent probes in various microheterogeneous systems where water is always a solvent component, and the dye aggregation in these systems, if overlooked, can easily lead to a misinterpretation of the observed results.     

Dual Fluorescence and Photoprototropic Characteristics of 2-Aminodiphenylsulphone-β-Cyclodextrin Inclusion Complex

The absorption and fluorescence spectral characteristics of 2-aminodiphenylsulphone (2ADPS) have been investigated in the presence of β-cyclodextrin (β-CDx) in water. Dual emission is observed upon the complexation of 2ADPS in β-CDx. The stoichiometry of the host:guest inclusion complex is found to be 2:1. Steady state and time-resolved fluorescence spectral analysis support the formation of 2:1 complex between β-CDx and 2ADPS. The large enhancement in fluorescence intensity of twisted intramolecular charge transfer (TICT) band in aqueous β-CDx solution is due to the decrease in non-radiative processes. The ground and the excited state pK _a values for the monocation-neutral equilibrium of 2ADPS in β-CDx are found to be different from the pK _a values in aqueous solution. In the presence of β-CDx, 2ADPS is found to be less basic in the ground and the excited states.     

分子内扭转电荷转移的新荧光探针的研究

选用对二乙氨基苯甲酸钠(SDEAB)和对二甲氨基苯甲酸钠(SDMAB)作为分子内扭转电荷转移(TICT)荧光探针, 表征了甲基修饰化所引起的β-CD空腔微环境的变化, 得出环糊精的非极性空腔有利于对二烷氨基苯甲酸型分子的TICT态形成的结论。以甲醇的测定为例, 展示了分子内扭转电荷转移作为荧光探针的分析应用。     

An Ultrafast Molecular-Rotor-Based Fluorescent Turn-On Sensor for the Perrhenate Anion in Aqueous Solution

Devising sensors for the perrhenate anion in aqueous media is extremely challenging, and has seldom been reported in the literature. Herein, we report a fluorescence turn-on sensor for the perrhenate anion in aqueous media based on the aggregation-induced emission of a popular ultrafast molecular rotor dye, Thioflavin-T. The selective response towards the perrhenate anion has been rationalized in terms of matching water affinity, with the weakly hydrated perrhenate anion spontaneously forming a contact ion pair with the weakly hydrated ultrafast molecule-rotor-based organic cation, Thioflavin-T, which in turn leads to an aggregate assembly that provides a fluorescence turn-on response towards perrhenate. The sensing response of Thioflavin-T has been found to be quite selective towards the perrhenate anion when tested against anions that are ubiquitously present in the environment, such as chloride, nitrate, and sulfate anions. The formation of self-assembled Thioflavin-T aggregates has also been investigated by time-resolved emission and temperature-dependent measurements.     

Dual fluorescence of omeprazole: effect of solvents and pH

Absorption, steady state fluorescence and time-resolved fluorescence spectra of omeprazole (OMP) have been studied in solvents of different polarity and pH. With an increase in the polarity of the solvents, blue shift is observed in the longer wavelength whereas red shift is noticed in the shorter wavelength band. The dual emission observed in non-polar solvents suggests that the energy of the twisted intramolecular charge transfer (TICT) state is lower than that of the locally excited (LE) state. The normal Stokes-shifted band originates from the LE state, and the large Stokes-shifted band is due to the emission from a TICT state. The Stokes shift of OMP is correlated with various solvent polarity scales like E_T(30) and f?(D,n).     

Host–guest complexation between 5-aminoisoquinoline and β-cyclodextrin and its effect on spectral and prototropic characteristics

The effects of the addition of β-cyclodextrin (β-CDx) on the absorption and emission properties of the 5-aminoisoquinoline (5AIQ) have been investigated in aqueous media. The formation of host–guest inclusion complex with 1:1 stoichiometry was revealed by absorption, steady state and time-resolved emission spectroscopy. The complex formation has also been confirmed by FT-IR spectra and SEM image analysis of the solid inclusion complex between 5AIQ and β-CDx. No significant change was observed in the ground and excited state pKa values in β-CDx medium. Based on photophysical and prototropic characteristics of 5AIQ in β-CDx, the structure of the 1:1 inclusion complex is proposed.     

Photophysical properties of DASPMI as revealed by spectrally resolved fluorescence decays

Photophysical properties of 2-(4-(dimethylamino)styryl)-1-methylpyridinium iodide (DASPMI) in various solvents were investigated using time- and space-correlated single photon counting. DASPMI is known to selectively stain mitochondria in living cells.1,2 The uptake and fluorescence intensity of DASPMI in mitochondria is a dynamic measure of membrane potential. Hence, an endeavor has been made to elucidate the mechanism of DASPMI fluorescence by obtaining spectrally resolved fluorescence decays in different solvents. A biexponential decay model was sufficient to globally describe the wavelength-dependent fluorescence in ethanol and chloroform. While in glycerol, a three-exponential decay model was necessary for global analysis. In the polar low-viscous solvent water, a monoexponential decay model fitted the decay data. The sensitivity of DASPMI to solvent viscosity was analyzed using various proportions of glycerol-ethanol mixtures. The lifetimes were found to increase with increasing solvent viscosity. The negative amplitudes of the short lifetime component found in chloroform and glycerol at the longer wavelengths validated the formation of new excited-state species from the initially excited state. Time-resolved emission spectra in chloroform and glycerol showed a biphasic increase of spectral width and emission maxima. The spectral width had an initial fast increase within 150 ps and a near constant thereafter. A three-state model of generalized scheme, on the basis of successive formation of locally excited state (LE), intramolecular charge transfer state (ICT), and twisted intramolecular charge transfer (TICT) state, has been proposed to explain the excited-state kinetics. The presumed role of solvation dynamics of ICT and TICT states leading to the asymmetrical broadening and structureless fluorescence has been substantiated by the decomposition of time-resolved emission spectra in chloroform, glycerol, and ethanol/glycerol mixtures.     

Femtosecond excited-state dynamics of 4-nitrophenyl pyrrolidinemethanol: evidence of twisted intramolecular charge transfer and intersystem crossing involving the nitro group

Ultrafast excited-state relaxation dynamics of a nonlinear optical (NLO) dye, (S)-(-)-1-(4-nitrophenyl)-2-pyrrolidinemethanol (NPP), was carried out under the regime of femtosecond fluorescence up-conversion measurements in augmentation with quantum chemical calculations. The primary concern was to trace the relaxation pathways which guide the depletion of the first singlet excited state upon photoexcitation, in such a way that it is virtually nonfluorescent. Ground- and excited-state (singlet and triplet) potential energy surfaces were calculated as a function of the -NO(2) torsional coordinate, which revealed the perpendicular orientation of -NO(2) in the excited state relative to the planar ground-state conformation. The fluorescence transients in the femtosecond regime show biexponential decay behavior. The first time component of a few hundred femtoseconds was ascribed to the ultrafast twisted intramolecular charge transfer (TICT). The occurrence of charge transfer (CT) is substantiated by the large dipole moment change during excitation. The construction of intensity- and area-normalized time-resolved emission spectra (TRES and TRANES) of NPP in acetonitrile exhibited a two-state emission on behalf of decay of the locally excited (LE) state and rise of the CT state with a Stokes shift of 2000 cm(-1) over a time scale of 1 ps. The second time component of a few picoseconds is attributed to the intersystem crossing (isc). In highly polar solvents both the processes occur on a much faster time scale compared to that in nonpolar solvents, credited to the differential stability of energy states in different polarity solvents. The shape of frontier molecular orbitals in the excited state dictates the shift of electron density from the phenyl ring to the -NO(2) group and is attributed to the charge-transfer process taking place in the molecule. The viscosity dependence of relaxation dynamics augments the proposition of considering the -NO(2) group torsional motion as the main excited-state relaxation coordinate.     

Global and distribution analysis of fluorescence decays and spectrofluorimetric determination of stoichiometry and association constant of the inclusion complex of 2-amino-5,6-dimethyl-benzimidazole with beta-cyclodextrin

The steady state and time-resolved fluorescence study of 2-amino-5,6-dimethyl-benzimidazole (ADBI) have been studied in aqueous solution of β-cyclodextrin (β-CD). The fluorescence decays were analyzed by global analysis and distribution analysis in order to get insight about the inclusion process. The fluorescence lifetime of ADBI is increased in β-CD and an enhancement of the emission, is observed, together with negligible changes in the energy of ADBI in β-CD. The experimental data show that β-CD reacts with ADBI to form a 1:1 host-guest complex with association constant was determined to be 2074±77 M⁻¹. Both global analysis and distribution analysis of the fluorescence decays support the formation of only 1:1 inclusion complex. AM1 calculation shows that the size of ADBI was appropriate for good insertion within the β-CD cavity and the inclusion of ADBI inside the β-CD cavity should takes place from the side of the amino group substituent.     

Encapsulation of alkylparabens with natural and modified α- and β-cyclodextrins

The inclusion complexation between methylparaben, ethylparaben, propylparaben, butylparaben with α-CD, β-CD, hydroxypropyl α-cyclodextrin and hydroxypropyl β-cyclodextrin were carried out by UV–Vis, steady state and time-resolved fluorescence, FT-IR, ¹H NMR techniques and semi-empirical method (PM3). The drug molecules are all given one emission maximum in water where as dual emission in all the CDs. CDs study revealed that the paraben molecules were formed 1:1 inclusion complex. The aliphatic side chain is present in the hydrophilic part whereas hydroxyl group is present in the hydrophobic part of the CD cavity. Nanosecond time-resolved studies indicated that paraben exhibited biexponential decay in water whereas triexponential decay in CDs solution. The complexation energy, thermodynamic parameters and HOMO–LUMO energy structure were calculated using quantum chemical calculation.     

环糊精诱导胶束形成的TICT荧光探针法研究

环糊精与表面活性剂的相互作用研究已有诸多报导，但主要涉及环糊精与表面活性剂的包络物的稳定常数和其中主客体的化学剂量比［１，２］，至于环糊精对表面活性剂胶束化性质的影响则少见涉猎，这或许与研究方法有关，分子内扭转电荷转移激发态涉及到一个完整的电荷转移，具有很高的极性，其荧光特性显示出显著的介质性质敏感性［３］，因此将ＴＩＣＴ荧光探针法用于环糊精－表面活性剂相互作用研究可能会提供一些新的信息，本文的结     

Energy transfer in calixarene-based cofacial-positioned perylene bisimide arrays

The synthesis of multichromophoric perylene bisimide-calix[4]arene arrays with up to five perylene units (containing orange, violet, and green perylene bisimide chromophores) and of monochromophoric model compounds was achieved by subsequent imidization of mono-Boc functionalized calix[4]arene linkers with three different types of perylene bisimide dye units. The optical properties of all compounds were studied with UV/vis absorption and steady state and time-resolved fluorescence spectroscopy. Upon excitation of the inner orange dye at 490 nm of array 3, strong fluorescence emission of the outer green perylene bisimide (PBI) chromophore at 744 nm is observed. The fluorescence excitation spectra of compounds 3 and 4 (lambdadet = 850 nm) show all absorption bands of the parent chromophores (e.g., all perylene units contribute to the emission from S1 state of the green PBI). Thus, the fluorescence emission and excitation spectra as well as time-resolved data of fluorescence lifetimes in the absence (tauD = 5.1 ns) and in the presence of an acceptor (tauDA = 0.8 ns) suggest efficient energy transfer processes between the perylene bisimide dye units. For the bichromophoric array 4, the energy transfer rate is calculated to a value of 1.05 x 109 s-1. These results demonstrate highly efficient energy transfer in cofacially assembled dye arrays.     

Multifaceted Excited State Dynamics of Coumarin Dyes Anchored on Al2O3 Film

The co-facially stacked dyes on semiconductor films serve as an alternative model to elucidate the photo-driven exciton dynamics occurring in a molecular assembly. In this study, we report the unique emission properties of coumarin dye adsorbed on the surface of the semiconductor film, measured by ultrafast time-resolved fluorescence. When a rigid coumarin derivative, 7-hydroxycoumarin-3-carboxylic acid (OHCCA), is anchored on the Al₂O₃ film, the dye manifests dual emissions from the two lowest excited states. Various anchoring modes of a carboxylic acid group on the Al₂O₃ surface are invoked to account for the unusual emission process. Additionally, we identified characteristic transition dipole interactions in the well-stacked dye aggregates, which leads to discernible excitonic splitting in the electronic transitions. Femtosecond time-resolved fluorescence reveals that the excimer formation in the aggregate occurs with the time constant of 550 fs. Picosecond time-resolved emission spectra confirm the subsequent structural relaxations of the nascent excimer. The enhanced transition dipole via the electronic coupling between OHCCA and metal oxide can be responsible for the dual emission and the ultrafast excimer formation.     

Role of dipole moment of solvents in formation and stabilization of the TICT states in Coumarin 445 under nitrogen laser excitation

In this paper we report the observation of dual Amplified Spontaneous Emission (ASE) from solutions of 7-ethylamino-4-methyl coumarin dye (Coumarin 445) in certain solvents such as n-butyl acetate, dioxane etc. when pumped by high power nitrogen laser. The two ASE bands appear to be from two different excited species (ICT and TICT conformation) one of which is the precursor of the other. The spectral characteristics of dye Coumarin 445 depend upon the solvent environment. The TICT coumarin photoisomers, which form exciplexes with the solvent molecules, have enough gain to produce amplified spontaneous emission even when there is apparently no detectable fluorescence. The behaviour of this dye in the excited state is studied by measuring the small signal gain and variation of the gain slope with temperature in different solvents. It is observed that polarity of the solvent plays a more dominant role in formation and stabilization of TICT states.     

Photophysical studies of dipolar organic dyes that feature a 1,3-cyclohexadiene conjugated linkage: the implication of a twisted intramolecular charge-transfer state on the efficiency of dye-sensitized solar cells

A detailed study of the synthesis and photophysical properties of a new series of dipolar organic photosensitizers that feature a 1,3‐cyclohexadiene moiety integrated into the π‐conjugated structural backbone has been carried out. Dye‐sensitized solar cells (DSSCs) based on these structurally simple dyes have shown appreciable photo‐to‐electrical energy conversion efficiency, with the highest one up to 4.03 %. Solvent‐dependent fluorescence studies along with the observation of dual emission on dye 4 b and single emission on dyes 4 a and 32 suggest that dye 4 b possesses a highly polar emissive excited state located at a lower‐energy position than at the normal emissive excited state. A detailed photophysical investigation in conjunction with computational studies confirmed the twisted intramolecular charge‐transfer (TICT) state to be the lowest emissive excited state for dye 4 b in polar solvents. The relaxation from higher‐charge‐injection excited states to the lowest TICT state renders the back‐electron transfer process a forbidden one and significantly retards the charge recombination to boost the photocurrent. The electrochemical impedance under illumination and transient photovoltage decay studies showed smaller charge resistance and longer electron lifetime in 4 b ‐based DSSC compared to the DSSCs with reference dyes 4 a and 32 , which further illustrates the positive influence of the TICT state on the performance of DSSCs.     

Photophysics of TICT states of 7-diethylamino-4-methyl coumarin dye by energy transfer techniques

Coumarin 1 exhibited dual amplified spontaneous emission (ASE) in certain solvents under nitrogen laser excitation. These emissions are known as normal and anomalous emissions. The anomalous emission corresponds to TICT state and it does not have a corresponding fluorescence peak. Energy transfer techniques have been used to study the photophysics of TICT states and the characteristics of dual ASE bands of the dye coumarin 1.     

Twisting dynamics in the excited singlet state of Michler's ketone

Ultrafast relaxation dynamics of the excited singlet (S(1)) state of Michler's ketone (MK) has been investigated in different kinds of solvents using a time-resolved absorption spectroscopic technique with 120 fs time resolution. This technique reveals that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state because of twisting of the N,N-dimethylanilino groups with respect to the central carbonyl group is the major relaxation process responsible for the multi-exponential and probe-wavelength-dependent transient absorption dynamics of the S1 state of MK, but solvation dynamics does not have a significant role in this process. Theoretical optimization of the ground-state geometry of MK shows that the dimethylanilino groups attached to the central carbonyl group are at a dihedral angle of about 51 degrees with respect to each other because of steric interaction between the phenyl rings. Following photoexcitation of MK to its S1 state, two kinds of twisting motions have been resolved. Immediately after photoexcitation, an ultrafast "anti-twisting" motion of the dimethylanilino groups brings back the pretwisted molecule to a near-planar geometry with high mesomeric interaction and intramolecular charge transfer (ICT) character. This motion is observed in all kinds of solvents. Additionally, in solvents of large polarity, the dimethylamino groups undergo further twisting to about 90 degrees with respect to the phenyl ring, to which it is attached, leading to the conversion of the ICT state to the TICT state. Similar characteristics of the absorption spectra of the TICT state and the anion radical of MK establish the nearly pure electron transfer (ET) character of the TICT state. In aprotic solvents, because of the steep slope of the potential energy surface near the Franck-Condon (FC) or LE state region, the LE state is nearly nonemissive at room temperature and fluorescence emission is observed from only the ICT and TICT states. Alternatively, in protic solvents, because of an intermolecular hydrogen-bonding interaction between MK and the solvent, the LE region is more flat and stimulated emission from this state is also observed. However, a stronger hydrogen-bonding interaction between the TICT state and the solvent as well as the closeness between the two potential energy surfaces due to the TICT and the ground states cause the nonradiative coupling between these states to be very effective and, hence, cause the TICT state to be weakly emissive. The multi-exponentiality and strong wavelength-dependence of the kinetics of the relaxation process taking place in the S1 state of MK have arisen for several reasons, such as strong overlapping of transient absorption and stimulated emission spectra of the LE, ICT, and TICT states, which are formed consecutively following photoexcitation of the molecule, as well as the fact that different probe wavelengths monitor different regions of the potential energy surface representing the twisting motion of the excited molecule.     

Effect of nanocavity confinement on the relaxation of anesthetic analogues: relevance to encapsulated drug photochemistry

We report on UV-vis absorption and picosecond emission studies of methyl 2-amino-4,5-dimethoxy benzoate in neutral water and complexed to alpha-, beta-, and gamma-cyclodextrin (CD). Upon encapsulation, the emission intensity and the fluorescence lifetime increase, indicating a hydrophobic effect of the nanocages on the photophysical behavior of the guest. beta-CD confinement shows the largest effect. The time-dependent frequency shift of the emission (approximately 720 cm(-1)) in beta-CD nanocavity is larger than the one observed in water (approximately 490 cm(-1)) due to the hydrophobic and polarity effect of the nanocage and reflects a strengthening of the intramolecular H-bond of the encapsulated dye upon electronic excitation. Anisotropy measurements indicate a free motion of the guest into the nanocavity. The observed results are relevant to the hydrophobic as well as hydrophilic interactions which govern photochemistry and photophysics of caged drugs, organic, and biological systems.     

Time-dependent density functional theory study on hydrogen-bonded intramolecular charge-transfer excited state of 4-dimethylamino-benzonitrile in methanol

The time-dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen-bonded intramolecular charge-transfer (ICT) excited state of 4-dimethylaminobenzonitrile (DMABN) in methanol (MeOH) solvent. We demonstrated that the intermolecular hydrogen bond C[triple bond]N...H-O formed between DMABN and MeOH can induce the C[triple bond]N stretching mode shift to the blue in both the ground state and the twisted intramolecular charge-transfer (TICT) state of DMABN. Therefore, the two components at 2091 and 2109 cm(-1) observed in the time-resolved infrared (TRIR) absorption spectra of DMABN in MeOH solvent were reassigned in this work. The hydrogen-bonded TICT state should correspond to the blue-side component at 2109 cm(-1), whereas not the red-side component at 2091 cm(-1) designated in the previous study. It was also demonstrated that the intermolecular hydrogen bond C[triple bond]N...H-O is significantly strengthened in the TICT state. The intermolecular hydrogen bond strengthening in the TICT state can facilitate the deactivation of the excited state via internal conversion (IC), and thus account for the fluorescence quenching of DMABN in protic solvents. Furthermore, the dynamic equilibrium of these electronically excited states is explained by the hydrogen bond strengthening in the TICT state.     

Twisted intramolecular charge-transfer state of trans-3-(N,N-Dimethylamino)-4'-cyanostilbene: The C−C bond twisting

Whether a twisted intramolecular charge-transfer (TICT) state is formed is an important issue for understanding the fluorescence properties of a push-pull organic dye. Here we report a position effect of the donor substituent on the TICT state formation of aminostilbenes: namely, trans-3-(N,N-dimethylamino)-4′-cyanostilbene ( mDCS ) behaves differently from its TICT-free para isomer DCS and forms a TICT state in acetonitrile (MeCN). The TICT state of mDCS also differs from that of many previously reported aminostilbenes by twisting a C−C bond instead of a C−N bond. In addition, among the ring-bridged model compounds mDCS-N , mDCS-C1 , and mDCS-C2 , we observed an unusual electronic effect of the ring-bridging in mDCS-C2 that mitigates the impact of the TICT state on the fluorescence properties. Both the C−C bond twisting in mDCS and the ring-bridging electronic effect in mDCS-C2 provide new insights into the TICT chemistry of aminostilbenes.