A combined laser flash photolysis,density functional theory and atoms in molecules study of the photochemical hydrogen abstraction by pyrene-4,5-dione

The photochemical hydrogen abstraction reaction of pyrene-4,5-dione (3) has been investigated by laser flash photolysis. Excitation (λ = 355 nm) of a degassed solution of 3 in acetonitrile resulted in the formation of a detectable transient with absorption maxima at 380 and 470 nm. This transient decays with a lifetime of around 4.8 μs and is quenched by oxygen. This transient is most probably a triplet state of 3. Addition of hydrogen donors, such as 2-propanol; 1,4-cyclohexadiene or 4-methoxyphenol led to the formation of a new transient with λ_max at 380, 500 nm and a broad absorption at 640 nm. This new transient slowly decays with second order kinetics and was assigned to the semiquinone radical obtained from the hydrogen abstraction reaction. Using DFT and AIM calculations the reactivity of 3 and 9,10-phenanthrenequinone (1) is best interpreted as a proton coupled electron transfer like mechanism for the hydrogen abstraction from 2-propanol.     

Formation of Stilbene Azo-Dimer by Direct Irradiation of p-Azidostilbene†

Triplet arylnitrenes may provide direct access to aryl azo-dimers, which have broad commercial applicability. Herein, the photolysis of p-azidostilbene ( 1 ) in argon-saturated methanol yielded stilbene azo-dimer ( 2 ) through the dimerization of triplet p-nitrenostilbene (³ 1N ). The formation of ³ 1N was verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λ_max ~ 375 nm) in cryogenic 2-methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of 1 in methanol formed ³ 1N (λ_max ~ 370 nm, 2.85 × 10⁷ s⁻¹). On shorter timescales, a transient absorption (λ_max ~ 390 nm) that decayed with a similar rate constant (3.11 × 10⁷ s⁻¹) was assigned to a triplet excited state (T) of 1 . Density functional theory calculations yielded three configurations for T of 1 , with the unpaired electrons on the azido (T_A) or stilbene moiety (T_Tw, twisted and T_Fl, flat). The transient was assigned to T_Tw based on its calculated spectrum. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal mol⁻¹ for 1 N ; thus, intersystem crossing of ¹ 1N to ³ 1N is unlikely at ambient temperature, supporting the formation of ³ 1N from T of 1 . Thus, sustainable synthetic methods for aryl azo-dimers can be developed using the visible-light irradiation of aryl azides to form triplet arylnitrenes.     

Acrylic-urethane/modified Rhodamine-B eco-friendly UV-curable anticounterfeiting ink

Counterfeiting is an ever-growing global problem challenging companies, governments, and customers. In recent decades, as a potential remedy, anticounterfeiting technology and information security have gained a great deal of attention from academia and industry. In this work and for the first time, Rhodamine B (RhB), an efficient and enticing fluorescent material, was modified and used as a reactive stimuli-responsive component in the formulation of an eco-friendly ink. Additionally, a UV-curable polyurethane dispersion (UCPUD) with zero volatile organic compound was synthesized and employed as the matrix for the fluorescent ink. The modified RhB and UCPUD were thoroughly characterized using Fourier-transform infrared and proton nuclear magnetic resonance analyses. Exploiting the fluorescent monomer in the ink formulations could enhance the absorption intensity (λ_max = 552 nm) of the prepared ink up to 7 with respect to its solution (λ_max = 519 nm). The printed pattern was immediately illuminated with brilliant red-pink fluorescence emission upon UV irradiation. It has been shown that the prepared fluorescent ink has potential applications in the encryption, security marking, and optical authentication of confidential cellulose substrates.     

Photophysical properties of 5-hydroxyflavone

To investigate the role of the excited triplet state in the deactivation process of 5-hydroxyflavone (5HF), the photophysical process of 5HF was studied by transient absorption, phosphorescence spectroscopies, and semiempirical calculations. The triplet–triplet absorption (T–T) spectra of 5HF and 5-methoxyflavone (5MF) were observed upon direct and triplet-sensitized excitation. The T–T spectrum of 5HF (λ_max=350 nm, τ_T=2.8 μs) was different from that of 5MF (λ_max=360 nm, τ_T=6.8 μs). Estimations of the triplet energies of 5HF and 5MF by quenching experiments, phosphorescence, and semiempirical (PM3/CI4) calculation revealed that 5HF underwent an intramolecular hydrogen atom transfer and formed the tautomer in the excited triplet state. The triplet energy of the normal form of 5HF was 260 kJ mol⁻¹, while that of the tautomer form (5HF′) was 197 kJ mol⁻¹. The triplet energy of 5MF, the model compound of the normal form of 5HF, was 261 kJ mol⁻¹. The PM3/CI4 calculation supported the experimental observations and suggested that the most stable conformer in the triplet state of 5HF is the tautomer form.     

2‐(2,4‐Dinitrobenzyl)pyridine (DNBP): A Potential Light‐Activated Proton Shuttle

The well‐known photochromic tautomerism of 2‐(2,4‐dinitrobenzyl)pyridine ( 1 ; CH; Scheme 1) was re‐investigated by flash photolysis in aqueous solution in view of its potential application as a light‐activated proton pump. Irradiation of 1 yields the enamine tautomer NH (λ_max=520 nm) that rapidly equilibrates with its conjugate base CNO^? (λ_max=420 nm). The pH–rate profile for the first‐order decay of NH and CNO^? provides a direct determination of the acidity constant of NH, pK =5.94±0.12 (I=0.1M ) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH⁺), pK =4.18±0.02, was determined by spectrophotometric titration.     

Exploring weak interactions to assemble a nitrosyl–ruthenium compound able to release NO under visible light irradiation

This work reports on a novel nitrosyl–ruthenium complex bearing the azanaphthalene ligand quinazoline (qui) in its coordination sphere. The product crystallizes with an additional quinazoline molecule, yielding the compound cis-[Ru(bpy)₂(qui)NO](PF₆)₃ · (qui). This feature leads to an absorption band at λ_max = 430 nm in CH₃CN and λ_max = 420 nm in phosphate buffer, which promotes the photorelease of nitric oxide under visible light irradiation (λ > 400 nm), in 1 ethanol:1 water (v/v) mixture or under physiological pH. Both the intensity and energy of this transition are dependent on solvent and solution pH, suggesting that the transition has a charge transfer nature, and that the association of the second quinazoline molecule with the complex is driven by weak interactions, possibly of the π-stacking type.     

Optical properties of bis(cyclopentadienyl)magnesium: excimer-type luminescence of the bis(cyclopentadienyl) ligand frame

The electronic spectra of solid MgCp₂ (Cp = cyclopentadienyl) show features which indicate the presence of intramolecular interligand interactions. The fluorescence of MgCp₂ (λ_max = 363 nm) undergoes a considerable Stokes shift which is apparently caused by a bonding attraction between both Cp rings in the excited state. An additional phosphorescence of the (Cp⁻)₂ fragment (λ_max = 535 nm) appears at 77 K.     

Dendrimers with a cyclam core. Absorption spectra, multiple luminescence, and effect of protonation

We have synthesized two dendrimers (4 and 5) consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam) core appended with four dimethoxybenzene and eight naphthyl units (4) and 12 dimethoxybenzene and 16 naphthyl units (5). The absorption and luminescence spectra of these compounds and the changes taking place upon protonation of their cyclam core have been investigated. In acetonitrile-dichloromethane 1:1 v/v solution they exhibit three types of emission bands, assigned to naphthyl localized excited states (λ_max=337 nm), naphthyl excimers (λ_max ca 390 nm), and naphthyl-amine exciplexes (λ_max=480 nm). The tetraamine cyclam core undergoes only two protonation reactions, whose constants have been obtained by fitting the spectral changes. Protonation not only prevents exciplex formation for electronic reasons, but also causes strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers.     

Synthesis,properties and quantum chemical evaluation of solvatochromic pyridinium-phenyl-1,3-thiazol-4-olate betaine dyes

The syntheses and characterizations of four zwitterionic betaines are presented. These dyes possess an uncommon heterocyclic 1,3-thiazol-4-olate donor moiety. The natures of the HOMO/LUMO transitions and of the intramolecular charge-transfer state were assigned with the help of quantum chemical calculations. Multiple intermolecular solute/solvent interactions were discussed using linear solvation energy relationship (LSER) with Kamlet–Taft and Catalán parameters. The dyes show a pronounced negative solvatochromism ranging from λ_max 392 nm in TFE to 820 nm in THF (Δλ=428 nm or 1.65 eV). The X-ray structures of one of the tetrafluoroborate salts and of one of the betaines are discussed.     

Kinetic and spectral properties of the intermediates formed in the pulse radiolysis of 2-mercaptobenzimidazole

The reactions of primary species of water radiolysis such as e^- _aq, H* and *OH, and some specific one electron reductants and oxidants with 2-mercaptobenzimidazole have been studied at various pHs. *OH radical reaction with MBZ at pH 7 gave a transient species having absorption maxima (λ_max) at 330 and 590 nm. The transient species (pK_a = 3.6) was found to be neutral at this pH and was a mild oxidant. The initial transient species formed by the reaction of MBZ with e^{- aq} at pH 7 and with H atom at pH 0 were found to react with the parent molecule to form another transient species which has an absorption spectrum similar to that obtained by *OH radical reaction with λ_max at 590 nm. However the reaction is not quantitative. The kinetic, spectral, acid-base and redox properties of the transient species are reported.     

Multicolor Amplified Spontaneous Emissions Based on Organic Polymorphs That Undergo Excited‐State Intramolecular Proton Transfer

Two polymorphs emitting near‐infrared ( 1 R form: α phase, λ_em=702 nm, Φ_f=0.41) and orange‐red fluorescence ( 1 O form: β phase, λ_em=618 nm, Φ_f=0.05) were synthesized by finely controlling the crystallization conditions of compound 1 , a structurally simple excited‐state intramolecular proton transfer (ESIPT)‐active molecule. Multicolor amplified spontaneous emissions (ASEs) were realized, for the first time, based on these polymorphs. Notably, the 1 O crystal underwent heating‐induced phase transformation from the β phase to the α form in a single‐crystal to single‐crystal (SCSC) manner accompanied with an unprecedented ASE changing. The ASE behavior of polymorphs 1 R , 1 O as well as the ASE changing during SCSC was investigated. The feasibility of multicolor lasing based on the present organic polymorphs was confirmed, which may provide a new development strategy for organic laser science and technology.     

Photophysical Properties of Fluorescent 2‐(Phenylamino)‐1,10‐phenanthroline Derivatives†

The present study details the experimental and theoretical characterization of the photophysical properties of 14 examples of 2‐(phenylamino)‐1,10‐phenanthrolines ( 1 ). The absorption spectra of 1 are substituent‐dependent but in a general manner present absorption bands at wavelengths of ~230; ~300; ~335 and a shoulder at ~380 nm. Electron‐donating groups (EDG) and electron‐withdrawing groups (EWG), respectively, result in bathochromic and hypsochromic shifts. Compounds 1 are highly luminescent, in contrast to phenanthroline, and emit in the region between 350 and 500 nm with substituent‐dependent λ_max emission. The emission spectra show a redshift for EDG (4‐OMe 62 nm; 4‐Me 19 nm) and a blueshift for EWG (4‐CN 41 nm; 4‐CF₃ 38 nm) relative to the emission of the unsubstituted parent compound 1a . Plotting the ${\mathbf{\lambda }}_{\mathbf{max}}^{\mathbf{\text{EM}}}$ against Hammett σ+ constants gave an excellent linear correlation demonstrating the electron‐deficient nature of the excited state and how the substituents (de)stabilize S₁. Theoretical calculations revealed a HOMO‐LUMO π‐π* electronic transition to S₁ which in combination with difference (S₁–S₀) in electron density maps revealed charge‐transfer character. Strongly electron‐withdrawing substituents switch off the charge transfer to give rise to a local excitation.     

Blue two-photon excited fluorescence of several D-π-D, A-π-A, and D-π-A compounds featuring dimesitylboryl acceptor

Several donor-π-donor (D-π-D), acceptor-π-acceptor (A-π-A), and donor-π-acceptor (D-π-A) types of organic compounds with fluorene as π bridge and dimesitylboryl group as electron acceptor, which show strong two-photon excited blue fluorescence, have been synthesized and structurally investigated. The symmetric A-π-A type of compound exhibits the shortest wavelength of two-photon excited fluorescence (TPEF) at λ_em=405 nm under the excitation of λ_ex=730 nm; the unsymmetric D-π-A type of compound with diphenylamino as donor exhibits the most intense TPEF at blue region (λ_em=484 nm) with a two-photon absorption cross-section of 425 GM under λ_ex=800 nm.     

Characterization of Locally Excited and Charge-Transfer States of the Anticancer Drug Lapatinib by Ultrafast Spectroscopy and Computational Studies

Lapatinib (LAP) is an anticancer drug, which is metabolized to the N- and O-dealkylated products (N-LAP and O-LAP, respectively). In view of the photosensitizing potential of related drugs, a complete experimental and theoretical study has been performed on LAP, N-LAP and O-LAP, both in solution and upon complexation with human serum albumin (HSA). In organic solvents, coplanar locally excited (LE) emissive states are generated; they rapidly evolve towards twisted intramolecular charge-transfer (ICT) states. By contrast, within HSA only LE states are detected. Accordingly, femtosecond transient absorption reveals a very fast switching (ca. 2 ps) from LE (λ_max=550 nm) to ICT states (λ_max=480 nm) in solution, whereas within HSA the LE species become stabilized and live much longer (up to the ns scale). Interestingly, molecular dynamics simulation studies confirm that the coplanar orientation is preferred for LAP (or to a lesser extent N-LAP) within HSA, explaining the experimental results.     

[Bis(trispivalatodimolybdenum (II))-μ-bis(4′-carboxylato-2,2′:6′,2″-terpyridine) ruthenium (II)] (2+) Tetrafluoroborate: Photophysical Studies

The photophysical properties of the title compound have been studied by fs and ns transient absorption spectroscopy. The electronic absorption spectrum consists of three principle absorptions assigned to terpy ¹LLCT at ~300 nm, ruthenium (II) t_2g⁶ to terpy ¹MLCT at ~470 nm and Mo₂ δ to terpycarboxylate at ~670 nm. The compound shows weak room temperature emission in THF solution at ~1,100 nm when excited into each of the aforementioned bands. This emission is assigned to the T₁ state, ³MMδδ*. Transient absorption spectroscopy indicates a lifetime for T₁ of 9.6 μs. This paper is dedicated to Prof. C. N. R. Rao.     

Phosphorescence at Low Temperature by External Heavy-Atom Effect in Zinc(II) Clusters

Luminescent Zn^II clusters [Zn₄L₄(μ₃-OMe)₂X₂] (X=SCN ( 1 ), Cl ( 2 ), Br ( 3 )) and [Zn₇L₆(μ₃-OMe)₂(μ₃-OH)₄]Y₂ (Y=I⁻ ( 4 ), ClO₄⁻ ( 5 )), HL=methyl-3-methoxysalicylate, exhibiting blue fluorescence at room temperature (λ_max=416≈429 nm, Φ_em=0.09–0.36) have been synthesised and investigated in detail. In one case the external heavy-atom effect (EHE) arising the presence of iodide counter anions yielded phosphorescence with a long emission lifetime (λ_max=520 nm, τ=95.3 ms) at 77 K. Single-crystal X-ray structural analysis and time-dependent density-functional theory (TD-DFT) calculations revealed that their emission origin was attributed to the fluorescence from the singlet ligand-centred (¹LC) excited state, and the phosphorescence observed in 4 was caused by the EHE of counter anions having strong CH−I interactions.     

Light‐Controlled Orthogonal Covalent Bond Formation at Two Different Wavelengths

We report light‐induced reactions in a two‐chromophore system capable of sequence‐independent λ‐orthogonal reactivity relying solely on the choice of wavelength and solvent. In a solution of water and acetonitrile, LED irradiation at λ_max=285 nm leads to full conversion of 2,5‐diphenyltetrazoles with N‐ethylmaleimide to the pyrazoline ligation products. Simultaneously present o‐methylbenzaldehyde thioethers are retained. Conversely, LED irradiation at λ_max=382 nm is used to induce ligation of the o‐methylbenzaldehydes in acetonitrile with N‐ethylmaleimide via o‐quinodimethanes, while 2,5‐diphenyltetrazoles also present are retained. This unprecedented photochemical selectivity is achieved through control of the number and wavelength of incident photons as well as favorable optical properties and quantum yields of the reactants in their environment.     

Photokinetic,Biochemical and Structural Features of Chimeric Photoactive Yellow Protein Constructs

Of the 10 photoactive yellow protein (PYPs) that have been characterized, the two from Rhodobacter species are the only ones that have an additional intermediate spectral form in the resting state (λ_max = 375 nm), compared to the prototypical Halorhodospira halophila PYP. We have constructed three chimeric PYP proteins by replacing the first 21 residues from the N‐terminus (Hyb1PYP), 10 from the β4–β5 loop (Hyb2PYP) and both (Hyb3PYP) in Hhal PYP with those from Rb. capsulatus PYP. The N‐terminal chimera behaves both spectrally and kinetically like Hhal PYP, indicating that the Rcaps N‐terminus folds against the core of Hhal PYP. A small fraction shows dimerization and slower recovery, possibly due to interaction at the N‐termini. The loop chimera has a small amount of the intermediate spectral form and a photocycle that is 20 000 times slower than Hhal PYP. The third chimera, with both regions exchanged, resembles Rcaps PYP with a significant amount of intermediate spectral form (λ_max = 380 nm), but has even slower kinetics. The effects are not strictly additive in the double chimera, suggesting that what perturbs one site, affects the other as well. These chimeras suggest that the intermediate spectral form has its origins in overall protein stability and solvent exposure.     

Photoreactions of aromatic compounds—XXIX : Pathway and intermediates of the photoreactions of 3,5-dinitroanisole with nucleophiles

The photoreactions of 3,5-dinitroanisole with nucleophiles (mostly hydroxide ion) have been studied by sensitization and quenching as well as by flash photolysis experiments. The photosubstitution starts mainly from the π-π* triplet excited state (λ_max ～ 475 nm; τ 5·10^?8s or shorter depending on the concentration of nucleophile). The formation of substitution product is completed within ～ 10^?6s. The occurrence of the radical anion (λ_max ～ 550 nm; τ ～ 10^?2-10^?4s depending on the nature of the nucleophile) could be established. This cannot be intermediate in the photosubstitution reaction but is so in the formation of reduction products. The radical anion and, probably also, the substitution product seem to originate from a complex (λ_max ～ 410 nm; τ ～ 5·10^?7s) between the triplet excited aromatic compound and its nucleophilic reaction partner. The formation of this complex is a very fast process.     

Pyridine-cored V-shaped π-conjugated oligomers: synthesis and optical properties

A new series of V-shaped pyridine-cored π-conjugated oligomers are synthesized utilizing two-fold Heck/Suzuki coupling reactions. Optical properties of these compounds (λ_max=390–449 nm, Φ_fl=79–5%, in solutions) are discussed. They are shown to be thermally stable and soluble in common organic solvents. Stilbenoid oligomers exhibited much higher fluorescence quantum yields than tri- and tetra-phenylethylene substituted oligomers in solutions.