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While the photophysics of closed-shell organic molecules is well established, much less is known about open-shell systems containing interacting radical pairs. In this work, we investigate the ultrafast excited state dynamics of a singlet verdazyl diradical system in solution using transient absorption (TA) spectroscopy for the first time. Following 510 nm excitation of the excitonic S0 → S1 transition, we detected TA signals in the 530–950 nm region from the S1 population that decayed exponentially within a few picoseconds to form a vibrationally hot S0* population via internal conversion. The dependence of the S1 decay rate on solvent and radical–radical distance revealed that the excited state possesses charge-transfer character and likely accesses the S0 state via torsional motion. The ultrafast internal conversion decay mechanism at play in our open-shell verdazyl diradicals is in stark contrast with other closed-shell, carbonyl-containing organic chromophores, which exhibit ultrafast intersystem crossing to produce long-lived triplet states as the major S1 decay pathway. 

Abstract Conjugated polymers (CPs) play a central role in electronic applications due to their easily tuned electronic and ionic conductivities via chemical or electrochemical doping. Although doping improves charge conduction by introducing high densities of carriers into the CP, the accompanying structural changes and their impact on carrier mobility remain elusive. Methods capable of probing carrier distributions and their dependence on polymer morphology are needed to better understand how to improve conductivity. Here, a transient absorption (TA) spectroscopy approach is demonstrated, capable of directly probing mobile and trapped carriers in doped CPs and that is also sensitive to polymer nanostructure by using a model polythiophene system with tuned crystallinity. Exciting polarons in the polymer films produces distinct photoinduced absorption signals in the near‐infrared spectrum that decay during the picosecond timescale in the form of biphasic, stretched exponential kinetics, which reflect a distribution of mobile (free) and trapped polarons. The kinetic analysis provides evidence for mobile polarons irrespective of polymer film crystallinity, whereas polarons located in impure amorphous phases with reduced chain ordering exist within a deeper distribution of trap states. Altogether, these observations suggest a stronger correlation of carrier trapping with local chain ordering (planarity or aggregation) rather than polymer crystallinity. 

Two-dimensional infrared (2D IR) spectroscopy, infrared pump–infrared probe spectroscopy, and density functional theory calculations were used to study vibrational relaxation by ring and carbonyl stretching modes in a series of methylated xanthine derivatives in acetonitrile and deuterium oxide (heavy water). Isotropic signals from the excited symmetric and asymmetric carbonyl stretch modes decay biexponentially in both solvents. Coherent energy transfer between the symmetric and asymmetric carbonyl stretching modes gives rise to a quantum beat in the time-dependent anisotropy signals. The damping time of the coherent oscillation agrees with the fast decay component of the carbonyl bleach recovery signals, indicating that this time constant reflects intramolecular vibrational redistribution (IVR) to other solute modes. Despite their similar frequencies, the excited ring modes decay monoexponentially with a time constant that matches the slow decay component of the carbonyl modes. The slow decay times, which are faster in heavy water than in acetonitrile, approximately match the ones observed in previous UV pump–IR probe measurements on the same compounds. The slow component is assigned to intermolecular energy transfer to solvent bath modes from low-frequency solute modes, which are populated by IVR and are anharmonically coupled to the carbonyl and ring stretch modes. 2D IR measurements indicate that the carbonyl stretching modes are weakly coupled to the delocalized ring modes, resulting in slow exchange that cannot explain the common solvent-dependence. IVR is suggested to occur at different rates for the carbonyl vs ring modes due to differences in mode-specific couplings and not to differences in the density of accessible states. 

Abstract Eumelanin is a brown-black biological pigment with sunscreen and radical scavenging functions important to numerous organisms. Eumelanin is also a promising redox-active material for energy conversion and storage, but the chemical structures present in this heterogeneous pigment remain unknown, limiting understanding of the properties of its light-responsive subunits. Here, we introduce an ultrafast vibrational fingerprinting approach for probing the structure and interactions of chromophores in heterogeneous materials like eumelanin. Specifically, transient vibrational spectra in the double-bond stretching region are recorded for subsets of electronic chromophores photoselected by an ultrafast excitation pulse tuned through the UV-visible spectrum. All subsets show a common vibrational fingerprint, indicating that the diverse electronic absorbers in eumelanin, regardless of transition energy, contain the same distribution of IR-active functional groups. Aggregation of chromophores diverse in oxidation state is the key structural property underlying the universal, ultrafast deactivation behavior of eumelanin in response to photoexcitation with any wavelength. 

Here, we investigate the photochemistry of a catechol : o-quinone heterodimer as a model system for uncovering the photoprotective roots of eumelanin. 

Abstract Redox is emerging as an alternative modality for bio‐device communication. In contrast to the more familiar ionic electrical modality: (i) redox involves the flow of electrons through oxidation–reduction reactions; (ii) the aqueous medium is an “insulator” to this electron flow since free electrons do not normally exist in water; and (iii) redox states are intrinsically digital (oxidized and reduced). By exploiting these unique features, a catechol‐based molecular memory film is reported. This memory is fabricated by electrochemically grafting catechol to a chitosan–agarose polysaccharide network to generate a redox‐active but non‐conducting matrix. The redox state of the grafted catechol moieties serves as the 2‐state memory. It is shown that these redox states: can be repeatedly switched by diffusible mediators (electron shuttles); can be easily read electrically or optically; are stable for at least 2 h in the absence of energy; are sensitive to biologically relevant oxidizing and reducing contexts; and can be switched enzymatically. This catechol‐based molecular memory film is a simple circuit element for redox linked bioelectronics.