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.

Energy-dependent timescales in the dissociation of diiodothiophene dication

Photodissociation molecular dynamics of gas-phase 2,5-diiodothiophene molecules was studied in an electron-energy-resolved electron-multi-ion coincidence experiment performed at the FinEstBeAMS beamline of MAX IV synchrotron. Following the photoionization of the iodine 4d subshell and the Auger decay, the dissociation landscape of the molecular dication was investigated as a function of the Auger electron energy. Concentrating on an major dissociation pathway, C₄H₂I₂S²⁺ → C₄H₂S⁺ + I⁺ + I, and accessing the timescales of the process via ion momentum correlation analysis, it was revealed how this three-body process changes depending on the available internal energy. Using a generalized secondary dissociation model, the process was shown to evolve from secondary dissociation regime towards concerted dissociation as the available energy increased, with the secondary dissociation time constant changing from 1.5 ps to 129 fs. The experimental results were compared with simulations using a stochastic charge-hopping molecular mechanics model. It represented the observed trend and also gave a fair quantitative agreement with the experiment.

Accurate Calculation of Excited-State Absorption for Small-to-Medium-Sized Conjugated Oligomers: Multiconfigurational Treatment vs Quadratic Response TD-DFT

Excited-state absorption (ESA) spectra of π-conjugated compounds are frequently calculated by (quadratic response) time-dependent density functional theory, (QR) TD-DFT, often giving a reasonable representation of the experimental results despite the (known) incomplete electronic description. To investigate whether this is inherent to the method, we calculate here the ESA spectra of small-to-medium-sized oligophenylenevinylenes (nPV) and oligothiophenes (nT) using QR TD-DFT as well as CASPT2 based on CASSCF geometries. CASPT2 gives indeed a reliable, theoretically correct description of the ESA features for all compounds; the computational effort can be reduced without significant loss of accuracy using TD-DFT geometries. QR TD-DFT, based on BHandHLYP and CAM-/B3LYP functionals, fails on short nTs but provides a reasonable description for spectral positions of nPVs and long nTs. The failure on short nTs is, however, only partly due to the incomplete configuration description but, in particular, related to an improper MO description, resulting in an asymmetric energy spacing of the occupied vs unoccupied MOs in the DFT scheme. Longer nTs, on the other side, adapt approximately the MO scheme for alternant hydrocarbons just like in nPVs, while contributions by two triplet excitations combined to a singlet (which inhibits an accurate treatment of polyenes with standard TD-DFT) do not play a relevant role in the current case. For such "well-behaved" systems, a reasonable representation of ESA spectra is found at the QR TD-DFT level due to the rather small energy shifts when including higher-order excitations.

On the inclusion of one double within CIS and TDDFT

We present an improved approach for generating a set of optimized frontier orbitals (HOMO and LUMO) that minimizes the energy of one double configuration. We further benchmark the effect of including such a double within a rigorous configuration interaction singles or a parameterized semi-empirical time-dependent density functional theory Hamiltonian for a set of test cases. Although we cannot quite achieve quantitative accuracy, the algorithm is quite robust and routinely delivers an enormous qualitative improvement to standard single-reference electronic structure calculations.

Tailoring CIPSI Expansions for QMC Calculations of Electronic Excitations: The Case Study of Thiophene

The perturbatively selected configuration interaction scheme (CIPSI) is particularly effective in constructing determinantal expansions for quantum Monte Carlo (QMC) simulations with Jastrow–Slater wave functions: fast and smooth convergence of ground-state properties and balanced descriptions of ground and excited states of different symmetries have been reported. In particular, accurate excitation energies have been obtained by the pivotal requirement of using CIPSI expansions with similar second-order perturbation corrections for each state, that is, a similar estimated distance to the full configuration interaction limit. Here, we elaborate on the CIPSI selection criterion for excited states of the same symmetry as the ground state, generating expansions from a common orbital set. Using these expansions in QMC as determinantal components of Jastrow–Slater wave functions, we compute the lowest, bright excited state of thiophene, which is challenging due to its significant multireference character. The resulting vertical excitation energies are within 0.05 eV of the best theoretical estimates, already with expansions of only a few thousand determinants. Furthermore, we relax the ground- and excited-state structures following the corresponding root in variational Monte Carlo and obtain bond lengths that are accurate to better than 0.01 Å. Therefore, while the full treatment at the CIPSI level of this system is quite demanding, in QMC, we can compute high-quality excitation energies and excited-state structural parameters building on affordable CIPSI expansions with relatively few, well-chosen determinants.

Exploring the effects of quantum decoherence on the excited-state dynamics of molecular systems

Fewest-switches surface hopping (FSSH) is employed in order to investigate the nonadiabatic excited-state dynamics of thiophene and related compounds and hence to establish a connection between the electronic system, the critical points in configuration space and the deactivation dynamics. The potential-energy surfaces of the studied molecules were calculated with complete active space self-consistent field and time-dependent density-functional theory. They are analyzed thoroughly to locate and optimize minimum-energy conical intersections, which are essential to the dynamics of the system. The influence of decoherence on the dynamics is examined by employing different decoherence schemes. We find that irrespective of the employed decoherence algorithm, the population dynamics of thiophene give results which are sound with the expectations grounded on the analysis of the potential-energy surface. A more detailed look at single trajectories as well as on the excited-state lifetimes, however, reveals a substantial dependence on how decoherence is accounted for. In order to connect these findings, we describe how ensemble averaging cures some of the overcoherence problems of uncorrected FSSH. Eventually, we identify carbon–sulfur bond cleavage as a common feature accompanying electronic transitions between different states in the simulations of all thiophene-related compounds studied in this work, which is of interest due to their relevance in organic photovoltaics.

The influence of spin-orbit coupling, Duschinsky rotation and displacement vector on the rate of intersystem crossing of benzophenone and its fused analog fluorenone: a time dependent correlation function based approach

To understand the effect of structural rigidity or flexibility on the intersystem crossing rate, herein we have adopted a time dependent correlation function based approach, an appropriate method for a harmonic oscillator under Condon approximation. Following this technique, we have developed generalized codes for calculating the rate of intersystem crossing (ISC) both at 0 K and at finite temperature. Since the rate of ISC is a measurable quantity, we have separated the real and imaginary parts of the complex correlation function carefully and eliminated the imaginary part by exploiting the odd nature of this function. Using this simplified method, we have calculated the ISC rate constant (kISC) of two molecules, namely, benzophenone and its fused analog, fluorenone. The calculations clearly elucidate that kISC of benzophenone is 103 times larger compared to that of fluorenone. Interestingly, our analyses reveal that the combined effect of spin-orbit coupling and the number of normal modes could increase the rate of ISC of benzophenone by three orders in comparison to that of fluorenone. Furthermore, the Duschinsky rotation matrix (J) and displacement vectors (D) could influence the rate of ISC by one order each, indicating that the overall rate of ISC of benzophenone could have been 105 times higher than that of fluorenone if the latter two factors, namely, J and D have practically no impact on the rate of ISC of fluorenone. However, it has been found that albeit J can't alter the rate of ISC of fluorenone, D indeed can change the rate by two orders, thereby keeping the overall ratio of the rate of ISC of benzophenone and fluorenone as 103. The present study elucidates that none of the above mentioned factors alone can explain the relative rate of ISC of the studied systems; rather a complex interplay between all these factors makes the rate of ISC of benzophenone 103 times higher than that of fluorenone.

Optoelectronic Properties of A-π-D-π-A Thiophene-Based Materials with a Dithienosilole Core: An Experimental and Theoretical Study

Two A-π-D-π-A thiophene-based small molecules with a central dithienosilole core and dicyanovinyl (DCV) end groups were synthesized. These compounds differ only by the presence of alkyl and alkylsulfanyl chains, respectively, on the thiophene beta positions. Computational data together with the spectroscopic and electrochemical findings (obtained by means of absorption, steady-state/time-resolved emission techniques, and cyclic voltammetry) revealed that both molecules possess low electronic and optical band gaps, broad absorption spectra, and good stability both in p and n-doping states, which make them suitable for optoelectronic applications. In both compounds, the HOMO-LUMO transition involves an intramolecular charge transfer from the electron-donor dithienosilole unit to the two terminal electron-acceptor DCV groups. A marked positive emission solvatochromism was observed for both molecules and was interpreted on the basis of the symmetry breaking in the S₁ excited state. The two synthesized compounds were also compared to their shorter precursors and to similar oligothiophenes to understand how the nature of the building block influences the characteristics of the final materials. Furthermore, it was possible to better understand the contribution of the sulfur atom in modulating the optical properties of the small molecules studied.

Large Excited-State Conformational Displacements Expedite Triplet Formation in a Small Conjugated Oligomer

Intersystem crossing in conjugated organic molecules is most conveniently viewed from pure electronic perspectives; yet, vibrational displacements may often drive these transitions. We investigate an alkyl-substituted thienylene-vinylene dimer (dTV) displaying efficient triplet formation. Steady-state electronic and Raman spectra display large Stokes shifts (∼4000 cm^-1) involving high-frequency skeletal symmetric stretching modes (∼900-1600 cm^-1) in addition to large displacements of low-frequency torsional motions (∼300-340 cm^-1). Transient absorption spectroscopy reveals the emergence of distorted singlet (S₁) and triplet signatures following initial vibrational relaxation dynamics that dominate spectral dynamics on time scales > 100 ps, with the latter persisting on time scales up to ca. 7 μs. Potential energy surfaces calculated along the dominant displaced out-of-plane torsional mode reveal shallow energy barriers for entering the triplet manifold from S₁. We propose that dTV is a good model system for understanding vibrational contributions to intersystem crossing events in related polymer systems.

Electron-Conducting PbS Nanocrystal Superlattices with Long-Range Order Enabled by Terthiophene Molecular Linkers

PbS nanocrystals are surface-functionalized with the organic semiconductor 5,5″-dithiol-[2,2':5,2″-terthiophene] and assembled to afford hybrid nanostructured thin films with a large structural coherence and an electron mobility of 0.2 cm²/(V s). Electrochemistry, optical spectroscopy, and quantum mechanical calculations are applied to elucidate the electronic structure at the inorganic/organic interface, and it is established that electron injection into the molecule alters its (electronic) structure, which greatly facilitates coupling of the neighboring PbS 1S_e states. This is verified by field-effect and electrochemically gated transport measurements, and evidence is provided that carrier transport occurs predominantly via the 1S_e states. The presented material allows studying structure-transport correlations and exploring transport anisotropies in semiconductor nanocrystal superlattices.

Conformationally controlled ultrafast intersystem crossing in bithiophene systems

Bithiophenes serve as model systems for larger polythiophenes used in solar cell applications and molecular electronics. We report a study of ultrafast dynamics of two bithiophene systems measured with femtosecond time-resolved photoelectron spectroscopy, and show that their intersystem crossing takes place within the first few picoseconds after excitation, in line with previous studies. We show that the intersystem crossing rate can be explained in terms of arguments based on symmetry of the S1 minimum energy geometry, which depends on the specific conformation of bithiophene. Furthermore, this work shows that the minor cis-conformer contributes to an even higher intersystem crossing rate than the major trans conformer. The work presented here can provide guiding principles towards the design of solar cell components with even faster formation of long-lived excited states for solar energy harvesting.

Effect of thiophene substitution on the intersystem crossing of arene photosensitizers

Thiophene substitution gives energy level-matched S₁/T₂ states and the ISC is enhanced, which was not observed with phenyl substitution.

Spectroscopic Study of Thiophene-Pyrrole-Containing S,N-Heteroheptacenes Compared to Acenes and Phenacenes

In this study, we report a detailed spectroscopic study concerning the energy levels and vibrational structure of thiophene-pyrrole-containing S,N-heteroacenes. The aim of the study is first, to understand the differences in the photoluminescence (PL) efficiencies in this structurally similar series and second, to compare the electronic structure of S,N-heteroacenes to that of linear acenes and phenacenes, with a view to derive guidelines for the design of singlet fission materials. For S,N-heteroacenes comprising seven fused heterocyclic rings, we observe a higher PL quantum yield for derivatives with terminal thienothiophene units than for thienopyrrole-capped ones. This is assigned to a stronger tendency of the thienopyrrole-capped derivatives to form nonemissive associates in dilute solution, producing emissive excimers at higher concentration. By conducting time-resolved PL studies at 77 K, we further determine the lowest singlet and triplet energies for the S,N-heteroacenes with three, five, and seven fused rings. We show that their energies evolve with oligomer length analogously to those of phenacenes, yet in a fundamentally different way from that of linear acenes. This difference in evolution is attributed to the increasingly biradical character in acenes with increasing chain length in contrast to the S,N-heteroacenes and phenacenes.

Ab initio molecular dynamics of thiophene: the interplay of internal conversion and intersystem crossing

Ab initio on-the-fly molecular dynamics reveals that excited thiophene decays via low lying conical intersections and via intersystem crossing. Open-ring structures are responsible for the observed long life times.

Ultrafast Raman Spectroscopy as a Probe of Local Structure and Dynamics in Photoexcited Conjugated Materials

An important challenge in the study of conjugated organic materials is to relate the properties of transient states underlying macroscopic material responses directly with intra- and intermolecular structure. We discuss recent efforts using the vibrational sensitivity of time-resolved Raman spectroscopy to interrogate structural properties of transient excited and charge-separated states in conjugated oligomers and polymers in order to relate them to molecular conformations and material microstructures. We focus on recent work with excited-state Raman spectroscopy that provides mode-specific signatures of structural relaxation in oligo- and polythiophenes, examination of structural heterogeneities associated with exciton localization in different structural motifs of amorphous polymers, and interrogation of correlations between microstructure and properties and dynamics of charge-separated states within polymer aggregates. On the basis of these recent efforts, we provide an outlook for further applying this method to elucidate relationships between the structure and properties of transient states and the photoresponses of conjugated materials.

Visualizing Excited-State Dynamics of a Diaryl Thiophene: Femtosecond Stimulated Raman Scattering as a Probe of Conjugated Molecules

Conjugated organic polymers based on substituted thiophene units are versatile building blocks of many photoactive materials, such as photochromic molecular switches or solar energy conversion devices. Unraveling the different processes underlying their photochemistry, such as the evolution on different electronic states and multidimensional structural relaxation, is a challenge critical to defining their function. Using femtosecond stimulated Raman scattering (FSRS) supported by quantum chemical calculations, we visualize the reaction pathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initial wavepacket dynamics of the reaction coordinates occurs within the first ≈1.5 ps, followed by a ≈10 ps thermalization. Subsequent slow opening of the thiophene ring through a cleavage of the carbon-sulfur bond triggers an intersystem crossing to the triplet excited state. Our work demonstrates how a detailed mapping of the excited-state dynamics can be obtained, combining simultaneous structural sensitivity and ultrafast temporal resolution of FSRS with the chemical information provided by time-dependent density functional theory calculations.