Single-atom substitutions in nucleobases can cause drastic changes in their photophysical response to ultraviolet radiation, in particular their propensity for intersystem crossing. Azabases are formed by replacing an endocyclic carbon atom with a nitrogen atom. 5-Azacytosine (5AC), an aza-derivative of cytosine with substitution at the C5 position of the ring, exists in both keto and enol tautomeric forms in the gas phase. However, most prior investigations have focused exclusively on the keto tautomer. In the present study, time-resolved photoelectron spectroscopy (TRPES) and extended multistate complete active space second-order perturbation theory, with double ξ-basis sets (XMS-CASPT2/cc-pVDZ) are employed to investigate the photophysics of both keto and enol forms of 5AC. The study finds that both tautomers preferentially undergo internal conversion (IC) to the ground state, with a measurable contribution from an intersystem crossing (ISC) pathway only in the keto form.

In an effort to obtain a fundamental understanding of substitution effects on the photodynamic response of the canonical nucleobases, recent spectroscopic studies have focused on derivatives with single atom substitutions. Uracil undergoes ultrafast internal conversion to the ground state facilitated by various ring distortions at the ethylenic bond. In 6-azauracil, site-specific nitrogen substitution at this carbon double-bond restricts access to ethylenic conical intersections that lead back to the ground state. Instead, intersystem crossing into the triplet manifold becomes highly efficient. This study uses time-resolved photoelectron spectroscopy to investigate the photodynamics of 6-azauracil with particular focus on the role of the lowest singlet excited state and hindrance of ethylenic deactivation coordinates in promoting intersystem crossing.

Molecular tautomerism is ubiquitous in nature and plays a crucial role in regulating biological function. In nucleobases, for example, structural tautomerism not only influences base pairing and genetic coding in DNA but also modulates the molecular response to UV irradiation. The photostability of the nucleobases depends on efficient internal conversion and is highly sensitive to structural variations and micro-environmental effects. Among the pyrimidine bases, cytosine exhibits the greatest number of tautomeric forms, offering a rich landscape to explore diverse structural scenarios, while simultaneously posing significant experimental challenges. This study builds on that context by unveiling the gas-phase photophysics of 2-thiocytosine (2TC) from a unique tautomer perspective. Specifically, it elucidates the decay mechanism in the absence of a thiocarbonyl group but under the influence of chalcogen heavy atom substitution. In solution, 2TC exists in its thion form, whose photodynamics are characterized by efficient intersystem crossing to the triplet manifold. Electronic and structural factors associated with the thiocarbonyl group play a crucial role in suppressing internal conversion pathways to the ground state-pathways that are otherwise active in canonical cytosine. This ISC mechanism is tautomer specific and does not apply to the thiol form, which dominates in the gas phase. Time-resolved photoelectron spectroscopy of thiol-2TC reveals ultrafast internal conversion dynamics, alongside the emergence of a long-lived state with nanosecond lifetime. The latter distinguishes the photodynamics from its canonical counterpart, enol cytosine. Ab initio calculations provide detailed insights into the deactivation mechanism of thiol 2TC and clarify the differences on the effect of thionation on both tautomeric forms of cytosine. Finally, we discuss how protonation (and hydrogen bonding) can modulate intersystem crossing in thiobases, with broader implications to other thiocarbonyl-containing compounds.

Understanding the mechanism of charge dynamics in photocatalysts is the key to design and optimize more efficient materials for renewable energy applications. In this study, the charge dynamics of a CuO thin film are unraveled via transient absorption spectroscopy (TAS) on the picosecond to microsecond timescale for three different excitation energies, i.e., above, near, and below the band gap, to explore the role of incoherent broadband light sources. The shape of the ps-TAS spectra changes with the delay time, while that of the ns-TAS spectra is invariant for all the excitation energies. Regardless of the excitations, three time constants, τ1 ∼ 0.34–0.59 ps, τ2 ∼ 162–175 ns, and τ3 ∼ 2.5–3.3 μs, are resolved, indicating the dominating charge dynamics at very different timescales. Based on these observations, the UV–vis absorption spectrum, and previous findings in the literature, a compelling transition energy diagram is proposed. Two conduction bands and two defect (deep and shallow) states dominate the initial photo-induced electron transitions, and a sub-valence band energy state is involved in the subsequent transient absorption. By solving the rate equations for the pump-induced population dynamics and implementing the assumed Lorentzian absorption spectral shape between two energy states, the TAS spectra are modeled which capture the main spectral and time-dependent features for t > 1 ps. By further considering the contributions from free-electron absorption during very early delay times, the modeled spectra reproduce the experimental spectra very well over the entire time range and under different excitation conditions.

Sulfur-substituted nucleobases are DNA and RNA base derivatives that exhibit extremely efficient photoinduced intersystem crossing (ISC) dynamics into the lowest-energy triplet state. The long-lived and reactive triplet states of sulfur-substituted nucleobases are crucial due to their wide range of potential applications in medicine, structural biology, and the development of organic light-emitting diodes (OLEDs) and other emerging technologies. However, a comprehensive understanding of non-negligible wavelength-dependent changes in the internal conversion (IC) and ISC events is still lacking. Here, we study the underlying mechanism using joint experimental gas-phase time-resolved photoelectron spectroscopy (TRPES) and theoretical quantum chemistry methods. We combine 2,4-dithiouracil (2,4-DTU) TRPES experimental data with computational analysis of the different photodecay processes, which are induced by increasing excitation energies along the entire linear absorption (LA) ultraviolet (UV) spectrum. Our results show how the double-thionated uracil (U), i.e., 2,4-DTU, appears as a versatile photoactivatable instrument. Multiple decay processes can be initiated with different ISC rates or triplet-state lifetimes that resemble the distinctive behavior of the singly substituted 2- or 4-thiouracil (2-TU or 4-TU). We obtained a clear partition of the LA spectrum based on the dominant photoinduced process. Our work clarifies the reasons behind the wavelength-dependent changes in the IC, ISC, and triplet-state lifetimes in doubly thionated U, becoming a biological system of utmost importance for wavelength-controlled applications. These mechanistic details and photoproperties are transferable to closely related molecular systems such as thionated thymines.

The ultrafast internal conversion and intersystem crossing dynamics of 2-thiouracil (2TU) and 2-thiothymine (2TT) are studied using time-resolved photoelectron spectroscopy to investigate the effect of methylation on the deactivation mechanism. Like other thiobases, the triplet manifold is populated with high quantum yields via the lowest singlet excited state, which is dark in absorption. This study focuses on the lowest triplet state and the role of two minima, with sulfur-out-of-plane and slightly boat-like geometries, in the intersystem crossing dynamics back to the ground state.

Porphyrins play pivotal roles in many crucial biological processes including photosynthesis. However, there is still a knowledge gap in understanding electronic and excited state implications associated with functionalization of the porphyrin ring system. These effects can have electrochemical and spectroscopic signatures that reveal the complex nature of these somewhat minor substitutions, beyond simple inductive or electronic effect correlations. To obtain a deeper insight into the influences of porphyrin functionalization, four free-base, meso-substituted porphyrins: tetraphenyl porphyrin (TPP), tetra(4-hydroxyphenyl) porphyrin (THPP), tetra(4-carboxyphenyl) porphyrin (TCPP), and tetra(4-nitrophenyl) porphyrin (TNPP), were synthesized, characterized, and investigated. The influence of various substituents, (-hydroxy,-carboxy, and -nitro) in the para position of the meso-substituted phenyl moieties were evaluated by spectroelectrochemical techniques (absorption and fluorescence), femtosecond transient absorption spectroscopy, cyclic and differential pulse voltammetry, ultraviolet photoelectron spectroscopy (UPS), and time-dependent density functional theory (TD-DFT). Spectral features were evaluated for the neutral porphyrins and differences observed among the various porphyrins were further explained using rendered frontier molecular orbitals pertaining to the relevant transitions. Electrochemically generated anionic and cationic porphyrin species indicate similar absorbance spectroscopic signatures attributed to a red-shift in the Soret band. Emissive behavior reveals the emergence of one new fluorescence decay pathway for the ionic porphyrin, distinct from the neutral macrocycle. Femtosecond transient absorption spectroscopy analysis provided further analysis of the implications on the excited-state as a function of the para substituent of the free-base meso-substituted tetraphenyl porphyrins. Herein, we provide an in-depth and comprehensive analysis of the electronic and excited state effects associated with systematically varying the induced dipole at the methine bridge of the free-base porphyrin macrocycle and the spectroscopic signatures related to the neutral, anionic, and cationic species of these porphyrins.

The ultrafast dynamics of photo-OxaDiBenzocycloOctyne (photo-ODIBO) photo-dissociation was studied using femtosecond transient absorption spectroscopy. Steady-state UV-Vis, time-dependent density functional theory, and 350 nm and 321 nm transient absorption studies are reported. Photo-ODIBO excitation with 321 nm and 350 nm light-induced photodecarbonylation of the cyclopropenone functional group results in the formation of ODIBO. The presence of the photoproduct was confirmed by the results of steady-state photolysis experiments and the observation of absorption signatures of ODIBO in the photo-ODIBO transient absorption spectra. Analysis of the latter revealed the underlying photochemical mechanisms and associated time constants, following excitation of the samples. The dynamics show a multi-exponential decay process, following the dissociation of photo-ODIBO into an excited state of the photoproduct ODIBO within <294 fs after 321 nm excitation. 350 nm excitation, on the other hand, is shown to produce ground state ODIBO via an intermediate species. Additional transient absorption measurements were performed directly on the photoproduct ODIBO to help distinguish spectral signatures associated with these processes.

Photo-oxa-dibenzocyclooctyne (Photo-ODIBO) undergoes photodecarbonylation under UV excitation to its bright S2 state, forming a highly reactive cyclooctyne, ODIBO. Following 321 nm excitation with sub-50 fs actinic pulses, the excited state evolution and cyclopropenone bond cleavage with CO release were characterized using femtosecond stimulated Raman spectroscopy and time-dependent density functional theory Raman calculations. Analysis of the photo-ODIBO S2 CO Raman band revealed multi-exponential intensity, peak splitting and frequency-shift dynamics. This suggests a stepwise cleavage of the two C-C bonds in the cyclopropenone structure that is completed within <300 fs after excitation. Evidence of intramolecular vibrational relaxation on the S2 state, concurrent with photodecarbonylation, with dynamics matching previous electronic transient absorption spectroscopy, was also observed. This confirms an excited state, as opposed to ground state, photodecarbonylation mechanism resulting in a vibronically excited photoproduct, ODIBO.

The photophysical properties of 2,4-dithiouracil (2,4-DTU) in the gas phase are studied by time-resolved photoelectron spectroscopy (TRPES) with three different excitation wavelengths in direct extension of previous work on uracil (U), 2-thiouracil (2-TU) and 4-thiouracil (4-TU). Non-radiative deactivation in the canonical nucleobases like uracil mainly occurs via internal conversion (IC) along singlet excited states, although intersystem crossing (ISC) to a long-lived triplet state was confirmed to play a minor role. In thionated uracils, ISC to the triplet state becomes ultrafast and highly efficient with a quantum yield near unity; however, the lifetime of the triplet state is strongly dependent on the position of the sulfur atom. In 2-TU, ISC back to the ground state occurs within a few hundred picoseconds, whereas the population remains trapped in the lowest triplet state in the case of 4-TU. Upon doubling the degree of thionation, ISC remains highly efficient and dominates the photophysics of 2,4-DTU. However, several low-lying excited states contribute to competing IC and ISC pathways and a complex deactivation mechanism, which is evaluated here based on TRPES measurements and discussed in the context of the singly thionated uracils.

Cu2O, CuO, and mixed-phase Cu2O/CuO thin films with different relative compositions were prepared by oxidizing Cu films at temperatures 150-380 degrees C for a time period ranging from 2 to 24 h, and their ultrafast transient absorption spectra have been characterized to understand the carrier dynamics of the heterostructured Cu2O/CuO system. The absorption dynamics of a pure p-type Cu2O sample followed a biexponential decay, with a fast time similar to 0.3 ps and a long life >150 ps, while a pure p-type CuO sample showed triexponential decay dynamics, with three time constants, 0.25, 2.5, and >150 ps. For the mixed-phased Cu2O/CuO thin films, their absorption dynamics all followed the triexponential decay, and the two ultrafast time constants showed strong composition dependence. Possible energy band structures and electron transition processes are proposed to understand both the spectroscopic and dynamics behaviors of these samples.

The photophysics of thionated uracils are investigated using time-resolved photoelectron spectroscopy with emphasis on evaluating differences in intersystem crossing dynamics with respect to substituent position.

Femtosecond time-resolved spectroscopy has been used to study the light-induced bioenergetics in isolated cyanobacterial photosystem I (PSI) reaction centers from Synechocystis sp. PCC 6803 at 77 K. A method was developed to produce optically clear samples at 77 K without the use of cryprotectants, which considerably simplifies the experiments. Relatively intense laser pulses were used for sample excitation. Following 400 nm excitation, predominantly chlorophyll a pigments in PSI with Qy absorption maxima in the 670–685 nm region are excited. This initially excited distribution of pigments transfers energy in ~400 fs to pigments absorbing near 690 nm, and to pigments absorbing near 715 nm. Further equilibration processes occur and are characterized by a 2.8 ps time constant. Following equilibration, energy trapping and formation of the secondary radical pair state, P700A1, occurs with a time constant of 32.5 ps.

The ultraviolet (UV) photophysics of the natural and modified nucleobases can be surprisingly different. In response to UV radiation, the natural pyrimidine nucleobases undergo ultrafast internal conversion back to the ground state, whereas their thiobase analogues, in which an oxygen has been replaced by sulfur, instead display efficient intersystem crossing to the triplet manifold. Here, the effect of the substituent position is investigated with time-resolved photoelectron spectroscopy on 4-thiouracil, which is contrasted to previous work on 2-thiouracil. Although the photophysical pathway of both structural isomers is similar, i.e., leading from the S2 (1ππ∗) state, via S1 (1nπ∗), to the triplet manifold and subsequently back to the ground state, the intersystem crossing dynamics are strongly influenced by the surrounding intramolecular environment of the sulfur atom.

We report time-dependent photoelectron spectra recorded with a single-photon ionization setup and extensive simulations of the same spectra for the excited-state dynamics of 2-thiouracil (2TU) in the gas phase. We find that single-photon ionization produces very similar results as two-photon ionization, showing that the probe process does not have a strong influence on the measured dynamics. The good agreement between the single-photon ionization experiments and the simulations shows that the norms of Dyson orbitals allow for qualitatively describing the ionization probabilities of 2TU. This reasonable performance of Dyson norms is attributed to the particular electronic structure of 2TU, where all important neutral and ionic states involve similar orbital transitions and thus the shape of the Dyson orbitals do not strongly depend on the initial neutral and final ionic state. We argue that similar situations should also occur in other biologically relevant thio-nucleobases, and that the time-resolved photoelectron spectra of these bases could therefore be adequately modeled with the techniques employed here.