Insights into the Complex Photophysics and Photochemistry of the Simplest Nitroaromatic Compound: A CASPT2//CASSCF Study on Nitrobenzene

Nitrobenzene is the simplest nitroaromatic compound and yet is characterized by a challenging and rich photophysics and photochemistry. In the present contribution, the main decay paths undertaken by the system after UV absorption from both the brightest ¹(L_aππ*) and the lowest ¹(n_Aπ*) singlet excited states have been characterized by means of CASPT2//CASSCF computations. The obtained results match with the main photophysical properties experimentally reported: the lack of fluorescence and phosphorescence emission is justified by the presence of accessible conical intersections and intersystem crossing regions between, respectively, the ¹(n_Aπ*) and ³(n_Aπ*) states and the ground state, while the high triplet quantum yield is attributable to the strong coupling between the ¹(n_Aπ*) and ³(π_Oπ*) states along the main decay path of the former. Two not previously reported singlet–triplet crossing regions, termed (T1/S0)_stc‑NO and (T1/S0)_stc‑ep, have been here documented, from which the ground state can decay toward NO and phenoxy radical production and toward the formation of an epoxide ring structure, respectively. A possible mechanism leading to the photoisomerization of the nitro into the nitrite group, believed to be a key step in the photodegradation of nitrobenzene, has been proposed, based on the geometrical deformation recorded along the decay path leading from the ¹(n_Aπ*) state back to the original ground state through a conical intersection characterized by a significant shortening of the carbon–nitrogen bond

Relaxation Mechanisms of 5‑Azacytosine

The photophysics and deactivation pathways of the noncanonical 5-azacytosine nucleobase were studied using the CASPT2//CASSCF protocol. One of the most significant differences with respect to the parent molecule cytosine is the presence of a dark ¹(<i>n</i>_Nπ*) excited state placed energetically below the bright excited state ¹(ππ*) at the Franck–Condon region. The main photoresponse of the system is a presumably efficient radiationless decay back to the original ground state, mediated by two accessible conical intersections involving a population transfer from the ¹(ππ*) and the ¹(<i>n</i>_Nπ*) states to the ground state. Therefore, a minor contribution of the triplet states in the photophysics of the system is expected, despite the presence of a deactivation path leading to the lowest ³(ππ*) triplet state. The global scenario on the photophysics and photochemistry of the 5-azacytosine system gathered on theoretical grounds is consistent with the available experimental data, taking especially into account the low values of the singlet–triplet intersystem crossing and fluorescence quantum yields observed

Photoinduced Formation Mechanism of the Thymine–Thymine (6–4) Adduct

The photoinduced mechanism leading to the formation of the thymine–thymine (6–4) photolesion has been studied by using the CASPT2//CASSCF approach over a dinucleotide model in vacuo. Following light absorption, localization of the excitation on a single thymine leads to fast singlet–triplet crossing that populates the triplet ³(nπ*) state of thymine. This state, displaying an elongated C₄O bond, triggers (6–4) dimer formation by reaction with the C₅C₆ double bond of the adjacent thymine, followed by a second intersystem crossing, which acts as a gate between the excited state of the reactant and the ground state of the photoproduct. The requirement of localized excitation on just one thymine, whose main decay channel (by radiationless repopulation of its ground state) is nonphotochemical, can rationalize the experimentally observed low quantum yield of formation for the thymine–thymine (6–4) adduct

Combined Theoretical and Experimental Study of the Photophysics of Asulam

The photophysics of the neutral molecular form of the herbicide asulam has been described in a joint experimental and theoretical, at the CASPT2 level, study. The unique π → π* aromatic electronic transition (f, ca. 0.5) shows a weak red-shift as the polarity of the solvent is increased, whereas the fluorescence band undergoes larger red-shifts. Solvatochromic data point to higher dipole moment in the excited state than in the ground state (μ_g < μ_e). The observed increase in p<i>K</i>_a in the excited state (p<i>K</i>_a* – p<i>K</i>_a, ca. 3) is consistent with the results of the Kamlet–Abboud–Taft and Catalán et al. multiparametric approaches. Fluorescence quantum yield varies with the solvent, higher in water (ϕ_f = 0.16) and lower in methanol and 1-propanol (approx. 0.02). Room temperature fluorescence lifetime in aqueous solution is (1.0 ± 0.2) ns, whereas the phosphorescence lifetime in glassy EtOH at 77 K and the corresponding quantum yield are (1.1 ± 0.1) s and 0.36, respectively. The lack of mirror image symmetry between modified absorption and fluorescence spectra reflects different nuclear configurations in the absorbing and emitting states. The low value measured for the fluorescence quantum yield is justified by an efficient nonradiative decay channel, related with the presence of an easily accessible conical intersection between the initially populated singlet bright ¹(L_a ππ*) state and the ground state (gs/ππ*)_CI. Along the main decay path of the ¹(L_a ππ*) state the system undergoes an internal conversion process that switches part of the population from the bright ¹(L_a ππ*) to the dark ¹(L_b ππ*) state, which is responsible for the fluorescence. Additionally, singlet–triplet crossing regions have been found, a fact that can explain the phosphorescent emission detected. An intersystem crossing region between the phosphorescent state ³(L_a ππ*) and the ground state has been characterized, which contributes to the nonradiative deactivation of the excitation energy

OpenMolcas: From source code to insight

In this article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with post calculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory and new electronic and muonic basis sets