Bioluminescence is a phenomenon that has fascinated mankind for centuries. Today the phenomenon and its sibling, chemiluminescence, have impacted society with a number of useful applications in fields like analytical chemistry and medicine, just to mention two. In this review, a molecular-orbital perspective is adopted to explain the chemistry behind chemiexcitation in both chemi- and bioluminescence. First, the uncatalyzed thermal dissociation of 1,2-dioxetane is presented and analyzed to explain, for example, the preference for triplet excited product states and increased yield with larger nonreactive substituents. The catalyzed fragmentation reaction and related details are then exemplified with substituted 1,2-dioxetanone species. In particular, the preference for singlet excited product states in that case is explained. The review also examines the diversity of specific solutions both in Nature and in artificial systems and the difficulties in identifying the emitting species and unraveling the color modulation process. The related subject of excited-state chemistry without light absorption is finally discussed. The content of this review should be an inspiration to human design of new molecular systems expressing unique light-emitting properties. An appendix describing the state-of-the-art experimental and theoretical methods used to study the phenomena serves as a complement.

Watasenia scintillans (W. scintillans) is a deep-sea luminescent squid with a popular name of firefly squid. It produces flashes of blue light via a series of complicated luciferin-luciferase reactions involving ATP, Mg, and molecular oxygen. Tsuji has proposed a hypothetical scheme for this mysterious bioluminescence (BL) process, but the proposal is short of strong evidence experimentally or theoretically, especially for two key steps. They are the addition of molecular oxygen to luciferin and the formation of light emitter. For the first time, the present study investigates the two steps by reliable density functional theory (DFT) and time-dependent DFT. The results of calculated energetics, charge transfer process, electronic structures, and molecular dynamics give convincing support for Tsuji's proposal. The oxygenation reaction occurs with a single electron-transfer (SET) mechanism, and the light emitter is produced via the mechanism of gradually reversible charge-transfer-induced luminescence (GRCTIL). The simulation of nonadiabatic molecular dynamics further confirms the GRCTIL mechanisms and evaluates the quantum yield of the light emitter to be 43%. The knowledge obtained in the current study will help to understand a large amount of BL systems in nature, since the core structure of W. scintillans luciferin, imidazopyrazinone, is common in the luciferins of about eight phyla of luminescent organisms.

Understanding the nature of charge transfer mechanisms in 3-dimensional metal-organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn(II) frameworks containing cofacial thiazolo[5,4- d]thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus-Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both in situ and ex situ electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative.

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Two new glycolated semiconducting polymers PgBT(F)2gT and PgBT(F)2gTT of differing backbone curvatures were designed and synthesised for application as p-type accumulation mode organic electrochemical transistor (OECT) materials. Both polymers demonstrated stable and reversible oxidation, accessible within the aqueous electrochemical window, to generate polaronic charge carriers. OECTs fabricated from PgBT(F)2gT featuring a curved backbone geometry attained a higher volumetric capacitance of 170 F cm À3 . However, PgBT(F)2gTT with a linear backbone displayed overall superior OECT performance with a normalised peak transconductance of 3.00 10 4 mS cm À1 , owing to its enhanced order, expediting the charge mobility to 0.931 cm 2 V À1 s À1 . Currentdeviceresearchwithintheemergentfieldoforganicbioelectronics is centred around the organic electrochemical transistor (OECT), [1] which is recognised as a functional amplifier for biosensing [2] as well as a materials testbed device from which we can springboard to other bioelectronic functionalities. [3][4][5] Unlike organic field-effect transistors (OFETs), the modulation of charge carrying polarons/bipolarons in an OECT active material is achieved throughout the bulk of the film by gate potential induced electrochemical oxidation or reduction, giving rise to its superior volumetric capacitance. [6] The electrochemical redox switching of OECTs necessitates volumetric and stoichiometric active material counterion accessibility, raising unique challenges associated with the design of OECT active materials. [7] Earlier OECT conjugated polymers integrated ionic components either onto the sidechains (e.g. poly(6-(thiophene-3-yl)hexane-1-sulfonate); PTHS) [8] or within separate but intimately mixed domains (e.g. poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); PEDOT:PSS), [9,10] to engender mixed ionic and electronic conductivity. Aqueous solubility was a drawback of these ionic OECT materials, requiring performance diminishing cross-linkers to be implemented into the active layer. [11,12] Thus, OECT active material designs pivoted towards the incorporation of oligomeric glycol sidechains, as these facilitate ionic diffusion without conferring aqueous solubility. [2,13,14] Several all donor thiophene-centric glycolated conjugated polymers have been reported for p-type OECT applications. [15] Recently, developments in OECT polymer designs have progressed towards donor-acceptor (D-A) conjugated backbones. [16][17][18][19] Refined energy level tuning is an important advantage of applying D-A backbones, which can be exploited to improve electrochemical/OECT stability by avoidance of undesirable redox side-reactions, [20,21] as well as to ensure OECT operation in favourable accumulation mode, where channel conductivity is negligible at resting gate potential, and grows with increased gate bias (c.f. depletion mode with vice versa operational characteristics). [22] However, these advantages have come at the cost of lower OECT transconductances than those observed for (less s...