Acceptor Chromophores Research Articles

Photoelectrochemical oxidation of nonoxidative methane into the value-added product over FRET-induced ZnO/ Ag@Ag4V2O7 donor-acceptor heterojunction

Direct photoelectrochemical oxidation of methane gas into oxygenates derivatives through pristine photocatalysts is a great challenge. Suitable band potential(eV) energy with a large charge carrier life span is required to break the C-H bond. Herein, a novel Förster Resonance Energy Transfer (FRET) active ZnO/Ag@Ag4V2O7 hybrid is employed to drive this photoredox reaction. FRET-enabled dipole-dipole charge transfer between ZnO donor and Ag4V2O7 acceptor chromophore for enhancing the photoredox ability of the heterojunction. DFT study also unveils the electronic density defect into ZnO, Zn-s orbital, and V-p orbital of Ag4V2O7 photo center which induces FRET into heterojunction. Meanwhile, LSPR increases the charge density into the heterojunction observed through Pl quenching. The higher positive potential of heterojunction (2.36eV) facilitated the exergonic methane oxidation. The photocurrent density of heterojunction upon visible illumination is found to be 300 times greater than the bare materials. The apparent quantum efficiency for methane photooxidation is found to be 11% along with 1.2 mW/cm2 of power production. The solar to fuel efficiency is more than 13.2% providing an opportunity to oxidise the methane under the ambient environment into energy-rich derivatives along with electricity generation.

Exciton dynamics in conjugated polymer systems

Exciton dynamics in π-conjugated polymers systems encompass multiple time and length scales. Ultrafast femtosecond processes are intrachain and involve a quantum mechanical correlation of the exciton and nuclear degrees of freedom. In contrast, post-picosecond processes involve the incoherent Förster transfer of excitons between polymer chains. Exciton dynamics is also strongly determined by the spatial and temporal disorder that is ubiquitous in conjugated polymers. Since excitons are delocalized over hundreds of atoms, a theoretical understanding of these processes is only realistically possible by employing suitably parametrized coarse-grained exciton-phonon models. Moreover, to correctly account for ultrafast processes, the exciton and phonon modes must be treated on the same quantum mechanical basis and the Ehrenfest approximation must be abandoned. This further implies that sophisticated numerical techniques must be employed to solve these models. This review describes our current theoretical understanding of exciton dynamics in conjugated polymer systems. We begin by describing the energetic and spatial distribution of excitons in disordered polymer systems, and define the crucial concept of a “chromophore” in conjugated polymers. We also discuss the role of exciton-nuclear coupling, emphasizing the distinction between “fast” and “slow” nuclear degrees of freedom in determining “self-trapping” and “self-localization” of exciton-polarons. Next, we discuss ultrafast intrachain exciton decoherence caused by exciton-phonon entanglement, which leads to fluorescence depolarization on the timescale of 10-fs. Interactions of the polymer with its environment causes the stochastic relaxation and localization of high-energy delocalized excitons onto chromophores. The coupling of excitons with torsional modes also leads to various dynamical processes. On sub-ps timescales it causes exciton-polaron formation (i.e., exciton localization and local polymer planarization). Conversely, on post-ps timescales stochastic torsional fluctuations cause exciton-polaron diffusion along the polymer chain and at higher temperatures to transient exciton delocalization via extended exciton states. We next describe a first-principles, Förster-type model of interchain exciton transfer and diffusion in the condensed phase, whose starting point is a realistic description of the donor and acceptor chromophores. Finally, we discuss condensed phase transient exciton delocalization in highly-ordered nanofibers. We survey experimental results and explain how they can be understood in terms of our theoretical description of exciton dynamics coupled to information on polymer multiscale structures. The review also contains a brief critique of computational methods to simulate exciton dynamics.

Copper(I) Donor–Chromophore–Acceptor Assembly for Light-Driven Oxidation on a Zinc Oxide Nanowire Electrode

Dye-sensitized photoelectrochemical cells have emerged as a potential candidate for solar-to-fuel conversion. Herein, we report a Cu(I)-based donor–chromophore–acceptor triad comprising a triphenylamine electron donor and a dipyrido[3,2-a:2′,3′-c]phenazine electron acceptor as the active material for photoanodes. Energy levels of this triad are carefully aligned for thermodynamically favorable photoinduced electron transfer. Once this triad is surface-grafted onto zinc oxide nanowires, photoelectrochemical studies confirm the utility of this architecture for oxidative processes such as alcohol oxidation with modest yields.

Development of Rotaxanes as E-Field-Sensitive Superstructures in Plasmonic Nano-Antennas

We present the concept of electrostatic field-driven supramolecular translation within electrically connected plasmonic nano-antennas. The antenna serves as an anchoring point for the mechanically interlocked molecules, as an electrode for the electrostatic field, and as an amplifier of the antenna-enhanced fluorescence. The synthesis of a push–pull donor–π–acceptor chromophore with optical properties aligned to the antenna resonance is described and its immobilization on the surface is demonstrated. Photoluminescence experiments of the chromophore on a gold nano-antenna are shown, highlighting the molecule–antenna coupling and resulting emission intensity increase. The successful synthesis of an electrostatic field-sensitive [2]rotaxane in water is described and the tightrope walk between functionality and water solubility is illustrated by unsuccessful designs. In solution, an enhanced fluorescence quantum yield is observed for the chromophore comprising the mechanically interlocked [2]rotaxane in water and DMSO compared to the reference rod, ideal for future experiments in plasmonic nano-antennas.

Distinctive Excited State Symmetry Breaking Dynamics in Typical Donor-Acceptor-Donor Fluorophore: Strong Photoluminescence and Ultrafast Charge Separation from a Partial Charge Transfer State.

Understanding the structure-property relationships in organic semiconductors is crucial for controlling their photophysical properties and developing new optoelectronic materials. Quadrupolar molecules, donor-acceptor-donor (DAD), have attracted extensive attention in various optoelectronic applications. However, the systematic studies on the differences on photophysical properties between DAD and simple donor-acceptor (DA) chromophores are rarely reported. Herein we present a comparative study on the excited state dynamics of DA and DAD fluorescence systems using theoretical calculation and transient absorption spectroscopy. Results show that DA and DAD molecules exhibit similar excited state dynamics, which are attributed to the distinctive excited-state symmetry breaking (ESSB) phenomenon observed in a DAD system. The strong photoluminescence (PL) and ultrafast charge separation (CS) from an ESSB-induced partial charge transfer (CT) state were clearly detected in different solvent environments. These results not only offer insight into the excited state dynamics of the DAD fluorescence system but also provide some basic guidelines for designing new optoelectronic materials.

Key Steps to Follow in a FRET Experiment

Förster resonance energy transfer (FRET) is a versatile part of the toolbox of fluorescence methods. This through-space, photon-less energy transfer process between a donor fluorophore and an acceptor chromophore is perhaps most famous for its utility as a “molecular ruler” that can resolve nanometer-scale distances. FRET is also a popular and advantageous basis for biomolecular assays and sensors.

AIE-active asymmetric A-π-A′ molecule with mechanoluminescent property

It is of great significance to explore the inherent relationship between organic molecular structure and photoluminescence mechanism. As we known, the push-pull electron groups of organic molecules have an obvious impact on molecular charge transfer. In this work, an asymmetric A-π-A′ molecule architecture was formed by using electron-rich carbazolyl as π-spacer (π) and p-fluorobenzesulfonyl and pyridinyl as acceptor (A) chromophores. The molecule has aggregation-induced emission (AIE) and mechanoluminescent features. Notably, the switching of the excited states from the locally excited state to the twisted intramolecular charge transfer state plays important roles in the AIE properties. Based on single crystal data and DFT calculations, we found that the change of intermolecular charge transfer (CT) to intramolecular charge transfer (ICT) states is the main cause of the mechanoluminescent performance. These results indicate that the molecular structure of A-π-A′ plays an important role in AIE and mechanoluminescence.

Nonlinear Optical and Ion Sensor Properties of Novel Molecules Conjugated by Click Chemistry

The molecular structure, luminescence behavior, and electronic energy level of an organic optoelectronic materials are important parameters for its synthesis. The electro-optical properties can be changed by modifying the structure of the molecule to make the electronic energy level adjustable. In this article, a series of organic conjugated micro-molecules are successfully synthesized by linking small compound units. This metal-free [2 + 2] click chemistry process generates donor–acceptor chromophore substances with high yield, high solubility, and adjustable energy levels, which can be widely used for sensors and nonlinear optics in different fields. A-TCNE, A-TCNQ, and A-F4-TCNQ molecules are characterized comprehensively via UV-Vis-NIR spectra, 1H NMR spectra, infrared spectroscopy, and mass spectrometry. The unique nonlinear optical phenomena and powerful intra-molecular charge–transfer interactions of these new materials give them fascinating potential for application as optoelectronic materials.

Exploration of the Intriguing Photovoltaic Behavior for Fused Indacenodithiophene-Based A-D-A Conjugated Systems: A DFT Model Study.

Many researchers are engaged nowadays in developing efficient photovoltaic materials to accomplish the demand of modern technology. Nonfullerene small molecular acceptors (NF-SMAs) show potential photovoltaic performance, accelerating the development of organic solar cells (OSCs). Herein, the first theoretical designing of a series of indacenodithiophene-based (IDIC1–IDIC6) acceptor chromophores was done by structural tailoring with various well-known acceptors from the recently synthesized IDICR molecule. For the selection of the best level of density functional theory (DFT), various functionals such as B3LYP, M06-2X, CAM-B3LYP, and ωB97XD with the 6-311G(d,p) basis set were used for the UV–visible analysis of IDICR. Consequently, UV–visible results revealed that an interesting agreement was found between experimental and DFT-based values at the B3LYP level. Therefore, quantum chemical investigations were executed at the B3LYP/6-311G(d,p) level to evaluate the photovoltaic and optoelectronic properties. Structural tailoring with various acceptors resulted in a narrowing of the energy gap (2.245–2.070 eV) with broader absorption spectra (750.919–660.544 nm). An effective transfer of charge toward lowest unoccupied molecular orbitals (LUMOs) from highest occupied molecular orbitals (HOMOs) was studied, which played a crucial role in conducting materials. Further, open circuit voltage (Voc) analysis was performed with respect to HOMOPBDB-T–LUMOACCEPTOR, and all of the derivatives exhibited a comparable value of voltage with that of the parent chromophore. Lower reorganization energies in titled chromophores for holes and electrons were examined, which indicated the higher rate of mobility of charges. Interestingly, all of the designed chromophores exhibited a preferable optoelectronic response compared to the reference molecule. Therefore, this computed framework demonstrates that conceptualized chromophores are preferable and might be used to build high-performance organic solar cells in the future.

Aggregation induced emission (AIE)-active ferrocene conjugated linear π−extended multi donor−π−acceptor (D-D′-π-A) chromophores: Synthesis, structural, theoretical, linear and nonlinear optical studies

A new series of ferrocene (D) and methoxyphenyl (D′) conjugated D-D′-π-A push-pull chromophores [Fc-(RPh)C=CH–CH=CN-R’{R, R’ = H, CN (1), OCH3, CN (2), H, C6H4NO2 (3), OCH3, C6H4NO2 (4)}] were synthesized and spectroscopically characterized. The structure of the chromophore 3 was confirmed by single crystal X-ray diffraction studies, and it was crystallized in triclinic crystal system with P-1 centrosymmetric space group. In the crystal packing, several non-covalent interactions (H-bonding and C–H·····π) were observed. The thermal behaviour of the chromophores 3 and 4 has been investigated using thermogravimetric analysis and was found to be stable up to 230 °C. The solvatochromic studies of the chromophores 1–4 showed a positive solvatochromism with red-shift from non-polar to polar solvents due to the high dipolar excited state in the presence of strong electron-donor and acceptor moieties. The electrochemical performance of the chromophores revealed the one-electron transfer from the ferrocene to ferrocenium ion Fe2+⇌Fe3+. The chromophores 1–4 exhibited low fluorescence intensity and quantum yield as a consequence of the quenching nature of ferrocene, it was further enhanced through the aggregation-induced emission (AIE) process in THF/H2O mixtures by the restriction of intramolecular rotation (RIR), and the chromophores 1–4 showed 2 times higher quantum yield in the aggregated state. The second-order nonlinear optical response was attained by Kurtz-Perry powder technique. The second harmonic generation (SHG) efficiencies of the chromophore 4 was close to the standard KDP, owing to the presence of multi-donor [Ferrocene (D) and methoxyphenyl (D′)] and acceptor groups with extended π-conjugation. In addition, the mesomeric effect (+M) of the methoxy group makes an effective charge transfer process. The optical and nonlinear optical properties of the chromophores were further analyzed by density functional theory (DFT) with different functionals (B3LYP, CAM-B3LYP and LC-BLYP). The hyperpolarizability values were underestimates in the B3LYP-hybrid functional because of the corrected long-range charge transfer between donor and acceptor in the D-π-A systems. The frontier molecular orbital (FMO) level helps for better understanding the charge transfer, nonlinear optical (NLO) properties, and the calculated HOMO-LUMO energy levels were in good agreement with the experimental results.

Interaction between miR4749 and Human Serum Albumin as Revealed by Fluorescence, FRET, Atomic Force Spectroscopy and Computational Modelling.

The interaction of Human Serum Albumin (HSA) with the microRNA, miR4749, was investigated by Atomic Force Spectrscopy (AFS), static and time-resolved fluorescence spectroscopy and by computational methods. The formation of a HSA/miR4749 complex with an affinity of about 104 M−1 has been assessed through a Stern–Volmer analysis of steady-state fluorescence quenching of the lone Trp residue (Trp214) emission of HSA. Förster Resonance Energy Transfer (FRET) measurements of fluorescence lifetime of the HSA/miR4749 complex were carried out in the absence and in the presence of an acceptor chromophore linked to miR4749. This allowed us to determine a distance of 4.3 ± 0.5 nm between the lone Trp of HSA and the dye bound to miR4749 5p-end. Such a distance was exploited for a screening of the possible binding sites between HSA and miR4749, as predicted by computational docking. Such an approach, further refined by binding free energy calculations, led us to the identification of a consistent model for the structure of the HSA/miR4749 complex in which a positively charged HSA pocket accommodates the negatively charged miRNA molecule. These results designate native HSA as a suitable miRNA carrier under physiological conditions for delivering to appropriate targets.

Molecular engineering of indenoindene-3-ethylrodanine acceptors with A2-A1-D-A1-A2 architecture for promising fullerene-free organic solar cells

Considering the increased demand and potential of photovoltaic devices in clean, renewable electrical and hi-tech applications, non-fullerene acceptor (NFA) chromophores have gained significant attention. Herein, six novel NFA molecules IBRD1–IBRD6 have been designed by structural modification of the terminal moieties from experimentally synthesized A2-A1-D-A1-A2 architecture IBR for better integration in organic solar cells (OSCs). To exploit the electronic, photophysical and photovoltaic behavior, density functional theory/time dependent-density functional theory (DFT/TD-DFT) computations were performed at M06/6-311G(d,p) functional. The geometry, electrical and optical properties of the designed acceptor molecules were compared with reported IBR architecture. Interestingly, a reduction in bandgap (2.528–2.126 eV), with a broader absorption spectrum, was studied in IBR derivatives (2.734 eV). Additionally, frontier molecular orbital findings revealed an excellent transfer of charge from donor to terminal acceptors and the central indenoindene-core was considered responsible for the charge transfer. Among all the chromophores, IBRD3 manifested the lowest energy gap (2.126 eV) with higher λmax at 734 and 745 nm in gaseous phase and solvent (chloroform), respectively due to the strong electron-withdrawing effect of five end-capped cyano groups present on the terminal acceptor. The transition density matrix map revealed an excellent charge transfer from donor to terminal acceptors. Further, to investigate the charge transfer and open-circuit voltage (Voc), PBDBT donor polymer was blended with acceptor chromophores, and a significant Voc (0.696–1.854 V) was observed. Intriguingly, all compounds exhibited lower reorganization and binding energy with a higher exciton dissociation in an excited state. This investigation indicates that these designed chromophores can serve as excellent electron acceptor molecules in organic solar cells (OSCs) that make them attractive candidates for the development of scalable and inexpensive optoelectronic devices.

Laponite elementary sheets assisted fluorescence resonance energy transfer: A demonstration by Langmuir-Blodgett technique

The fluorescence resonance energy transfer (FRET) between donor-acceptor molecules of fluorescein (FL) and rhodamine B (RhB) was enhanced by clay sheets of laponite. This enhancement was implemented by the Langmuir-Blodgett (LB) technique. Hybrid films were prepared by incorporating elementary clay sheets into dye molecules (FL and RhB for different molar ratios) at the air/water interface. The spectroscopic measurements (UV–Vis absorption and photoluminescence) of these mixture systems (FL, RhB and clay mineral) were analyzed both in colloidal dispersion and LB films. A high efficiency (92%) of FRET from FL to RhB was achieved with presence of laponite. Characterizations such as surface pressure versus area (π-A) isotherms, atomic force microscopy (AFM) and contact angle measurements were used to confirm the successful incorporation of clay sheets in hybrid films with loaded dye molecules. The clay incorporation were indicated to shorten the distance between donor and acceptor chromophores (Förster radius), which was realized by the tightly bounded dye molecules on clay sheets and self-assembly of dye molecule pairs.

Triplet‐Forming Thionated Donor‐Acceptor Chromophores for Electrochemically Amphoteric Photosensitization

AbstractA series of piperidine‐naphthalene monoimide, donor−acceptor (D−A) chromophores and their thionated derivatives were prepared. The physical properties of the chromophores were studied by spectroscopic and electrochemical measurements and DFT calculations. Relative to their non‐thionated counterparts, the thionated D−A chromophores display enhanced visible‐light absorption and 1O2‐sensitization quantum yields and negligible fluorescence, suggesting facile triplet formation of this class of molecules. This behavior originates from the significant n‐π* character in the singlet excited state, resulting in large spin‐orbit coupling between the singlet/triplet manifolds regardless of the degree of D/A interactions. With the electrochemically active D/A moieties and high triplet energy, the thionated chromophores can be used as amphoteric photosensitizers to catalyze reductive and oxidative photoreactions with efficiency comparable to Ru(bpy)32+ as the sensitizer. Our results demonstrate that thiocarbonylation can be utilized in amide/imide‐containing D−A chromophores to provide a wide range of heavy‐atom‐free redox‐active photosensitizers.

Synthesis of benzo[b]phosphole‐based alkynylgold(I) complexes with resistive memory properties modulated by donor–acceptor chromophores

AbstractA class of benzo[b]phosphole‐based alkynylgold(I) complexes has been synthesized and characterized. These complexes share a similar benzo[b]phosphole ligand, in which the phosphole moiety is substituted with various π‐conjugated units with different donor strengths, namely phenoxazinylphenyl, tris(di‐tert‐butylcarbazolyl)phenyl and 2,4‐dimethylphenyl moieties. These phosphole‐containing gold(I) complexes are found to be strongly luminescent in toluene with tunable emission maxima and possess solvatochromic behaviors, suggesting an emission of metal‐perturbed intraligand charge transfer origin. Cyclic voltammetry studies reveal that the presence of gold(I) metal center strongly perturbs the electronic properties of the phosphole moiety of the resultant complexes, which can be further fine‐tuned by the auxiliary ligand on the gold(I) center. In the resistive memory studies, devices based on these alkynylgold(I) complexes exhibit satisfactory binary memory behaviors, demonstrating low threshold voltages in narrow distributions, high durability and low misreading rates. Such performances are believed to be originated from a field‐induced charge transfer of the alkynylgold(I) complexes, in which the electron‐accepting phosphole‐gold(I) unit plays a crucial role in stabilizing the charge transfer state and that led to the observed resistive switching and memory behavior.

Efficient tuning of small acceptor chromophores with A1-π-A2-π-A1 configuration for high efficacy of organic solar cells via end group manipulation

Now a days, non-fullerene organic compounds are considered with keen interest for the greener energy and potential photovoltaic devices. Herein, novel acceptor chromophores (DPDVD1-DPDVD4) were designed via small acceptor DPDVR compound with A1-π-A2-π-A1 architecture using promising approach by redistribution of end-capped acceptor moieties. The effect of end-capped acceptor moieties on designed architecture for the photophysical, photovoltaic and electronic behavior was explored using quantum chemical study. Frontier molecular orbital (FMO) findings revealed that all the molecules (DPDVD1-DPDVD4) contained narrow band gaps. The central acceptor moiety (PzDP) was responsible for the transformation of charge in DPDVD1-DPDVD4. UV–Vis data disclosed that λmax values of DPDVD1-DPDVD4 compounds were existed in visible region (537.36–517.05 nm). Interestingly, DPDVD2 exhibited the reduced band gap with broader absorption spectra among all chromophores might be due to the negative inductive effect (−I) of the terminal acceptors. The open circuit voltage (Voc) analysis was performed at the interface between standard donor polymer (PTB7) and DPDVD2. The higher value (3.34 V) of Voc was observed for DPDVD2 among reference and designed compounds. Furthermore, reorganization energy findings exploited DPDVR-DPDVD4 shown lower values of λe than λh which indicated that these chromophores were the promising candidates for electron transportation mobilities. The larger charge mobilities with greater exciton dissociation in excited state for DPDVD2 were examined, subsequently this compound showed minimum value of λe (0.015739 eV) among all chromophores. Detailed studies reveal that the entitled molecules can behave as efficient electron acceptor molecules in organic solar cells (OSCs) that make them interesting candidates for the development of inexpensive optoelectronic devices.

Dual-Encryption in a Shape-Memory Hydrogel with Tunable Fluorescence and Reconfigurable Architecture.

Materials capable of shape-morphing and/or fluorescence imaging have practical significances in the fields of anti-counterfeiting, information display, and information protection. However, it's challenging to realize these functions in hydrogels due to the poor mechanical properties and lack of tunable fluorescence. A tough hydrogel with good shape-memory ability and phototunable fluorescence is reported here, which affords reprogrammable shape designing and information encoding for dual-encryption. This hydrogel is prepared by incorporating donor-acceptor chromophore units into a poly(1-vinylimidazole-co-methacrylic acid) network, in which the dense intra- and interchain hydrogen bonds lead to desirable features including high stiffness, high toughness, and temperature-mediated shape-memory property. Additionally, the hydrogel shows photomediated tunable fluorescence through a unimer-to-dimer transformation of the chromophores. By combining photolithography and origami/kirigami designs, hydrogel sheets encoded with fluorescent patterns can deform into specific 3D configurations. The geometrically encrypted fluorescent information in the architected hydrogels is readable only after sequential shape recovery and UV light irradiation. As demonstrated by proof-of-concept experiments, both the fluorescent pattern and the 3D configuration are reprogrammable, facilitating repeated information protection and display. The design of tough hydrogels with rewritable fluorescent patterns and reconfigurable shapes should guide the future development of smart materials with improved security and wider applications in aqueous environments.

Engineering couplings for exciton transport using synthetic DNA scaffolds

Summary Control over excitons enables electronic energy to be harnessed and transported for light harvesting and molecular electronics. Such control requires nanoscale precision over the molecular components. Natural light-harvesting systems achieve this precision through sophisticated protein machinery, which is challenging to replicate synthetically. Here, we introduce a DNA-based platform that spatially organizes cyanine chromophores to construct tunable excitonic systems. We synthesized DNA-chromophore nanostructures and characterized them with ensemble ultrafast and single-molecule spectroscopy and structure-based modeling. This synthetic approach facilitated independent control over the coupling among the chromophores and between the chromophores and the environment. We demonstrated that the coupling between the chromophores and the environment could enhance exciton transport efficiency, highlighting the key role of the environment in driving exciton dynamics. Control over excitons, as reported here, offers a path toward the development of designer nanophotonic devices.

Unravelling the role of charge transfer state during ultrafast intersystem crossing in compact organic chromophores.

When organic electron donor (D) and acceptor (A) chromophores are linked together, an electron transfer (ET) state can take place. When a short bridge such as one Sigma bond is used to link the donor and the acceptor, complete charge separation is difficult to access and one usually observes an intramolecular charge transfer (CT) state instead. Due to the inevitable coupling between the donor and the acceptor in compact organic chromophores, the most common decay pathway for the CT state is charge recombination, which may lead to a distinct longer wavelength fluorescence emission or non-radiative dissipation of the excited state energy. However, recent studies have shown that unique excited state dynamics can be observed when the CT state is involved during both forward and backward intersystem crossing (ISC) from singlet excited states to triplet excited states in organic chromophores. Analysis of the mechanism for ISC involving the CT state has received much attention over the last decade. In this perspective, we present a collection of molecular design rationales, spectroscopy and theoretical investigations that provide insights into the mechanism of the ISC involving the CT state in compact organic chromophores. We hope that this perspective will prove beneficial for researchers to design novel compact organic chromophores with a predictable ISC property for future biochemical and optoelectronic applications.

Spectroelectrochemical and Computational Analysis of a Series of Cycloaddition-Retroelectrocyclization-Derived Donor-Acceptor Chromophores.

The [2+2] cyclcoaddition (CA) and subsequent retroelectrocyclization (RE) reactions are useful in constructing nonplanar donor-acceptor chromophores that exhibit nonlinear optical properties and intramolecular charge-transfer transitions. However, both the infrared (IR) and visible-near IR (vis-NIR) spectroelectrochemical responses of CA-RE-derived chromophores are rarely explored in depth. Reported in this contribution is a comprehensive IR and vis-NIR spectroelectrochemical study of the CA-RE adducts of DMAP-C2n-NAPiPr of both tetracyanoethene (TCNE) and tetracyanoquinodimethane (TCNQ) and companion time-dependent density functional theory (TD-DFT) analysis of the bands observed. Specifically, DMAP-C2n-NAPiPr (1a, n = 1; 1b n = 2; DMAP = N,N-dimethylaniline; NAPiPr = N-isopropyl-1,8-naphthalimide) react with TCNE to yield the tetracyanobutadiene (TCBD) derivatives (2a and 2b, respectively) and with TCNQ to yield the dicyanoquinodimethane (DCNQ) derivatives (3a and 3b, respectively). IR spectroelectrochemical studies showed the emergence/intensification of new CN stretches upon reductions. Ultraviolet-vis-NIR (UV-vis-NIR) spectroelectrochemical study of 3 revealed a partial bleach of the charge-transfer (CT) bands, originally appearing in the neutral species, and the emergence of new CT bands originating from NAPiPr to the reduced DCNQ moiety. UV-vis-NIR spectroelectrochemical study of 2, surprisingly, indicated a very minimal change upon reductions. Dynamic changes were observed in the mid-IR absorption for C≡C and C≡N for both 2 and 3, indicative of enhanced asymmetry and the formation of ion pairs on the dicyano bridge. DFT and TD-DFT analyses were used to obtain the semi-quantitative pictures of the frontier orbitals of 1-3 and elucidate the origin of the transient features observed spectroelectrochemically for the 1e- and 2e- reduced species.