Predicting Nonradiative Decay Rate Constants of Cyclometalated Ir(III) Complexes.

The theoretical calculation of the temperature-dependent nonradiative decay rate constant is fundamental for predicting the usefulness of transition-metal complexes for technological applications. Such a computation implies the determination of the barriers separating the emitting triplet state from metal-centered states, which are key mediators of this type of radiationless relaxation. We here do so for the two green-emitting cyclometalated Ir(III) complexes, [Ir(ppy)(pyim)] and [Ir(diFppy)(dtb-bpy)], of general formula [Ir(CN)(NN)], performing DFT calculations with both B3LYP and PBE0 functionals.

Insights into the anticancer photodynamic activity of Ir(III) and Ru(II) polypyridyl complexes bearing β-carboline ligands.

Ir(III) and Ru(II) polypyridyl complexes are promising photosensitizers (PSs) for photodynamic therapy (PDT) due to their outstanding photophysical properties. Herein, one series of cyclometallated Ir(III) complexes and two series of Ru(II) polypyridyl derivatives bearing three different thiazolyl-β-carboline N^N' ligands have been synthesized, aiming to evaluate the impact of the different metal fragments ([Ir(C^N)] or [Ru(N^N)]) and N^N' ligands on the photophysical and biological properties. All the compounds exhibit remarkable photostability under blue-light irradiation and are emissive (605 < λ < 720 nm), with the Ru(II) derivatives displaying higher photoluminescence quantum yields and longer excited state lifetimes.

XMS-CASPT2//XMS-CASPT2 and XMS-CASPT2//CASSCF at comparison: The impact of dynamic correlation in the excited state optimization of nitronaphthalene.

Here, analytical extended multi-state complete active space second-order perturbation method (XMS-CASPT2) gradients are used to rationalize the decreasing triplet quantum yield trend in 2-nitronaphthalene, 1-nitronaphthalene, and 2-methyl-1-nitronaphthalene, a series of nitro-substituted aromatic compounds. Comparison with the XMS-CASPT2//CASSCF (where CASSCF stands for complete active space self-consistent field method) results highlights the importance of dynamic correlation in geometry optimization and challenges the validity of an XMS-CASPT2//CASSCF approach: XMS-CASPT2 S1 minima leads to planar structures, while CASSCF optimizations trigger a pyramidalization of the nitro group. The XMS-CASPT2 results correlate the reported decreasing triplet quantum yield trend in these species to a decrease in S1 to T2 population transfer and an increase in S1-S0 decay, while no such correlation is observed when using XMS-CASPT2//CASSCF data.

Quantum-Chemistry Study of the Photophysical Properties of 4-Thiouracil and Comparisons with 2-Thiouracil.

DNA in living beings is constantly damaged by exogenous and endogenous agents. However, in some cases, DNA photodamage can have interesting applications, as it happens in photodynamic therapy. In this work, the current knowledge on the photophysics of 4-thiouracil has been extended by further quantum-chemistry studies to improve the agreement between theory and experiments, to better understand the differences with 2-thiouracil, and, last but not least, to verify its usefulness as a photosensitizer for photodynamic therapy.

A First Proposal on the Nitrobenzene Photorelease Mechanism of NO and Its Relation to NO Formation through a Roaming Mechanism.

Despite the fact that NO is considered to be the main photoproduct of nitrobenzene photochemistry, no mechanism has ever been proposed to rationalize its formation. NO photorelease is instead a more studied process, probably due to its application in the drug delivery sector and the study of roaming mechanisms. In this contribution, a photoinduced mechanism accounting for the formation of NO in nitrobenzene is theorized based on CASPT2, CASSCF, and DFT electronic structure calculations and CASSCF classical dynamics.

Governing the emissive properties of 4-aminobiphenyl-2-pyrimidine push-pull systems via the restricted torsion of N,N-disubstituted amino groups.

Donor-acceptor-substituted biphenyl derivatives are particularly interesting model compounds, which exhibit intramolecular charge transfer because of the extent of charge transfer between both substituents. The connection of a 4-[1,1'-biphenyl]-4-yl-2-pyrimidinyl) moiety to differently disubstituted amino groups at the biphenyl terminal can offer push-pull compounds with distinctive photophysical properties. Herein, we report a comprehensive study of the influence of the torsion angle of the disubstituted amino group on the emissive properties of two pull-push systems: 4-[4-(4-,-dimethylaminophenyl)phenyl]-2,6-diphenylpyrimidine () and 4-[4-(4-,-diphenylaminophenyl)phenyl]-2,6-diphenylpyrimidine ().

On the importance of equatorial metal-centered excited states in the photophysics of cyclometallated Ir(III) complexes.

In the present contribution, the following three cyclometallated Ir(III) complexes were theoretically investigated using density functional theory calculations to explain their different photophysical properties: [Ir(ppy)(bpy)], where Hppy is 2-phenylpyridine and bpy is 2,2'-bipyridine, [Ir(ppy)(pbpy)], where pbpy is 6-phenyl-2,2'-bipyridine, and [Ir(ppy)(dpbpy)], where dpbpy is 6,6'-diphenyl-2,2'-bipyridine. Despite sharing the same molecular skeleton, with the only difference being the addition of one or two phenyl groups attached to the ancillary bpy ligand, the complexes show different emission quantum yields in CHCl solution (0.196, 0.

On the photorelease of nitric oxide by nitrobenzene derivatives: A CASPT2//CASSCF model.

Nitroaromatic compounds can photorelease nitric oxide after UV absorption. The efficiency of the photoreaction depends on the molecular structure, and two features have been pointed out as particularly important for the yield of the process: the presence of methyl groups at the ortho position with respect to the nitro group and the degree of conjugation of the molecule. In this paper, we provide a theoretical characterization at the CASPT2//CASSCF (complete active space second-order perturbation theory//complete active space self-consistent field) level of theory of the photorelease of NO for four molecules derived from nitrobenzene through the addition of ortho methyl groups and/or the elongation of the conjugation.

On the Importance of Ligand-Centered Excited States in the Emission of Cyclometalated Ir(III) Complexes.

The photophysical behavior of the cyclometalating Ir(III) complexes [Ir(ppy)(bpy)], where Hppy is 2-phenylpyridine and bpy is 2,2'-bipyridine (complex ), and [Ir(diFppy)(dtb-bpy)], where diFppy is 2-(2,4-difluorophenyl)pyridine and dtb-bpy is 4,4'-di--butyl-2,2'-bipyridine (complex ), has been theoretically investigated by performing density functional theory calculations. The two complexes share the same molecular skeleton, complex being derived from complex through the addition of fluoro and -butyl substituents, but present notable differences in their photophysical properties. The remarkable difference in their emission quantum yields (0.

Building a Functionalizable, Potent Chemiluminescent Agent: A Rational Design Study on 6,8-Substituted Luminol Derivatives.

Luminol is a prominent chemiluminescent (CL) agent, finding applications across numerous fields, including forensics, immunoassays, and imaging. Different substitution patterns on the aromatic ring can enhance or decrease its CL efficiency. We herein report a systematic study on the synthesis and photophysics of all possible 6,8-disubstituted luminol derivatives bearing H, Ph, and/or Me substituents.

On the chemiluminescence emission of luminol: protic and aprotic solvents and encapsulation to improve the properties in aqueous solution.

Luminol is a popular molecule that is currently gaining further interest due to its potential role for non-invasive cancer treatments. Design of more efficient derivatives in this context would benefit from a clear knowledge on the origin of the distinct intensity and spectroscopic properties in protic and aprotic solvents observed experimentally, which are still not rationalized. By efficiently combining molecular dynamics, quantum methodologies based on density functional theory and multiconfigurational quantum chemistry and hybrid approaches, and developing herein a computational approach for accurately determining "molar negative extinction (or gain) coefficients of emission", we firstly demonstrate that the amino and imino forms of the 3-aminophthalate dianion are responsible for the chemiluminescence in protic and aprotic medium, respectively.

Regiochemical memory in the adiabatic photolysis of thymine-derived oxetanes. A combined ultrafast spectroscopic and CASSCF/CASPT2 computational study.

The photoinduced cycloreversion of oxetanes has been thoroughly investigated in connection with the photorepair of the well-known DNA (6-4) photoproducts. In the present work, the direct photolysis of the two regioisomers arising from the irradiation of benzophenone (BP) and 1,3-dimethylthymine (DMT), namely the head-to-head (HH-1) and head-to-tail (HT-1) oxetane adducts, has been investigated by combining ultrafast spectroscopy and theoretical multiconfigurational quantum chemistry analysis. Both the experimental and computational results agree with the involvement of an excited triplet exciplex 3[BPDMT]* for the photoinduced oxetane cleavage to generate 3BP* and DMT through an adiabatic photochemical reaction.

How important is roaming in the photodegradation of nitrobenzene?

At low excitation energies nitrobenzene photoreleases NO with low translational and rotational energy, while at higher excitation energies NO is photoreleased with both low and high translational and rotational energy. The fast products are formed through a singlet-triplet crossing (STC) region featuring an oxaziridine ring, while a ground state roaming mechanism was suggested to produce the slow molecules. Computing translational and rotational energies performing CASSCF classical dynamics, we here prove how the same oxaziridine STC can account for both fast and slow photoproducts, depending on the region of the seam through which the ground state is populated.

On the Intrinsically Low Quantum Yields of Pyrimidine DNA Photodamages: Evaluating the Reactivity of the Corresponding Minimum Energy Crossing Points.

The low quantum yield of photoformation of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) adducts in DNA bases is usually associated with the presence of more favorable nonreactive decay paths and with the unlikeliness of exciting the system in a favorable conformation. Here, we prove that the ability of the reactive conical intersection to bring the system either back to the absorbing conformation or to the photoproduct must be considered as a fundamental factor in the low quantum yields of the mentioned photodamage. In support of the proposed model, the one order of magnitude difference in the quantum yield of formation of the cyclobutane thymine dimer with respect to the thymine-thymine (6-4) adduct is rationalized here by comparing the reactive ability of the seam of intersections leading respectively to the cyclobutane thymine dimer and the oxetane precursor of the thymine-thymine (6-4) adduct at the CASPT2 level of theory.

Induced Night Vision by Singlet-Oxygen-Mediated Activation of Rhodopsin.

In humans, vision is limited to a small fraction of the whole electromagnetic spectrum. One possible strategy for enhancing vision in deep-red or poor-light conditions consists of recruiting chlorophyll derivatives in the rod photoreceptor cells of the eye, as suggested in the case of some deep-sea fish. Here, we employ all-atom molecular simulations and high-level quantum chemistry calculations to rationalize how chlorin e6 (Ce6), widely used in photodynamic therapy although accompanied by enhanced visual sensitivity, mediates vision in the dark, shining light on a fascinating but largely unknown molecular mechanism.

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.

Pyrene, a Test Case for Deep-Ultraviolet Molecular Photophysics.

We determined the complete relaxation dynamics of pyrene in ethanol from the second bright state, employing experimental and theoretical broadband heterodyne detected transient grating and two-dimensional photon echo (2DPE) spectroscopy, using pulses with duration of 6 fs and covering a spectral range spanning from 250 to 300 nm. Multiple lifetimes are assigned to conical intersections through a cascade of electronic states, eventually leading to a rapid population of the lowest long-living excited state and subsequent slow vibrational cooling. The lineshapes in the 2DPE spectra indicate that the efficiency of the population transfer depends on the kinetic energy deposited into modes required to reach a sloped conical intersection, which mediates the decay to the lowest electronic state.

Similar chemical structures, dissimilar triplet quantum yields: a CASPT2 model rationalizing the trend of triplet quantum yields in nitroaromatic systems.

The photophysics of nitroaromatic compounds is characterized by an ultrafast decay into the triplet manifold and by significant triplet quantum yields. The latter quantity changes drastically depending on the system, as shown for 2-nitronaphthalene, 1-nitronaphthalene, and 2-methyl-1-nitronaphthalene, whose triplet quantum yields have been previously measured to be 0.93 ± 0.

Azole-containing cationic bis-cyclometallated iridium(iii) isocyanide complexes: a theoretical insight into the emission energy and emission efficiency.

Using a density functional theory approach, we explore the emission properties of a family of bis-cyclometallated cationic iridium(iii) complexes of general formula [Ir(C^N)2(CN-tert-Bu)2]+ that have tert-butyl isocyanides as neutral auxiliary ligands. Taking the [Ir(ppy)2(CN-tert-Bu)2]+ complex (Hppy = 2-phenylpyridine) as a reference, the effect of replacing the pyridine ring in the cyclometallating ppy ligand by a five-membered azole ring has been examined. To this end, two series of complexes differing by the nature of the atom (either nitrogen or carbon) linking the azole to the phenyl ring of the cyclometallating ligand have been designed.

Molecular Basis of the Chemiluminescence Mechanism of Luminol.

Light emission from luminol is probably one of the most popular chemiluminescence reactions due to its use in forensic science, and has recently displayed promising applications for the treatment of cancer in deep tissues. The mechanism is, however, very complex and distinct possibilities have been proposed. By efficiently combining DFT and CASPT2 methodologies, the chemiluminescence mechanism has been studied in three steps: 1) luminol oxygenation to generate the chemiluminophore, 2) a chemiexcitation step, and 3) generation of the light emitter.

Photoinduced formation mechanism of the thymine-thymine (6-4) adduct in DNA; a QM(CASPT2//CASSCF):MM(AMBER) study.

The UVB-induced photomechanism leading the carbonyl group of a thymine nucleobase to react with the carbon-carbon double bond of a consecutive thymine nucleobase in a DNA strand to form the thymine-thymine (6-4) photodamage adduct remains poorly understood. Key questions remain unanswered, concerning both the intrinsic features of the photoreaction (such as the contribution (or not) of triplet states, the nature of the involved states and the time-scale of the photoprocess) and the role played by the non-reactive surroundings of the two reactive pyrimidine nucleobases (such as the nature of the flanked nucleobases and the flexibility of the whole DNA molecule). A small number of theoretical studies have been carried out on the title photoreaction, most of which have used reduced model systems of DNA, consequently neglecting potential key parameters for the photoreaction such as the constraints due to the double strain structure and the presence of paired and stacked nucleobases.

Two-dimensional electronic spectroscopy as a tool for tracking molecular conformations in DNA/RNA aggregates.

A computational strategy to simulate two-dimensional electronic spectra (2DES) is introduced, which allows us to analyse ground state dynamics and to sample and measure different conformations attained by flexible molecular systems in solution. An explicit mixed quantum mechanics/molecular mechanics (QM/MM) approach is employed for the evaluation of the necessary electronic excited state energies and transition dipole moments. The method is applied towards a study of the highly flexible water-solvated adenine-adenine monophosphate (ApA), a system featuring two interacting adenine moieties that display various intermolecular arrangements, known to deeply affect their photochemical outcome.

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 (Lππ*) and the lowest (nπ*) 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 (nπ*) and (nπ*) states and the ground state, while the high triplet quantum yield is attributable to the strong coupling between the (nπ*) and (ππ*) states along the main decay path of the former.

On the Simulation of Two-dimensional Electronic Spectroscopy of Indole-containing Peptides.

A benchmark study of low-cost multiconfigurational CASSCF/CASPT2 schemes for computing the electronic structure of indole is presented. This facilitates the simulation of near-ultraviolet (UV) pump visible (VIS) probe (i.e.