Ultrafast Relaxation Dynamics from the S2 State of Malachite Green Studied with Femtosecond Upconversion Spectroscopy

Insights into the Photodynamics of Fluorescence Emission and Singlet Oxygen Generation of Fluorogen Activating Protein-Malachite Green Systems

The self-assembled fluorogen activating protein (FAP)-malachite green (MG) complex is a well-established protein-ligand system, which can realize binding-caused fluorescence turn-on of MG and singlet oxygen (¹ O₂ ) generation by MG iodination. To clarify the mechanism of fluorescence activation and ¹ O₂ generation, the photodynamics of different halogen-substituted MG derivatives and their corresponding FAP-MG complexes were studied by femtosecond transient absorption spectroscopy and theoretical computations. The results show that the rotation of MG is restricted by FAP binding, which prevents a rapid internal conversion to allow a longer lifetime for the excited MG to undergo fluorescence emission and intersystem crossing. Moreover, these FAP-MG complexes exhibit notably varied fluorescence quantum yields (Φ_FL ) and ¹ O₂ yields. The study on the decay pathways indicates that such an anti-heavy atom effect predominately stems from the lifetimes of the excited-state species. The photodynamic mechanism study here will lead to more advanced FAP-MG systems with high spatiotemporal resolution.

Dual Emission: Classes, Mechanisms, and Conditions

There has been much interest in dual-emission materials in the past few years for materials and life science applications; however, a systematic overview of the underlying processes is so-far missing. We resolve this issue herein by classifying dual-emission (DE) phenomena as relying on one emitter with two emitting states (DE1), two independent emitters (DE2), or two correlated emitters (DE3). Relevant DE mechanisms for materials science are then briefly described together with the electronic and/or geometrical conditions under which they occur. For further reading, references are given that offer detailed insight into the complex mechanistic aspects of the various DE processes or provide overviews on materials families or their applications. By avoiding ambiguities and misinterpretations, this systematic, insightful Review might inspire future targeted designs of DE materials.

Upper Excited State Photophysics of Malachite Green in Solution and Films

Relaxation pathways of upper excited electronic states of malachite green (MG) in ethanol and in films are studied by steady-state and time-resolved spectroscopic techniques. In contrast to ethanol, where MG emits weak short-lived spectrally well separated S₂ and S₁ fluorescence with the lifetimes ∼0.3 and ∼0.9 ps, MG films show a much stronger broadband fluorescence within 430-700 nm, revealing multiexponential kinetics with the characteristic decay times τ₁ ≈ 1 ps, τ₂ ≈ 10 ps, τ₃ ≈ 0.05-0.8 ns, and τ₄ ≈ 2-3 ns. By the analysis of spectroscopic responses of MG in ethanol and in films as well as by theoretical modeling, we demonstrate that significant increase of fluorescence lifetimes and substantial enhancement of fluorescence intensity in MG films are stipulated by the decrease of efficiency of the S₂ → S₁ and S₁ → S₀ internal conversion, which in turn is caused by hindrance of rotation of MG's phenyl rings controlling the S₂/S₁ and S₁/S₀ conical intersections. These findings indicate that MG films may become promising non-Kasha materials (with reasonable S₂ emission) with numerous photophysical and photochemical applications.

O-H vibrational motions promote sub-50 fs nonadiabatic dynamics in 3-hydroxypyran-4-one: interplay between internal conversion and ESIPT

A theoretical study is used to explore the involvement of O-H vibrational motions in the S0 → S2 photoinduced dynamics of 3-hydroxypyran-4-one (3-HOX). Two transitions, S0 → S1 and S0 → S2, are attributed to the experimentally observed electronic absorption spectral features in the range of 3.5-5.5 eV. We compute model potential energy surfaces of vibronically coupled S1 (nπ*) and S2 (ππ*) states with the aid of extensive electronic structure calculations. The S1-S2 conical intersection is characterized in the O-H bend and O-H stretch vibrational coordinate space. Quantum wavepacket dynamics simulations reveal an ultrafast S2 → S1 internal conversion decay, where about 90% of the S2 population disappears within the first 50 fs of the propagation time. The participation of O-H vibrational motions in the early events of nonadiabatic dynamics is analyzed based on the time evolution of nuclear densities on S2. We discuss the implications of these observations to provide fundamental insights into the nonadiabatic excited-state intramolecular proton transfer in 3-HOX and its derivatives.

Two-dimensional electronic-vibrational spectroscopic study of conical intersection dynamics: an experimental and electronic structure study

The relaxation from the lowest singlet excited state of the triphenylmethane dyes, crystal violet and malachite green, is studied via two-dimensional electronic-vibrational (2DEV) spectroscopy.

Ethyl violet-bovine serum albumin fluorescent protein nanovessels target to lysosomes and mitochondria

AIM

Organelles are essential in maintaining homeostasis of mammalian cells. Monitoring the morphology and dynamics of organelles is of significance in cell state determination and disease diagnosis.

MATERIALS & METHODS

We describe here a new material called ethyl violet-bovine serum albumin fluorescent protein nanovessel (EV-BSA FPN). Upon heating, BSA was denatured to form higher polyhedral structures, which was prone to EV binding. These dye-protein hybrid materials were red fluorescence emissive upon excitation.

RESULTS

EV-BSA FPNs can be readily internalized by mammalian cells and dual localized in lysosomes and mitochondria. Besides, EV-BSA FPN can serve as carriers and efficiently deliver drug into cells.

CONCLUSION

EV-BSA FPNs can be dual function fluorescent vessels for both dual-organelle imaging and drug delivery.

Reversible Insulin Hexamer Assembly Promoted by Ethyl Violet: pH-Controlled Uptake and Release

Therapeutically improved long-acting insulin preparations require in-depth understanding of the hexamer assembly, structural selectivity, and its stability in solution. This Letter demonstrates, for the first time, an efficient method for the hexamerization of human insulin by a structure-specific triphenylmethane (TPM) dye, Ethyl Violet (EV), particularly, in the absence of Zn²⁺. Upon detailed spectroscopic evaluation and comparison with other TPM homologues, we establish that the diethylamino phenyl arms of EV are specific and effective in clipping the three dimer helices in a hexameric assembly. We establish that at physiological pH 7.4 and in the presence of the EV, insulin exists predominantly in its hexameric form, a condition appropriate for storage and preparation of long-acting insulin formulations. On the other hand, the disassembly of the hexamer into the monomeric form is accomplished at pH 5, highlighting its potential as a delivery vehicle for such custom-modified dyes/drugs.

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.

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.

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.

S2 Fluorescence and Ultrafast Relaxation Dynamics of the S2 and S1 States of a Ketocyanine Dye

Dynamics of the excited singlet (both the S2 and S1) states of a ketocyanine dye, namely, 2,5-bis[(2,3-dihydroindolyl)-propylene]-cyclopentanone (KCD), have been investigated in different kinds of media using steady-state absorption and emission as well as femtosecond transient absorption spectroscopic techniques. Steady-state fluorescence measurements, following photoexcitation of KCD to its second excited singlet state, reveal dual fluorescence (emission from both the S2 and S1 states) behavior. Although the intensity of the S2 --> S0 fluorescence is weaker than that of the S1 --> S0 fluorescence in solutions at room temperature (298 K), the former becomes as much as or more intense than the latter in rigid matrixes at 77 K. The lifetime of the S2 state is short and varies between 0.2 and 0.6 ps in different solvents. After its creation, the S2 state undergoes two simultaneous processes, namely, S2 --> S0 fluorescence and S2 --> S1 internal conversion. Time-resolved measurements reveal the presence of an ultrafast component in the decay dynamics of the S1 state. A good correlation between the lifetime of this component and the longitudinal relaxation times (tauL) of the solvents suggests that this component arises due to solvation in polar solvents. More significant evolution of the spectroscopic properties of the S1 state in alcoholic solvents in the ultrafast time domain has been explained by the occurrence of the repositioning of the hydrogen bonds around the carbonyl group in the excited state of KCD. In 2,2,2-trifluoroethanol, a strongly hydrogen bond donating solvent, it has even been possible to establish the existence of two distinct forms of the S1 state, namely, the non-hydrogen-bonded (or free) molecule and the hydrogen-bonded complex.