Controlling the spatial distribution of electronic excitation in asymmetric D-A-D' and symmetric D'-A-D-A-D' electron donor-acceptor molecules

Understanding how electronic energy is funnelled towards a specific location in a large conjugated molecule is of primary importance for the development of a site-specific photochemistry. To this end, we investigate here how electronic excitation redistributes spatially in a series of electron donor-acceptor (D-A) molecules containing two different donors, D and D', and organised in both linear D-A-D' and symmetric double-branch D'-A-D-A-D' geometries. Using transient IR absorption spectroscopy to probe the alkyne spacers, we show that for both types of systems in non-polar solvents, excitation remains delocalised over the whole molecule. In polar media, charge-transfer (CT) exciton in the linear D-A-D' systems localises rapidly at the end with the strongest donor. For the double-branch systems, excited-state symmetry breaking occurs and the CT exciton localises at the end of one of the two branches, even if the D' terminal donor is not the strongest one. This unexpected behaviour is explained by considering that the energy of a CT state depends not only on the electron donating and withdrawing properties of the donor and acceptor constituents, but also on the solvation energy. This study demonstrates the possibility to control the location of CT excitons in large conjugated systems by varying the nature of the donors and acceptors, the distance between them as well as the environment.

Design, synthesis, and redox properties of ferrocene-functionalized phenothiazine and phenothiazine sulfone isomers

A set of ferrocene-functionalized phenothiazine (PTZ) derivatives 1-3 was designed and synthesized via the Pd-catalyzed Buchwald-Hartwig cross-coupling reaction in good yields. 1-3 were treated with m-chloroperbenzoic acid (m-CPBA), which oxidizes the sulfur (S) atom in the thiazine ring to a sulfone, resulting in ferrocene substituted phenothiazine sulfone derivatives 4-6. The influence of the sulfur (S) oxidation state on the photophysical, redox, and spectroelectrochemical properties, and thermal stability of the phenothiazine and phenothiazine sulfone derivatives, containing the ferrocene unit at the para, meta, and ortho positions of the phenylene ring attached to the N-position of phenothiazine and phenothiazine sulfone, was examined. The photophysical behaviour indicates that the phenothiazine sulfone derivatives 4-6 exhibit a hypsochromic shift in the deep UV region of the UV-vis absorption spectrum as compared to 1-3. The ortho-ferrocene-functionalized phenothiazine derivative 1 exhibits greater thermal stability than its sulfone counterpart 4. The cyclic voltammetry studies on 1-6 show two oxidation waves. The redox behavior of 1-6 reveals that phenothiazine sulfone derivatives (4-6) exhibit shifts towards more positive oxidation potentials compared to their phenothiazine counterparts (1-3). Density functional theory (DFT) and TD-DFT calculations were performed on 1-6 to analyze the molecular geometry, frontier molecular orbitals, molecular electrostatic potentials (MEPs), and density of states (DOS). The spectroelectrochemical behavior of redox-active species of 1-3 shows a new and intense absorption band centred around 525 nm and three new red-shifted broad peaks centred at around 700, 790, and 895 nm in the NIR region. The molecular structures of 1-6 were confirmed by single crystal X-ray diffraction. Phenothiazine sulfone derivatives (4-6) exhibit intermolecular hydrogen bonding within their crystal lattice, contributing to stabilizing their solid-state architecture.

Synthesis of Nonplanar Push-Pull Chromophores with Various Heterocyclic Moieties via [2 + 2] Cycloaddition-Retroelectrocyclization Reaction

A series of 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives with various heterocyclic moieties, including pyridine, carbazole, indole, and benzothiadiazole, was newly synthesized through a [2 + 2] cycloaddition-retroelectrocyclization reaction. Symmetric electron-rich 1,3-butadiynes with end-capped heterocyclic substituents were reacted with tetracyanoethylene (TCNE), yielding the target TCBD products in 60-80% yields under ambient or mild heating conditions. The thermal stability and optical and electrochemical properties of both 1,3-butadiyne precursors and the corresponding TCBD derivatives were investigated by using thermogravimetric analysis (TGA), UV-vis spectroscopy, and cyclic voltammetry (CV). The TCBD derivatives featured narrow bandgaps due to the intramolecular charge transfer interactions of push-pull chromophores. In addition, the optimized structures and frontier molecular orbitals with their energy levels were determined by density functional theory (DFT) calculations.

NIR absorbing ferrocenyl perylenediimide-based donor-acceptor chromophores

A set of ferrocenyl-functionalized perylenediimide (PDI) compounds and their 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives 1-5 were designed and synthesized using palladium-catalyzed Sonogashira cross-coupling, followed by a thermally activated [2 + 2] cycloaddition-retroelectrocyclization [CA-RE] reaction with a 1,1,2,2-tetracyanoethylene (TCNE) acceptor in good yields. The TCBD group works as an acceptor, whereas the ferrocenyl group acts as a donor at the central PDI core. The effects of varying the number of ferrocenyl and TCNE groups on the photophysical, thermal, electrochemical, and spectroelectrochemical properties were studied. The di-substituted PDI derivatives 3, 4, and 5 exhibit bathochromic shifts in the absorption spectra compared to 1 and 2, attributed to the extended π-conjugation. The electrochemical analysis of derivatives 2, 4, and 5 shows multiple reduction waves in the low potential region due to the presence of TCBD and perylenediimide acceptor units. Spectroelectrochemical studies were performed, showing that upon applying redox potentials, the absorption spectra shifted from the visible to the near-infrared (NIR) region. Computational calculations indicate that in the HOMO, the electron density is localized on the ferrocene unit, while in the LUMO, it is distributed over the PDI-TCBD unit, indicating a strong D-A interaction.

Near-IR Capturing N-Methylbenzene Sulfonamide-Phenothiazine Incorporating Strong Electron Acceptor Push-Pull Systems: Photochemical Ultrafast Carrier Dynamics

Unraveling the intriguing aspects of the intramolecular charge transfer (ICT) phenomenon of multi-modular donor-acceptor-based push-pull systems are of paramount importance considering their promising applications, particularly in solar energy harvesting and light-emitting devices. Herein, a series of symmetrical and unsymmetrical donor-acceptor chromophores 1-6, are designed and synthesized by the Corey-Fuchs reaction via Evano's condition followed by [2+2] cycloaddition retroelectrocyclic ring-opening reaction with strong electron acceptors TCNE and TCNQ in good yields (~60-85 %). The photophysical, electrochemical, and computational studies are investigated to explore the effect of incorporation of strong electron acceptors 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and dicyanoquinodimethane (DCNQ) with phenothiazine (PTZ) donor. An additional low-lying broad absorption band extended towards the near-infrared (NIR) region suggests charge polarization after the introduction of the electron acceptors in both symmetrical and asymmetrical systems, leading to such strong ICT bands. The electrochemical properties reveal that reduction potentials of 3 and 6 are lower than those of 2 and 5, suggesting DCNQ imparts more on the electronic properties and hence largely contributes to the stabilization of LUMO energy levels than TCBD, in line with theoretical observations. Relative positions of the frontier orbitals on geometry-optimized structures further support accessing donor-acceptor sites responsible for the ICT transitions. Eventually, ultrafast carrier dynamics of the photoinduced species are investigated by femtosecond transient absorption studies to identify their spectral characteristics and target analysis further provides information about different excited states photophysical events including ICT and their associated time profiles. The key findings obtained here related to excited state dynamical processes of these newly synthesized systems are believed to be significant in advancing their prospect of utilization in solar energy conversion and related photonic applications.

Perspectives on push-pull chromophores derived from click-type [2 + 2] cycloaddition-retroelectrocyclization reactions of electron-rich alkynes and electron-deficient alkenes

Various push-pull chromophores can be synthesized in a single and atom-economical step through [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reactions involving diverse electron-rich alkynes and electron-deficient alkenes. In this review, a comprehensive investigation of the recent and noteworthy advancements in the research on push-pull chromophores prepared via the [2 + 2] CA-RE reaction is conducted. In particular, an overview of the physicochemical properties of the family of these compounds that have been investigated is provided to clarify their potential for future applications.

NIR-Absorbing 1,1,4,4-Tetracyanobuta-1,3-diene- and Dicyanoquinodimethane-Functionalized Donor-Acceptor Phenothiazine Derivatives: Synthesis and Characterization

A series of symmetrical and unsymmetrical donor-acceptor type phenothiazine derivatives 1-18 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition-retroelectrocyclization reactions. The incorporation of cyano-based acceptors 1,1,4,4-tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) in the phenothiazine derivatives resulted in systematic variation in the photophysical, thermal, and electrochemical properties. The electronic absorption spectra of the phenothiazine derivatives with strong acceptors 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 show red-shifted absorption as compared to phenothiazine derivatives 1, 4, 7, 10, 13, and 16 in the near-IR region due to a strong intramolecular charge transfer (ICT) transition. The electrochemical analysis of the phenothiazine derivatives 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 reveals two reduction waves at low potential due to the TCBD and DCNQ acceptors. The mono-TCBD-functionalized phenothiazine 2 shows higher thermal stability compared to other phenothiazine derivatives. The computational studies on phenothiazines 1-18 reveal the LUMO is substantially stabilized as acceptor strength increases, which lowers the HOMO-LUMO gap.