Vibronic coupling and ultrafast relaxation dynamics in the first five excited singlet electronic states of bithiophene

The vibronic structure and nuclear dynamics in the first five excited singlet electronic states of bithiophene (2T) are investigated here. Specifically, considerations are given to comprehend the first two structureless and broad electronic absorption bands and the role of nonadiabatic coupling in the excited state relaxation mechanism of 2T in the gas phase. Associated potential energy surfaces (PESs) are established by constructing a model vibronic coupling Hamiltonian using 18 vibrational degrees of freedom and extensive ab initio electronic structure calculations. The topographies of these PESs are critically examined, and multiple conical intersections are established. The nuclear dynamics calculations are performed by propagating wave packets on the coupled electronic manifold. The present theoretical results are in good agreement with the experimental observations. It is found that strong nonadiabatic coupling between the S1-S4 and S1-S5 states along totally symmetric modes is predominantly responsible for the structureless and broad first absorption band, and overlapping S2, S3, S4, and S5 states form the second absorption band. Photorelaxation from the highly excited S5 to the lowest S1 state takes place through a cascade of diabatic population transfers among the S1-S4-S5 electronic manifold within the first ∼100 fs. Totally symmetric C=C stretching, C-S stretching, C-H wagging, ring puckering, and inter-ring bending modes collectively drive such relaxation dynamics.

Evidence of Excited-State Vibrational Mode Governing the Photorelaxation of a Charge-Transfer Complex

Modern, nonlinear, time-resolved spectroscopic techniques have opened new doors for investigating the intriguing but complex world of photoinduced ultrafast out-of-equilibrium phenomena and charge dynamics. The interaction between light and matter introduces an additional dimension, where the complex interplay between electronic and vibrational dynamics needs the most advanced theoretical-computational protocols to be fully understood on the molecular scale. In this study, we showcase the capabilities of ab initio molecular dynamics simulation integrated with a multiresolution wavelet protocol to carefully investigate the excited-state relaxation dynamics in a noncovalent complex involving tetramethylbenzene (TMB) and tetracyanoquinodimethane (TCNQ) undergoing charge transfer (CT) upon photoexcitation. Our protocol provides an accurate description that facilitates a direct comparison between transient vibrational analysis and time-resolved spectroscopic signals. This molecular level perspective enhances our understanding of photorelaxation processes confined in the adiabatic regime and offers an improved interpretation of vibrational spectra. Furthermore, it enables the quantification of anharmonic vibrational couplings between high- and low-frequency modes, specifically the TCNQ "rocking" and "bending" modes. Additionally, it identifies the primary vibrational mode that governs the adiabaticity between the ground state and the CT state. This comprehensive understanding of photorelaxation processes holds significant importance in the rational design and precise control of more efficient photovoltaic and sensor devices.

Density Functional Calculation and Evaluation of the Spectroscopic Properties and Luminescent Material Application Potential of the N-Heterocyclic Platinum(II) Tetracarbene Complexes

A series of reported Pt(II) carbene complexes possibly have the ability to serve as the new generation of blue emitters in luminescent devices because of their narrow emission spectra, high photoluminescence quantum yields (PLQYs), and rigid molecular skeleton. However, the combination of all carbene ligands with different multidentate structures will affect the overall planarity and horizontal dipole ratio to varying degrees, but the specific extent of this effect has not previously been analyzed in detail. In this work, density functional computation is used to study a class of platinum tetracarbene bidentate complexes with similar absorption and emission band characteristics, which is the main reason for the remarkable difference in quantum efficiency due to subtle differences in electronic states caused by different ligands. From the calculation results, the major reason, which results in significantly decrease in quantum efficiency for [Pt(cyim)2]2+, is that [Pt(cyim)2]2+ can reach the non-radiative deactivation metal-centered d-d excited state through an easier pathway compared with [Pt(meim)2]2+. The result, based on changes in the dihedral angle between ligands, can achieve the goal of improving and designing materials by adjusting the degree of the dihedral angle. (meim: bis(1,1'-dimethyl-3,3'-methylene-diimidazoline-2,2'-diylidene); cyim: bis(1,1'-dicyclohexyl-3,3'-methylene-diimidazoline-2,2'-diylidene).

Exciton Bimolecular Annihilation Dynamics in Push-Pull Semiconductor Polymers

Exciton-exciton annihilation is a ubiquitous nonlinear dynamic phenomenon in materials hosting Frenkel excitons. In this work, we investigate the nonlinear exciton dynamics of an electron push-pull conjugated polymer by fluence-dependent transient absorption and excitation-correlation photoluminescence spectroscopy, where we can quantitatively show the latter to be a more selective probe of the nonlinear dynamics. Simulations based on a time-independent exciton annihilation model show a decreasing trend for the extracted annihilation rates with excitation fluence. Further investigation of the fluence-dependent transients suggests that the exciton-exciton annihilation bimolecular rates are not constant in time, displaying a t-1/2 time dependence, which we rationalize as reflective of one-dimensional exciton diffusion, with a diffusion length estimated to be 9 ± 2 nm. In addition, exciton annihilation gives rise to a long-lived species that recombines on a nanosecond time scale. Our conclusions shed broad light onto nonlinear exciton dynamics in push-pull conjugated polymers.

Strong Bathochromic Shift of Conjugated Polymer Nanowires Assembled with a Liquid Crystalline Alkyl Benzoic Acid via a Film Dispersion Process

We present aqueous dispersions of conjugated polymer nanowires (CPNWs) with improved light absorption properties aimed at aqueous-based applications. We assembled films of a donor-acceptor-type conjugated polymer and liquid crystalline 4-n-octylbenzoic acid by removing a cosolvent of their mixture solutions, followed by annealing of the films, and then formed aqueous-dispersed CPNWs with an aspect ratio >1000 by dispersing the films under ultrasonication at a basic pH. X-ray and spectroscopy studies showed that the polymer and liquid crystal molecules form independent domains in film assemblies and highly organized layer structures in CPNWs. Our ordered molecular assemblies in films and aqueous dispersions of CPNWs open up a new route to fabricate nanowires of low-band-gap linear conjugated polymers with the absorption maximum at 794 nm remarkably red-shifted from 666 nm of CPNWs prepared by an emulsion process. Our results suggest the presence of semicrystalline polymorphs β1 and β2 phases in CPNWs due to long-range π-π stacking of conjugated backbones in compactly organized lamellar structures. The resulting delocalization with a reduced energy bang gap should be beneficial for enhancing charge transfer and energy-conversion efficiencies in aqueous-based applications such as photocatalysis.

Identification of a bridge-specific intramolecular exciton dissociation pathway in donor-π-acceptor alternating conjugated polymers

Intramolecular exciton dissociation is critical for high efficient mobile charge carrier generations in organic solar cells. Yet despite much attention, the effects of π bridges on exciton dissociation dynamics in donor-π-acceptor (D-π-A) alternating conjugated polymers remain still unclear. Here, using a combination of femtosecond time-resolved transient absorption (TA) spectroscopy and steady-state spectroscopy, we track ultrafast intramolecular exciton relaxation dynamics in three D-π-A alternating conjugated polymers which were synthesized by Qin's group and named HSD-A, HSD-B, HSD-C. It is found that the addition of thiophene unit as π bridges will lead to the red shift of steady-state absorption spectrum. Importantly, we reveal the existence of a new intramolecular exciton dissociation pathway mediated by a bridge-specific charge transfer (CT') state with the TA fingerprint peak at 1200 nm in π-bridged HSD-B and HSD-C. This CT' state results in higher electron capture rates for HSD-B and HSD-C as compared to HSD-A. Depending on the proportion of CT' state and nongeminate recombination are important step for the understanding of high power conversion efficiencies in HSD-B than in HSD-C. We propose that this bridge-specific exciton dissociation pathway plays an important role in ultrafast intramolecular exciton dissociation of organic photovoltaic material D-π-A alternating conjugated polymers.

Ultrastable and Biofunctionalizable Conjugated Polymer Nanoparticles with Encapsulated Iron for Ferroptosis Assisted Chemodynamic Therapy

We report the development of novel tumor-targeted conjugated polymer nanoparticles (CPNPs) carrying iron for chemodynamic therapy (CDT). Tumor cell killing proceeds through ferroptosis, a reactive oxygen species (ROS) mechanism that is not dependent on external activation by, for example, light, as is the case in photodynamic therapy (PDT). The ferroptosis mechanism is also not heavily reliant on oxygen availability and is, therefore, promising for the treatment of hypoxic tumors. In this work, we apply this development to the case study of melanoma, a difficult to treat cancer in advanced stages due to resistance to chemotherapy. The iron-carrying CPNPs reported here are targeted to endothelin-B receptors (EDNRB) through endothelin-3 surface moieties (EDN3-CPNPs). Our results show excellent targeting to tumor cells that overexpress EDNRB, specifically for melanoma and bladder tumor cells. In these cases, efficient cell killing, over 80% at higher doses, was found. Conversely, tumor cells not targeted by the EDN3-CPNPs show little effects of CDT, with tumor cell death under 20% in most cases. The outcomes of our work demonstrate that EDN3-CPNPs enable ferroptosis-assisted CDT and present a new therapeutic avenue for tumor treatment.

E to Z Photoisomerization of Phytochrome Cph1Δ Exceeds the Born-Oppenheimer Adiabatic Limit

The Born-Oppenheimer adiabatic limit applies broadly in chemistry because most reactions occur on the ground electronic state. Photochemical reactions involve two or more electronic states and need not be subject to this adiabatic limit. The spectroscopic signatures of nonadiabatic processes are subtle, and therefore, experimental investigations have been limited to the few systems dominated by single photochemical outcomes. Systems with branched excited-state pathways have been neglected, despite their potential to reveal insights into photochemical reactivity. Here we present experimental evidence from coherent three-dimensional electronic spectroscopy that the E to Z photoisomerization of phytochrome Cph1 is strongly nonadiabatic, and the simulations reproduce the measured features only when the photoisomerization proceeds nonadiabatically near, but not through, a conical intersection. The results broaden the general understanding of photoisomerization mechanisms and motivate future studies of nonadiabatic processes with multiple outcomes arising from branching on excited-state potential energy surfaces.

Morphological Contributions to Interfacial Charge Trapping and Nongeminate Recombination in Polymer Solar Cells Revealed by UV Light Soaking

Nongeminate charge recombination occurs over a broad range of time scales in polymer solar cells and represents a serious loss channel for the performance and lifetime of devices. Multiple factors influence this process, including changes in morphology and formation of permanent defects, but individual contributions are often difficult to resolve from conventional experiments. We use intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) to investigate nongeminate charge recombination in blends of poly[2,6-(4,4-bis-(2-ethylhexyl)-4 H-cyclopenta [2,1- b;3,4- b']dithiophene)- alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) solar cells. PCPDTBT/PCBM devices are exposed to varying doses of UV light resonant with PCBM to induce small perturbations in the thin film morphology, namely local heating. IMPS/IMVS sweeps display signatures unique to degradation, that is, photocurrent and photovoltage leading the excitation light modulation appearing as positive phase shifts or 1st quadrant features in Bode and Nyquist representations, respectively. We assign this component to interface charging at purified PCPDTBT/PCBM phase boundaries that trap mobile charges and facilitate nongeminate recombination. Time- and frequency-domain drift-diffusion simulations are then used to model the perturbed photocurrent responses that show good agreement with experiments. Trap occupancies and their impact of photocurrent production are investigated using variable background (dc) excitation light intensities revealing increases of the 1st quadrant component in devices irradiated for longer times. No evidence of chemical degradation was observed from molecular spectroscopy and imaging experiments, and we conclude that morphological changes are chiefly responsible for larger nongeminate charge recombination yields as devices age. Lastly, we propose that the 1st quadrant IMPS/IMVS is a universal signature of morphology-related degradation, although its relative contribution may vary between material systems.

Theoretical Modeling of Singlet Fission

Singlet fission is a photophysical reaction in which a singlet excited electronic state splits into two spin-triplet states. Singlet fission was discovered more than 50 years ago, but the interest in this process has gained a lot of momentum in the past decade due to its potential as a way to boost solar cell efficiencies. This review presents and discusses the most recent advances with respect to the theoretical and computational studies on the singlet fission phenomenon. The work revisits important aspects regarding electronic states involved in the process, the evaluation of fission rates and interstate couplings, the study of the excited state dynamics in singlet fission, and the advances in the design and characterization of singlet fission compounds and materials such as molecular dimers, polymers, or extended structures. Finally, the review tries to pinpoint some aspects that need further improvement and proposes future lines of research for theoretical and computational chemists and physicists in order to further push the understanding and applicability of singlet fission.

Theoretical Studies of Singlet Fission: Searching for Materials and Exploring Mechanisms

In this Review article, a survey is given for theoretical studies in the subject of singlet fission. Singlet fission converts one singlet exciton to two triplet excitons. With the doubled number of excitons and the longer lifetime of the triplets, singlet fission provides an avenue to improve the photoelectric conversion efficiency in organic photovoltaic devices. It has been a subject of intense research in the past decade. Theoretical studies play an essential role in understanding singlet fission. This article presents a Review of theoretical studies in singlet fission since 2006, the year when the research interest in this subject was reignited. Both electronic structure and dynamics studies are covered. Electronic structure studies provide guidelines for designing singlet fission chromophores and insights into the couplings between single- and multi-excitonic states. The latter provides fundamental knowledge for engineering interchromophore conformations to enhance the fission efficiency. Dynamics studies reveal the importance of vibronic couplings in singlet fission.

Ultrafast bridge planarization in donor-π-acceptor copolymers drives intramolecular charge transfer

Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.

Hot and Cold Charge-Transfer Mechanisms in Organic Photovoltaics: Insights into the Excited States of Donor/Acceptor Interfaces

The evolution of the excited-state manifold in organic D/A aggregates (e.g., the prototypical P3HT/PCBM) is investigated through a bottom-up approach via first-principles calculations. We show how the excited-state energies, the charge transfer (CT) states, and the electron-hole density distributions are strongly influenced by the size, the orientation, and the position (i.e., on-top versus on-edge phases) of P3HT/PCBM domains. We discuss how the structural order influences the excited-state electronic structure, providing an atomistic interpretation of the photophysics of organic blends. We show how the simultaneous presence of on-top and on-edge phases does not alter the optical absorption spectrum of the blend but does affect the photophysics. Photovoltaic processes such as (i) the simultaneous charge generation obtained from hot and cold excitations, (ii) the instantaneous and delayed charge separation, and (iii) the pump-push-probe charge generation can be interpreted based on our study.

Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad

Understanding electronic energy transfer (EET) is an important ingredient in the development of artificial photosynthetic systems and photovoltaic technologies. Although EET is at the heart of these applications and crucially influences their light-harvesting efficiency, the nature of EET over short distances for covalently bound donor and acceptor units is often not well understood. Here we investigate EET in an orthogonal molecular dyad (BODT4), in which simple models fail to explain the very origin of EET. On the basis of nonadiabatic ab initio molecular dynamics calculations and ultrafast fluorescence experiments, we gain detailed microscopic insights into the ultrafast electrovibrational dynamics following photoexcitation. Our analysis offers molecular-level insights into these processes and reveals that it takes place on time scales ≲100 fs and occurs through an intermediate charge-transfer state.

Electronic structure dynamics in a low bandgap polymer studied by time-resolved photoelectron spectroscopy

The excited electronic structure of PCPDTBT was measured by combining a high harmonic generation source and a time-of-flight spectrometer.

Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have a stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.

Modeling ultrafast exciton deactivation in oligothiophenes via nonadiabatic dynamics

Nonadiabatic excited-state dynamics reveal the exciton relaxation processes in oligothiophenes. Ultrafast deactivation and exciton localization are predicted to occur within 200 fs, involving bond stretching, ring puckering, and torsional oscillations.

The critical role of interfacial dynamics in the stability of organic photovoltaic devices

By combining optical and magnetic spectroscopy we found that the long-lived emissive interfacial CT state is responsible for the generation of reactive and harmful species, affecting the robustness of the photovoltaic active layer.

Panchromatic "Dye-Doped" Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response

There has been phenomenal effort synthesizing new low-band gap polymer hole-conductors which absorb into the near-infrared (NIR), leading to >10% efficient all-organic solar cells. However, organic light absorbers have relatively narrow bandwidths, making it challenging to obtain panchromatic absorption in a single organic semiconductor. Here, we demonstrate that (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)] (PCPDTBT) can be "photo-sensitized" across the whole visible spectrum by "doping" with a visible absorbing dye, the (2,2,7,7-tetrakis(3-hexyl-5-(7-(4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-9,9-spirobifluorene) (spiro-TBT). Through a comprehensive sub-12 femtosecond-nanosecond spectroscopic study, we demonstrate that extremely efficient and fast energy transfer occurs from the photoexcited spiro-TBT to the PCPDTBT, and ultrafast charge injection happens when the system is interfaced with ZnO as a prototypal electron-acceptor compound. The visible photosensitization can be effectively exploited and gives panchromatic photoresponse in prototype polymer/oxide bilayer photovoltaic diodes. This concept can be successfully adopted for tuning and optimizing the light absorption and photoresponse in a broad range of polymeric and hybrid solar cells.

Hot exciton dissociation in polymer solar cells

The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC(60)BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200 fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.

Interfacial States in Donor-Acceptor Organic Heterojunctions: Computational Insights into Thiophene-Oligomer/Fullerene Junctions

Donor-acceptor heterojunctions composed of thiophene oligomers and C60 fullerene were investigated with computational methods. Benchmark calculations were performed with time-dependent density functional theory. The effects of varying the density functional, the number of oligomers, the intermolecular distance, the medium polarization, and the chemical functionalization of the monomers were analyzed. The results are presented in terms of diagrams where the electronic states are classified as locally excited states, charge-transfer states, and delocalized states. The effects of each option for computational simulations of realistic heterojunctions employed in photovoltaic devices are evaluated and discussed.