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Biindenylidenediones (BIDs) are a family of compounds that have been studied for a relatively short time. The crystals of these compounds are yellowish, and become purplish when they are irradiated and return back to their original color slowly in the dark or quickly when they are heated up. BIDs can be classified into different subfamilies depending on the nature of their substituents. BID-II crystals show a thermally dependent electron paramagnetic resonance (EPR) signal that is a characteristic of chemical species with unpaired electrons. These properties make BIDs very attractive for industrial applications but the mechanisms responsible for their photochromism and photomagnetism are still under debate. In this article, a computational study focused on the BID-II subfamily is presented. A variety of multiconfigurational methods (CASSCF, CASPT2 and IDDCI) have been used to study exhaustively the topography of the potential energy surfaces of the lower electronic states of a single BID molecule. Methods based on density functional theory (DFT) were then used to model the most important structures in a periodic crystal system. Our results suggest that delta-hydrogen abstraction could explain the observed experimental phenomena. After the initial excitation to the (1)pi pi* state, non-symmetric n pi* minima are populated, which are adiabatically connected to the photoproduct zone through a barrier along the reaction coordinate. Based on our set of results, we propose that an epoxide constitutes the most stable and accessible photoproduct preceded by the population of a triplet biradical of pi(O)pi* nature which has only small geometrical differences in comparison with the reactant. The spin-orbit coupling indicates that the EPR signal arises due to the population of a l

Organic luminogens constitute promising prototypes for various optoelectronic applications. Since gaining distinct color emissions normally requires the alternation of the conjugated backbone, big issues remain in material synthetic cost and skeleton compatibility while pursuing full-color luminescence. Upon a facile one-step coupling, three simple but smart perchalcogenated (O, S, and Se) arenes are synthesized. They exhibit strong luminescent tricolor primaries (i.e., blue, green, and red, respectively) in the solid state with a superior quantum yield up to >40% (5–10 times higher than that in corresponding solutions). The properties originate from a fluorescence–phosphorescence–phosphorescence triple-channel emission effect, which is regulated by S and Se heavy atoms–dependent intersystem crossing upon molecular packing, as well as Se–Se atom interaction–caused energy splittings. Consequently, full-color luminescence, including a typical white-light luminescence with a Commission Internationale de I'Eclairage coordinate of (0.30, 0.35), is realized by complementarily incorporating these tricolor luminescent materials in the film. Moreover, mechanochromic luminescent color conversions are also observed to achieve the fine-tuning of the luminescent tints. This strategy can be smart to address full-color luminescence on the same molecular skeleton, showing better material compatibility as an alternative to the traditional multiple-luminophore engineering., QC 20200701

In nanoscience, chirality has shown a significant ability to tune materials' electronic properties, whereas imposing macrochirality into the regulation of singlet-triplet features of organic optoelectronics remains a challenging research topic. Since the tuning for singlet and triplet excited-state properties in a single π-functional molecule connects to its multicolor luminescent application and potential improvement of internal quantum efficiency, we here report that supramolecular chirality can be employed to toggle the singlet and triplet emissions in a welldesigned asterisk-shaped molecule. Employing a hexathiobenzene-based single luminophore as a prototype and functionalizing it with chiral α-lipoiate side groups, we find that helical nanoarchitectures can accordingly form in mixed DMF/H20 solution. On this basis, switching between fluorescence and phosphorescence of the material can be realized upon helical self-assembly and dissociation. Such a behavior can be attributed to a helical-conformation-dependent manipulation of the intersystem crossing. Furthermore, reversible mechanoluminescence of the corresponding solid sample was also observed to rely on an analogous molecular self-assembly alternation. These results can probably provide new visions for the development of next-generation supramolecular chiral functional materials., QC 20171108

Electronic transitions from one excited state to another excited state of different spin symmetry play important roles in many biochemical reactions. Although recent years have seen much progress in the elucidation of nonradiative (intersystem crossing) relaxation mechanisms for such transitions, there is presently a scarcity of data available to assess whether also radiative (phosphorescence) mechanisms are relevant for these processes. Here, we demonstrate that the well-established ability of quantum chemical methods to describe intersystem crossing events between excited states can be supplemented by the ability to also describe inter-excited-state phosphorescence. Specifically, by performing four-component relativistic time-dependent density functional theory calculations, we obtain rate constants for the radiative transitions from the absorbing 1(πHπL∗) singlet state of lumiflavin to the 3(πHπL∗), 3(nN2πL∗), and 3(πH-1πL∗) triplet states, and subsequently, we compare these results with rate constants calculated for the corresponding nonradiative transitions. Thereby, it is found that the radiative rate constants for these particular transitions are typically 2 to 5 orders of magnitude smaller than the nonradiative ones., QC 20170118