Time-resolved spectroscopic and computational study of the initial events in doxazosin photochemistry

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 S0 is present in two prototropic forms, with the protonated form dominating (∼91 %, pKa = 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 (pKa* = 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, ΦISC = 0.17, ε520 = 11600 ± 100 M-1 cm-1 and τ = 11 μs, 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.

Synthesis of D-π-A'-π-A Chromophores with Quinoxaline Core as Auxiliary Acceptor and Effect of Various Silicon-Substituted Donor Moieties on Thermal and Nonlinear Optical Properties at Molecular and Material Level

Novel D-π-A'-π-A chromophores with quinoxaline cores as auxiliary acceptors and various donor moieties (aniline, carbazole, phenothiazine, tetrahydroquinoline) containing bulky tert-butyldimethylsilyloxy (TBDMSO) groups and tricyanofuranyl (TCF) acceptors with bulky cyclohexylphenyl substituents were synthesized via eight- to nine-step procedures, and their photo-physical and thermal properties were investigated. The values of the chromophores' first hyperpolarizabilities were calculated in the framework of DFT at the M06-2X/aug-cc-pVDZ computational level; the effect of the introduction of the TBDMSO group into the donor fragment is shown to be inessential, as this group is not coupled to the π-conjugated system of the chromophore. The chromophore with the tetrahydroquinoline donor has a first hyperpolarizability value of 937 × 10^-30 esu, which is the highest for the studied chromophores. Atomistic modeling of composite materials with the studied chromophores as guests demonstrated that the presence of bulky substituent in the donor fragment prevents notable aggregation of chromophores, even at high chromophore content (40 wt.%). The nonlinear optical performance of guest-host materials with 25 and 40 wt.% of suggested chromophore content was studied using a second harmonic generation technique to give the NLO coefficient, d₃₃ up to 52 pm/V.

Push-Pull Structures Based on 2-Aryl/thienyl Substituted Quinazolin-4(3H)-ones and 4-Cyanoquinazolines

Design and synthesis of 2-(aryl/thiophen-2-yl)quinazolin-4(3H)-ones and 4-cyano-2-arylquinazolines with Et2N-, Ph2N- or carbazol-9-yl- electron donating fragment are described. The key photophysical properties of these compounds have been studied by UV/Vis absorption and fluorescence spectroscopy in solvents of different polarity (toluene and MeCN). 2-(Aryl/thiophen-2-yl)quinazolin-4(3H)-ones show fluorescence in blue-green region in toluene solution with quantum yields up to 89% in the case of 2-(4’-N,N-diphenylamino[1,1’-biphenyl]-4-yl)-quinazolin-4(3H)-one. Moreover, triphenylamino derivative based on quinazolin-4(3H)-one with para-phenylene linker displays the highest quantum yield of 40% in powder. The fluorescence QY of Et2N and Ph2N derivatives decrease when going from toluene to MeCN solution, whereas carbazol-9-yl counterparts demonstrate strengthening of intensity that emphasizes the strong influence of donor fragment nature on photophysical properties. 4-Cyanoquinazolines are less emissive in both solvents, as well as, in solid state. The introduction of cyano group into position 4 leads to orange/red colored powder and dual emission bands. Some molecules demonstrate the increase in emission intensity upon addition of water to MeCN solution. According to frontier molecular orbitals (HOMO, LUMO) calculations, the energy gap of 4-cyanoquinazoline decreases by more than 1 eV compared to quinazolin-4-one, that is consistent with experimental data.

Push–Pull Derivatives Based on 2,4′-Biphenylene Linker with Quinoxaline, [1,2,5]Oxadiazolo[3,4-B]Pyrazine and [1,2,5]Thiadiazolo[3,4-B]Pyrazine Electron Withdrawing Parts

A series of novel V-shaped quinoxaline, [1,2,5]oxadiazolo[3,4-b]pyrazine and [1,2,5]thiadiazolo[3,4-b]pyrazine push–pull derivatives with 2,4′-biphenylene linker were designed and their electrochemical, photophysical and nonlinear optical properties were investigated. [1,2,5]Oxadiazolo[3,4-b]pyrazine is the stronger electron-withdrawing fragment as shown by electrochemical, and photophysical data. All compounds are emissive in a solid-state (from the cyan to red region of the spectrum) and quinoxaline derivatives are emissions in DCM solution. It has been found that quinoxaline derivatives demonstrate important solvatochromism and extra-large Stokes shifts, characteristic of twisted intramolecular charge transfer excited state as well as aggregation induced emission. The experimental conclusions have been justified by theoretical (TD-)DFT calculations.

2-(4-Chlorophenyl)-4-(3,4-dimethoxy-phenyl)-6-methoxy-3-methylquinoline

A 2,4-diarylquinoline derivative, 2-(4-chlorophenyl)-4-(3,4-dimethoxyphenyl)-6-methoxy-3-methylquinoline, was synthesized in a conventional two-step procedure from p-anisidine, p-chlorobenzaldehyde and methyl isoeugenol as available starting reagents through a sequence of BF3·OEt2-catalyzed Povarov cycloaddition reaction/oxidative dehydrogenation aromatization processes under microwave irradiation conditions in the presence of a green oxidative I2-DMSO system. The structure of the compound was fully characterized by FT-IR, 1H and 13C-NMR, ESI-MS, and elemental analysis. Its physicochemical parameters (Lipinski’s descriptors) were also calculated using the Molinspiration Cheminformatics software. The diarylquinoline molecule obtained is an interesting model with increased lipophilicity and thus permeability, an important descriptor for quinoline-based drug design. Such types of derivatives are known for their anticancer, antitubercular, antifungal, and antiviral activities.

FTIR spectroscopic evidence for new isomers of 3-aminopyrazine-2-carboxylic acid formed in argon matrices upon UV irradiations

The UV-induced photochemistry and molecular structure of 3-aminopyrazine-2-carboxylic acid were studied in argon matrices by Fourier-transform infrared spectroscopy and B3LYP/6-311++G(2d,2p) calculations. Out of seventeen possible isomers of this molecule located on the singlet potential energy surface the most stable one, APA1 comprising intramolecular O-H···N and N-H···O hydrogen bonds, was detected experimentally in the matrix after deposition. Two new conformers APA2 and APA3 were generated upon irradiation with λ = 280 nm by trans/cis-COOH isomerization and at λ = 360 nm by COOH group rotamerization, respectively, whereas an amino-imino tautomerization leading to IPA1 and IPA2 structures occurred at λ = 305 nm. The reverse reactions were also observed upon irradiation of the matrices at 265, 230 and 400 nm. Simultaneously with the photoisomerizations, a cleavage of the pyrazine ring along with CO₂ elimination was observed leading to the formation of carbodiimide and cyanamide derivatives.

Substituted 2-(2-hydroxyphenyl)-3H-quinazolin-4-ones and their difluoroboron complexes: Synthesis and photophysical properties

2-(2-Hydroxyphenyl)-3H-quinazolin-4-ones with diverse substituents at phenol ring and their six-membered difluoroboron complexes have been synthesized via few-stage approach. The photophysical properties of target compounds have been investigated in two solvents as well as in the solid state. The nature of substituents and substitution point in the phenol moiety of ligands and resulting BF₂-complexes on the photophysical properties of dyes have been explored. The complex bearing two t-Bu groups proved to be the most emissive in solid state, whereas its 5-methoxy and 4-diethylamino counterparts possess strong emission in toluene solution. The dyes exhibited large Stokes shifts which was attributed to excited state intramolecular proton transfer (ESIPT). Additionally, fluorescence of quinazolinones in the mixture of THF/water was studied. All ligands demonstrated emission enhancement with increase of water fraction which was due to aggregation induced emission.

Diastereodivergent synthesis of dispiroheterocyclic structures comprising pyrrolidinyloxindole and imidazothiazolotriazine moieties

Highly diastereoselective methods for the synthesis of two different diastereomers of polynuclear dispiroheterocyclic compounds with five chiral centers comprising pyrrolidinyloxindole and imidazothiazolotriazine moieties (dispiro[imidazo[4,5-e]thiazolo[2,3-c]-1,2,4-triazine-7,3'-pyrrolidine-2',3''-indoles]) have been developed on the basis of a dipolar cycloaddition of azomethine ylides to benzylidene derivatives of imidazothiazolotriazines and an alkali-induced rearrangement of the thiazolotriazine fragment. The different sequence of the cycloaddition and rearrangement stages allows us to perform the targeted synthesis of two diastereomerically pure products from the same starting compounds.