Near-IR Intramolecular Charge Transfer in Strongly Interacting Diphenothiazene-TCBD and Diphenothiazene-DCNQ Push-Pull Triads

Novel Benzothiadiazole-based Donor-Acceptor Systems: Synthesis, Ultrafast Charge Transfer and Separation Dynamics

Near-infrared (NIR) absorbing electron donor-acceptor (D-A) chromophores have been at the forefront of current energy research owing to their facile charge transfer (CT) characteristics, which are primitive for photovoltaic applications. Herein, we have designed and developed a new set of benzothiadiazole (BTD)-based tetracyanobutadiene (TCBD)/dicyanoquinodimethane (DCNQ)-embedded multimodular D-A systems (BTD1-BTD6) and investigated their inherent photo-electro-chemical responses for the first time having identical and mixed terminal donors of variable donicity. Apart from poor luminescence, the appearance of broad low-lying optical transitions extendable even in the NIR region (>1000 nm), particularly in the presence of the auxiliary acceptors, are indicative of underlying nonradiative excited state processes leading to robust intramolecular CT and subsequent charge separation (CS) processes in these D-A constructs. While electrochemical studies identify the moieties involved in these photo-events, orbital delocalization and consequent evidence for the low-energy CT transitions have been achieved from theoretical calculations. Finally, the spectral and temporal responses of different photoproducts are obtained from femtosecond transient absorption studies, which, coupled with spectroelectrochemical data, identify broad NIR signals as CS states of the compounds. All the systems are found to be susceptible to ultrafast (~ps) CT and CS before carrier recombination to the ground state, which is, however, significantly facilitated after incorporation of the secondary TCBD/DCNQ acceptors, leading to faster and thus efficient CT processes, particularly in polar solvents. These findings, including facile CT/CS and broad and intense panchromatic absorption over a wide window of the electromagnetic spectrum, are likely to expand the horizons of BTD-based multimodular CT systems to revolutionize the realm of solar energy conversion and associated photonic applications.

Excited-State Dynamics in Segregated Donor-Acceptor Stacks Versus a Peri-Bisdonor-Acceptor System

The investigation of impact of through-space/through-bond electronic interaction among chromophores on photoexcited-state properties has immense potential owing to the distinct emergent photophysical pathways. Herein, the photoexcited-state dynamics of homo-sorted π-stacked aggregates of a naphthalenemonoimide and perylene-based acceptor-donor (NI-Pe) system and a fork-shaped acceptor-bisdonor (NI-Pe2) system possessing integrally stacked peri-substituted donors was examined. Femtosecond transient absorption (fsTA) spectra of NI-Pe monomer recorded in chloroform displayed spectroscopic signatures of the singlet state of Pe; 1Pe*, the charge-separated state; NI-⋅-Pe+⋅, and the triplet state of Pe; 3Pe*. The examination of ultrafast excited-state processes of NI-Pe aggregate in chloroform revealed faster charge recombination (τ C R a ${{\tau }_{CR}^{a}}$ =1.75 ns) than the corresponding monomer (τ C R m ${{\tau }_{CR}^{m}}$ =2.46 ns) which was followed by observation of a broad structureless band attributed to an excimer-like state. The fork-shaped NI-Pe2 displayed characteristic spectroscopic features of the NI radical anion (λmax~450 nm) and perylene dimer radical cation (λmax~520 nm) upon photoexcitation in non-polar toluene solvent in the nanosecond transient absorption (nsTA) spectroscopy. The investigation highlights the significance of intrinsic close-stacked arrangement of donors in ensuring a long-lived photoinduced charge-separated state (τ C R ${{\tau }_{CR}}$ =1.35 μs) in non-polar solvents via delocalization of radical cation between the donors.

Modular Construction of Vinylene-Linked Covalent Organic Frameworks with Tunable Emission for Tumor Visualization

Covalent organic frameworks (COFs) with ordered π structures are very promising in porous light-emitting materials. However, most of these COFs are either poor in luminescence or lack of water-stability. Herein, a series of isostructural D-A vinylene-linked COFs were constructed based a new D2h symmetric linker 1,4-bis(4,6-dimethyl-1,3,5-triazin-2-yl)benzene (TMTA) with high crystallinity, comparative high surface area and excellent chemical/thermal stability. Impressively, their adsorption and luminescence wavelength vary with respect to the density of π-systems in the electron-donating group, which constitute the foundation for molecular engineering the luminescent properties of vinylene-linked COFs. The DFT calculations further established the relationship between the luminescence properties and the donor electronic structure. Moreover, one of representative COF named FZU-203 showed inspiring applications in bioimaging, which may further provide strategic guidance for the use of vinylene-linked COFs as fluorescent nanoprobes in non-invasive medical diagnosis and visualization therapy of tumors.

Zinc Tetrapyrrole Coordinated to Imidazole Functionalized Tetracyanobutadiene or Cyclohexa-2,5-diene-1,4-diylidene-expanded-tetracyanobutadiene Conjugates: Dark vs. Light-Induced Electron Transfer

Using the popular metal-ligand axial coordination self-assembly approach, donor-acceptor conjugates have been constructed using zinc tetrapyrroles (porphyrin (ZnP), phthalocyanine (ZnPc), and naphthalocyanine (ZnNc)) as electron donors and imidazole functionalized tetracyanobutadiene (Im-TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded-tetracyanobutadiene (Im-DCNQ) as electron acceptors. The newly formed donor-acceptor conjugates were fully characterized by a suite of physicochemical methods, including absorption and emission, electrochemistry, and computational methods. The measured binding constants for the 1 : 1 complexes were in the order of 104 -105  M-1 in o-dichlorobenzene. Free-energy calculations and the energy level diagrams revealed the high exergonicity for the excited state electron transfer reactions. However, in the case of the ZnNc:Im-DCNQ complex, owing to the facile oxidation of ZnNc and facile reduction of Im-DCNQ, slow electron transfer was witnessed in the dark without the aid of light. Systematic transient pump-probe studies were performed to secure evidence of excited state charge separation and gather their kinetic parameters. The rate of charge separation was as high as 1011  s-1 suggesting efficient processes. These findings show that the present self-assembly approach could be utilized to build donor-acceptor constructs with powerful electron acceptors, TCBD and DCNQ, to witness ground and excited state charge transfer, fundamental events required in energy harvesting, and building optoelectronic devices.

Linear and Nonlinear Optical Responses of Nitrobenzofurazan-Sulfide Derivatives: DFT-QTAIM Investigation on Twisted Intramolecular Charge Transfer

In this study, we report on the green fluorescence exhibited by nitrobenzofurazan-sulfide derivatives (NBD-Si, i = 1-4). The optical responses of these studied compounds in a polar methanol solvent were simulated by the use of time-dependent density functional theory (TD-DFT) employing the Becke-3-Parameter-Lee-Yang-Parr (B3LYP) functional along with the 6-31G(d,p) basis set. The computed energy and oscillator strength (f) results complement the experimental results. The band gap was calculated as the difference between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). Additionally, the density of states (DOS) was computed, providing a comprehensive understanding of the fundamental properties of these materials and further corroborating the experimental data. When the experimental data derived from ultraviolet/visible (UV/visible) and fluorescence spectroscopic techniques and those from simulated spectra are analyzed, the extracted values match up adequately. In addition, the NBD-sulfide compounds exhibit a large Stokes shift up to 85 nm in a polar methanol solvent. They are hypothesized to represent a novel paradigm of excited-state intramolecular charge transfer (ICT). To understand the intrinsic optical properties of NBD-Si materials, an ICT was identified, and its direction within the molecule was evaluated using the ratio of βvect and βtotal, values extracted from the computed nonlinear optical (NLO) properties. Moreover, the reduced density gradient (RDG)-based noncovalent interactions (NCIs) were employed to characterize the strength and type of NBD-Si interactions. Furthermore, noncovalent interactions were identified and categorized using the Quantum Theory of Atoms in Molecules (QTAIM) analysis. Ultimately, the combination of Hirshfeld surface analysis and DFT calculations was utilized to enhance the characterization and rationalization of these NCIs.

Accelerated Intramolecular Charge Transfer in Tetracyanobutadiene- and Expanded Tetracyanobutadiene-Incorporated Asymmetric Triphenylamine-Quinoxaline Push-Pull Conjugates

The excited-state properties of an asymmetric triphenylamine-quinoxaline push-pull system wherein triphenylamine and quinoxaline take up the roles of an electron donor and acceptor, respectively, are initially investigated. Further, in order to improve the push-pull effect, powerful electron acceptors, viz., 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded tetracyanobutadiene (also known as expanded-TCBD or exTCBD), have been introduced into the triphenylamine-quinoxaline molecular framework using a catalyst-free [2 + 2] cycloaddition-retroelectrocyclization reaction. The presence of these electron acceptors caused strong ground-state polarization extending the absorption well into the near-IR region accompanied by strong fluorescence quenching due to intramolecular charge transfer (CT). Systematic studies were performed using a suite of spectral, electrochemical, computational, and pump-probe spectroscopic techniques to unravel the intramolecular CT mechanism and to probe the role of TCBD and exTCBD in promoting excited-state CT and separation events. Faster CT in exTCBD-derived compared to that in TCBD-derived push-pull systems has been witnessed in polar benzonitrile.

Near-IR absorbing 1,1,4,4-tetracyanobutadiene-functionalized phenothiazine sulfones

Triphenylamine (TPA) substituted π-conjugated chromophores TPA1-TPA5 were designed and synthesized via Pd-catalysed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reactions. The effects of acceptor 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (DCNQ) units in the photophysical studies and the HOMO-LUMO energy levels of the phenothiazine sulfones TPA1-TPA5 were evaluated. The absorption spectra of chromophores TPA4 and TPA5 show a significant change due to the incorporation of DCNQ units, resulting in bathochromically shifted broad absorption in the NIR region. The photophysical studies revealed that DCNQ-based chromophores TPA4 and TPA5 have a better D-A interaction than the TCBD functionalized TPA2 and TPA3. Density functional theory calculations and electrochemical studies were performed to examine the molecular geometry and frontier energy levels of the sulfone-based chromophores. Systematic structural modification of the chromophore TPA1 modulated the electrochemical properties and successively tuned the energy gaps for TPA2-TPA5. The theoretically estimated HOMO-LUMO gaps for TPA1-TPA5 exhibit good agreement with the experimental data calculated from the electrochemical studies. The chromophore TPA1 exhibits solvatochromism and aggregation-induced emission (AIE) behavior owing to the emission in the solid state.

Charge Resonance and Photoinduced Charge Transfer in Bis(N,N-dimethylaminophenyl-tetracyanobutadiene)-diketopyrrolopyrrole Multimodular System

Intervalence charge transfer (IVCT) or charge resonance is often observed in redox-active systems encompassed of two identical electroactive groups, where one of the groups is either oxidized or reduced and serves as a model system to improve our fundamental understanding of charge transfer. This property has been explored in the present study in a multimodular push-pull system carrying two N,N-dimethylaminophenyl-tetracyanobutadiene (DMA-TCBD) entities covalently linked to the opposite ends of bis(thiophenyl)diketopyrrolopyrrole (TDPP). Electrochemical or chemical reduction of one of the TCBDs promoted electron resonance between them, exhibiting an IVCT absorption peak in the near-infrared area. The comproportionation energy, -ΔG_com, and equilibrium constant, K_com, evaluated from the split reduction peak were, respectively, 1.06 × 10⁴ J/mol and 72.3 M^-1. Excitation of the TDPP entity in the system promoted the thermodynamically feasible sequential charge transfer and separation of charges in benzonitrile, wherein the IVCT peak formed upon charge separation served as a signature peak in characterizing the product. Further, transient data analyzed using Global Target Analysis revealed the charge separation to take place in a ps time scale (k ∼ 10¹⁰ s^-1) as a result of close positioning and strong electronic interaction between the entities. The significance of IVCT in probing excited-state processes is evidenced by the present study.

Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor-Acceptor Constructs

Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor-acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push-pull systems, PTS2-PTS6 have been newly designed and synthesized via the Pd-catalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO₂) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2-PTS6, and the subsequent pump-probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push-pull systems.

Tetracyanobutadiene Bridged Push-Pull Chromophores: Development of New Generation Optoelectronic Materials

This review describes the design strategies used for the synthesis of various tetracyanobutadiene bridged donor-acceptor molecular architectures by a click type [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction sequence. The photophysical and electrochemical properties of the tetracyanobutadiene bridged molecular architectures based on various moieties including diketopyrrolopyrrole, isoindigo, benzothiadiazole, pyrene, pyrazabole, truxene, boron dipyrromethene (BODIPY), phenothiazine, triphenylamine, thiazole and bisthiazole are summarized. Further, we discuss some important applications of the tetracyanobutadiene bridged derivatives in dye sensitized solar cells, bulk heterojunction solar cells and photothermal cancer therapy.

Phenothiazine and phenothiazine-5,5-dioxide based push-pull Derivatives: Synthesis, photophysical, electrochemical and computational Studies

A set of phenothiazine (PTZ) and phenothiazine-5,5-dioxide based π-conjugated push–pull chromophores PTZ 1–6 were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition retroelectrocyclic ring opening reaction in...