Ultrafast photophysics of para-substituted 2,5-bis(arylethynyl) rhodacyclopentadienes: thermally activated intersystem crossing.

2,5-Bis(phenylethynyl) rhodacyclopentadienes (RCPDs), as a type of Rh(iii) complex, exhibit unusually intense fluorescence and slow intersystem crossing (ISC) due to weak metal-ligand interactions. However, details on their ultrafast photophysics and ISC dynamics are limited. In this work, electronic relaxation upon photoexcitation of two substituted RCPDs with two -CO2Me (A-RC-A) or -NMe2/-CO2Me (D-RC-A) end groups are comprehensively investigated using femtosecond transient absorption spectroscopy and theoretical analysis. Upon ultraviolet and visible excitation, dephasing of vibrational coherence, charge transfer, conformation relaxation, and ISC are observed experimentally. By calculating the spin-orbit coupling, reorganization energy, and adiabatic energy gap of plausible ISC channels, semi-classical Marcus theory revealed the dominance of thermally activated ISC (S1 → T2) for both D-RC-A and A-RC-A, while S1 → T1 channels are largely blocked due to high ISC barriers. With weak spin-orbit coupling, such differences in plausible ISC channels are predominately tuned by energetic parameters. Singlet oxygen sensitization studies of A-RC-A provide additional insight into the excited-state behavior of this complex.

Dynamics of reduced perylene bisimide cyclophane redox species by ultrafast spectroelectrochemistry.

Charged molecules play essential roles in many natural and artificial functional processes, ranging from photosynthesis to photovoltaics to chemical reactions and more. It is often difficult to identify the optical dynamic properties of relevant redox species because they cannot be easily prepared, their spectra overlap, or they evolve on a femtosecond timescale. Here, we address these challenges by combining spectroelectrochemistry, ultrafast transient absorption spectroscopy, and suitable data analysis. We illustrate the method with the various redox species of a cyclophane composed of two perylene bisimide subunits. While singular-value decomposition is a well-established tool in the analysis of time-dependent spectra of a single molecular species, we here use it additionally to separate transient maps of individual redox species. This is relevant because at any specific applied electrochemical potential, several redox species coexist in the ensemble, and our procedure allows disentangling their spectroscopic response. In the second step, global analysis is then employed to retrieve the excited-state lifetimes and decay-associated difference spectra. Our approach is generally suitable for unraveling ultrafast dynamics in materials featuring charge-transfer processes.

Time-resolved circular dichroism of excitonic systems: theory and experiment on an exemplary squaraine polymer.

Experimental and theoretical foundations for femtosecond time-resolved circular dichroism (TRCD) spectroscopy of excitonic systems are presented. In this method, the system is pumped with linearly polarized light and the signal is defined as the difference between the transient absorption spectrum probed with left and with right circularly polarized light. We present a new experimental setup with a polarization grating as key element to generate circularly polarized pulses. Herein the positive (negative) first order of the diffracted light is left-(right-)circularly polarized and serves as a probe pulse in a TRCD experiment. The grating is capable of transferring ultrashort broadband pulses ranging from 470 nm to 720 nm into two separate beams with opposite ellipticity. By applying a specific chopping scheme we can switch between left and right circular polarizations and detect transient absorption (TA) and TRCD spectra on a shot-to-shot basis simultaneously. We perform experiments on a squaraine polymer, investigating excitonic dynamics, and we develop a general theory for TRCD experiments of excitonically coupled systems that we then apply to describe the experimental data in this particular example. At a magic angle of 54.7° between the pump-pulse polarization and the propagation direction of the probe pulse, the TRCD and TA signals become particularly simple to analyze, since the orientational average over random orientations of complexes factorizes into that of the interaction with the pump and the probe pulse, and the intrinsic electric quadrupole contributions to the TRCD signal average to zero for isotropic samples. Application of exciton theory to linear absorption and to linear circular dichroism spectra of squaraine polymers reveals the presence of two fractions of polymer conformations, a dominant helical conformation with close interpigment distances that are suggested to lead to short-range contributions to site energy shifts and excitonic couplings of the squaraine molecules, and a fraction of unfolded random coils. Theory demonstrates that TRCD spectra of selectively excited helices can resolve state populations that are practically invisible in TA spectroscopy due to the small dipole strength of these states. A qualitative interpretation of TRCD and TA spectra in the spectral window investigated experimentally is offered. The 1 ps time component found in these spectra is related to the slow part of exciton relaxation obtained between states of the helix in the low-energy half of the exciton manifold. The dominant 140 ps time constant reflects the decay of excited states to the electronic ground state.

Higher-Order Multidimensional and Pump-Probe Spectroscopies.

Transient absorption and coherent two-dimensional spectroscopy are widely established methods for the investigation of ultrafast dynamics in quantum systems. Conventionally, they are interpreted in the framework of perturbation theory at the third order of interaction. Here, we discuss the potential of higher-(than-third-)order pump-probe and multidimensional spectroscopy to provide insight into excited multiparticle states and their dynamics. We focus on recent developments from our group. In particular, we demonstrate how phase cycling can be used in fluorescence-detected two-dimensional spectroscopy to isolate higher-order spectra that provide information about highly excited states such as the correlation of multiexciton states. We discuss coherently detected fifth-order 2D spectroscopy and its power to track exciton diffusion. Finally, we show how to extract higher-order signals even from ordinary pump-probe experiments, providing annihilation-free signals at high excitation densities and insight into multiexciton interactions.

High-order pump-probe and high-order two-dimensional electronic spectroscopy on the example of squaraine oligomers.

Time-resolved spectroscopy is commonly used to study diverse phenomena in chemistry, biology, and physics. Pump-probe experiments and coherent two-dimensional (2D) spectroscopy have resolved site-to-site energy transfer, visualized electronic couplings, and much more. In both techniques, the lowest-order signal, in a perturbative expansion of the polarization, is of third order in the electric field, which we call a one-quantum (1Q) signal because in 2D spectroscopy it oscillates in the coherence time with the excitation frequency. There is also a two-quantum (2Q) signal that oscillates in the coherence time at twice the fundamental frequency and is fifth order in the electric field. We demonstrate that the appearance of the 2Q signal guarantees that the 1Q signal is contaminated by non-negligible fifth-order interactions. We derive an analytical connection between an nQ signal and (2n + 1)th-order contaminations of an rQ (with r < n) signal by studying Feynman diagrams of all contributions. We demonstrate that by performing partial integrations along the excitation axis in 2D spectra, we can obtain clean rQ signals free of higher-order artifacts. We exemplify the technique using optical 2D spectroscopy on squaraine oligomers, showing clean extraction of the third-order signal. We further demonstrate the analytical connection with higher-order pump-probe spectroscopy and compare both techniques experimentally. Our approach demonstrates the full power of higher-order pump-probe and 2D spectroscopy to investigate multi-particle interactions in coupled systems.

Intersystem crossing of perylene bisimide neutral, radical anion, and dianion derivatives compared via ultrafast spectroelectrochemistry.

Perylene bisimides are widely studied due to their various applications. Most research is carried out on neutral molecules but charged species are essential in the context of organic electronics and photovoltaics. In this study, we carry out ultrafast transient absorption spectroelectrochemistry and coherent two-dimensional electronic spectroelectrochemistry on three different neutral perylene bisimide derivatives as well as their singly and doubly reduced species. We compare an unsubstituted, flat parent perylene bisimide with two twisted ones that introduce spin-orbit coupling, leading to enhanced intersystem crossing. The internal conversion from higher excited states to the lowest electronically excited state occurs in the picosecond regime with lifetimes significantly shorter for the charged species compared to the neutral ones. Coherent oscillations of the transients for the flat dianion of parent perylene bisimide indicate the occurrence of Fermi resonance. The corresponding vibrational coupling and the superposition of the participating vibrational modes may lead to an enhanced charge separation and triplet formation.

Excitonic coupling of RNA-templated merocyanine dimer studied by higher-order transient absorption spectroscopy.

We report the synthesis and spectroscopic analysis of RNA containing the barbituric acid merocyanine rBAM2 as a nucleobase surrogate. Incorporation into RNA strands by solid-phase synthesis leads to fluorescence enhancement compared to the free chromophore. In addition, linear absorption studies show the formation of an excitonically coupled H-type dimer in the hybridized duplex. Ultrafast third- and fifth-order transient absorption spectroscopy of this non-fluorescent dimer suggests immediate (sub-200 fs) exciton transfer and annihilation due to the proximity of the rBAM2 units.

femtoPro: virtual-reality interactive training simulator of an ultrafast laser laboratory.

The huge field of optics and photonics research and development is in constant demand of well-trained experts. However, it is challenging to teach efficiently the setup process of complicated optical experiments due to limited hardware availability and eye-safety concerns, in particular, in the case of femtosecond lasers. We have developed an interactive simulation of an ultrafast laser laboratory ("femtoPro") for teaching and training, implementing physical models for the calculation and visualization of Gaussian laser beam propagation, ultrashort optical pulses, their modulation by typical optical elements, and linear as well as nonlinear light-matter interaction. This facilitates the setup and simulated measurement procedure, in virtual reality (VR) and at real-time speeds, of various typical optical arrangements and spectroscopy schemes such as telescopes, interferometers, or pulse characterization. femtoPro can be employed to supplement academic teaching in connection with regular courses in optics or spectroscopy, to train future scientists and engineers in the field of (ultrafast) optics in practical skills, to communicate to other researchers how to set up and align a particular experiment, to "test-build" and simulate new designs of optical setups, to simulate ultrafast spectroscopy data, to offer practical exercises to high-school students, and to reach out to the general public.

Single-Molecule Ultrafast Fluorescence-Detected Pump-Probe Microscopy.

We introduce fluorescence-detected pump-probe microscopy by combining a wavelength-tunable ultrafast laser with a confocal scanning fluorescence microscope, enabling access to the femtosecond time scale on the micrometer spatial scale. In addition, we obtain spectral information from Fourier transformation over excitation pulse-pair time delays. We demonstrate this new approach on a model system of a terrylene bisimide (TBI) dye embedded in a PMMA matrix and acquire the linear excitation spectrum as well as time-dependent pump-probe spectra simultaneously. We then push the technique toward single TBI molecules and analyze the statistical distribution of their excitation spectra. Furthermore, we demonstrate the ultrafast transient evolution of several individual molecules, highlighting their different behavior in contrast to the ensemble due to their individual local environment. By correlating the linear and nonlinear spectra, we assess the effect of the molecular environment on the excited-state energy.

Separating single- from multi-particle dynamics in nonlinear spectroscopy.

Quantum states depend on the coordinates of all their constituent particles, with essential multi-particle correlations. Time-resolved laser spectroscopy1 is widely used to probe the energies and dynamics of excited particles and quasiparticles such as electrons and holes2,3, excitons4-6, plasmons7, polaritons8 or phonons9. However, nonlinear signals from single- and multiple-particle excitations are all present simultaneously and cannot be disentangled without a priori knowledge of the system4,10. Here, we show that transient absorption-the most commonly used nonlinear spectroscopy-with N prescribed excitation intensities allows separation of the dynamics into N increasingly nonlinear contributions; in systems well-described by discrete excitations, these N contributions systematically report on zero to N excitations. We obtain clean single-particle dynamics even at high excitation intensities and can systematically increase the number of interacting particles, infer their interaction energies and reconstruct their dynamics, which are not measurable via conventional means. We extract single- and multiple-exciton dynamics in squaraine polymers11,12 and, contrary to common assumption6,13, we find that the excitons, on average, meet several times before annihilating. This surprising ability of excitons to survive encounters is important for efficient organic photovoltaics14,15. As we demonstrate on five diverse systems, our procedure is general, independent of the measured system or type of observed (quasi)particle and straightforward to implement. We envision future applicability in the probing of (quasi)particle interactions in such diverse areas as plasmonics7, Auger recombination2 and exciton correlations in quantum dots5,16,17, singlet fission18, exciton interactions in two-dimensional materials19 and in molecules20,21, carrier multiplication22, multiphonon scattering9 or polariton-polariton interaction8.

Fluorescence-Detected Two-Quantum Photon Echoes via Cogwheel Phase Cycling.

Two-dimensional (2D) electronic spectroscopy can separate homogeneous and inhomogeneous broadening. While established methods usually probe a photon-echo signal, i.e., a third-order response, to access the homogeneous line width of singly excited states, the homogeneous line width of doubly excited states remained spectroscopically inaccessible. Here we demonstrate the acquisition of two-quantum (2Q) photon echoes using fluorescence-detected 2D spectroscopy. In these eighth-order signals, 2Q coherences are rephased with themselves, leading to line-narrowed 2Q-2Q 2D spectra. By using cogwheel phase cycling, adapted from nuclear magnetic resonance spectroscopy, we isolate the 2Q-2Q 2D spectra of a squaraine dimer and a squaraine polymer and verify the same selectivity of cogwheel phase cycling compared to traditional "nested" phase cycling. The observed difference, between the two systems, in the homogeneous line width of the biexciton can be rationalized as a signature of the interplay of exciton-exciton annihilation, exciton diffusion, and exciton delocalization.

Direct comparison of molecular-beam vs liquid-phase pump-probe and two-dimensional spectroscopy on the example of azulene.

Although azulene's anomalous fluorescence originating from S2 rather than from S1 is a textbook example for the violation of Kasha's rule, an understanding of the underlying processes is still a subject of investigation. Here, we use action-based coherent two-dimensional electronic spectroscopy (2DES) to measure a single Liouville-space response pathway from S0 via S1 to the S2 state of azulene. We directly compare this sequential excitation in the liquid phase detecting S2 fluorescence and in a molecular beam detecting photoionized cations, using the S2 anomalous emission to our advantage. We complement the 2DES study with pump-probe measurements of S1 excitation dynamics, including vibrational relaxation and passage through a conical intersection. A direct comparison of the liquid and gas phase allows us to assess the effect of the solvent and the interplay of intra- and intermolecular energy relaxation.

Hybridized Exciton-Photon-Phonon States in a Transition Metal Dichalcogenide van der Waals Heterostructure Microcavity.

Excitons in atomically thin transition-metal dichalcogenides (TMDs) have been established as an attractive platform to explore polaritonic physics, owing to their enormous binding energies and giant oscillator strength. Basic spectral features of exciton polaritons in TMD microcavities, thus far, were conventionally explained via two-coupled-oscillator models. This ignores, however, the impact of phonons on the polariton energy structure. Here we establish and quantify the threefold coupling between excitons, cavity photons, and phonons. For this purpose, we employ energy-momentum-resolved photoluminescence and spatially resolved coherent two-dimensional spectroscopy to investigate the spectral properties of a high-quality-factor microcavity with an embedded WSe_{2} van der Waals heterostructure at room temperature. Our approach reveals a rich multibranch structure which thus far has not been captured in previous experiments. Simulation of the data reveals hybridized exciton-photon-phonon states, providing new physical insight into the exciton polariton system based on layered TMDs.

Substituent-dependent absorption and fluorescence properties of perylene bisimide radical anions and dianions.

Perylene-3,4:9,10-bis(dicarboximides) (PBIs) rank among the most important functional dyes and organic semiconductors, but only recently have their radical anions and dianions attracted interest for a variety of applications. Here, we systematically elucidate the functional properties (redox, absorption, and emission) of five PBI anions and dianions bearing different bay-substituents attached to the chromophore core. Cyclic voltammetry measurements reveal the influence of the substituents ranging from electron-withdrawing cyano to electron-donating phenoxy groups on the oxidation and reduction potentials that relate to the HOMO and LUMO levels ranging from -7.07 eV to -6.05 eV and -5.01 eV to -4.05 eV, respectively. Spectroelectrochemical studies reveal a significant number of intense absorption bands in the NIR-spectral range (750-1400 nm) for the radical anions, whereas the dianionic species are characterized by similar spectra to those for the neutral dyes, however being bathochromically shifted and with increased molar extinction coefficients of approximately 100 000 M-1 cm-1. The increase of the transition dipole moment is up to 56% and accompanied by an almost cyanine-like red-shifted (by 300 nm) absorption spectrum for the most electron-poor tetracyanotetrachloro PBI. Whilst the outstanding fluorescence properties of the neutral PBIs are lost for the radical anions, an appreciable near-infrared (NIR) fluorescence with a quantum yield of up to 18% is revealed for the dianions by utilizing a custom-built flow-cell spectroelectrochemistry setup. Time-dependent density functional theory calculations help to assign the absorption bands to the respective electronic transitions.

Fluorescence-Detected Pump-Probe Spectroscopy.

We introduce a new approach to transient spectroscopy, fluorescence-detected pump-probe (F-PP) spectroscopy, that overcomes several limitations of traditional PP. F-PP suppresses excited-state absorption, provides background-free detection, removes artifacts resulting from pump-pulse scattering, from non-resonant solvent response, or from coherent pulse overlap, and allows unique extraction of excited-state dynamics under certain conditions. Despite incoherent detection, time resolution of F-PP is given by the duration of the laser pulses, independent of the fluorescence lifetime. We describe the working principle of F-PP and provide its theoretical description. Then we illustrate specific features of F-PP by direct comparison with PP, theoretically and experimentally. For this purpose, we investigate, with both techniques, a molecular squaraine heterodimer, core-shell CdSe/ZnS quantum dots, and fluorescent protein mCherry. F-PP is broadly applicable to chemical systems in various environments and in different spectral regimes.

Anisotropy in fifth-order exciton-exciton-interaction two-dimensional spectroscopy.

Exciton-exciton-interaction two-dimensional (EEI2D) spectroscopy is a fifth-order variant of 2D electronic spectroscopy. It can be used to probe biexciton dynamics in molecular systems and to observe exciton diffusion in extended systems such as polymers or light-harvesting complexes. The exciton transport strongly depends on the geometrical and energetic landscape and its perturbations. These can be of both local character, such as molecular orientation and energetic disorder, and long-range character, such as polymer kinks and structural domains. In the present theoretical work, we investigate the anisotropy in EEI2D spectroscopy. We introduce a general approach for how to calculate the anisotropy by using the response-function formalism in an efficient way. In numerical simulations, using a Frenkel exciton model with Redfield-theory dynamics, we demonstrate how the measurement of anisotropy in EEI2D spectroscopy can be used to identify various geometrical effects on exciton transport in dimers and polymers. Investigating a molecular heterodimer as an example, we demonstrate the utility of anisotropy in EEI2D spectroscopy for disentangling dynamic localization and annihilation. We further calculate the annihilation in extended systems such as conjugated polymers. In a polymer, a change in the anisotropy provides a unique signature for exciton transport between differently oriented sections. We analyze three types of geometry variations in polymers: a kink, varying geometric and energetic disorder, and different geometric domains. Our findings underline that employing anisotropy in EEI2D spectroscopy provides a way to distinguish between different geometries and can be used to obtain a better understanding of long-range exciton transport.

Femtosecond dynamics of diphenylpropynylidene in ethanol and dichloromethane.

Carbon chains with an odd number of C atoms are reactive intermediates with a high biradical character. Here we report a joint experimental and computational investigation of the dynamics of diphenylpropynylidene, C6H5-C3-C6H5, in dichloromethane and ethanol. The biradical is generated by ultraviolet light from 1,3-diphenyldiazopropyne. Electron paramagnetic resonance spectra are recorded to elucidate the spin multiplicity and geometry of the biradical. In both solvents a triplet ground state at 4 K is verified. Transient absorption spectra provide insight into the fate of the biradical. A study in deaerated dichloromethane permits us to follow the photophysics of diphenylpropynylidene and to extract time constants for its vibrational as well as electronic relaxation. In the presence of oxygen, a more complex photochemistry is observed that permits us to derive a model for the reaction of the biradical with O2. In ethanol, the spectra recorded in the presence and absence of O2 are very similar, which can be explained by the similarity of the chromophores of the reaction products.

Coherent 2D electronic spectroscopy with complete characterization of excitation pulses during all scanning steps.

Coherent two-dimensional (2D) electronic spectroscopy has become a standard tool in ultrafast science. Thus it is relevant to consider the accuracy of data considering both experimental imperfections and theoretical assumptions about idealized conditions. It is already known that chirped excitation pulses can affect 2D line shapes. In the present work, we demonstrate performance-efficient, automated characterization of the full electric field of each individual multipulse sequence employed during a 2D scanning procedure. Using Fourier-transform spectral interferometry, we analyze how the temporal intensity and phase profile varies from scanning step to scanning step and extract relevant pulse-sequence parameters. This takes into account both random and systematic variations during the scan that may be caused, for example, by femtosecond pulse-shaping artifacts. Using the characterized fields, we simulate and compare 2D spectra obtained with idealized and real shapes obtained from an LCD-based pulse shaper. Exemplarily, we consider fluorescence of a molecular dimer and multiphoton photoemission of a plasmonic nanoslit. The deviations from pulse-shaper artifacts in our specific case do not distort strongly the population-based multidimensional data. The characterization procedure is applicable to other pulses-shaping technologies or excitation geometries, including also pump-probe geometry with multipulse excitation and coherent detection, and allows for accurate consideration of realistic optical excitation fields at all inter-pulse time-delays.

Coherent two-dimensional electronic spectroelectrochemistry.

We report the development of a new spectroscopic scheme, coherent two-dimensional (2D) electronic spectroelectrochemistry. Conventional 2D electronic spectroscopy has become well established to investigate molecular energy transfer, charge transfer, or structural dynamics with femtosecond time resolution following electronic excitation, providing frequency resolution for both the excitation and the detection step. Here we combine this method with electrochemistry in a flow cell. Thus we have established access to the dynamics of various oxidized and reduced molecular species in solution. We investigate the photophysics of a tetraphenoxy-substituted perylene bisimide dye and its reduced species as a proof of principle and find substantially different dynamics for the neutral and the twofold reduced compound. The electrochemical flow cell is furthermore applied in conventional transient absorption spectroscopy and photoluminescence spectroscopies as an application in different setups.