Symmetry-Breaking Charge Separation in a Chiral Bis(perylenediimide) Probed at Ensemble and Single-Molecule Levels.

Chiral molecular assemblies exhibiting symmetry-breaking charge separation (SB-CS) are potential candidates for the development of chiral organic semiconductors. Herein, we explore the excited-state dynamics of a helically chiral perylenediimide bichromophore (Cy-PDI2) exhibiting SB-CS at the ensemble and single-molecule levels. Solvent polarity-tunable interchromophoric excitonic coupling in chiral Cy-PDI2 facilitates the interplay of SB-CS and excimer formation in the ensemble domain. Analogous to the excited-state dynamics of Cy-PDI2 at the ensemble level, single-molecule fluorescence lifetime traces of Cy-PDI2 depicted long-lived off-states characteristic of the radical ion pair-mediated dark states. The discrete electron transfer and charge separation dynamics in Cy-PDI2 at the single-molecule level are governed by the distinct influence of the local environment. The present study aims at understanding the fundamental excited-state dynamics in chiral organic bichromophores for designing efficient chiral organic semiconductors and applications toward charge transport materials.

Excitation-Wavelength-Dependent Functionalities of Temporally Controlled Sensing and Generation of Singlet Oxygen by a Photoexcited State Engineered Rhodamine 6G-Anthracene Conjugate.

The present study provides design guidance for unique multipotent molecules that sense and generate singlet oxygen (1 O2 ). A rhodamine 6G-aminomethylanthracene-linked donor-acceptor molecule (RA) is designed and synthesized for demonstrating wavelength-dependent functionalities as follows; (i) RA acts as a conventional fluorogenic 1 O2 sensor molecule like the commercially available reagent, singlet oxygen sensor green (SOSG), when it absorbs ultraviolet (UV)-visible light and reacts with 1 O2 . (ii) RA acts as a temporally controlled 1 O2 sensing reagent under the longer wavelength (∼700 nm) photosensitization. RA enters an intermediate state after capturing 1 O2 and does not become strongly fluorescent until it is exposed to UV, blue, or green light. (iii) RA acts as an efficient photosensitizer to generate 1 O2 under green light illumination. The spin-orbit charge transfer mediated intersystem crossing (SOCT-ISC) process achieves this function, and RA shows a potential cancer-killing effect on pancreatic cancer cells. The wavelength-switchable functionalities in RA offer to promise molecular tools to apply 1 O2 in a spatiotemporal manner.

Caging and photo-triggered uncaging of singlet oxygen by excited state engineering of electron donor-acceptor-linked molecular sensors.

Singlet oxygen (1O2), one of the most sought-after species in oxidative chemical reactions and photodynamic cancer therapy, is activated and neutralized in the atmosphere and living cells. It is essential to see "when" and "where" 1O2 is produced and delivered to understand and utilize it. There is an increasing demand for molecular sensor tools to capture, store, and supply 1O2, controlled by light and engineered singlet and triplet states, indicating the 1O2-capturing-releasing state. Here, we demonstrate the outstanding potential of an aminocoumarin-methylanthracene-based electron donor-acceptor molecule (1). Spectroscopic measurements confirm the formation of an endoperoxide (1-O2) which is not strongly fluorescent and remarkably different from previously reported 1O2 sensor molecules. Moreover, the photoexcitation on the dye in 1-O2 triggers fluorescence enhancement by the oxidative rearrangement and a competing 1O2 release. The unique ability of 1 will pave the way for the spatially and temporally controlled utilization of 1O2 in various areas such as chemical reactions and phototherapies.

Modulating singlet fission through interchromophoric rotation.

Singlet fission (SF) is a spin-allowed, exciton-multiplying phenomenon that can be utilized to improve the efficiency of organic solar cells. It is well-understood that SF is sensitive to the local crystal morphology and an appropriately balanced coupling is essential to facilitate efficient SF. In this study, we show how the interchromophoric rotation selectively modulates the interaction between the monomer frontier molecular orbitals, promoting both fast and exothermal SF. We evaluate the effective electronic coupling for SF (VSF), the square of which is proportional to the SF rate, and the effective energies of the Frenkel exciton (FE/S1S0) and triplet pair exciton (TT) in a terrylene dimer model. Optimal interplanar rotation of the chromophoric moieties in slip-stacked arrangements pulls the effective energy of the TT state below that of the FE state. Consequently, SF is favored over competing pathways such as excimer formation, thereby enhancing the overall triplet yield. This work represents a step towards improvising the molecular design guidelines for SF and understanding the importance of interchromophoric rotation over the conventional slip-stacked arrangements for achieving favorable intermolecular electronic coupling towards efficient SF.

Exciton interactions in helical crystals of a hydrogen-bonded eumelanin monomer.

Eumelanin, a naturally occurring group of heterogeneous polymers/aggregates providing photoprotection to living organisms, consist of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) building blocks. Despite their prevalence in the animal world, the structure and therefore the mechanism behind the photoprotective broadband absorption and non-radiative decay of eumelanin remain largely unknown. As a small step towards solving the incessant mystery, DHI is crystallized in a non-protic solvent environment to obtain DHI crystals having a helical packing motif. The present approach reflects the solitary directional effect of hydrogen bonds between the DHI chromophores for generating the crystalline assembly and filters out any involvement of the surrounding solvent environment. The DHI single crystals having an atypical chiral packing motif (P212121 Sohncke space group) incorporate enantiomeric zig-zag helical stacks arranged in a herringbone fashion with respect to each other. Each of the zig-zag helical stacks originates from a bifurcated hydrogen bonding interaction between the hydroxyl substituents in adjacent DHI chromophores which act as the backbone structure for the helical assembly. Fragment-based excited state analysis performed on the DHI crystalline assembly demonstrates exciton delocalization along the DHI units that connect each enantiomeric helical stack while, within each stack, the excitons remain localized. Fascinatingly, over the time evolution for generation of single-crystals of the DHI-monomer, mesoscopic double-helical crystals are formed, possibly attributed to the presence of covalently connected DHI trimers in chloroform solution. The oligomeric DHI (in line with the chemical disorder model) along with the characteristic crystalline packing observed for DHI provides insights into the broadband absorption feature exhibited by the chromophore.

Access to the triplet excited states of organic chromophores.

Over the last several decades, exploring the pathways to access the triplet excited states of organic chromophores has been a stimulating area of research. Among the numerous photoinduced processes in organic chromophores, analysis of intersystem crossing (ISC) dynamics has received immense attention. The ISC process involves a spin-forbidden horizontal transition from an excited singlet state to a higher vibrational level of the isoenergetic triplet state. Generally, ISC necessitates a strong driving force from efficient spin-orbit coupling (SOC) between the singlet and triplet wavefunctions. The magnitude of SOC can be tuned by the substituent groups (e.g. heavy atoms, carbonyl moieties) or by the out-of-plane vibrational modes in the chromophores. Besides the SOC induced ISC pathway, triplet excited states are also realised in organic chromophores through singlet fission or via charge recombination. Accessing the triplet manifold in π-conjugated systems would also include a possible evolution to more aromatically stable configurations in the excited states, an emerging area that needs attention. In the aforesaid mechanisms, the molecular architecture and/or packing arrangement of the chromophores are vital for the effective population of triplet states. We, herein, present a collection of synthetic, spectroscopic and theoretical investigations that provide insights into the diverse pathways to access triplet excited states in organic chromophores. We believe this tutorial review would prove beneficial for researchers to achieve triplet excited states of organic chromophores for numerous biochemical and optoelectronic applications.

Photoinduced Betaine Generation for Efficient Photothermal Energy Conversion.

The conversion of solar energy to thermal, chemical, or electrical energy attracts great attention in chemistry and physics. There has been a considerable effort for the efficient extraction of photons throughout the entire solar spectrum. In this work light energy was efficiently harvested by using a long-lived betaine photogenerated from an acridinium-based electron donor-acceptor dyad. The photothermal energy-conversion efficiency of the dyad is significantly enhanced by simultaneous illumination with blue (420-440 nm) and yellow (>480 nm) light in comparison with the sum of the conversion efficiencies for individual illumination with blue or yellow light. The enhanced photothermal effect is due to the photogenerated betaine, which absorbs longer-wavelength light than the dyad, and thus the dyad-betaine combination is promising for efficient photothermal energy conversion. The mechanisms of betaine generation and energy conversion are discussed on the basis of steady-state and transient spectral measurements.

Metastable Chiral Azobenzenes Stabilized in a Double Racemate.

The self-assembly of chiral organic chromophores is gaining huge significance due to the abundance of supramolecular chirality found in natural systems. We report an interdigitated molecular assembly involving axially chiral octabrominated perylenediimide (OBPDI) which transfers chiral information to achiral aromatic moieties. The crystalline two-component assemblies of OBPDI and electron-rich aromatic units were facilitated through π-hole⋅⋅⋅π donor-acceptor interactions, and the charge-transfer characteristics in the ground and excited states of the OBPDI cocrystals were established through spectroscopic and theoretical techniques. The OBPDI cocrystals entail a remarkable homochiral segregation of P and M enantiomers of both molecular entities in the same crystal system, leading to twisted double-racemic arrangements. Synergistically engendered cavities with the stored chiral information of the twisted OBPDI stabilize higher-energy P/M enantiomers of trans-azobenzene through non-covalent interactions.

Deciphering the Multifarious Charge-Transport Behaviour of Crystalline Propeller-Shaped Triphenylamine Analogues.

A collection of para-substituted propeller-shaped triphenylamine (TPA) derivatives have been computationally investigated for charge-transport characteristics exhibited by the derivatives by using the Marcus-Hush formalism. The various substituents chosen herein, with features that range from electron withdrawing to electron donating in nature, play a key role in defining the reorganisation energy and electronic coupling properties of the TPA derivatives. The TPA moiety is expected to possess weak electronic coupling on the basis of poor orbital overlap upon aggregation, owing to the restriction imposed by the propeller shape of the TPA core. However, the substituent groups attached to the TPA core can significantly dictate the crystal-packing motif of the TPA derivatives, wherein the variety of noncovalent intermolecular interactions subsequently generated drive the packing arrangement and influence electronic coupling between the neighbouring orbitals. Intermolecular interactions in the crystalline architecture of TPA derivatives were probed by using Hirshfeld and quantum theory of atoms-in-molecules techniques. Furthermore, symmetry-adapted perturbation theory analysis of the TPA analogues has revealed that a periodic arrangement of energetically stable dimers with significant electronic coupling is essential to contribute high charge-carrier mobility to the overall crystal.

Extending the scope of the carbonyl facilitated triplet excited state towards visible light excitation.

A series of extended π-conjugated benzophenone analogs was synthesized through a facile Lewis-acid catalyzed Friedel-Crafts reaction in order to exploit the integral triplet state properties of benzophenone. Extending the π-conjugated plane of the phenyl ring of benzophenone allowed tuning of the excitation wavelength from the far-UV end (∼260 nm) to the visible spectrum (∼446 nm). Compared to benzophenone, significant red-shifts in the absorption (up to 450 nm in solution) with high photostability were observed for the synthesized benzophenone analogs. As is evident from the density functional theory calculations, expansion of the ring size of the aromatic part of the benzophenone analogs induces a decrease in the HOMO-LUMO gap. The considerable extension of the electron density to the carbonyl group in the LUMO substantiates the triplet nature associated with the benzophenone analogs. By virtue of the properties of the carbonyl functionality, an apparent increase in the triplet quantum yield (ΦT = 5.4% to 87.7%) was observed for the benzophenone analogs when compared to the corresponding bare polyaromatic hydrocarbon. The spin orbit coupling was computationally estimated for the benzophenone analogs to propose pathways for the observed intersystem crossing process. The plausibility to photoexcite the aromatic-ring-fused benzophenone frameworks for triplet activation in the visible range opens the door for a new class of materials for photonic application.