Photochemistry of <Emphasis Type="Italic">N</Emphasis>-substituted salicylic acid amides

The products of photolysis of N-substituted salicylic acid amides, viz., 2-hydroxy-3-tert-butyl-5-ethylbenzoic acid N-(4-hydroxy-3,5-di-tert-butylphenyl)amide (1) and 2-hydroxybenzoic acid N-[3-(4-hydroxy-3,5-di-tert-butylphenyl)prop-1-yl]amide (2), in heptane were studied by optical spectroscopy and stationary and nanosecond laser photolysis (Nd: YAG laser, 355 nm). It was shown by the method of partial deuteration of amides 1 and 2 that they exist in both the unbound state and as complexes with intraand intermolecular hydrogen bond. Amides 1 and 2 are subjected to photolysis, which results in the formation of a triplet state and phenoxyl radicals RO? presumably due to the absorption of the second photon by the excited singlet state. The formation of radical products due to N–H bond ionization was not observed. The main channel of decay of the triplet state and radicals RO? is triplet–triplet annihilation and recombination (k _r ≈ 2.3?10⁸ L mol^–1 s^–1), respectively. The UV irradiation of compounds 1 and 2 leads to the excitation of the amide groups, and no formation of radical products due to N–H bond ionization was observed.     

Influence of medium viscosity on photophysical properties of kynurenic acid and kynurenine yellow

Dependences of the fluorescence and triplet state quantum yields of kynurenic acid (1) and kynurenine yellow (2) in water—glycerol mixtures on medium viscosity have been studied. The main channel of the singlet excited state decay of compound 1 is the intersystem crossing, which rate weakly depends on the viscosity; only a small (approximately 1.5-fold) increase in the fluorescence yield was found for this compound with the increase of the solution viscosity from 0.84 cP (aqueous solution) to 78 cP (86% glycerol). The deactivation of the S₁ state of compound 2 is caused mainly by the internal conversion, and a noticeable increase of the fluorescence yield (approximately 3-fold), as well as the change in the photolysis product yields, was observed with the increasing percentage of glycerol in the mixture. The triplet state quantum yields for compounds 1 and 2 remained unchanged with the variation of the glycerol content in the mixture.     

Triarylamines with branched multi-pyridine groups: modulation of emission properties by structural variation,solvents, and tris(pentafluorophenyl)borane

Six triarylamine derivatives 1–6 with branched multi-pyridine substituents were prepared and characterized. These compounds are distinguished by the substituent on one of the phenyl group with NO2 for 1, CN for 2, Cl for 3, p-C6 H4 OMe for 4, OMe for 5,and NMe2 for 6, respectively. As revealed by single crystal X-ray analysis, these substituents play an important role in determining the configuration of the triarylamine framework and the crystal packing of 1–6. The emission properties of these compounds were examined in different solvents(toluene, CH2 Cl2, acetone, tetrahydrofuran(THF), and N,N-dimethylformamide(DMF)) and in solid states. Distinct dual emissions from the localized emissive state and the intramolecular charge transfer state were observed for compound 5 in CH2 Cl2. Compounds 1 and 6 show apparent aggregated enhanced emissions in acetone/H2 O.The emission properties of these compounds were further modulated by the addition of tris(pentafluorophenyl)borane. In addition, density functional theory(DFT) and time-dependent DFT(TDDFT) calculations have been performed on the ground and singlet excited states to complement the experimental findings.     

The first observation of the effect of dendritic structure to produce the triplet excited state of the core stilbene by dendron excitation

We report that both singlet and triplet energy transfers in stilbene-cored benzophenone dendrimers (trans-BPST) took place quite efficiently. On excitation (290 nm) of stilbene group, the intramolecular singlet energy transfer from the excited core stilbene to the benzophenone part (99.7%) was confirmed by quenching of the fluorescence from the core stilbene. The benzophenone in the excited singlet state is known to undergo intersystem crossing to give its excited triplet state quantitatively. However, the very weak phosphorescence from benzophenone part in trans-BPST was observed even at 77 K. The phosphorescence intensity of trans-BPST is only 1% of that of model compound (4-methylbenzophenone) at 77 K. During the irradiation, the absorption spectra also changed due to the trans-cis isomerization. This is probably due to the ultrafast triplet energy transfer from the benzophenone to produce the triplet state stilbene.     

Nitrogen-containing analogues of boron difluoride benzoylacetonate: synthesis,structure, luminescence,and quantum chemical modeling

Nitrogen-containing analogues of boron difluoride benzoylacetonate (1), i.e., boron difluoride ketoiminates (boron difluoride 3-amino-1-phenyl-2-buten-1-onate (2) and 3-methylamino-1-phenyl-2-buten-1-onate (3)), were taken as examples to study the influence of a substituent (H, Me) at the nitrogen atom on structure and spectral-luminescent properties. The replacement of the oxygen atom with the nitrogen atom in compound 1 led to a bathochromic shift of the monomeric luminescence maximum. Quantum chemical modeling of absorption spectra for compound 2 was carried out. In contrast to compounds 1 and 3, crystals of compound 2 are characterized by the formation of J-aggregates and excimers on their basis, which leads to a bathochromic shift of the luminescence spectrum and an increase in the luminescence intensity.     

Rhenium(I) carbonyl complex of 4,7-diphenyl-1,10-phenanthroline-Spectroscopic properties, X-Ray structure, theoretical studies of ground and excited electronic states

The paper presents structural studies of the tricarbonyl complexes incorporating 4,7-diphenyl-1,10-phenanthroline (dpphen) - [Re(CO)₃(dpphen)Cl]₂·Me₂CO (1) and [Re(CO)₃(dpphen)Cl] (2). The absorption and emission spectra of [Re(CO)₃(dpphen)Cl] were discussed with the aid of DFT and time-dependent TD-DFT theories. The theoretical studies reveal mixing between the singlet (S₁) and triplet (T₁) excited states of MLCT character.     

Air-stable diradical dications with ferromagnetic interaction exceeding the thermal energy at room temperature: from a monomer to a dimer

Two tetraazacyclophane dications (1²⁺ and 2²⁺) with different remote substituents have been synthesized, isolated and characterized. Their electronic structures and physical property were studied by various spectroscopic techniques, single crystal X-ray diffraction, super conducting quantum interference device (SQUID) measurements and theoretical calculations. The dications have triplet ground states with ferromagnetic interaction exceeding the thermal energy at room temperature. The solid-state structures of these species were tunable by substituent effect, with 1²⁺ as a monomer and 2²⁺ as a dimer.     

Theoretical Study and Experimental Analysis on 2-(1-Ethyl-4-hydroxy-2-oxo-1,2-dihydroquinolin-3-yl)-2-oxoacetic Acid (<Emphasis Type="Bold">3</Emphasis>) Using the DFT Approach

Riley oxidation of 3-acetyl-4-hydroxyquinolin-2(1H)-one (1) with selenium dioxide furnished the unexpected product 2-(1-ethyl-4-hydroxy-2-oxo-1,2-dihydroquinolin-3-yl)-2-oxoacetic acid (3). The expected product 2-(1-ethyl-4-hydroxy-2-oxo-1,2-dihydroquinolin-3-yl)-2-oxoacetaldehyde (2) was excluded based on the spectral data. The elemental analysis and spectral data (IR, and ¹H NMR) was used to deduce the structure of compound 3. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations of the electronic structure at the B3LYP/6-311G (d,p) level of theory have been employed for compound 3 to investigated its geometry, linear polarizability 〈Δα〉, first-order hyperpolarizability 〈β〉, natural bonding orbital (NBO), molecular electrostatic potential contours (MEP and ESP), electrophilicity, and UV–Vis spectra in both ethanol and dioxane solvents. The geometrical and energetic parameters have been extensively investigated to reveal the reason behind the selective formation of compound 3, rather than the expected product 2. FT-IR spectra in the solid phase were recorded for compound 3. The thermo-chemical parameters, harmonic vibration frequencies, and the equilibrium geometries have been calculated at the DFT/B3LYP/6-311G (d,p) level. Time-dependent density functional theory (TD-DFT) was used to calculate the excited states of compound 3. Changes in the solvent cause changes in the band intensities and in the positions of the band maxima (λ_max). The excited state was identified and contributes to the electronic configurations, and it was characterized in terms of the relevant MOs. The theoretical spectra were computed using the Coulomb-attenuating method (CAM-B3LYP), using the basis set 6-311G (d,p) in the gas phase, and the polarizable continuum model (PCM) in dioxane and ethanol. The results indicate good agreement with the observed spectra.     

Synthesis and conformational analysis of some cyanomethylene derivatives of piperidines

Five 4-dicyanomethylene derivatives 6–10, N-cyanoacetyl-cis-2,6-diphenylpiperidin-4-one 11 and 4-cyano(ethoxycarbonyl)-methylene-cis-2,6-bis(o-chlorophenyl)piperidine 12 were synthesised by condensing the appropriate piperidin-4-ones 13–17 with malononitrile/ethylcyanoacetate and their ¹H and ¹³C NMR spectra were recorded. The ¹H-¹H COSY spectrum for 6 and NOESY spectra for 8, 10, and 11 were also recorded. Based on coupling constants and the results obtained from NOESY spectra boat conformation for 10 and epimerised chair conformations for 8 and 9 have been proposed. Other derivatives adopt normal chair conformations. Theoretical calculations and the ¹H and ¹³C chemical shifts also support the above conformations. Mass spectra were also recorded for 6–12.     

Heteroatom effects on allenic seven-membered ring X2C4H4C (X = CH,N, P,and As)

Stable configurations of seven-membered rings X₂C₄H₄C (1 _X , X = CH, N, P, and As) in the singlet (s) and triplet (t) states are found at B3LYB/6-311++G** level of theory. Thermal energy gaps, ΔE _s-t; enthalpy gaps, ΔH _s-t; Gibbs free energy gaps, ΔG _s-t, between the singlet and triplet states of 1 _X were estimated at the same level of theory. The ΔG _s-t gap between the singlet and triplet states of 1 _X changes in the order: 1 _P > 1 _As > 1 _N , respectively.     

Photochemistry of a naphthalene-thymine dyad in the presence of acetone

Irradiation of dyad 1 in aqueous acetone leads to the introduction of an acetonyl substituent at the naphthalene 5-position, to give photoproduct 2. The proposed reaction mechanism involves electron transfer from the naphthalene excited singlet state to the ketone. Neither thymine dimers, nor acetone photoadducts involving the thymine ring were detected. These photoproducts would arise from the thymine triplet excited state, which in dyad 1 must be efficiently depopulated via a fast intramolecular energy transfer to the naphthalene chromophore, due to the lower energy of its excited triplet state.     

Pure aromatic hydrocarbons with <Emphasis Type="Italic">meta</Emphasis>-linked phenyl-core and perihedral fluorene substitutions with/without inert groups of <Emphasis Type="Italic">tert</Emphasis>-butyl: bipolar hosts for blue phosphorescence

Two pure hydrocarbon molecules of 1,3,5-tris(9-phenyl-9H-fluoren-9-yl)benzene (m TPFB) and 1,3,5-tris(2-tert-butyl-9-phenyl-9H-fluoren-9-yl)benzene (t Bu-m TPFB) were synthesized. Due to the conjugation blocked connection mode and rigid/bulky substitutions, these two materials possess high triplet energy, enabling them as good hosts for blue phosphor in PhOLEDs. By studying their thermal, electrochemical, electronic absorption and photoluminescent properties, it was found that the influence of the inert tert-butyl group on material photoelectrical properties is negligible. For instance, m TPFB and t Bu-m TPFB showed very similar absorption and emission profiles, with almost the same bandgap, triplet energy and energy levels. However, the encapsulation of tert-butyl on the 2-position of 9-phenylfluorene enhanced material thermal stability. Most importantly, carrier transport properties were improved dramatically, as proved by the mono carrier device. Blue phosphorescent OLEDs hosted by t Bu-m TPFB showed external quantum efficiency of 15.2% and current efficiency of 23.0 cd/A, which were much higher than that of the OLEDs based on m TPFB with the analogous structure.     

Triplet states of 3,3′-alkylsubstituted thiacarbocyanine dimers and dimeric complexes with cucurbit[8]uril in water

Alkylsubstituted thiacarbocyanines (3,3′-diethylthiacarbocyanine, D1, and 3,3′-disulfopropylthiacarbocyanine, D2), existing in water as monomers and dimers, manifest the ability to transition to the triplet state. The spectrum of triplet-triplet (T–T) absorption of the D2 dimers is shifted in the range higher than 590 nm by 20 nm to the red in comparison with the T–T spectrum of monomers. The D1 dimers in the presence of cucurbit[8]uril form a dimeric complex with two bands in the differential absorption spectrum. The band at 550 nm belongs to the triplet-triplet absorption of the dimeric complexes, and the band in the range of 620–700 nm is the result of charge transfer in the triplet state. The rate constants of deactivation for these triplet states coincide.     

Copper complexes of pyridyl–tetrazole ligands with pendant amide and hydrazide arms: synthesis,characterization, DNA-binding and antioxidant properties

Pyridyl–tetrazole ligands 2-(5-(pyridin-2-yl)-1H-tetrazol-1-yl)acetamide (L1), 2-(5-(pyridin-2-yl)-2H-tetrazol-2-yl)acetamide (L2), 2-(5-(pyridin-2-yl)-1H-tetrazol-1-yl)acetohydrazide (L3) and 2-(5-(pyridin-2-yl)-2H-tetrazol-2-yl)acetohydrazide (L4) have been prepared and coordinated with CuCl₂·2H₂O to furnish the corresponding complexes [Cu(L1) ₂ ]–[Cu(L4) ₂ ]. EPR spectra of the complexes are characteristic of square planar geometries, with nuclear hyperfine spin 3/2. DNA-binding studies using UV–Vis absorption spectroscopy, viscosity and thermal denature studies revealed that all of these complexes are avid binders of calf thymus DNA. The antioxidant properties of the free ligands and the Cu(II) complexes were investigated using the p-nitrosodimethyl aniline hydroxyl radical scavenging method, and [Cu(L4) ₂ ] was found to show the highest activity.     

Ruthenium(II) complexes of azoimine and α-diimine ligands: synthesis,spectroscopic and electrochemical properties,crystal structures and DFT calculations

Five octahedral ruthenium(II) complexes with azoimine–quinoline (Azo) and α-diimine (L) ligands having the general formula [Ru^II(L)(Azo)Cl](PF₆) (1–5) {Azo: PhN=NC(COMe)=NC₉H₆N, L = 4,4′-dimethoxy-2,2′-bipyridine (dmeb) (1), 4,4′-di-tertbutyl-2,2′-bipyridine (dtb) (2), 1,10-phenanthroline (phen) (3), 5-chlorophenanthroline (Clphen) (4), or 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen) (5)} were prepared by stepwise addition of the tridentate azoimine (H₂Azo) and α-diimine (L) pro-ligands to RuCl₃ in refluxing EtOH. The tridentate azoimine–quinoline ligands coordinate to ruthenium via the Azo-N′, N′-imine and N″-quinolone nitrogen atoms. The spectroscopic properties (IR, UV/Vis, ¹H, ¹³C and ¹⁹F NMR) and electrochemical behavior of complexes 1–5 and the X-ray crystal structures of complexes 2 and 3 are presented. The coordination of Ru(II) to these strong π-acceptor ligands (Azo and L) results in a large anodic shift for the Ru(III/II) couples of 1.63–1.72 V versus NHE. The electronic spectra in MeCN and IR spectra in CH₂Cl₂ for complex 3 in its oxidized 3 ⁺ and reduced 3 ^? forms were investigated. The calculated absorption spectrum of 3 in MeCN was used to assign the UV–Vis absorption bands.     

Theoretical studies on the electronic structure and spectral properties of versatile diarylethene-containing 1,10-phenanthroline ligands and their rhenium(I) complexes

The structures of versatile diarylethene-containing 1,10-phenanthroline ligands (L₁ and L₂) and their rhenium(I) complexes [Re(CO)₃(L)Cl] (1 and 2) in the ground and low-lying excited states have been optimized at the B3LYP functional and the ab initio configuration interaction singlets (CIS) level, respectively. The spectral properties are predicted with use of time-dependent density functional theory (TDDFT). As shown, the transition character of the strongest absorption band and luminescent spectrum for closed-ring complex 1 is different from that of 2, the former has ππ^∗ character and the latter has MLCT and LLCT character. We presume the second triplet excited state contributes to the phosphorescence of 1, while the lowest triplet excited state accounts for the phosphorescence of 2. Spin-orbit coupling influences the excitation energies for d(Re)-joined transitions whereas it has negligible effect on the transition character for complexes 1 and 2.     

Paramagnetic Cu<Superscript>II</Superscript> complexes with 1-(hetarylmethyl)silatranes

The structures of paramagnetic complexes CuCl₂?L with L = HetCH₂Si(OCH₂CH₂)₃N [Het = pyrrol-1-yl (1), indol-1-yl (2), carbazol-9-yl (3), imidazole-1-yl (4), 3,5-dimethylpyrazol-1-yl (5), 1,2,4-triazol-1-yl (6), benzimidazol-1-yl (7), and 1,2,3-benzotriazol-1-yl (8)] were studied by the ESR and quantum chemical methods in terms of the density functional theory (DFT) approximation. The difference in structures of complexes CuCl₂?1—CuCl₂?8 is mainly determined by the nature of the five-membered heterocycle. The ESR spectra at room temperature are typical of the mononuclear complexes of Cu^II of axial symmetry with various types of distortions. In complexes CuCl₂?1—CuCl₂?3, narrow symmetric signals of zero-valent copper are recorded. These signals remain in the spectra for at least six months, which can characterize them as efficient stabilizing matrices of nanoparticles.     

Effect of N-pyridyl substitution and hydrogen bonding on the deactivation of singlet excited 1,2-naphthalimide

Photophysical properties of 1,2-naphthalimide (1) and N-(4-pyridyl)-1,2-naphthalimide (2) as well as the effect of their hydrogen bonding with phenols have been studied in toluene. Fluorescence emission is the dominant energy dissipation pathway of the singlet excited 1. Introduction of the 4-pyridyl substituent into the imide moiety significantly accelerates the internal conversion due to the efficient vibronic coupling between close-lying S1 and S2 excited states, however, the rate of triplet formation exhibits negligible change. In contrast with the behavior of the corresponding substituted phenyl derivatives, 2 does not emit dual fluorescence because of the less extensive conjugation within the molecule. Fluorescence quenching with phenols takes place both in dynamic and static processes. Electron transfer is slower in the hydrogen bonded complex where phenols are linked to the pyridyl moiety due to the larger distance between the electron donor and acceptor components.     

Resolving electron injection from singlet fission-borne triplets into mesoporous transparent conducting oxides

Photoinduced electron transfer into mesoporous oxide substrates is well-known to occur efficiently for both singlet and triplet excited states in conventional metal-to-ligand charge transfer (MLCT) dyes. However, in all-organic dyes that have the potential for producing two triplet states from one absorbed photon, called singlet fission dyes, the dynamics of electron injection from singlet vs. triplet excited states has not been elucidated. Using applied bias transient absorption spectroscopy with an anthradithiophene-based chromophore (ADT-COOH) adsorbed to mesoporous indium tin oxide (nanoITO), we modulate the driving force and observe changes in electron injection dynamics. ADT-COOH is known to undergo fast triplet pair formation in solid-state films. We find that the electronic coupling at the interface is roughly one order of magnitude weaker for triplet vs. singlet electron injection, which is potentially related to the highly localized nature of triplets without significant charge-transfer character. Through the use of applied bias on nanoITO:ADT-COOH films, we map the electron injection rate constant dependence on driving force, finding negligible injection from triplets at zero bias due to competing recombination channels. However, at driving forces greater than −0.6 eV, electron injection from the triplet accelerates and clearly produces a trend with increased applied bias that matches predictions from Marcus theory with a metallic acceptor.

The rate of photoinduced electron transfer from triplet excited states after singlet fission in molecules adsorbed to mesoporous oxide substrates is shown through transient absorption studies to depend systematically on applied bias.     

Alternative preparation and characterization of platinum(II) organometallic dinuclear complexes and oligomers with 3,8-diethynylphenanthroline derivatives

Novel platinum(II) organometallic dinuclear complexes and oligomers with two types of phenanthroline ligands, namely 3,8-diethynylphenanthroline (L1) and 3,8-bis-(4-ethynyl-phenylethynyl)-1,10-phenanthroline (L2), were synthesized from trans-Pt(PBu₃)₂(1-ethynyl-4-methyl-benzene)Cl and trans-Pt(PBu₃)₂Cl₂ by transmetalation of copper ion. The alternative procedure targeted platinum oligomer termination selection of either chloride or respective phenanthrolines and was successfully performed with different purifications by extraction and column chromatography. The structural formulae of these platinum complexes and oligomers were revealed with by analysis of both ³¹P{¹H}-NMR and ¹H-NMR spectral data. Alternative preparations of platinum oligomers with two types between chloride and respective phenanthroline termination are very useful for the selective synthesis for hybrid polymers with the coupling reaction with two different platinum oligomers with different diethynylaryl ligands. The platinum organometallic compounds showed similar absorption bands in the UV–Vis region. Those prepared with L1 had a strong absorption band at around 400 nm, assignable to the lowest energy metal-perturbed ¹[π–π*] transitions, while in compounds prepared with L2, the strong band appeared around 410 nm, because L2 has an extended π conjugation relative to L1. No distinct differences were observed in the absorption spectra of these platinum oligomers between the different terminal structures, chloride or various phenanthrolines. The luminescence spectra of the platinum compounds prepared with either L1 or L2, however, showed a distinct difference. Those with L1 showed only a phosphorescence assignable to a typical metal-perturbed ³[π–π*] transition with vibronic progressions centered at around 530 nm in deoxygenated CH₂Cl₂ at room temperature, while those with L2 showed weak dual emissions assignable to a mixture of typical metal-perturbed ¹[π–π*] and ³[π–π*] transitions in the visible region.