Torsional dynamics of molecules on barrierless potentials in liquids. I. Temperature and wavelength dependent picosecond studies of triphenyl‐methane dyes

Torsional Disorder, Symmetry Breaking, and the Crystal Violet Shoulder Controversy

The nature of the lowest-energy electronic absorption band of crystal violet (CV) and particularly the origin of its high-energy shoulder have been debated since the middle of the past century. The most recent studies invoke a splitting of the S₁ state upon symmetry breaking induced by interactions with the solvent and/or the counterion. Using a combination of stationary and time-resolved polarized spectroscopy together with quantum-chemical calculations, we show that torsional disorder in the ground-state results in an inhomogeneous broadening of the absorption band of CV. The center of the band is mostly due to symmetric molecules with a degenerate S₁ state, whereas the edges originate from transitions to the S₁ and S₂ states of distorted symmetry-broken molecules. Transient-absorption measurements with different excitation wavelengths reveal that these two groups of molecules interconvert rapidly in liquid but not in a rigid environment.

Solvent Tuning Excited State Structural Dynamics in a Novel Bianthryl

Symmetry breaking charge separation (SBCS) is central to photochemical energy conversion. The widely studied 9,9-bianthryl (9,9'BA) is the prototype, but the role of bianthryl structure is hardly investigated. Here we investigate excited state structural dynamics in a bianthryl of reduced symmetry, 1,9-bianthryl (1,9'BA), through ultrafast electronic and vibrational spectroscopy. Resonance selective Raman in polar solvents reveals a Franck-Condon state mode that disappears concomitant with the rise of ring breathing modes of radical species. Solvent-dependent dynamics show that CS is driven by solvent orientational motion, as in 9,9'BA. In nonpolar solvents the excited state undergoes multistep structural relaxation, including subpicosecond Franck-Condon state decay and biexponential diffusion-controlled structural evolution to a distorted slightly polar state. These data suggest two possible routes to SBCS; the established solvent driven pathway in rapidly relaxing polar solvents and, in slowly relaxing media, initial intramolecular reorganization to a polar structure which drives solvent orientational relaxation.

Valence-bond non-equilibrium solvation model for a twisting monomethine cyanine

We propose and analyze a two-state valence-bond model of non-equilibrium solvation effects on the excited-state twisting reaction of monomethine cyanines. Suppression of this reaction is thought responsible for environment-dependent fluorescence yield enhancement in these dyes. Fluorescence is quenched because twisting is accompanied via the formation of dark twisted intramolecular charge-transfer (TICT) states. For monomethine cyanines, where the ground state is a superposition of structures with different bond and charge localizations, there are two possible twisting pathways with different charge localizations in the excited state. For parameters corresponding to symmetric monomethines, the model predicts two low-energy twisting channels on the excited-state surface, which leads to a manifold of TICT states. For typical monomethines, twisting on the excited state surface will occur with a small barrier or no barrier. Changes in the solvation configuration can differentially stabilize TICT states in channels corresponding to different bonds, and that the position of a conical intersection between adiabatic states moves in response to solvation to stabilize either one channel or the other. There is a conical intersection seam that grows along the bottom of the excited-state potential with increasing solvent polarity. For monomethine cyanines with modest-sized terminal groups in moderately polar solution, the bottom of the excited-state potential surface is completely spanned by a conical intersection seam.

Organic nanoparticles of malachite green with enhanced far-red emission: size-dependence of particle rigidity

The “rigidity” of malachite green nanoparticles is strongly size-dependent; the smaller the size is, the greater the particle rigidity inside.

On the interaction of triarylmethane dye crystal violet with LAPONITE® clay: using mineral nanoparticles to control the dye photophysics

The combination of an organic dye with clays leads to very interesting hybrid materials with original properties.

Ultrafast excited state dynamics of S2 and S1 states of triphenylmethane dyes

Excited state dynamics of S2 and S1 states for a series of TPM dyes, pyrogallol red (PGR), bromopyrogallol red (Br-PGR) and aurin tricarboxylic acid (ATC), have been monitored by using ultrafast transient absorption and fluorescence up-conversion techniques.

Barrierless electronic relaxation in solution: An analytically solvable model

We propose an analytical method for understanding the problem of electronic relaxation in solution, modelled by a particle undergoing diffusive motion under the influence of two potentials. The coupling between the two potentials is assumed to be represented by a Dirac delta function. The diffusive motion in this paper is described by the Smoluchowski equation. Our solution requires the knowledge of the Laplace transform of the Green's function for the motion in both the uncoupled potentials. Our model is more general than all the earlier models, because we are the first one to consider the effect of ground state potential energy surface explicitly.

Ultrafast Studies of the Photophysics of Cis and Trans States of the Green Fluorescent Protein Chromophore

Cis-trans photoisomerization is proposed as a key process in the photoswitching of some photoactivatable fluorescent proteins. Here we present ultrafast fluorescence measurements of the model GFP chromophore (HBDI) in the cis state and in a mixture of the cis and trans states. Our results demonstrate that the mean lifetimes of the cis and trans states are remarkably similar. Therefore, the specific isomer of the chromophore cannot be solely responsible for the different photophysics of the bright and dark states of photoactive proteins, which must therefore be due to differential interactions between the different isomers of the chromophore and the protein.

Excited state relaxation dynamics of model green fluorescent protein chromophore analogs: evidence for cis-trans isomerism

Two green fluorescent protein (GFP) chromophore analogs (4Z)-4-(N,N-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DMPI) and (4Z)-4-(N,N-diphenylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5H-imidazolin-5-one (DPMPI) were investigated using femtosecond fluorescence up-conversion spectroscopy and quantum chemical calculations with the results being substantiated by HPLC and NMR measurements. The femtosecond fluorescence transients are found to be biexponential in nature and the time constants exhibit a significant dependence on solvent viscosity and polarity. A multicoordinate relaxation mechanism is proposed for the excited state relaxation behavior of the model GFP analogs. The first time component (τ(1)) was assigned to the formation of twisted intramolecular charge transfer (TICT) state along the rotational coordinate of N-substituted amine group. Time resolved intensity normalized and area normalized emission spectra (TRES and TRANES) were constructed to authenticate the occurrence of TICT state in subpicosecond time scale. Another picosecond time component (τ(2)) was attributed to internal conversion via large amplitude motion along the exomethylenic double bond which has been enunciated by quantum chemical calculations. Quantum chemical calculation also forbids the involvement of hula-twist because of high activation barrier of twisting. HPLC profiles and proton-NMR measurements of the irradiated analogs confirm the presence of Z and E isomers, whose possibility of formation can be accomplished only by the rotation along the exomethylenic double bond. The present observations can be extended to p-HBDI in order to understand the role of protein scaffold in reducing the nonradiative pathways, leading to highly luminescent nature of GFP.

Alkaline Fading of Triarylmethyl Carbocations in Self-Assembly Microheterogeneous Media

This review reports on the alkaline fading of crystal violet and other related carbocations in the presence of different microheterogeneous media (micelles, microemulsions and vesicles).

Microviscosity inside a nanocavity: a femtosecond fluorescence up-conversion study of malachite green

Femtosecond fluorescence up-conversion measurements of malachite green (MG) have been carried out to confirm the relaxation pathway and subsequently to probe the microviscosity of water trapped in a nanoconfined environment using an AOT (sodium dioctylsulfosuccinate, aerosol-OT) reverse micelle as a model system. The experimental results reveal a strong dependence of S(1) state relaxation dynamics of MG on solvent viscosity while a very weak dependence has been observed for the S(2) state relaxation. The time-dependent density functional theory (TD-DFT) calculations have been used to construct potential energy surfaces of MG by pursuing an intramolecular rotation along the torsional coordinate of the phenyl rings. On synchronization with the experimental observations, the computational results comprehend the existence of a conical intersection along the S(1) and S(0) potential energy surfaces, which leads to mixed vibrational levels of S(1) and S(0) characteristics. The results suggest that the conical intersection is along the torsional coordinate of N,N-dimethyl substituted phenyl ring. Correlating the observed dynamics of MG in a confined system with the relaxation time of MG in different glycerol-water mixtures, we assert the determination of the microviscosity of water inside the AOT reverse micelle. The data confer that the microviscosity of water in an AOT water pool of w(0) = 2 (9 cP) is almost 9 times higher than the bulk water. As we increase the w(0) from 2 to 40, the microviscosity decreases monotonically to 5.68 cP, and the decrease is observed to be exponential in nature.

Comparison of the ATP binding sites of protein kinases using conformationally diverse bisindolylmaleimides

The conformation of a bisindolylmaleimide may be controlled by the size of a macrocyclic ring in which it is constrained. A range of techniques were used to demonstrate that the tether controls both the ratio of the two limiting conformers (syn and anti) in solution and the extent of conjugation between the maleimide and indole rings. Screening the conformationally diverse bisindolylmaleimides against a panel of protein kinases allowed their ATP binding sites to be compared using a chemical approach which, like sequence alignment, does not require detailed structural information. This approach lead to the conclusion that several AGC group protein kinases (including PKCalpha, PKCbeta, MSK1, p70 S6K, PDK-1, and MAPKAP-K1alpha) may be best inhibited by bisindolylmaleimides which adopt a compressed approximately C2-symmetric anti conformation; in constrast, GSK3beta may be best inhibited by bisindolylmaleimides whose ground state is a distorted syn conformation. It is concluded that PDK-1, whose structure has been determined by X-ray crystallography, and its mutants, may serve as particularly useful surrogates for the study of PKC inhibitors.

Photoisomerization and photoionization of the photoactive yellow protein chromophore in solution

Dispersed pump-dump-probe spectroscopy has the ability to characterize and identify the underlying ultrafast dynamical processes in complicated chemical and biological systems. This technique builds on traditional pump-probe techniques by exploring both ground- and excited-state dynamics and characterizing the connectivity between constituent transient states. We have used the dispersed pump-dump-probe technique to investigate the ground-state dynamics and competing excited-state processes in the excitation-induced ultrafast dynamics of thiomethyl p-coumaric acid, a model chromophore for the photoreceptor photoactive yellow protein. Our results demonstrate the parallel formation of two relaxation pathways (with multiple transient states) that jointly lead to two different types of photochemistry: cis-trans isomerization and detachment of a hydrated electron. The relative transition rates and quantum yields of both pathways have been determined. We find that the relaxation of the photoexcited chromophores involves multiple, transient ground-state intermediates and the chromophore in solution does not generate persistent photoisomerized products, but instead undergoes photoionization resulting in the generation of detached electrons and radicals. These results are of great value in interpreting the more complex dynamical changes in the optical properties of the photoactive yellow protein.

Excited state dynamics and rapid internal conversion in a stable dipole molecule

The excited singlet state of an azomethine ylide or 'stable dipole' exhibits an ultrafast radiationless relaxation after femtosecond laser excitation. These transients are observed before the excited state decays in an almost activationless manner, the barrier is 440 cm-1, to the ground state with a 1.5 ps lifetime. Cooling of the hot ground state is also apparent in the transient absorption data and in methanol decays with a 5.7 ps lifetime. The viscosity dependence of the fluorescence yield and lifetime in different solvents is small and far less pronounced than in triphenylmethane dyes. Surprisingly, the excited state decay is not due to twisting about the C-N bond of the ylide but it is caused by buckling of one of the rings as the nitrogen atom changes character from sp2 to sp3 hybridisation.