Nile Red Fluorescence: Where's the Twist?

Nile Red is a fluorescent dye used extensively in bioimaging due to its strong solvatochromism. The photophysics underpinning Nile Red's fluorescence has been disputed for decades, with some studies claiming that the dye fluoresces from two excited states and/or that the main emissive state is twisted and intramolecular charge-transfer (ICT) in character as opposed to planar ICT (PICT). To resolve these long-standing questions, a combined experimental and theoretical study was used to unravel the mechanism of Nile Red's fluorescence.

Efficient Ground-State Recovery of UV-Photoexcited -Nitrophenol in Aqueous Solution by Direct and Multistep Pathways.

Nitroaromatic compounds are found in brown carbon aerosols emitted to the Earth's atmosphere by biomass burning, and are important organic chromophores for the absorption of solar radiation. Here, transient absorption spectroscopy spanning 100 fs-8 μs is used to explore the pH-dependent photochemical pathways for aqueous solutions of -nitrophenol, chosen as a representative nitroaromatic compound. Broadband ultrafast UV-visible and infrared probes are used to characterize the excited states and intermediate species involved in the multistep photochemistry, and to determine their lifetimes under different pH conditions.

Femtosecond to Microsecond Observation of Photochemical Pathways in Nitroaromatic Phototriggers Using Transient Absorption Spectroscopy.

The synthetic accessibility and tolerance to structural modification of phototriggered compounds (PTs) based on the - nitrobenzene (ONB) protecting group have encouraged a myriad of applications including optimization of biological activity, and supramolecular polymerization. Here, a combination of ultrafast transient absorption spectroscopy techniques is used to study the multistep photochemistry of two nitroaromatic phototriggers based on the ONB chromophore, -(4,5-dimethoxy-2-nitrobenzyl)-l-serine (DMNB-Ser) and [(2-nitrophenyl)methyl]-l-tyrosine hydrochloride (NB-Tyr), in DMSO solutions on femtosecond to microsecond time scales following the absorption of UV light. From a common -S excited state, the PTs can either undergo excited state intramolecular hydrogen transfer (ESIHT) to an -S isomer within the singlet state manifold, leading to direct S → S internal conversion through a conical intersection, or competitive intersystem crossing (ISC) to access the triplet state manifold on time scales of (1.

Orchestrating Nuclear Dynamics in a Permanganate Doped Crystal with Chirped Pump-Probe Spectroscopy.

Pump-probe spectroscopy is a powerful tool to investigate light-induced dynamical processes in molecules and solids. Targeting vibrational excitations occurring on the time scales of nuclear motions is challenging, as pulse durations shorter than a vibrational period are needed to initiate the dynamics, and complex experimental schemes are required to isolate weak signatures arising from wavepacket motion in different electronic states. Here, we demonstrate how introducing a temporal delay between the spectral components of femtosecond beams, namely a chirp resulting in the increase of their duration, can counterintuitively boost the desired signals by 2 orders of magnitude.

The use of NADH anisotropy to investigate mitochondrial cristae alignment.

Life may be expressed as the flow of electrons, protons, and other ions, resulting in large potential difference. It is also highly photo-sensitive, as a large proportion of the redox capable molecules it relies on are chromophoric. It is thus suggestive that a key organelle in eukaryotes, the mitochondrion, constantly adapt their morphology as part of the homeostatic process.

Retinal photoisomerization versus counterion protonation in light and dark-adapted bacteriorhodopsin and its primary photoproduct.

Discovered over 50 years ago, bacteriorhodopsin is the first recognized and most widely studied microbial retinal protein. Serving as a light-activated proton pump, it represents the archetypal ion-pumping system. Here we compare the photochemical dynamics of bacteriorhodopsin light and dark-adapted forms with that of the first metastable photocycle intermediate known as "K".

Phototrophy by antenna-containing rhodopsin pumps in aquatic environments.

Energy transfer from light-harvesting ketocarotenoids to the light-driven proton pump xanthorhodopsins has been previously demonstrated in two unique cases: an extreme halophilic bacterium and a terrestrial cyanobacterium. Attempts to find carotenoids that bind and transfer energy to abundant rhodopsin proton pumps from marine photoheterotrophs have thus far failed. Here we detected light energy transfer from the widespread hydroxylated carotenoids zeaxanthin and lutein to the retinal moiety of xanthorhodopsins and proteorhodopsins using functional metagenomics combined with chromophore extraction from the environment.

Bidirectional Photochemistry of Antarctic Microbial Rhodopsin: Emerging Trend of Ballistic Photoisomerization from the 13- Resting State.

The decades-long ultrafast examination of nearly a dozen microbial retinal proteins, ion pumps, and sensory photoreceptors has not identified structure-function indicators which predict photoisomerization dynamics, whether it will be sub-picosecond and ballistic or drawn out with complex curve-crossing kinetics. Herein, we report the emergence of such an indicator. Using pH control over retinal isomer ratios, photoinduced transient absorption is recorded in an inward proton pumping Antarctic microbial rhodopsin (AntR) for 13- and retinal resting states.

The impact of twisting on the intersystem crossing in acenes: an experimental and computational study.

Due to their unique excited state dynamics, acenes play a dominant role in optoelectronic and light-harvesting applications. Their optical and electronic properties are typically tailored by side-group engineering, which often result in distortion of the acene core from planarity. However, the effect of such distortion on their excited state dynamics is not clear.

Ring Size Determines the Conformation, Global Aromaticity and Photophysical Properties of Macrocyclic Oligofurans.

In π-conjugated macrocycles, there is a trade-off between the global and local expression of effects such as aromaticity, with the outcome of the trade-off determined by the geometry and aromaticity of the constituent units. Compared with other aromatic rings, the aromatic character of furan is relatively small, and therefore global effects in macrocyclic furans are expected to be more pronounced. Following our introduction of macrocyclic oligofuran, we present the first synthesis of a series of π-conjugated bifuran macrocycles of various ring sizes, from trimer to hexamer, and characterize them using both computational and experimental methods.

The elusive dynamics of aqueous permanganate photochemistry.

Despite decades of investigation, mechanistic details of aqueous permanganate photo-decomposition remain unclear. Here we follow photoinduced dynamics of aqueous permanganate with femtosecond spectroscopy. Photoexcitation of KMnO(aq) in the visible unleashes a sub-picosecond cascade of non-radiative transitions, leading to a distinct species which relaxes to S with a lifetime of 16 ps.

Phenothiazine-Based Oligo(p-phenylenevinylene)s: Substituents Affected Self-Assembly, Optical Properties, and Morphology-Induced Transport.

Designing intramolecular charge-transfer (ICT)-based luminogenic ordered assemblies exhibiting significant electrical transport is a challenging task in the field of organic optoelectronics. In this context, a series of novel phenothiazine-based oligo(p-phenylenevinylene) (OPV1-6) derivatives were designed and their structure-property relationship was investigated. Upon examining their photophysical properties, all the OPVs were found to exhibit significant intramolecular charge-transfer characteristics in organic solvents.

Pure white light emission from organic molecules using solvent induced selective self-assembly.

Organic materials generating pure white light emission have attracted considerable attention in recent times due to their potential applications in lighting and display devices. We have used anthracene carboxylic acid (AnA) and anthracene carbaldehyde (AnC) to generate white light using solvent induced selective self-assembly (SISSA). Mechanistically, SISSA prevents a fast intersystem crossing of the AnC monomer and generates an emissive state from the self-assembled AnC.

Self-assembly and gelation of poly(aryl ether) dendrons containing hydrazide units: factors controlling the formation of helical structures.

Self-assembly of AB2 and AB3 type low molecular weight poly(aryl ether) dendrons that contain hydrazide units were used to investigate mechanistic aspects of helical structure formation during self-assembly. The results suggest that there are three important aspects that control helical structure formation in such systems with acyl hydrazide/hydrazone linkage: i) J-type aggregation, ii) the hydrogen-bond donor/acceptor ability of the solvent, and iii) the dielectric constant of the solvent. The monomer units self-assemble to form dimer structures through hydrogen-bonding and further assembly of the hydrogen-bonded dimers leads to macroscopic chirality in the present case.

Metal induced gelation from pyridine cored poly(aryl ether) dendrons with in situ synthesis and stabilization of hybrid hydrogel composites.

Metal induced gel formation from pyridine cored poly(aryl ether) dendrons has been achieved. The gel is successfully utilized as a template for the in situ synthesis of silver nanoparticles (AgNPs) in hydrogel networks without using any external reducing or capping agents for the development of an exceptionally well-ordered AgNP-hybrid hydrogel soft composite.