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Principles Of Fluorescence Spectroscopy

The traditional design strategies for highly bright solid-state luminescent materials rely on weakening the intermolecular π-π interactions, which may limit diversity when developing new materials. Herein, we propose a strategy of tuning the molecular packing mode by regioisomerization to regulate the solid-state fluorescence. TBP-e-TPA with a molecular rotor in the end position of a planar core adopts a long-range cofacial packing mode, which in the solid state is almost non-emissive. By shifting molecular rotors to the bay position, the resultant TBP-b-TPA possesses a discrete cross packing mode, giving a quantum yield of 15.6±0.2 %. These results demonstrate the relationship between the solid-state fluorescence efficiency and the molecule's packing mode. Thanks to the good photophysical properties, TBP-b-TPA nanoparticles were used for two-photon deep brain imaging. This molecular design philosophy provides a new way of designing highly bright solid-state fluorophores.

ACQ-to-AIE Transformation: Tuning Molecular Packing by Regioisomerization for Two-Photon NIR Bioimaging.

We report a novel example of electro-mediated photoredox catalysis (e-PRC) in the reductive cleavage of C(sp 3 )-O bonds of phosphinated alcohols to alkyl carbanions. As well as deoxygenations, olefinations are reported which are E -selective and can be made Z -selective in a tandem reduction/photosensitization process where both steps are photoelectrochemically promoted. Spectroscopy, computation and catalyst structural variations reveal that our new naphthalene monoimide-type catalyst allows for a more intimate dispersive precomplexation of its radical anion form with the phosphinate substrate, facilitating a reactivity-determining C(sp 3 )-O cleavage. Surprisingly and in contrast to previously reported photoexcited radical anion chemistries, our conditions i) tolerate aryl chlorides/bromides and ii) do not give rise to Birch-type reductions.

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Electro-mediated PhotoRedox Catalysis for Selective C(sp3)-O Cleavages of Phosphinated Alcohols to Carbanions

The fulfillment of a high quantum efficiency for photocatalytic CO2 reduction presents a key challenge, which can be overcome by developing strategies for dynamic attachment between photosensitizer and catalyst. In this context, we exploit the use of coordinate bond to connect a pyridine-appended iridium photosensitizer and molecular catalysts for CO2 reduction, which is systematically demonstrated by 1H nuclear magnetic resonance titration, theoretical calculations, and spectroscopic measurements. The mechanistic investigations reveal that the coordinative interaction between the photosensitizer and an unmodified cobalt phthalocyanine significantly accelerates the electron transfer and thus realizes a remarkable quantum efficiency of 10.2% ± 0.5% at 450 nm for photocatalytic CO2-to-CO conversion with a turn-over number of 391 ± 7 and nearly complete selectivity, over 4 times higher than a comparative system with no additional interaction (2.4%±0.2%). Moreover, the decoration of electron-donating amino groups on cobalt phthalocyanine can optimize the quantum efficiency up to 27.9% ± 0.8% at 425 nm, which is more attributable to the enhanced coordinative interaction rather than the intrinsic activity. The control experiments demonstrate that the dynamic feature of coordinative interaction is important to prevent the coordination occupancy of labile sites, also enabling the wide applicability on diverse non-noble-metal catalysts. Positioning photosensitizer and catalyst complexes in photocatalytic systems is a promising method to direct desired electron transfers. Here, authors employ a dynamic coordinative interaction between molecular components to improve CO2 photoreduction to CO with a high quantum efficiency of 27.9%.

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Rapid electron transfer via dynamic coordinative interaction boosts quantum efficiency for photocatalytic CO2 reduction

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates.

Visible-light-mediated photoredox catalysis has evolved as an efficient and mild alternative to conventional organic synthesis. Inspired by the elegance and sophistication of natural photosynthetic...

Supramolecular Engineering and Self-Assembly Strategies in Photoredox Catalysis

Design and synthesis of molecular assemblies which exhibit aggregation-induced emission (AIE) have attracted persistent attention in recent years. However, studies on the impact of the substitution position in isostructural molecules to regulate aggregation-induced emission (AIE) or convert aggregation-caused quenching (ACQ) to AIE have not been reported to date. In the present study, aggregation properties of two isostructural fluorophores viz. (E)-4-(2-(anthracen-9-yl)vinyl)benzonitrile (p-AnCN) and (E)-3-(2-(anthracen-9-yl)vinyl)benzonitrile (m-AnCN), with different substitution positions have been evaluated. It was observed that m-AnCN exhibits an ACQ behavior, whereas, p-AnCN exhibits enhanced emission upon aggregation. Upon aggregation, c.a. 1.4-fold decrease in luminescence quantum yield has been observed for m-AnCN, whereas, c.a. 11-fold enhancement in luminescence quantum yield has been achieved for p-AnCN. Results from the study indicate that absence of interplay between the excited states from a dark Intramolecular Charge Transfer (ICT) state to Locally Excited state (LE) in m-AnCN is the primary reason behind the observed difference in the emission properties of the aggregates. Thus, modulation of ACQ-AIE properties has been achieved using isostructural organic fluorophores, which in turn can be used as a versatile strategy to achieve AIE-luminophores. Further, the AIE active p-AnCN aggregates have been utilized to generate three-component white light emission with CIE coordinates of (0.33, 0.33).

Achieving ACQ-AIE modulation using isostructural organic fluorophores

The present study investigates the photophysical properties of two highly conjugated pyrene and anthracene derivatives viz ( E )-4-(2-(pyren-1-yl) vinyl) benzonitrile (PyCN) and ( E )-4-(2-(anthracen-9-yl) vinyl) benzonitrile (AnCN) in the presence and absence of electron donors such as N , N -diethylaniline (DEA) and N , N -diphenylamine (DPA). The observed rates of back electron transfer (BET) in the case of PyCN-DEA and AnCN-DEA are 1.3 × 10 5 and 4.8 × 10 4 s −1 , respectively, which are 3–4 orders of magnitude lower compared to the well-known pyrene–amine and anthracene–amine systems. Mechanistic investigations indicate that the donor-acceptor systems undergo different pathways in presence of tertiary and the secondary amine. The results indicate that the design strategy is highly efficient in reducing the rate of back electron transfer, which results in long lived charge-separated excited states.

Long lived charge separated states in vinylbenzonitrile substituted derivatives of pyrene and anthracene

In the present study, the stability of the photogenerated, solvent-separated charged states of graphene quantum dots (GQDs) in the presence of N,N-diethylaniline (DEA) has been evaluated in a series of organic solvents. The results indicate that the rate constant for back electron transfer (kBET) from GQD radical anion to DEA radical cation is diffusion-controlled. As a result of the diffusion-controlled back electron transfer (BET), kBET exhibits an inverse exponential relation to (a) the viscosity coefficient (η) of the solvent and (b) the average radius of the graphene quantum dots. An analytical expression for the diffusion-controlled back electron transfer rate constant has been formulated. The dependence of kBET on the diffusion of solvent-separated ion pairs has been evaluated for the first time for quantum dot systems and the results provide an efficient method for enhancing the lifetime of the photogenerated charge-separated states from graphene quantum dots. The present findings can potentially ...

Diffusion of Solvent-Separated Ion Pairs Controls Back Electron Transfer Rate in Graphene Quantum Dots

In the present study, photoinduced electron transfer (PET) dynamics between N,N-diethylaniline (DEA) and (E)-9-(4-nitrostyryl)anthracene (
 $$\hbox {An-NO}_{2}$$
 ) in a non-polar solvent medium {methylcyclohexane (MCH)}, has been investigated. The rate constant of back electron transfer (
 $$\hbox {k}_{{\mathrm{BET}}}$$
 ) for the $$\hbox {An-NO}_{2}$$
 – DEA pair was $$\sim 3.8\times 10^{5}~\hbox {s}^{-1}$$
 which is ca. 2 orders of magnitude less compared to the anthracene (An)-DEA (control) system. The results indicate that long-lived charge separated species can be generated using the design strategy used herein by achieving resonance stabilization of the excited state (acceptor) radical via conjugation. SYNOPSIS For N,N-diethylaniline/(E)-9-(4-nitrostyryl)anthracene donor-acceptor pair, the back electron transfer rate constant is $$\sim 2$$
 orders of magnitude slower compared to N,N-diethylaniline/anthracene system. The results indicate that organic molecules with extended $$\uppi $$
 -conjugation can be utilized for generating long-lived charge separated states via bimolecular PET, due to increased feasibility of charge delocalization.

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Photoinduced electron transfer processes of (E)-9-(4-nitrostyryl)anthracene in non-polar solvent medium: generation of long-lived charge-separated states $$^{\S }$$ §

Ayan Bhattacharyya

Papers

Supramolecular Engineering and Self-Assembly Strategies in Photoredox Catalysis

Achieving ACQ-AIE modulation using isostructural organic fluorophores

Long lived charge separated states in vinylbenzonitrile substituted derivatives of pyrene and anthracene

Diffusion of Solvent-Separated Ion Pairs Controls Back Electron Transfer Rate in Graphene Quantum Dots

Photoinduced electron transfer processes of (E)-9-(4-nitrostyryl)anthracene in non-polar solvent medium: generation of long-lived charge-separated states $$^{\S }$$ §