Dangling Water Molecules Bridge for ESIPT in Aggregated TMP: A Theoretical Study.

We present a theoretical study on the occurrence of excited-state proton transfer in an aggregated structure of 2-(benzo[d]thiazol-2-yl)-6-methoxyphenol (TMP) exclusively in water among polar solvents, as reported in a recent experiment (Bhattacharyya, A. New J. Chem. 2019, 43, 15087). Our extensive investigation of the TMP monomer and dimer implementing density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, in three different solvents, namely, water, methanol, and dimethyl sulfoxide (DMSO), with explicit inclusion of solvent molecules demonstrated the existence of both enol and keto forms of the TMP dimer in the excited state, but only in water; this confirmed the experimental emission spectra completely and simultaneously validated the aggregation-induced emission phenomenon. Further analysis of various parameters such as potential energy scan (PES) of the hydroxyl (O-H) bond involved in hydrogen bonding, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP), and infrared (IR) stretching frequencies of both the monomer and dimer forms of TMP in different solvents clearly indicated the geometry of the dimer, with the arrangement of the solvent molecules to be the sole reason for the excited-state charge transfer. The bridging alignment of water molecules in between the stacked units of the TMP dimer results in intermolecular interactions, ultimately leading to intermolecular proton transfer in the excited state.

New insights into self-structure induction in poly (rA) by Quinacrine through non-classical intercalation: Spectroscopic and theoretical perspectives.

Self-structure induction in a single stranded polyriboadenylic acid [poly (rA)] is an auspicious physiological phenomenon which switches off protein production in tumor cells. In the present study, the self-structure induction process in poly (rA) moiety was thoroughly investigated using various steady state and time resolved techniques. Optical melting pattern directly evidenced the formation of self-structured assembly in single stranded poly (rA) upon complexation with quinacrine. Further, UV-absorption spectroscopic studies revealed that quinacrine binds to poly (rA) in co-operative fashion and the indication of intercalative mode of binding first came out with the involvement of around two base pairs of poly (rA) in the complexation. Experimental observations established the unconventional or non-classical intercalation of quinacrine molecule inside self-structured duplex poly (rA) moiety. This complexation was accompanied with negative enthalpy change and positive entropy change; suggesting strong van der Waals and the H-bonding interactions as the major governing forces in the complexation. Moreover, ionic strength dependent binding study established that the non-polyelectrolytic forces were the dominating forces. Further, the photo physical behavior of QN was authenticated using time dependent density functional theory (TDDFT) where both the ground and excited states were exploited.

Strong Duschinsky Mixing Induced Breakdown of Kasha's Rule in an Organic Phosphor.

We present the novel observation that Duschinsky mixings can lead to the breakdown of Kasha's rule in a white light phosphor molecule, dibenzo[ b, d]thiophen-2-yl (4-chlorophenyl)methanone. Our theoretical analyses show the energy gap between the T1 and T2 states (0.48 eV) is too large to allow for any significant population of the T2 state at room temperature and instead the faster intersystem crossing (ISC) between the S1 and T2 states is rather due to strong Duschinsky mixing, leading to the emission from the T2 state as well. A second-order cumulant-based method has been used for the calculation of the ISC rate, which suggests 2 orders of magnitude faster ISC rates for S1 → T2 compared to those for S1 → T1. We found that the carbonyl moiety of the S1 and T2 states of the molecule is significantly different with respect to bond angle and dihedral angles, engendering large displacements in selective normal modes, thus giving rise to strong Duschinsky mixing.

Unraveling the Microscopic Origin of Triplet Lasing from Organic Solids.

We present a heuristic mechanism for the origin of the unusual triplet lasing from (E)-3-(((4-nitrophenyl)imino)methyl)-2H-thiochroman-4-olate·BF2.We demonstrate that whereas the moderate lifetime (1.03 µs) of the first triplet state (T1) prohibits triplet-triplet annihilation, the relatively faster S1 → T1 intersystem crossing and the 104 times smaller reverse intersystem crossing effectively help achieve population inversion in the T1 state. Furthermore, the triplet lasing wavelength (675 nm) for the tetramer does not overlap with the triplet-triplet absorptions wavelength, indicating that the spin-forbidden emission cross section is very large. Additionally, the almost complete absence of a vibrational progression in the vibronic phosphorescence spectrum of the monomer plays an important role in ensuring efficient triplet-state lasing from this organic material. Our results show that controlling the triplet-state lifetimes combined with lowering of the triplet-triplet absorption in the emission region and small vibronic coupling will be the key steps when designing novel organic triplet-lasing materials.

Anomalous Phosphorescence from an Organometallic White-Light Phosphor.

We report theoretical results on the possible violation of Kasha's rule in the phosphorescence process of (acetylacetonato)bis(1-methyl-2-phenylimidazole)iridium(III) and show that the anomalous emission from both the T1 and T2 states is key to its white-light phosphorescence. This analysis is supported by the calculated Boltzmann-averaged phosphorescence lifetime of 2.21 µs, estimated including both radiative and nonradiative processes and in excellent agreement with the experimentally reported value of 1.96 ± 0.1 µs. The T2 state is found to be of metal-to-ligand charge-transfer character (dπ → nπ), and the d orbital contribution comes from 5dz2 and 5dx2-y2, whereas the S1 and T1 states both have dπ-pπ character with significant 5dxz orbital contribution, allowing for efficient intersystem crossing from the S1 to the T2 state and, in turn, phosphorescence from the T2 state. Our results open new opportunities for tailoring the phosphorescence wavelength and thus the design of molecules with improved photovoltaic properties.

Origin of Dual-Peak Phosphorescence and Ultralong Lifetime of 4,6-Diethoxy-2-carbazolyl-1,3,5-triazine.

Recently, ultralong phosphorescence lifetime has been observed in 4,6-diethoxy-2-carbazolyl-1,3,5-triazine, and H-aggregation induced stabilization of the T1 state was suggested as its source. The response theory calculations demonstrate that the Davydov stabilization of the T1 state of the dimer is marginal with respect to the monomer and the corresponding transition moments are virtually the same. Moreover, the calculated radiative rate constant is far from the experimental value, indicating that the ultralong lifetime is not likely to be of electronic origin only. Our calculations reveal that the dual-peak emission from the T1 state is due to strong vibronic coupling between the T1 and S0 states along selected normal modes. Interestingly, the calculated vibronic radiative rate constant of the dimer (2.33 × 10-3 s-1) is comparable to the experimental value (4.7 × 10-3 s-1), supporting the notion that vibronic contributions to the transition moment are responsible for the ultralong lifetime observed in the bulk system.

Eye-Catching Dual-Fluorescent Dynamic Metal-Organic Framework Senses Traces of Water: Experimental Findings and Theoretical Correlation.

A guest-dependent dynamic fivefold interpenetrated 3D porous metal-organic framework (MOF) of ZnII ions has been synthesized that exhibits selective carbon dioxide adsorption. Furthermore, the MOF shows excellent luminescence behavior, which is supported by a systematic study on the guest-responsive multicolor emission of a suspension of the MOF. The dual-emission behavior arises from the excited-state intramolecular proton transfer (ESIPT), and the compound also shows remarkable potential to detect traces of water in various organic solvents. The experimental observations were also painstakingly authenticated by using time-dependent density-functional-theory (DFT) calculations.