Unraveling the mystery of solvation-dependent fluorescence of fluorescein dianion using computational study.

Fluorescein, one of the brightest fluorescent dye molecules, is a widely used fluorophore for various applications from biomedicine to industry. The dianionic form of fluorescein is responsible for its high fluorescence quantum yield. Interestingly, the molecule was found to be nonfluorescent in the gas phase. This characteristic is attributed to the photodetachment process, which out-competes the fluorescence emission in the gas phase. In this work, we show that the calculated vertical and adiabatic detachment energies of fluorescein dianion in the gas and solvent phases account for the drastic differences observed in their fluorescence characteristics. The functional dependence of these detachment energies on the dianion's microsolvation was systematically investigated. The performance of different solvent models was also assessed. The higher thermodynamic stability of fluorescein dianion over the monoanion doublet in the solvent phase plays a crucial role in quenching photodetachment and activating the radiative channel with a high fluorescence quantum yield.

Strong Dopant-Dopant Electronic Coupling in Emissive Codoped Two Dimensional Metal Halide Hybrid.

Multimetallic halide hybrids are attractive for the fundamental understanding of interacting excitons. However, realizing halide hybrids that incorporate multiple heterometal centers has been synthetically challenging. This further limits access to gaining physical insight into the electronic coupling mechanism between the constituent metal halide units. Reported herein is an emissive heterometallic halide hybrid, synthesized by codoping (with Mn2+, Sb3+) a 2D host (C6H22N4CdCl6) hybrid, that shows strong dopant-dopant interaction. Here, C6H22N4Sb0.003Mn0.128Cd0.868Cl6 codoped hybrid shows weak green emission (Sb3+ dopant based) and strong orange emission (Mn2+ dopant based). The observed dominance of the Mn2+ dopant emission, arising due to efficient energy transfer between the distant dopants (Sb3+ → Mn2+), highlights strong dopant-dopant electronic coupling. DFT calculations, supporting the observed dopant-dopant interaction, suggest that the electronic coupling between the dopant units (Mn-Cl; Sb-Cl) is mediated by the 2D networked host structure. This work reports physical insight into the coupling mechanism of interacting excitons in multimetallic halide hybrids synthesized through a codoping strategy.

Electronic Substitution Effect on the Ground and Excited State Properties of Indole Chromophore: A Computational Study.

Indole, being the main chromophore of amino acid tryptophan and several other biologically relevant molecules like serotonin, melatonin, has prompted considerable theoretical and experimental interest. The current work focuses on the investigation of substitution effect on the ground and excited electronic states of indole using computational quantum chemistry. Having three close-lying excited electronic states, the vibronic coupling effect becomes extremely important yet challenging for the photophysics and photochemistry of indole. Here, we have evaluated the performance of time-dependent density functional theory against available experimental and ab initio results from the literature. The electronic effects on the excited states of indole and indole derivatives e. g. tryptophan, serotonin and melatonin are reported. A bathochromic shift has been observed in the absorption spectrum for the La state. The absorption wavelength increases in the order of indole

Why intermolecular nitric oxide (NO) transfer? Exploring the factors and mechanistic aspects of NO transfer reaction.

Small molecule activation and their transfer reactions in biological or catalytic reactions are greatly influenced by the metal-centers and the ligand frameworks. Here, we report the metal-directed nitric oxide (NO) transfer chemistry in low-spin mononuclear {Co(NO)}8, [(12-TMC)CoIII(NO-)]2+ (1-CoNO, S = 0), and {Cr(NO)}5, ([(BPMEN)Cr(NO)(Cl)]+) (4-CrNO, S = 1/2) complexes. 1-CoNO transfers its bound NO moiety to a high-spin [(BPMEN)CrII(Cl2)] (2-Cr, S = 2) and generates 4-CrNOvia an associative pathway; however, we did not observe the reverse reaction, i.e., NO transfer from 4-CrNO to low-spin [(12-TMC)CoII]2+ (3-Co, S = 1/2). Spectral titration for NO transfer reaction between 1-CoNO and 2-Cr confirmed 1 : 1 reaction stoichiometry. The NO transfer rate was found to be independent of 2-Cr, suggesting the presence of an intermediate species, which was further supported experimentally and theoretically. The experimental and theoretical observations support the formation of µ-NO bridged intermediate species ({Cr-NO-Co}4+). Mechanistic investigations using 15N-labeled-15NO and tracking the 15N-atom established that the NO moiety in 4-CrNO is derived from 1-CoNO. Further, to investigate the factors deciding the NO transfer reactivity, we explored the NO transfer reaction between another high-spin CrII-complex, [(12-TMC)CrII(Cl)]+ (5-Cr, S = 2), and 1-CoNO, showing the generation of the low-spin [(12-TMC)Cr(NO)(Cl)]+ (6-CrNO, S = 1/2); however, again there was no opposite reaction, i.e., from Cr-center to Co-center. The above results advocate clearly that the NO transfer from Co-center generates thermally stable and low-spin and inert {Cr(NO)}5 complexes (4-CrNO & 6-CrNO) from high-spin and labile Cr-complexes (2-Cr & 5-Cr), suggesting a metal-directed NO transfer (cobalt to chromium, not chromium to cobalt). These results explicitly highlight that the NO transfer is strongly influenced by the labile/inert behavior of the metal-centers and/or thermal stability rather than the ligand architecture.

Unraveling the regioselectivity of odd electron halogen bond formation using electrophilicity index and chemical hardness parameters.

Odd electron halogen bonding of freons with five different free radicals have been investigated using M06-2X/6-311G++(d,p). Conceptual density functional theory parameters such as Fukui functions, electrophilicity index and chemical hardness have been utilised to examine the regioselectivity and strength of free radical induced halogen bonding. The strong electrophilic radical Cl˙ forms three-electron bonds upon interacting with the nucleophilic ring surrounding the sigma hole or fk- regions. This behavior is also observed for moderately electrophilic radicals OH˙ and OCl˙ for complexation with CHFCl2 only. In other complexes, these radicals along with least nucleophilic radicals ˙NO and ˙Ph form one-electron bond with sigma hole or fk+ regions of freons. This regioselectivity of radicals and the interaction energy upon complexation have been explained in terms of local electrophilicity index and chemical hardness. We hope that this finding will shed light on the understanding of non-covalent interactions in terms of conceptual density functional theory for halogen bonding as well as in pnictogen, chalcogen and tetrel bonding systems. This result may find applications in understanding the radical mediated reactions and ozone depletion in the stratosphere.