Robust Red-Absorbing Donor-Acceptor Stenhouse Adduct Photoswitches.

Donor-Acceptor Stenhouse Adduct (DASA), a class of push-pull negative photochrome, has received large interest lately owing to its versatile synthesis, modularity and excellent photoswitching in solutions. From a technological perspective, it is imperative for this class of photoswitches to work robustly in solid state, e. g. thin films. We feature a molecular framework for the optimized design of DASAs by introducing a new thioindoline donor (D3) and assessing its performance against known 2nd generation indoline-based donors. The systematic structure-function investigations suggest that to achieve robust reversible photoswitching, a ground state with low charge separation is desired. DASAs with stronger electron donors and a larger charge separation in the ground state result in a low population of the photothermalstationary state (PTSS) and reduced photostability. The DASA with thioindoline donor (D3A3) seems to be a special case among the donor series as it causes a red shift (ca. 15 nm), however with less polarization of the ground state and marginally better photostability as compared to the unsubstituted 2-methyl indoline (D1A3). We also emphasize the consideration of the key additional factors that can modulate the red-light photoswitching properties of DASA chromophores in polymer thin films, which might not be dominant in homogenous solution state.

Fast Charge Separation in Distant Donor-Acceptor Dyads Driven by Relaxation of a Hot Excited State.

A series of three perylenemonoimide-p-oligophenylene-dimethylaniline molecular dyads undergo photoinduced charge separation (CS) with anomalous distance dependence as a function of increasing donor-acceptor (DA) distances. A comprehensive experimental and computational investigation of the photodynamics in the donor-bridge-acceptor (DBA) chromophores reveals a clear demarcation concerning the nature of the CS accessed at shorter (bridgeless) and longer DA distances. At the shortest distance, a strong DA interaction and ground-state charge delocalization populate a hot excited state (ES) with prominent charge transfer (CT) character, via Franck-Condon vertical excitation. The presence of such a CT-polarized hot ES enables a subpicosecond CS in the bridgeless dyad. The incorporation of the p-oligophenylene bridge effectively decouples the donor and the acceptor units in the ground state and consequentially suppresses the CT polarization in the hot ES. Theoretically, this should render a slower CS at longer distances. However, the transient absorption measurement reveals a fast CS process at the longer distance, contrary to the anticipated exponential distance dependence of the CS rates. A closer look into the excited-state dynamics suggests that the hot ES undergoes ultrafast geometry relaxation (τ < 1 ps) to create a relaxed ES. As compared to a decoupled, twisted geometry in the hot ES, the geometry of the relaxed ES exhibits a more planar conformation of the p-oligophenylene bridges. Planarization of the bridge endorses an increased charge delocalization and a prominent CT character in the relaxed ES and forms the origin for the evident fast CS at the longest distance. Thus, the relaxation of the hot ES and the concomitantly enhanced charge delocalization adds a new caveat to the classic nature of distance-dependent CS in artificial DBA chromophores and recommends a cautious treatment of the attenuation factor (ß) while discussing anomalous CS trends.

Excited state dynamics of BODIPY-based acceptor-donor-acceptor systems: a combined experimental and computational study.

Donor-bridge-acceptor systems based on boron dipyrromethene (BODIPY) are attractive candidates for bio-imagining and sensing applications because of their sensitivity to temperature, micro-viscosity and solvent polarity. The optimization of the properties of such molecular sensors requires a detailed knowledge of the relation between the structure and the photophysical behavior in different environments. In this work we have investigated the excited-state dynamics of three acceptor-donor-acceptor molecules based on benzodithiophene and BODIPY in solvents of different polarities using a combination of ultrafast spectroscopy and DFT-based electronic structure calculations. Transient absorption spectra show that upon photoexcitation an initial excited species with an induced absorption band in the near-infrared regime is formed independent of the solvent polarity. The subsequent photophysical processes strongly depend on the solvent polarity. In non-polar toluene this initial excited state undergoes a structural relaxation leading to a delocalized state with partial charge transfer character, while in the more polar tetrahydrofuran a fully charge separated state is formed. The results clearly show how factors such as donor-acceptor distance and restricted rotational motion by steric hindrance can be used to tune the excited state photophysics to optimize such systems for specific applications.

Directing charge transfer in perylene based light-harvesting antenna molecules.

Directing energy and charge transfer processes in light-harvesting antenna systems is quintessential for optimizing the efficiency of molecular devices for artificial photosynthesis. In this work, we report a novel synthetic method to construct two regioisomeric antenna molecules (1-D2A2 and 7-D2A2), in which the 4-(n-butylamino)naphthalene monoimide energy and electron donor is attached to the perylene monoimide diester (PMIDE) acceptor at the 1- and 7-bay positions, respectively. The non-symmetric structure of PMIDE renders a polarized distribution of the frontier molecular orbitals along the long axis of this acceptor moiety, which differentiates the electron coupling between the donor, attached at either the 1- or the 7-position, and the acceptor. We demonstrate that directional control of the photo-driven charge transfer process has been obtained by engineering the molecular structure of the light-harvesting antenna molecules.

Efficacious elimination of intramolecular charge transfer in perylene imide based light-harvesting antenna molecules.

Two light-harvesting antenna molecules were obtained by positioning naphthalene monoimide energy donors at the imide position, instead of the bay positions, of perylene imide energy acceptors. Such rational design resulted in a complete suppression of parasitic intramolecular charge transfer without compromising the desired ultrafast rates of excitation energy transfer.

Decoding the Curious Tale of Atypical Intersystem Crossing Dynamics in Regioisomeric Acetylanthracenes.

Mapping the primary photochemical dynamics and transient intermediates in functional chromophores is vital for crafting archetypal light-harvesting materials. Although the excited state dynamics in 9-acetylanthracene is well explored, the origin of near-quantitative triplet population and the atypical intersystem crossing (ISC) rate as compared with the regioisomeric analogs (1-/2-acetylanthracene) have rarely been scrutinized. We present a comprehensive account of the photoinduced dynamics in three regioisomeric monoacetylanthracenes using ultrafast transient absorption and quantum chemical calculations. The conjoint experimental and computational investigations suggest that (i) greater stabilization of the 1nπ* relative to 1ππ* state, (ii) dissimilar 1ππ* → 1nπ* crossover barriers, and (iii) the strong spin-orbit coupling (νSO) of the 1nπ* state with the receiver 3ππ* state command the divergent triplet population in 1-/2-/9-acetylanthracenes. A tacit understanding of the subtle structural-alteration-facilitated contrasting ISC dynamics in carbonylated arenes can act as a stepping stone for the evolution of potent photofunctional materials.

Null Exciton Splitting in Chromophoric Greek Cross†(+) Aggregate.

Exciton interactions in molecular aggregates play a crucial role in tailoring the optical behaviour of π-conjugated materials. Though vital for optoelectronic applications, ideal Greek cross-dipole (α=90°) stacking of chromophores remains elusive. We report a novel Greek cross (+) assembly of 1,7-dibromoperylene-3,4,9,10-tetracarboxylic tetrabutylester (PTE-Br2 ) which exhibits null exciton coupling mediated monomer-like optical characteristics in the crystalline state. In contrast, nonzero exciton coupling in X-type (α=70.2°, PTE-Br0 ) and J-type (α=0°, θ=48.4°, PTE-Br4 ) assemblies have perturbed optical properties. Additionally, the semi-classical Marcus theory of charge-transfer rates predicts a selective hole transport phenomenon in the orthogonally stacked PTE-Br2 . Precise rotation angle dependent optoelectronic properties in crystalline PTE-Br2 can have consequences in the rational design of novel π-conjugated materials for photonic and molecular electronic applications.

Concerted Interplay of Excimer and Dipole Coupling Governs the Exciton Relaxation Dynamics in Crystalline Anthracenes.

A combined theoretical and experimental investigation into the role of concerted long- (dipole coupling) and short-range (orbital overlap mediated excimer) electronic interactions in modulating the emission of six crystalline acetylanthracenes (1-3) is reported. Friedel-Crafts acylation of anthracene rendered crystalline acetylanthracenes with discrete close packing, varied orbital overlap, and resultant distinct emission (blue-green-yellow) from cooperative excimer and dipole coupling. Time-resolved emission spectroscopy (TRES) studies and the Kasha's exciton theory based quantitative estimation of dipole coupling (mean-field approximation) substantiates the exciton dynamics in crystalline 1-3. Extension of the Kasha's exciton model beyond the traditional nearest-neighbor approach, and consistent agreement among the computed spectral shifts and TRES temporal components, corroborate a holistic approach to decipher the exciton relaxation dynamics in the molecular assembly of novel photonic materials.

Prolonged Charge Separated States in Twisted Stacks of All-Carbon Donor and Acceptor Chromophores.

Twisted donor-on-donor and acceptor-on-acceptor bicontinuous assembly in all-carbon pyren-1-ylaceanthrylene (PA) dyad extends the survival time of the photoinduced radical ion-pair intermediates. Aceanthrylene, a functional analog of C70, acts as a versatile electron acceptor owing to its high electron affinity and visible light absorption. Antithetical trajectories of the excitons in the nonparallel π-ways led to persistent radical ion-pair intermediates in aggregated (τcrA ∼ 1.28 ns) vs monomeric (τcrM ≤ 110 fs) PA dyad as observed using femtosecond transient absorption spectroscopy. Marcus theory of charge transfer rates predicts an ambipolar transport characteristic in crystalline PA, thereby endorsing PA as an all-carbon DA hybrid for nonfullerene photovoltaic applications.

On the origin of multiexponential fluorescence decays from 2-aminopurine-labeled dinucleotides.

The fluorescent probe 2-aminopurine (2Ap) has been used for decades to study local conformational fluctuations in DNA. Steady-state and time-resolved measurements of 2Ap fluorescence have been used to predict specific conformational states through suitable modeling of the quenching of the fluorescence of a 2Ap residue incorporated site-specifically into a DNA strand. The success of this approach has been limited by a lack of understanding of the precise factors responsible for the complex, multiexponential decays observed experimentally. In this study, dinucleotides composed of 2Ap and adenine were studied by the time-correlated single-photon counting technique to investigate the causes of heterogeneous emission kinetics. Contrary to previous reports, we argue that emission from 2Ap that is stacked with a neighboring base contributes negligibly to the emission signals recorded more than 50 ps after excitation, which are instead dominated by emission from unstacked 2Ap. We find that the decay kinetics can be modeled using a continuous lifetime distribution, which arises from the inherent distance dependence of electron transfer rates without the need to postulate a small number of discrete states with decay times derived from multiexponential fits. These results offer a new perspective on the quenching of 2Ap fluorescence and expand the information that can be obtained from experiments.

Progressive acylation of pyrene engineers solid state packing and colour via C-H···H-C, C-H···O and π-π interactions.

Quantum theory of atoms-in-molecules and Hirshfeld surface analyses indicated an increase in the extent of (i) C-H···H-C; (ii) C-H···O, (iii) π-π interactions and a decrease in the extent of (i) σ-π interaction, (ii) an interplanar angle between the vicinal pyrene units in a series of acetylpyrene derivatives offering blue-green-orange emissive crystals.