Triplet–Triplet Energy Transfer from a UV‐A Absorber Butylmethoxydibenzoylmethane to UV‐B Absorbers

The phosphorescence decay of a UV‐A absorber, 4‐tert‐butyl‐4′‐methoxydibenzolymethane (BMDBM) has been observed following a 355 nm laser excitation in the absence and presence of UV‐B absorbers, 2‐ethylhexyl 4‐methoxycinnamate (octyl methoxycinnamate, OMC) and octocrylene (OCR) in ethanol at 77 K. The lifetime of the lowest excited triplet (T₁) state of BMDBM is significantly reduced in the presence of OMC and OCR. The observed quenching of BMDBM triplet by OMC and OCR suggests that the intermolecular triplet–triplet energy transfer occurs from BMDBM to OMC and OCR. The T₁ state of OCR is nonphosphorescent or very weakly phosphorescent. However, we have shown that the energy level of the T₁ state of OCR is lower than that of the enol form of BMDBM. Our methodology of energy‐donor phosphorescence decay measurements can be applied to the study of the triplet–triplet energy transfer between UV absorbers even if the energy acceptor is nonphosphorescent. In addition, the delayed fluorescence of BMDBM due to triplet–triplet annihilation was observed in the BMDBM–OMC and BMDBM–OCR mixtures in ethanol at 77 K. Delayed fluorescence is one of the deactivation processes of the excited states of BMDBM under our experimental conditions.     

Synthesis of a [3.3]biphenylophane, and its photophysical and photochemical properties studied by emission and transient absorption measurements

[3.3](4,4′)Biphenylophane (BPP) is synthesized, and the photophysical and photochemical properties are studied by means of emission and transient absorption measurements. BPP emits excimer fluorescence at 295 and 77 K, and phosphorescence from the locally-excited (LE) triplet state at 77 K. Based on the transient absorption spectra of BPP, it is found that the excimeric triplet state of BPP is produced along with the LE triplet at 295 and 77 K. The triplet excimer of BPP is shown to be formed via intersystem crossing from the singlet excimer state, and concluded to be non-phosphorescent.     

Phase transition sensitive to interlayer defects in layered semiconductor TlGaSe2

The structural and thermal properties of ferroelectric–semiconductor TlGaSe₂ with layered crystalline structure have been investigated in 77–300 K temperature range. It is found that all outlined physical properties of TlGaSe₂ are significantly modified near 180 K due to the phase transition (PT). The nature of this PT has been analyzed and it is established that the main peculiarity of such PT is the extreme sensitivity to interlayer bonding. As a result, any manifestations of this PT depend strongly on defects and impurities located between the layers and can be changed by illumination, temperature annealing and applying electric field.     

Photoexcited States of UV Absorbers,Benzophenone Derivatives

The UV absorption, phosphorescence and phosphorescence‐excitation spectra of benzophenone (BP) derivatives used as organic UV absorbers have been observed in rigid solutions at 77 K. The triplet–triplet absorption spectra have been observed in acetonitrile at room temperature. The BP derivatives studied are 2,2′,4,4′‐tetrahydroxybenzophenone (BP‐2), 2‐hydroxy‐4‐methoxybenzophenone (BP‐3), 2,2′‐dihydroxy‐4,4′‐dimethoxybenzophenone (BP‐6), 5‐chloro‐2‐hydroxybenzophenone (BP‐7) and 2‐hydroxy‐4‐n‐octyloxybenzophenone (BP‐12). The energy levels and lifetimes of the lowest excited triplet (T₁) states of these BP derivatives were determined from the first peak of phosphorescence. The time‐resolved near‐IR emission spectrum of singlet oxygen generated by photosensitization with BP‐7 was observed in acetonitrile at room temperature. BP‐2, BP‐3, BP‐6 and BP‐12 show photoinduced phosphorescence enhancement in ethanol at 77 K. The possible mechanism of the observed phosphorescence enhancement is discussed. The T₁ states of 2‐hydroxy‐5‐methylbenzophenone, 4‐methoxybenzophenone and 2,4′‐dimethoxybenzophenone have been studied for comparison.     

Triplet Excited States and Singlet Oxygen Production by Analogs of Red Wine Pyranoanthocyanins

During the maturation of red wines, the anthocyanins of grapes are transformed into pyranoanthocyanins, which possess a pyranoflavylium cation as their basic chromophore. Photophysical properties of the singlet and triplet excited states of a series of synthetic pyranoflavylium cations were determined at room temperature in acetonitrile solution acidified with 0.10 mol dm^?3 trifluoroacetic acid (TFA, to inhibit competitive excited state proton transfer) and at 77 K in a rigid TFA‐acidified isopropanol glass. In solution, the triplet states of these pyranoflavylium cations are efficiently quenched by molecular oxygen, resulting in sensitized formation of singlet oxygen, as confirmed by direct detection of the triplet‐state decay by laser flash photolysis and of singlet oxygen monomol emission in the near infrared. The strong visible light absorption, the relatively small singlet‐triplet energy differences, the excited state redox potentials and the reasonably long lifetimes of pyranoflavylium triplet states in the absence of molecular oxygen suggest that they might be useful as triplet sensitizers and/or as cationic redox initiators in polar aprotic solvents like acetonitrile.     

One‐Component Thioxanthone Acetic Acid Derivative Photoinitiator for Free Radical Polymerization

Acetic acid–based thioxanthone (TXCH₂COOH) was synthesized and characterized and used as a photoinitiator for free radical photopolymerization of methyl methacrylate (MMA) in the absence and presence of a tertiary amine (MDEA) in different solvents. Different absorption properties were observed depending on the solvent. Fluorescence and phosphorescence experiments were also carried out successfully. The fluorescence quantum yield was found to be 0.09 and the phosphorescence lifetime was calculated as 138 ms at 77 K. The photoinitiator undergoes efficient intersystem crossing into the triplet state and the lowest triplet state possesses π–π* configuration. Laser flash photolysis experiments show that transient absorption of TXCH₂COOH is similar to the parent thioxanthone and the triplet lifetime was calculated as 2.3 μs at 630 nm.     

Synthesis of urea‐containing ABA triblock copolymers: Influence of pendant hydrogen bonding on morphology and thermomechanical properties

Reversible addition‐fragmentation chain transfer (RAFT) polymerization produced novel ABA triblock copolymers with associative urea sites within pendant groups in the external hard blocks. The ABA triblock copolymers served as models to study the influence of pendant hydrogen bonding on polymer physical properties and morphology. The triblock copolymers consisted of a soft central block of poly(di(ethylene glycol) methyl ether methacrylate) (polyDEGMEMA, 58 kg/mol) and hard copolymer external blocks of poly(2‐(3‐hexylureido)ethyl methacrylate‐co‐2‐(3‐phenylureido)ethyl methacrylate) (polyUrMA, 18‐116 kg/mol). Copolymerization of 2‐(3‐hexylureido)ethyl methacrylate (HUrMA) and 2‐(3‐phenylureido)ethyl methacrylate (PhUrMA) imparted tunable hard block T_g's from 69 to 134 °C. Tunable hard block T_g's afforded versatile thermomechanical properties for diverse applications. Dynamic mechanical analysis (DMA) of the triblock copolymers exhibited high modulus plateau regions (∼100 MPa) over a wide temperature range (−10 to 90 °C), which was indicative of microphase separation. Atomic force microscopy (AFM) confirmed surface microphase separation with various morphologies. Variable temperature FTIR (VT‐FTIR) revealed the presence of both monodentate and bidentate hydrogen bonding, and pendant hydrogen bonding remained as an ordered structure to higher than expected temperatures. This study presents a fundamental understanding of the influence of hydrogen bonding on polymer physical properties and reveals the response of pendant urea hydrogen bonding as a function of temperature as compared to main chain polyureas. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1844–1852     

Energy Dissipation Processes of singlet‐excited 1‐Hydroxyfluorenone and its Hydrogen‐bonded Complex with N‐methylimidazole¶

Effects of solvent, pH and hydrogen bonding with N‐methylimidazole (MIm) on the photophysical properties of 1‐hydroxyfluorenone (1HOF) have been studied. Fluorescence lifetime, fluorescence quantum yield and triplet yield measurements demonstrated that intersystem crossing was the dominant process in apolar media and its rate constant significantly diminished with increasing solvent polarity. The acceleration of internal conversion in alcohols paralleled the strength of intermolecular hydrogen bonding. The faster energy dissipation from the singlet‐excited state in cyclohexane was attributed to intramolecular hydrogen bonding. The pK_a of 1HOF decreased from 10.06 to 5.0 on light absorption, and H₃O⁺ quenched the singletexcited molecules in a practically diffusion‐controlled reaction. On addition of MIm in toluene, dual fluorescence was observed, which was attributed to reversible formation of excited hydrogen‐bonded ion pair. Rate constants for the various deactivation pathways were derived from the combined analysis of the steady‐state and the time‐resolved fluorescence results.     

Intramolecular Hydrogen Bonding in Benzoxazines: When Structural Design Becomes Functional

The future evolution of benzoxazines and polybenzoxazines as advanced molecular, structural, functional, engineering, and newly commercial materials depends to a great extent on a deeper and more fundamental understanding at the molecular level. In this contribution, the field of benzoxazines is briefly introduced along with a more detailed review of ortho‐amide‐functional benzoxazines, which are the main subjects of this article. Provided in this article are the detailed and solid scientific evidences of intramolecular five‐membered‐ring hydrogen bonding, which is supposed to be responsible for the unique and characteristic features exhibited by this ever‐growing family of ortho‐functionalized benzoxazines. One‐dimensional (1D) ¹H NMR spectroscopy was used to study various concentrations of benzoxazines in various solvents with different hydrogen‐bonding capability and at various temperatures to investigate in detail the nature of hydrogen bonding in both ortho‐amide‐functionalized benzoxazine and its para counterpart. These materials were further investigated by two‐dimensional (2D) ¹H–¹H nuclear Overhauser effect spectroscopy (NOESY) to verify and support the conclusions derived during the 1D ¹H NMR experiments. Only highly purified single‐crystal benzoxazine samples have been used for this study to avoid additional interactions caused by any impurities.     

Intermolecular Hydrogen Bonding Modulates O‐H Photodissociation in Molecular Aggregates of a Catechol Derivative

The catechol functional group plays a major role in the chemistry of a wide variety of molecules important in biology and technology. In eumelanin, intermolecular hydrogen bonding between these functional groups is thought to contribute to UV photoprotective and radical buffering properties, but the mechanisms are poorly understood. Here, aggregates of 4‐t‐butylcatechol are used as model systems to study how intermolecular hydrogen bonding influences photochemical pathways that may occur in eumelanin. Ultrafast UV‐visible and mid‐IR transient absorption measurements are used to identify the photochemical processes of 4‐t‐butylcatechol monomers and their hydrogen‐bonded aggregates in cyclohexane solution. Monomer photoexcitation results in hydrogen atom ejection to the solvent via homolytic O‐H bond dissociation with a time constant of 12 ps, producing a neutral semiquinone radical with a lifetime greater than 1 ns. In contrast, intermolecular hydrogen bonding interactions within aggregates retard O‐H bond photodissociation by over an order of magnitude in time. Excited state structural relaxation is proposed to slow O‐H dissociation, allowing internal conversion to the ground state to occur in hundreds of picoseconds in competition with this channel. The semiquinone radicals formed in the aggregates exhibit spectral broadening of both their electronic and vibrational transitions.     

Photophysical Characterization of the Naturally Occurring Dioxobacteriochlorin Tolyporphin A and Synthetic Oxobacteriochlorin Analogues

Tolyporphins are tetrapyrrole macrocycles produced by a cyanobacterium‐containing culture known as HT‐58‐2. Tolyporphins A–J are free base dioxobacteriochlorins, whereas tolyporphin K is an oxochlorin. Here, the photophysical characterization is reported of tolyporphin A and two synthetic analogues, an oxobacteriochlorin and a dioxobacteriochlorin. The characterization (in toluene, diethyl ether, ethyl acetate, dichloromethane, 1‐pentanol, 2‐butanone, ethanol, methanol, N,N‐dimethylformamide and dimethylsulfoxide) includes static absorption and fluorescence spectra, fluorescence quantum yields and time‐resolved data. The data afford the lifetime of the lowest singlet excited state and the yields of the nonradiative decay pathways (intersystem crossing and internal conversion). The three macrocycles exhibit only modest variation in spectroscopic and excited‐state photophysical parameters across the solvents. The long‐wavelength (Q_y) absorption band of tolyporphin A appears at ~680 nm and is remarkably narrow (full‐width‐at‐half‐maximum ~7 nm). The position of the long‐wavelength (Q_y) absorption band of tolyporphin A (~680 nm) more closely resembles that of chlorophyll a (662 nm) than bacteriochlorophyll a (772 nm). The absorption spectra of tolyporphins B–I, K (which were available in minute quantities) are also reported in methanol; the spectra of B–I closely resemble that of tolyporphin A. Taken together, tolyporphin A generally exhibits spectral and photophysical features resembling those of chlorophyll a.     

Probing the Hydrogen Bonding Structure in the Rieske Protein

The use of the far‐infrared spectral range presents a novel approach for analysis of the hydrogen bonding in proteins. Here it is presented for the analysis of Fe? S vibrations (500–200 cm^?1) and of the intra‐ and intermolecular hydrogen bonding signature (300–50 cm^?1) in the Rieske protein from Thermus thermophilus as a function of temperature and pH. Three pH values were adequately chosen in order to study all the possible protonation states of the coordinating histidines. The Fe? S vibrations showed pH‐dependent shifts in the FIR spectra in line with the change of protonation state of the histidines coordinating the [2Fe? 2S] cluster. Measurements of the low‐frequency signals between 300 and 30 K demonstrated the presence of a distinct overall hydrogen bonding network and a more rigid structure for a pH higher than 10. To further support the analysis, the redox‐dependent shifts of the secondary structure were investigated by means of an electrochemically induced FTIR difference spectroscopic approach in the mid infrared. The results confirmed a clear pH dependency and an influence of the immediate environment of the cluster on the secondary structure. The results support the hypothesis that structure‐mediated changes in the environment of iron? sulfur centers play a critical role in regulating enzymatic catalysis. The data point towards the role of the overall internal hydrogen bonding organization for the geometry and the electronic properties of the cluster.     

Phenylazide Hybrid‐Silica – Polarization Platform for Dynamic Nuclear Polarization at Cryogenic Temperatures

Hyperpolarization of NMR‐active nuclei is key to gather high quality spectra of rare species and insensitive isotopes. We have recently established that silica‐based materials containing regularly distributed nitroxyl radicals connected to the silica matrix by flexible linkers can serve as promising polarization matrices for dynamic nuclear polarization (DNP). Here we investigate the influence of the linker on the efficiency of the polarization. The materials were fully characterized and exhibit high surface areas and narrow pore size distributions with a tunable amount of phenyl azide groups over a broad range of concentrations. The phenyl azide groups can be easily functionalized via a two‐step procedure with 4‐carboxy‐2,2,6,6‐tetramethyl‐1‐oxylpiperidine (TEMPO) to give polarizing matrices with controllable radical content. The DNP efficiency was found to be similar as in materials with flexible linkers, both for magic angle spinning at 105 K and dissolution DNP at 4 K.     

Tuning the excited-state energy of the organic chromophore in bichromophoric systems based on the Ru(II) complexes of tridentate ligands

A series of new heteroleptic and homoleptic Ru(II) complexes containing variously substituted bis(pyridyl)triazine ligands has been prepared and their absorption spectra, redox behaviour and luminescence properties (both in fluid solution at room temperature and in a rigid matrix at 77 K) have been investigated. For some compounds, X-ray structures have also been determined. The new bis(pyridyl)triazines incorporate additional chromophores, such as biphenyl, phenanthrene, anthracene and bromoanthracene derivatives, so the Ru(II) species can be considered as multichromophoric species. The absorption spectra and redox properties of all the metal complexes have been assigned to features belonging to specific subunits, thus suggesting that these species can be regarded as multicomponent, supramolecular assemblies from an electronic coupling point of view. Whereas most of the complexes exhibit luminescence properties that can be attributed to metal-to-ligand charge-transfer (MLCT) states involving the metal-based subunit(s), the species containing the anthryl and, even more, the brominated anthryl chromophores exhibit complicated luminescence behaviour. For example, 2 d (the anthryl-containing heteroleptic metal compound) exhibits MLCT emission at room temperature and emission from the anthracene triplet at 77 K; 2 e (the bromo-substituted anthryl-containing heteroleptic metal compound) exhibits anthryl-based emission at 77 K and MLCT emission at room temperature, but with a prolonged lifetime, thus suggesting equilibration between two triplet states that belong to different chromophores. The equilibration regime between MLCT and aromatic hydrocarbon triplet states is therefore reached by suitable substitution on the organic chromophore.     

Effect of hydrogen bonding on the intramolecular charge transfer fluorescence of 6-dodecanoyl-2-dimethylaminonaphtalene

The effect of hydrogen bonding on the intramolecular charge transfer (ICT) of 6-dodecanoyl-2-dimethylaminonaphtalene (laurdan) in neat and binary solvent mixtures has been investigated by using steady-state and time-resolved spectroscopic techniques. The different features of ICT emission of laurdan in methylcyclohexane–tetrahydrofuran and methylcyclohexane–ethanol are explained by the absence and presence of hydrogen bonded ICT. The presence of isosbestic point in absorption spectra of laurdan in methylcyclohexane–ethanol confirms the formation of 1:1 complex between laurdan and ethanol. The obtained data were used to determine the stoichiometric equilibrium constants. In protic rigid (77 K) the fluorescence spectra of laurdan show excitation wavelength dependence (the red-edge effect). Moreover, we reported the decay characteristics of laurdan molecule in locally excited (LE) and ICT state in methylcyclohexane–ethanol.     

SPECTROSCOPIC STUDIES OF PURINES—I. FACTORS AFFECTING THE FIRST EXCITED LEVELS OF PURINE*

Abstract— Studies of purine absorption and emission in seven solvents differing greatly in dielectric constant and hydrogen bonding potential, reveal a variety of solvent effects. For example, the resolution of structure in the absorption spectrum, the position and/or intensity of the X₂ absorption band, the intensity of fluorescence, the magnitude of the long wave-lenth tail, and the position of the X₁ absorption band are differentially affected—in the order listed—by the solvents tested. Even though it is possible to correlate the extent of decrease in the n-π* tail with increasing solvent dielectric constant, probably alterations in all of these spectroscopic parameters depend most critically upon the ability of the various solvents to form hydrogen bonds with the hydrogen on N9 and/for with the non-bonding electrons on the purine nitrogens: it is tentatively concluded that the probability of hydrogen bonding is directly correlated with the electronegativity of the aza nitrogens (N7 > N3 > N1). In solvents like isopropanol not all of the non-bonding electrons must be solvated maximally in most purine molecules since there is appreciable fluorescence under conditions where a long wavelength tail is readily observed in the absorption spectrum (alternatively some noa-bonding electrons may not te relevant to fluorescence quenching.) Decreases in fluorescence yield are associated with red shifts in the fluorescence maximum, and in the solvents of highest polarity the fluorescence yield is again small indicating that glycerol and water can enhance radiationless tunneling—presumably by altering Franck-Condon configurations and/or improving electronic-vibrational coupling between solute and solvent. The quantum yield is uniform throughout the atsorption band for a given solvent, but studies in aqueous buffers varying from pH 1 to 11 show that the fluorescence yield is greater for charged than for neutral molecules. Further, the fluorescence excitation peak is red shifted in powders. Since phosphorescence is the predominant emission at 777deg;K and increases in fluorescence can be correlated with the presumed solvation of non-bonding electrons, the singlet excited state of lowest energy in ‘unperturbed’ purine must be n-π* in nature. The shape of the phosphorescence band and the decay lifetime of ? 1 sec at 77°K lead to the conclusion that the emitting triplet is a π-π* state. The eight vibrational structures in phosphorescence emission can be readily grouped into two progressions: there is an average separation of about 1300 cm^-1 between peaks within a given progression, and the two sets are mutually displaced by about 500 cm-^l. Individual vibrational peaks are favoured in different solvents and the whole band may be shifted up to 500 cm^-l. Even larger shifts are observed in charged purine molecules and in powders (up to 3000 cm^-l) and the presumed 0–0 band is not observed.     

Synthesis,Structure, and Properties of Scandium Dysprosium Antimonide ScDySb

Scandium dysprosium antimonide ScDySb was synthesized from scandium metal and DySb in an all‐solid state reaction at 1770 K. According to X‐ray analysis of the crystal structure [P4/nmm, Z = 4, a = 430.78(1) pm, c = 816.43(4) pm, R₁ = 0.0238, wR(all) = 0.0688, 268 independent reflections], ScDySb adopts the anti‐PbFCl type of structure, but with pronounced deviations in structural details, which are related to specific bonding interactions between the atoms involved. ScDySb shows antiferromagnetic ordering below 35.4 K, which was verified by susceptibility, heat capacity, and resistivity measurements. X‐ray structure determination, performed at 30 K, showed no significant structural changes to occur during the magnetic phase transition. The band structure was calculated in the framework of Density Functional Theory. The bonding properties are comparable to those of Sc₂Sb. Pronounced basins of the Electron Localization Function (ELF) appear in the tetragonal pyramidal Sc₄Dy voids.     

Thymine-substituted nitronyl nitroxide biradical as a triplet (S = 1) component for bio-inspired molecule-based magnets

As a novel crystal engineering approach to organic molecule-based magnets, we have proposed a strategy of bio-inspired molecular assemblage based on intermolecular hydrogen bonding. Complementary hydrogen bonding between nucleobases as found in DNA is a promising non-covalent interaction for controlling the molecular arrangement of open-shell building block molecules. The hydrogen bonding of complementary nucleobases substituted with radical entities of different spin quantum numbers S, e.g., S = 1 and S = 1/2, gives rise to a heteromolecular aggregation of the S = 1 and S = 1/2 entities, leading to organic ferrimagnetics. In this study, we have designed and synthesized a thymine-substituted nitronyl nitroxide biradical (1) as a triplet (S = 1) component for the bio-inspired ferrimagnetic system. The molecular ground state of 1 has been found to be triplet (S = 1) with a singlet–triplet energy gap of 2J/k_B = 21.4 K from magnetic susceptibility measurements. It has been found from X-ray structure analyses that the molecules form hydrogen-bonded aggregates in the crystalline solid state, in which the thymine moiety plays a primary role in the molecular packing. The ground-state triplet biradical serves as an S = 1 building block for bio-inspired molecule-based magnets with hydrogen-bonded nucleobase pairings.     

New Fluorescent Conjugates of Uridine Nucleoside and Substituted 1,8-Naphthalimide: Synthesis, Weak Interactions and Solvent Effects on Spectra

Summary.  Novel fluorescent conjugates of uridine nucleoside and 4-dimethylamino-1,8-naphthalimide via linkage with different length, and their precursors were synthesized. Their spectroscopic properties were examined in ten different solvents. It was found that the spectroscopic properties for these conjugates are strongly dependent on polarity and hydrogen bonding ability of solvents. Their fluorescence spectra are also strongly influenced by intramolecular aromatic stacking and hydrogen bonding between the base or sugar moiety of the uridine nucleoside and naphthalimide moiety, which is controlled by the length of the linker. Corresponding author. E-mail: xhqian@dlut.edu.cn; xhqian@ecust.edu.cn Received May 27, 2002; accepted (revised) July 16, 2002