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 photophysical and photochemical properties of indole and indole derivatives e.g. tryptophan, serotonin and melatonin using theoretical and computational methodologies. 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 used density functional theory (DFT) extensively and evaluated the performance of DFT in compared to available experimental and ab initio results from literature. The benchmarking of the method is followed by investigation of the chemical and geometrical effects of ring substitution in indole. A bathochromic shift has been observed in the HOMO-LUMO gap as well as vertical excitation energy from indole to melatonin. While the contribution of the in-plane small adjacent groups increases the electron density of the indole ring, the out-of-plane long substituent groups have minor effect. The comparison of singlet-triplet gaps suggests highest probability of inter-system crossing for tryptophan which is in line with previous experiment. The absorption spectra calculated including the vibronic coupling are in good agreement with experiment. These results can be used to estimate the error in photophysical observables of indole derivatives calculated considering indole as prototypical system. This study also demonstrates the merits and demerits of using DFT functionals to compute the photophysical properties of indole derivatives

A Computational Study on Light-Induced Spin Crossover in [Fe(Tp)(CN)3]-2 in Search of Potential Building Block for Single-Molecule Magnet

Light-induced spin crossover (LISCO) in transition metal complexes has drawn attention to the researcher due to its various application in science and technologies. The interplay of LISCO with single molecular magnetism (SMM) is interesting in view of its application towards photoregulated storage devices, magnetic photoswitches. Herein, we have studied the interplay between LISCO and SMM of a Fe(II) complex ([Fe(Tp)(CN)3]-2) which is an ultimate miniature of potential building block of SMM using density functional theory (DFT) and time-dependent DFT method. The molecular structure and energy in low-spin singlet, high-spin quintet as well as intermediate spin triplet is calculated. It is found that the molecule is stable in its LS state but can undergo spin crossover upon irradiation of UV-vis light via triplet excited states. The singlet excited states are close-lying, forming a band structure. The detailed mechanism of LISCO is proposed based on the calculated potential energy cuts and spin-orbit coupling values. While the LS state of the complex has Ms=0 and diamagnetic, the HS state has Ms=±2 and paramagnetic. The calculations suggest a positive zero field splitting parameter and a reasonably small E/D value with a high magnetic relaxation barrier of 96 cm-1. Therefore, for a good SMM, the complex has to be trapped in its HS state after the SCO and reverse spin-crossover (rSCO) has to be stopped. On the other hand, the complex can be used as photoregulated magnetic switch if both the SCO and rSCO happens at the similar time scale

Exclusive Solution Discharge in Li-O2 Batteries?

Capacity, rate performance, and cycle life of aprotic Li-O2 batteries critically depend on reversible electrodeposition of Li2O2. Current understanding states surface adsorbed versus solvated LiO2 to control Li2O2 growth as surface film or as large particles. Here we show that Li2O2 forms across a wide range of electrolytes, carbons, and current densities as particles via solution mediated LiO2 disproportionation, questioning the prevalence of any surface growth under practical conditions. We describe a unified O2 reduction mechanism, which can explain all found capacity relations and Li2O2 morphologies with exclusive solution discharge. Deciding for particle morphology and achievable capacities are species mobilities, true areal rate and the degree of LiO2 association in solution. Capacity is conclusively limited by mass transport through the tortuous Li2O2 rather than electron transport through a passivating Li2O2 film. Provided that species mobilities and surface are high, high capacities are also achieved with weakly solvating electrolytes, previously considered prototypical for low capacity via surface growth

Exclusive solution discharge in Li-O₂ batteries?

Capacity, rate performance, and cycle life of aprotic Li–O2 batteries critically depend on reversible electrodeposition of Li2O2. Current understanding states surface-adsorbed versus solvated LiO2 controls Li2O2 growth as surface film or as large particles. Herein, we show that Li2O2 forms across a wide range of electrolytes, carbons, and current densities as particles via solution-mediated LiO2 disproportionation, bringing into question the prevalence of any surface growth under practical conditions. We describe a unified O2 reduction mechanism, which can explain all found capacity relations and Li2O2 morphologies with exclusive solution discharge. Determining particle morphology and achievable capacities are species mobilities, true areal rate, and the degree of LiO2 association in solution. Capacity is conclusively limited by mass transport through the tortuous Li2O2 rather than electron transport through a passivating Li2O2 film. Provided that species mobilities and surface growth are high, high capacities are also achieved with weakly solvating electrolytes, which were previously considered prototypical for low capacity via surface growth

Exclusive Solution Discharge in Li–O2 Batteries?

Capacity, rate performance, and cycle life of aprotic Li-O2 batteries critically depend on reversible electrodeposition of Li2O2. Current understanding states surface-adsorbed versus solvated LiO2 controls Li2O2 growth as surface film or as large particles. Herein, we show that Li2O2 forms across a wide range of electrolytes, carbons, and current densities as particles via solution-mediated LiO2 disproportionation, bringing into question the prevalence of any surface growth under practical conditions. We describe a unified O2 reduction mechanism, which can explain all found capacity relations and Li2O2 morphologies with exclusive solution discharge. Determining particle morphology and achievable capacities are species mobilities, true areal rate, and the degree of LiO2 association in solution. Capacity is conclusively limited by mass transport through the tortuous Li2O2 rather than electron transport through a passivating Li2O2 film. Provided that species mobilities and surface growth are high, high capacities are also achieved with weakly solvating electrolytes, which were previously considered prototypical for low capacity via surface growth.ISSN:2380-819

To DISP or not? The far‐reaching reaction mechanisms underpinning Lithium‐air batteries

The short history of research on Li-O2 batteries has seen a remarkable number of mechanistic U-turns over the years. From the initial use of carbonate electrolytes, that were then found to be entirely unsuitable, to the belief that (su)peroxide was solely responsible for degradation, before the more reactive singlet oxygen was found to form, to the hypothesis that capacity depends on a competing surface/solution mechanism before a practically exclusive solution mechanism was identified. Herein, we argue for an ever-fresh look at the reported data without bias towards supposedly established explanations. We explain how the latest findings on rate and capacity limits, as well as the origin of side reactions, are connected via the disproportionation (DISP) step in the (dis)charge mechanism. Therefrom, directions emerge for the design of electrolytes and mediators on how to suppress side reactions and to enable high rate and high reversible capacity

Singlet oxygen in non-aqueous oxygen redox: Direct spectroscopic evidence for formation pathways and reliability of chemical probes

Singlet oxygen (1O2) formation is now recognised as a key aspect of non-aqueous oxygen redox chemistry. For identifying 1O2, chemical trapping via 9,10-dimethylanthracene (DMA) to form the endoperoxide (DMA-O2) has become the mainstay method due to its sensitivity, selectivity, and ease of use. While DMA has been shown to be selective for 1O2, rather than forming DMA-O2 with a wide variety of potentially reactive O-containing species, false positives might hypothetically be obtained in the presence of previously overlooked species. Here, we first give unequivocal direct spectroscopic proof by the 1O2-specific near infrared (NIR) emission at 1270 nm for the previously proposed 1O2 formation pathways, which centre around superoxide disproportionation. We then show that peroxocarbonates, common intermediates in metal-O2 and metal carbonate electrochemistry, do not produce false-positive DMA-O2. Moreover, we identify a previously unreported 1O2-forming pathway through the reaction of CO2 with superoxide. Overall, we give unequivocal proof for 1O2 formation in non-aqueous oxygen redox and show that chemical trapping with DMA is a reliable method to assess 1O2 formation