When designing novel chromophores to target specific properties and applications, there is no established procedure for predicting optical and photophysical properties. To facilitate the screening of promising candidates, we attempt at rationalizing the optical and photophysical properties of six quinazoline compounds by examining their S1 excited-state (anti)aromaticity via the nuclear-independent chemical shift (NICS) indices. The quinazolines were selected to achieve white light or specific wavelength emission, as they are insufficiently explored chromophores for materials chemistry applications. We found that the accumulation of charge density in the quinazoline π* levels in the S1 state, without the possibility of transferring it to a suitable electron-acceptor, tends to diminish the fluorescence yields in solution. A larger accumulated charge on the quinazoline core increases its antiaromaticity in the Franck-Condon geometry, thus enhancing the dynamical tendency to relieve it. The NICS indices correlate well with the differences in the interatomic distance matrices of the S1 and S0 minima (Δ(S0,S1)). Thus, they can be used to anticipate the extent of deformation undergone by the quinazoline core during relaxation from the Franck-Condon geometry towards the S1 minimum. The structure-property relationships are discussed for all quinazoline compounds, including three newly synthesized derivatives developed from readily available, similar precursors. In this work, we demonstrate that the synergy of experimental and theoretical approaches is essential for the rational design of new chromophores with desired properties.

Methyl ethers and acetyl esters of N,N-dimethylamino-hydroxymethylbiphenyls were synthesized with varying positions of the amino activating group and the methoxy or acetyl leaving group. Their photochemical reactivity was studied through photohydrolysis reactions to evaluate their potential as photocleavable protecting groups (PPGs) for the release of alcohols and carboxylic acids, respectively. The compounds were designed to investigate the ortho-meta-effects in biphenyl photochemistry. Photoreactivity was examined via irradiations in aqueous media, followed by isolation of photoproducts. The quantum yields of photoelimination (ΦR) showed correlation with the ortho-meta-effects when both functional groups were located on the same aromatic ring, but not when distributed across different rings. To rationalize differences, potential energy surfaces for the CH2-O bond cleavage were analyzed. Mechanistic insights were obtained using computational studies, fluorescence spectroscopy, and laser flash photolysis. The reaction proceeds via a heterolytic cleavage in the excited state, followed by the formation of benzyl radicals (τ = 25-50 μs) upon relaxation to the ground state. These radicals either recombine to form oligomers or undergo further electron transfer in aqueous solution, producing carbocations and acetate. The synthetic applicability was demonstrated through benzoyl and palmitoyl esters, which underwent photochemical cleavage with yields exceeding 70%.

Optical spectroscopy predominantly probes dipole-allowed singlet excitations, limiting access to spin-forbidden and non-dipole transitions. In polyatomic molecules, mixing between valence and Rydberg excitations further complicates spectral interpretation. Here, we combine...

BODIPY photocages (photocleavable protective groups) have stirred interest because they can release biologically active cargo upon visible light excitation. We conducted combined theoretical and experimental investigations on selected BODIPY photocages to elucidate the mechanism of the competing photocleavage at the boron and meso-position. Based on the computations, the former reaction involves elongation of the B-C bond, yielding a tight borenium cation and methyl anion. These ions are intercepted by CH3OH, enabling an efficient proton-coupled electron transfer (PCET) to produce the methane and isolated ether photoproducts. Singlet and triplet excited-state lifetimes were measured in CH3OH and CD3OD to probe the kinetic isotope effects (KIEs). The resulting KIEs are small, implying that the kinetic bottleneck is due to the C-B bond scission rather than the subsequent PCET. The introduction of a methoxy group in the meso-phenoxy substituent redirects the photosubstitution toward the meso-position. The corresponding regiochemistry was explained computationally. On elongating the C-O bonds in the S1 state, it is found that the unproductive conical intersection is encountered much earlier for the alkyl-O bond than for the phenyl-O bond. The current findings are valuable for the rational design of new BODIPY photocages with tailored biological applications.

Doxazosin is a quinazoline derivative widely used in medicine as a drug. In this study, a combined experimental and computational approach based on the time-dependent density functional theory was used to elucidate the primary events following the photoexcitation of DOX upon interaction with light. The photophysical properties and photochemical reactivity of DOX were investigated by steady-state and time-resolved absorption and fluorescence spectroscopy. DOX in H2O in S-0 is present in two prototropic forms, with the protonated form dominating (similar to 91 %, pK(a) = 6.75). The computations indicated that the most basic quinazoline nitrogen is at the position 1. Upon excitation, DOX deprotonates in the singlet excited state (pK(a)* = 1.31), and the decay times from the singlet excited state of 5 ns and 13 ns are attributed to the non-protonated and protonated forms of DOX, respectively. The quantum yield of fluorescence in H2O is 0.51 and 0.64 in basic media. The quantum yield of intersystem crossing along with triplet-triplet molar absorption coefficient at 520 nm and the lifetime of the triplet excited state were obtained by LFP, Phi(ISC) = 0.17, epsilon(520) = 11600 +/- 100 M-1 cm(-1) and tau = 11 mu s, respectively. Furthermore, LFP enabled detection of DOX radical formed by the photoinduced intramolecular electron transfer from the benzodioxane-carbamoyl to the protonated aminoquinazoline. Computations were used to back up the assignments of the detected transients and to construct an energy diagram with all plausible photophysical and photochemical pathways. These results elucidated the mechanisms of DOX photochemistry leading to DOX photodegradation which is relevant to environmental studies. They also provided insights into the potential use of such a quinazoline derivative in other applications such as push-pull chromophores or fluorescent probes.

Studies of persistent organic radical films on conductive metal surfaces can pave the way for diverse applications such as improved spin probes and labels, data control and storage, spintronics, and quantum computing. We grew monolayer films of three nitroxyl radicals (NRs), viz. TEMPO and two carbamoyl-proxyl radicals (nit8 and nit9) under ultra-high vacuum conditions on Au(111) and Cu(111) surfaces. The electronic properties of the films and NR adsorption mechanisms were analyzed by means of X-ray photoelectron (XPS) and absorption (NEXAFS) spectroscopies, with the aid of density functional theory (DFT) and time-dependent DFT computations performed on large unit cells (rev-PBE) and clusters (CAM-B3LYP). We found that all three NRs physisorb weakly on Au. In the case of nit8 and nit9, H-bonded monolayers are formed that recline parallel to the Au surface. Stronger interactions with Cu resulted in chemisorption and robust films, with nit8 and nit9 exhibiting upright orientation due to the amide group acting as an efficient binding anchor. Conversely, TEMPO binds to Cu necessarily via NO which is observed to lead to the destruction of the spin-carrying NO functionality. Computational evidence highlighted the decisive role of Cu surface defects in the partial fragmentation of the CONH2 anchor upon chemisorption of nit8 and nit9. Nitroxide radicals' adsorption mechanisms and film properties tunable by appropriately selecting the substrate.

The interactions of electrons with molecular systems under various conditions are essential to interdisciplinary research fields extending over the fundamental and applied sciences. In particular, investigating electron-induced ionization and dissociation of molecules may shed light on the radiation damage to living cells, the physicochemical processes in interstellar environments, and reaction mechanisms occurring in combustion or plasma. We have, therefore, studied electron-induced ionization and dissociation of the gas phase 3,4-dihydro-2H-pyran (DHP), a cyclic ether appearing to be a viable moiety for developing efficient clinical pharmacokinetics and revealing the mechanisms of biofuel combustion. The mass spectra in the m/z = 10–90 mass range were measured at several different energies of the ionizing electron beam using mass spectrometry. The mass spectra of DHP at the same energies were simulated using on-the-fly semi-classical molecular dynamics (MD) within the framework of the QCxMS formalism. The MD settings were suitably adjusted until a good agreement with the experimental mass spectra intensities was achieved, thus enabling a reliable assignment of cations and unraveling the plausible fragmentation channels. Based on the measurement of the absolute total ionization cross section of DHP (18.1 ± 0.9) × 10−16 cm2 at 100 eV energy, the absolute total and partial ionization cross sections of DHP were determined in the 5–140 eV electron energy. Moreover, a machine learning algorithm that was trained with measured cross sections from 25 different molecules was used to predict the total ionization cross section for DHP. Comparison of the machine learning simulation with the measured data showed acceptable agreement, similar to that achieved in past predictions of the algorithm.

Two BODIPY phenolic esters 1 and 2 were synthesized and their acid-catalyzed hydrolytic and esterase enzymatic stability was demonstrated. Their photophysical properties were investigated in solvents of different po-larity. Their spectral properties are the typical of BODIPY dyes with narrow absorption and emission spectra in the visible spectral region and large molar absorption coefficients (approximate to 500 nm, epsilon = 40000-50000 M-1cm- 1). The derivative with methyl substituted BODIPY core 2 shows quantum yields of fluorescence one magnitude higher (phi f = 0.30-0.55) compared to the BODIPY without methyl groups. Solvents affect very weakly spectral prop-erties, with more pronounced effect of the solvent polarizability than the solvent polarity. Both BODIPY dyes show pH responsive fluorescence in aqueous solution (pKa = 2.72 +/- 0.08 and 4.18 +/- 0.05), with quenching of fluorescence by protonation due to PET from the BODIPY to phenol, rendering them applicable for intracellular pH measurements. The feasibility of the PET in the protonated form of 1 was corroborated by electrochemical measurements and calculation of Delta ETGo for PET, and (time-dependent) density functional theory computations. This less frequently encountered quenching mechanism where the BODIPY is an electron donor provides new opportunities in the rational design of new generations of fluorescent sensors. The applicability for intracellular measurements was demonstrated by confocal fluorescence microscopy on H460 human cancer cell line.

The photochemical reactivity of diphenyldiazomethane 1 and phenyl 1- and 2-adamantyl diazomethanes 2 and 3, respectively, was investigated by transient absorption spectroscopy (TA). Photoelimination of N2 upon UV excitation takes place in the anti-Kasha ultrafast photochemical reaction from the upper excited singlet states to deliver singlet carbenes, which were, in the case of 1 and 2, detected by fs-TA. The reactivity of the carbenes differs with respect to the substituent at the carbene center. The singlet car-1 in a nonpolar solvent delivers the triplet carbene by intersystem crossing (ISC). Singlet car-2 does not undergo ISC but reacts in the intermolecular insertion reactions into C-H bonds. Car-3 has an α-C-H bond next to the carbene center and reacts rapidly in the intramolecular C-H insertion reaction to deliver alkene, precluding its detection by fs-TA. However, the isolation of ketone photoproducts from 3 is highly indicative of triplet car-3's intermediate formation. The TA spectra from the S1-S3 states of 1-3 were computed using time-dependent density functional theory, while the multiconfigurational perturbation theory to the second order was used for the absorption spectra of the corresponding singlet and triplet carbenes. The modeled and measured spectra are in good agreement, and the computations corroborate the assignments of the key short-lived intermediates.

The near-edge X-ray absorption fine structure (NEXAFS) spectra of the gas-phase isoxazole molecule have been measured by collecting total ion yields at the C, N, and O K-edges. The spectral structures have been interpreted using time-dependent density functional theory (TD-DFT) with the short-range corrected SRC2-BLYP exchange-correlation functional. Experimental and calculated energies of core excitations are generally in good agreement, and the nature of observed core-excitation transitions has been elucidated. The experimental C 1s, N 1s, and O 1s core electron binding energies (CEBEs) have additionally been estimated from another yield measurement where the neutral fragments in high-Rydberg (HR) states were ionized by the electric field. For comparison, theoretical CEBEs have been calculated at the ΔM06-2X//mixed basis set level. We have also calculated the vibrationally resolved spectra pertaining to the lowest C 1s and N 1s core-excited roots in the Franck-Condon-Herzberg-Teller (FCHT) approximation. These spectra correlate well with the observed spectral features and have proven useful in resolving certain ambiguities in the assignment of the low-lying C 1s NEXAFS bands.

We have investigated the frontier orbitals of persistent organic radicals known as nitroxyls by resonant photoelectron spectroscopy (ResPES) under inner shell excitation. By means of this site-specific technique, we were able to disentangle the different atomic contributions to the outer valence molecular orbitals and examine several core-hole relaxation pathways involving the singly occupied molecular orbital (SOMO) localized on the nitroxyl group. To interpret the ResPES intensity trends, especially the strong enhancement of the SOMO ionized state at the N K-edge, we computed the Dyson spin orbitals (DSOs) pertaining to the transitions between the core-excited initial states and several of the singly ionized valence final states. We found that the computed vertical valence ionization potentials and norms of the DSOs are reasonably reliable when based on the long-range corrected CAM-B3LYP density functional. Thanks to their unpaired electrons, nitroxyls have recently found application in technological fields implying a spin control, such as spintronics and quantum computing. The present findings on the electronic structure of nitroxyl persistent radicals furnish important hints for their implementation in technological devices and, more in general, for the synthesis of new and stable organic radicals with tailored properties.

We measured and computed the rate constants of the reaction between the α-hydroxyethyl radical (˙CH(CH3)OH) and bromoacetate (BrCH2CO2-) in the non-buffered (NB), as well as in the bicarbonate (HCO3-) and hydrogen phosphate (HPO42-) buffered aqueous solutions in the presence of ethanol. These complex multistep reactions are initiated by the proton-coupled electron transfer (PCET) which reduces BrCH2CO2- and incites its debromination. The PCET is followed by the step in which the resulting carboxymethyl radical propagates a radical chain reaction thus recovering ˙CH(CH3)OH and enhancing the debromination yields. It is found that the rate constants for the initial PCET step (k1) are raised by ca. an order of magnitude in the presence of the buffers (k1(NB) = 1.4 × 105 dm3 mol-1 s-1; k1(HCO3-) = 1.4 × 106 dm3 mol-1 s-1; k1(HPO42-) = 1.1 × 106 dm3 mol-1 s-1). To rationalize this, we used density functional theory at the M06-2X-D3/6-311+G(2d,p) level in conjunction with the polarizable continuum model (PCM) for an implicit description of the aqueous environment. To acceptably reproduce the measured rate constants, the minimal solute, consisting of ˙CH(CH3)OH, BrCH2CO2- and the buffer anion, has to be expanded by at least 2-3 explicit molecules of the water solvent. The used kinetic model consisting of a set of coupled differential equations indicates the sigmoid dependence of yields vs. k1 thereby confirming the autocatalytic trait of these reactions. The computations unravel the profound influence of the presence of buffers on these reaction systems. On the one hand, the buffer anions promote the PCET by accelerating the proton transfer; on the other hand, they slow down the propagation step by forming the strong hydrogen bonds with the carboxymethyl radical. The two opposing effects cancel out and cause the Br- yields to remain approximately comparable in the non-buffered and buffered media.

Compounds containing trivalent boron (TB) as the electron-deficient site(s) find numerous practical uses ranging from Lewis bases in organic synthesis to high-tech industry, with a number of novel applications anticipated. We present an experimental and theoretical study of the gas-phase valence photoionization (VUV-PES), core photoionization (XPS) and photoexcitation (NEXAFS) spectra of a representative TB compound catecholborane (CB). For modelling and assigning the spectra we used the ΔDFT and restricted single excitation space TD-DFT methods for the XPS and NEXAFS, and OVGF and EOM-CCSD for the VUV-PES. The vibrationally resolved structure was computed in the Franck-Condon (FC) and Herzberg-Teller (FCHT) approximations generally resulting in a good agreement with the observed spectral features. For the prediction of core-electron binding energies (CEBEs) several density functionals were tested. The best performance overall was furnished by ωB97X-D suggesting that including the dispersion correction is beneficial. The FCHT vibronic intensities are in clear discrepancy with the B 1s NEXAFS spectrum if the harmonic approximation is used for the B-H wagging mode both in the ground and in the first core-excited state. Instead, a much better agreement is obtained if the excited state potential is approximated to a symmetric double-well. The observed vibronic pattern could be a general fingerprint of the presence of TB centre(s), specifically, the transfer of the (core) density to the vacant boron p-orbital in the excited state.

We carried out an investigation of the performances of 18 density functionals (DFs) for modelling the mechanisms and kinetics of the aqueous phase reactions between the α-hydroxyisopropyl radical and 9 organic substrates. The primary goal was to evaluate the applicability of density functional theory specifically in conjunction with the polarizable continuum model (DFT/PCM) for a fully implicit description of the aqueous environment. Accordingly, the solute is augmented with the explicit molecule(s) of the water solvent only when it is confirmed that the water participates in the reaction mechanism directly and not just as a potential donor or acceptor of additional hydrogen bonds. The tested DFs are chosen by systematically ascending the Jacob's ladder of DFs with particular emphasis on the versatile Minnesota family. For most of the DFs we used the empirical corrections for the dispersion in accordance with Grimme's D3 or D3-BJ models. The optimum DFs are determined on the basis of the lowest mean absolute errors (MAEs) and the largest Pearson correlation coefficients (PCCs) with respect to the set of experimentally determined rate constants. The studied substrates are carbon tetrachloride (CCl4), chloroform (CHCl3), trichloroacetate (Cl3Ac-), chloral hydrate (ClH), iodoacetate (IAc-), iodoacetamide (IAm), 5-bromouracil (BrU), 5-nitrouracil (NO2U), and cysteamine (Cys+). The mechanisms that contribute dominantly to the observed rate constants are: chlorine abstractions for CCl4, CHCl3, Cl3Ac-, and ClH; proton-coupled electron transfer (PCET) for IAc-; water-assisted PCET and iodine abstraction for IAm; ortho-addition for BrU and NO2U; and hydrogen atom abstraction from the sulphur atom for Cys+. It is found that in the DFT/PCM setting climbing up the Jacob's ladder does not necessarily imply a systematically increasing accuracy. Thus, M06-D3 and PBE0-D3 exhibit the best performance according to the lowest MAEs (1.10 and 1.26 kcal mol-1 MAEs in the Gibbs free energies of activation), and M06-D3, M06-2X-D3 and MN15 according to the largest PCCs (0.95, 0.94, and 0.94). In a surprising contrast, the three tested double-hybrid DFs, B2PLYP-D3, DSD-PBEP86, and PBEQIDH, all exhibit comparatively large MAEs and poor PCCs, and therefore do not appear well-suited for use in the DFT/PCM framework.

Insights into the electronic structure of galvinoxyl - a prototype persistent free radical species - are of interest to elucidate its attractive photophysical and magnetic properties and to pave way for a sensible design of novel applications. To this end, we study the photoionization and photoexcitation UPS, XPS and NEXAFS spectra of the gas-phase galvinoxyl in the valence and core (C 1s and O 1s) regions using synchrotron X-ray radiation. We observe significant variations of relative band intensities with photon energy for valence ionizations below 10 eV which are rationalized in terms of the properties of the corresponding valence molecular orbitals. We calculate the core electron binding energies and core-excited states by employing the spin-unrestricted ΔDFT (B3LYP, M06-2X, and ωB97xD) and time-dependent DFT (SRC2-BLYP) methods. A good correlation between the calculations and the measured C 1s and O 1s XPS and NEXAFS spectra is obtained if one assumes that the galvinoxyl sample has undergone a partial degradation (around 50%) to the saturated (closed-shell) phenolic-quinonic derivative known as galvinol. We carry out a comparative theoretical analysis of the XPS and NEXAFS spectra of galvinoxyl and galvinol by assigning the relevant absorptions and pointing out the most important relative differences. The calculations identify a band in the O 1s NEXAFS spectrum whose diminishing intensity is a most manifest indicator of the extent of the degradation. Such a feature may thus prove useful in monitoring the scavenging dynamics of galvinoxyl using the core-excitation spectroscopy.

The reaction systems of α-hydroxyalkyl radicals with halogenated organics in aqueous solutions are uniquely suited for studying the fundamentally important proton-coupled electron transfer (PCET) mechanism in competition with alternatives such as substitution, hydrogen abstraction, halogen atom abstraction etc. We report experimental (steady state γ-radiolysis) and theoretical (density functional theory) studies of reactions of the α-hydroxyethyl radical (˙EtOH) with the four monohaloacetate anions (XAc-): fluoroacetate (FAc-), chloroacetate (ClAc-), bromoacetate (BrAc-) and iodoacetate (IAc-). The reactions are conducted in non-buffered and buffered (bicarbonate or phosphate) aqueous solutions of ethanol. In these conditions, only IAc- and BrAc- are reduced by ˙EtOH, and the PCET is predicted to be the most feasible reaction mechanism. In contrast to analogous reaction systems with alkyl halides, halophenols and 5-bromouracil, the radical-mediated one-electron reduction and subsequent dehalogenation of IAc- and BrAc- proceed regardless of the presence of buffers as the external proton acceptors. This implies that the proton can be efficiently transferred to the carboxyl group. The proton transfer is predicted to take place directly as interposition of one water molecule raises the barriers to the PCET. The addition of HCO3- or HPO42- accelerates the PCET owing to their larger proton affinities compared to that of the carboxyl group. The reduction of IAc- and BrAc- generates daughter carboxymethyl radicals thus initiating a radical chain reaction which considerably enhances the Br- and I- yields. In contrast, ClAc- and FAc- are not degraded by ˙EtOH even at elevated temperatures. These comparatively simple reaction systems enable general insights into PCET processes in which the carboxyl group may assume the role of proton acceptor.

Core excitation (NEXAFS) C 1s, N 1s, and O 1s gas-phase spectra of stable nitroxide free radical TEMPO and two of its amide-substituted analogues are assigned from the onset of the absorptions to the vicinity of the core-ionization thresholds using the theoretical TDDFT and ΔDFT methods.