Effects of Intermolecular Hydrogen Bonding and Solvation on Enol-Keto Tautomerism and Photophysics of Azomethine-BODIPY Dyads

Boron-dipyrromethene derivatives (BODIPYs) are a category of molecules with excellent photophysical properties and can be applied to various fields. This work investigates the fluorescent properties of two azomethine-BODIPY dyads in different solvents based on the time-dependent density functional theory (TD-DFT) method. The potential energy curves (PECs) show that the polar protic solvent and the enhanced π-conjugation effect can lower the proton-transfer (PT) barriers, causing the main configuration of NA-BODIPY in methanol to be the keto form, while the main configuration of NA-BODIPY in toluene and SA-BODIPY in methanol and toluene is the enol form. The keto forms of the two compounds possess the twisted intramolecular charge transfer (TICT) decay pathway in the excited state identified by the optimized twisted configurations and the appropriate barriers of the TICT process, whereas the twisted configurations of the enol forms are nonexistent. TICT successfully competes with excited-state proton transfer (ESIPT) of the keto form, which leads to the fluorescence quenching of NA-BODIPY in methanol. This work provides new ideas for the influence of enol-keto tautomerism and the competitiveness of TICT and ESIPT on the photophysical properties of BODIPYs and is expected to provide guidance for the design of new BODIPY functional molecules.

Spectroscopic and In Vitro Investigations of Boron(III) Complex with Meso-4-Methoxycarbonylpropylsubstituted Dipyrromethene for Fluorescence Bioimaging Applications

This study focuses on the behavior of a new fluorescent marker for labeling individual biomolecules and staining cell organelles developed on a meso-substituted BODIPY platform. Boron(III) complex with meso-4-methoxycarbonylpropylsubstituted 3,3’,5,5’-tetramethyl-2,2′-dipyrromethene has been synthesized and identified via visible, UV-, NMR- and MS-spectra X-ray. The behavior of fluorophore in solutions has been studied with various experimental techniques. It has been found that luminophore exhibits a high quantum yield (almost ~100–75%) in the blue-green region (513–520 nm) and has high photostability. In addition, biological analysis indicates that the fluorophore exhibits a tendency to effectively penetrate into cell membranes. On the other hand, the proposed BODIPY can be used to study the significant differences among a large number of pathogens of mycotic infections, as well as to visualize structural changes in the plasma membrane, which is necessary for the clearance of mammalian cells undergoing apoptotic cell death.

Live cell imaging by 3-imino-(2-phenol)-1,8-naphthalimides: The effect of ex vivo hydrolysis

A series of 3-amino-N-substituted-1,8-naphthalimides and their salicylic Schiff base derivatives were synthesized. The structure of the obtained compounds was confirmed using ¹H and ¹³C NMR, FT-IR spectroscopy and elemental analysis and COSY and HMQC for the representative molecules. The photophysical (UV-Vis, PL) and biological properties of all of the prepared compounds were studied. It was found that the amine with the n-hexyl group in EtOH had the highest PL quantum yield (Ф = 85%) compared to the others. Moreover, the chelating properties of the azomethines with the n-hexyl group (1a, 1b, 1c) were tested against various cations (Al³⁺, Ba²⁺, Co²⁺, Cu²⁺, Cr³⁺, Fe²⁺, Fe³⁺, Mn²⁺, Ni²⁺, Pb²⁺, Sr²⁺ and Zn²⁺) in an acetonitrile, acetone and PBS/AC mixture. Compounds that contained the electron withdrawing groups (-Br, -I) had the ability to chelate most of the studied cations, while the unsubstituted derivative chelated only the trivalent cations such as Al³⁺, Cr³⁺ and Fe³⁺ in acetonitrile. The effect of the environment on the keto-enol tautomeric equilibrium was also demonstrated, especially in the case of the derivative with a bromine atom. The biological studies showed that the tested molecules had no cytotoxicity. Additionally, the ability to image intracellular organelles such as the mitochondria and endoplasmic reticulum was revealed. The crucial role of the hydrolysis of imines for cellular imaging was presented.

Experimental and theoretical investigation of ground state intramolecular proton transfer (GSIPT) in salicylideneaniline Schiff base derivatives in polar protic medium

Ground state intramolecular proton transfer process has been comprehensively investigated in three salicylideneaniline Schiff base derivatives (SB1, SB2, and SB3) using experimental and theoretical methods. It has been confirmed that all the three Schiff base molecules in the ground electronic state exist in the enol form in non-polar and polar aprotic solvents. Keto form is being populated by the polar protic solvent through ground state intramolecular proton transfer (GSIPT) process. Ground state equilibrium between the enol and keto tautomers for SB1 and SB3 is mainly governed by the proton donating ability of the solvent. Ground state equilibria between the enol and keto tautomers of SB2 which is a positional isomer of SB3 is governed by the polarity and proton donating ability of the solvents. Excited state intramolecular proton transfer (ESIPT) process is also evidenced in all the three Schiff base molecules. Theoretical calculations at the B3LYP/cc-pVDZ level in the gas phase and in different solvents using polarisable continuum model (PCM) have failed to establish the GSIPT process. Microsolvation of individual enol and keto conformers has been investigated considering upto three solvent molecules. The energetics of the individual conformers together with the corresponding transition state have been calculated. It has been confirmed that the keto conformer is more stable compared to the enol conformer in microsolvated cluster of three methanol molecules. Lowering of activation energy for the enol to keto tautomerisation in the presence of methanol also supports the experimental observation for GSIPT process. TDDFT/B3LYP/cc-pVDZ single point calculations for microsolvated clusters of enol and keto form of the Schiff base molecules exhibit an excellent agreement with the experimentally obtained absorption spectra. Difference in spectral nature of the Schiff base molecules has been explained using natural bond orbital (NBO) analysis. Quantum theory of atoms in molecules (QTAIM) has also been utilised to understand the GSIPT process in detail.

Exploration of fluorescence behavior of an imidazolium-based chemosensor in solution and in the solid state and its turn-on response to Al3+ in pure aqueous medium

An imidazolium-based quinoline framework is constructed, and its fluorescence behaviour studies with fluorescence turn-on chemosensory response to the selective detection of Al³⁺ in aqueous medium are discussed in detail.

Tethered sensitizer-catalyst noble-metal-free molecular devices for solar-driven hydrogen generation

Inspired by natural photosynthesis in an organized assembly, compact H2-evolving molecular devices, which tether sensitizer and catalyst modules in one single molecule, present an opportunity to overcome the diffusion limit required for multi-component molecular systems, and increase intramolecular electron transfer rates from the photoactivated unit to the catalytic center to improve H2-evolving efficiency. Thereinto absolutely noble-metal free H2-evolving molecular devices are of particular interest because they don't contain precious and scarce noble-metal based components. This Frontier article focuses specifically on the recent advances in the design, synthesis, and photocatalytic properties of all-abundant-element molecular devices for photoinduced H2 generation via intramolecular processes. Some challenges and suggestions for future directions in this field are also illustrated.

A “turn-on-and-off” pH sensitive BODIPY fluorescent probe for imaging E. coli cells

Highly selective fluorescent BODIPY probes towards H⁺ ions displayed good photostability and reversibility and the utility of acid sensitive behavior to visualize extreme acidity in E. coli cells is also demonstrated.

A pH-induced conformational switch in a tyrosine kinase inhibitor identified by electronic spectroscopy and quantum chemical calculations

Tyrosine kinase inhibitors (TKIs) are a major class of drug utilised in the clinic. During transit to their cognate kinases, TKIs will encounter different pH environments that could have a major influence on TKI structure. To address this, we report UV-Vis spectroscopic and computational studies of the TKI, AG1478, as a function of pH. The electronic absorption spectrum of AG1478 shifted by 10 nm (from 342 nm to 332 nm) from acid to neutral pH and split into two peaks (at 334 nm and 345 nm) in highly alkaline conditions. From these transitions, the pKa value was calculated as 5.58 ± 0.01. To compute structures and spectra, time-dependent density functional theory (TD-DFT) calculations were performed along with conductor-like polarizable continuum model (CPCM) to account for implicit solvent effect. On the basis of the theoretical spectra, we could assign the AG1478 experimental spectrum at acidic pH to a mixture of two twisted conformers (71% AG1478 protonated at quinazolyl nitrogen N(1) and 29% AG1478 protonated at quinazolyl nitrogen N(3)) and at neutral pH to the neutral planar conformer. The AG1478 absorption spectrum (pH 13.3) was fitted to a mixture of neutral (70%) and NH-deprotonated species (30%). These studies reveal a pH-induced conformational transition in a TKI.

Synthesis and spectroscopic properties of β,β'-dibenzo-3,5,8-triaryl-BODIPYs

A series of β,β'-bicyclo-3,5-diaryl-BODIPYs were synthesized from the corresponding β,β'-bicyclo-3,5-diiodo-BODIPYs (1a,b) via Pd(0)-mediated Suzuki cross-coupling reactions in 82-92% yields. Subsequent aromatization with DDQ afforded the corresponding β,β'-dibenzo-aryl-BODIPYs, which showed red-shifted absorptions and emissions in the near-IR range. The dibenzo-appended BODIPYs showed characteristic 1H-, 13C-, 11B- and 19F-NMR shifts, and nearly planar conformations by X-ray crystallography.

Base-driven keto–enol anion tautomerism of a perylene diimide derivative in DMF solution

A perylene diimide derivative (d-THBPDI) containing twisted heptatomic biphenyl substituents in the bay positions exhibited base (hydrazine, 1,2-ethylenediamine, triethylamine and K₂CO₃)-driven keto–enol anion tautomerism in DMF solution.

Computational Study of Proton Transfer in Tautomers of 3- and 5-Hydroxypyrazole Assisted by Water

The tautomerism of 3- and 5-hydroxypyrazole is studied at the B3LYP, CCSD and G3B3 computational levels, including the gas phase, PCM-water effects, and proton transfer assisted by water molecules. To understand the propensity of tautomerization, hydrogen-bond acidity and basicity of neutral species is approached by means of correlations between donor/acceptor ability and H-bond interaction energies. Tautomerism processes are highly dependent on the solvent environment, and a significant reduction of the transition barriers upon solvation is seen. In addition, the inclusion of a single water molecule to assist proton transfer decreases the barriers between tautomers. Although the second water molecule further reduces those barriers, its effect is less appreciable than the first one. Neutral species present more stable minima than anionic and cationic species, but relatively similar transition barriers to anionic tautomers.

A new pyridoxal based fluorescence chemo-sensor for detection of Zn(ii) and its application in bio imaging

A new pyridoxal-based chemosensor (PydDmen) was reported for selective detection of Zn²⁺ in solution. The experimental and theoretical supports are provided to establish Zn²⁺ induced transformation of PydDmen to 3-pyridone tautomeric form.

The relationship between the boron dipyrromethene (BODIPY) structure and the effectiveness of homogeneous and heterogeneous solar hydrogen-generating systems as well as DSSCs

BODIPY photosensitizers were used to investigate the relationship between structure and effectiveness of visible-light-driven hydrogen production as well as DSSCs.

Noble-metal-free BODIPY–cobaloxime photocatalysts for visible-light-driven hydrogen production

Four BODIPY–cobaloxime systems Co-Bn (n = 1–4) were studied as single-component homogeneous photocatalytic systems for H₂ generation in aqueous media.