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Singlet fission (SF) is a process by which one excited singlet state yields two triplet states upon close interaction with a ground-state chromophore of the same kind. This photoreaction was first observed in solid state and has important implications in organic photovoltaics. Singlet fission was also reported in concentrated solutions, where the need for diffusion of the reaction partners slows the dynamics. This helps to single out reaction stages and to identify the involved species. In this work, ultrafast transient absorption spectroscopy and time-correlated single photon counting are applied to the concentration-dependent (from 10-1 to 102 mM) photodynamics of a tetrachlorinated phenazinothiadiazole in toluene. Time-resolved emission shows a monoexponential decay, which is constant across the emission band. The corresponding decay rate depends linearly on the concentration of the phenazinothiadiazole. Femtosecond transient absorption demonstrates that a concentration-dependent singlet-to-triplet conversion hides behind the emission decay which is diffusion controlled. Contrary to previous reports on SF in pentacenes and tetracenes, no indication of intermediate states has been found. Efficient, direct and barrierless SF is concluded. The strong enhancement of the triplet yield at increasingly higher concentrations of the thiadiazole indicates very efficient singlet fission with a triplet yield up to 189 ± 5%.

Singlet fission (SF) has the potential to boost solar energy conversion. Research has focused on designing new strategies to tune the electrochemistry, photophysics, and device architecture at the molecular level to improve the efficiency of SF sensitizers. These studies indicate that SF efficiency strongly depends on morphology, packing, and chemical structure. In this work, we use time-resolved spectroscopy to study intramolecular SF in three covalently linked azaarene dimers. Their rigid structure makes them promising model systems to investigate the effect of chemical modification on intramolecular SF without any potential contributions from geometrical factors. Our experimental results along with theoretical calculations show that SF occurs in all three dimers, confirming SF in perpendicularly oriented chromophores with negligible overlapping π-systems. Additionally, a complex branching mechanism is discovered for the evolution of the singlet (S0S1) and the correlated triplet pair 1(T1T1) states. Although chemical modification has only a minor effect on SF rate and generation of the correlated triplet pair, it plays a critical role in the evolution toward the formation of free triplets. Finally, comparison of deaerated and aerated solutions underpins the effect of oxygen in altering the 1(T1T1) dynamics by opening new decay pathways.

Time-resolved spectroscopies have been playing an essential role in the elucidation of the fundamental mechanisms of light-driven processes, particularly in exploring relaxation models for electronically excited molecules. However, the determination of such models from experimentally obtained time-resolved and spectrally resolved data still demands a high degree of intuition, frequently poses numerical challenges, and is often not free from ambiguities. Here, we demonstrate the analysis of time-resolved laser spectroscopy data via a deep learning network to obtain the correct relaxation kinetic model. In its current design, the presented Deep Spectroscopy Kinetic Analysis Network (DeepSKAN) can predict kinetic models (involved states and relaxation pathways) consisting of up to five states, which results in 103 possible different classes, by estimating the probability of occurrence of a given kinetic model class. DeepSKAN was trained with synthetic time-resolved spectra spanning over 4 orders of magnitude in time with a unitless time axis, thereby demonstrating its potential as a universal approach for analyzing data from various time-resolved spectroscopy techniques in different time ranges. By adding the probabilities of each pathway of the top

Generating two long-living low-energy excitations after absorption of a single high-energy photon has stoked interest in singlet fission (SF) to enhance solar energy conversion in photovoltaics. To this end, survival of the triplet states is critical. This process is investigated in diethynylbenzene-linked tetraaza-triisopropylsilylethynyl-pentacene dimers, for which SF is energetically feasible and facilitated by the close distances between the azapentacenes. The

The ultrafast structural changes during the photoinduced isomerization of the retinal-protonated Schiff base (RPSB) is still a poorly understood aspect in the retinal's photochemistry. In this work, we apply pump-degenerate four-wave mixing (pump-DFWM) to all- trans retinal (ATR) and retinal Schiff bases (RSB) to resolve coherent high- and low-frequency vibrational signatures from excited electronic states. We show that the vibrational spectra of excited singlet states in these samples exhibit pronounced differences compared to the relaxed ground state. Pump-DFWM results indicate three major features for ATR and RSB. (i) Excited state vibrational spectra of ATR and RSB consist predominately of low-frequency modes in the energetic range 100-500 cm-1. (ii) Excited state vibrational spectra show distinct differences for excitation in specific regions of electronic transitions of excited state absorption and emission. (iii) Low-frequency modes in ATR and RSB are inducible during the entire lifetime of the excited electronic states. This latter effect points to a transient molecular structure that, following initial relaxation between different excited electronic states, does not change anymore over the lifetime of the finally populated excited electronic state.

We apply pump-degenerate four-wave-mixing (pump-DFWM) for the investigation of the ultrafast internal relaxation of the excited states of lycopene. A unique feature in the pump-DFWM signal, appearing at small temporal delays between the initial pump pulse and the DFWM sequence, provides direct evidence for the participation of an additional excited state located between the S(2) and S(1) states. Our experimental findings are corroborated by a detailed numerical simulation of lycopene's pump-DFWM signal using the Brownian oscillator model. A very fast dynamics directly after excitation of the S(2) state manifests as a component populated with a time constant of about 20 fs and which decays to S(1) with a lifetime of 110 fs. This ultrafast dynamics is discussed under the light of several different models suggested for the relaxation pathway of carotenoids. In this context, we show that the dynamics can be explained in terms of a dark electronic state between the S(2) and S(1) states.