Photoisomerization of cis‐1,2‐di(1‐Methyl‐2‐naphthyl)ethene at 77 K in Glassy Media

cis‐1,2‐Di(1‐methyl‐2‐naphthyl)ethene, c‐ 1,1 , undergoes photoisomerization in methylcyclohexane, isopentane and diethyl ether/isopentane/ethanol glasses at 77 K. On 313 nm excitation the fluorescence of c‐ 1,1 is replaced by fluorescence from t‐ 1,1 . Singular value decomposition reveals that the spectral matrices behave as two component systems suggesting conversion of a stable c‐ 1,1 conformer to a stable t‐ 1,1 conformer. However, the fluorescence spectra are λ_exc dependent. Analysis of global spectral matrices shows that c‐ 1,1 is a mixture of two conformers, each of which gives one of four known t‐ 1,1 conformers. The λ_exc dependence of the c‐ 1,1 fluorescence spectrum is barely discernible. Structure assignments to the resolved fluorescence spectra are based on the principle of least motion and on calculated geometries, energy differences and spectra of the conformers. The relative shift of the c‐ 1,1 conformer spectra is consistent with the shift of the calculated absorption spectra. The calculated structure of the most stable conformer of c‐ 1,1 agrees well with the X‐ray crystal structure. Due to large deviations of the naphthyl groups from the ethenic plane in the conformers of both c‐ and t‐ 1,1 isomers, minimal motion of these bulky substituents accomplishes cis → trans interconversion by rotation about the central bond.     

Modulating Photo‐ and Electroluminescence in a Stimuli‐Responsive π‐Conjugated Donor–Acceptor Molecular System

Functional organic materials that display reversible changes in fluorescence in response to external stimuli are of immense interest owing to their potential applications in sensors, probes, and security links. While earlier studies mainly focused on changes in photoluminescence (PL) color in response to external stimuli, stimuli‐responsive electroluminescence (EL) has not yet been explored for color‐tunable emitters in organic light‐emitting diodes (OLEDs). Here a stimuli‐responsive fluorophoric molecular system is reported that is capable of switching its emission color between green and orange in the solid state upon grinding, heating, and exposure to chemical vapor. A mechanistic study combining X‐ray diffraction analysis and quantum chemical calculations reveals that the tunable green/orange emissions originate from the fluorophore's alternating excited‐state conformers formed in the crystalline and amorphous phases. By taking advantage of this stimuli‐responsive fluorescence behavior, two‐color emissive OLEDs were produced using the same fluorophore in different solid phases.     

Red/Near‐Infrared Thermally Activated Delayed Fluorescence OLEDs with Near 100 % Internal Quantum Efficiency

Developing red thermally activated delayed fluorescence (TADF) emitters, attainable for both high‐efficient red organic light‐emitting diodes (OLEDs) and non‐doped deep red/near‐infrared (NIR) OLEDs, is challenging. Now, two red emitters, BPPZ‐PXZ and mDPBPZ‐PXZ, with twisted donor–acceptor structures were designed and synthesized to study molecular design strategies of high‐efficiency red TADF emitters. BPPZ‐PXZ employs the strictest molecular restrictions to suppress energy loss and realizes red emission with a photoluminescence quantum yield (Φ_PL) of 100±0.8 % and external quantum efficiency (EQE) of 25.2 % in a doped OLED. Its non‐doped OLED has an EQE of 2.5 % owing to unavoidable intermolecular π–π interactions. mDPBPZ‐PXZ releases two pyridine substituents from its fused acceptor moiety. Although mDPBPZ‐PXZ realizes a lower EQE of 21.7 % in the doped OLED, its non‐doped device shows a superior EQE of 5.2 % with a deep red/NIR emission at peak of 680 nm.     

Peptide models XXIII. Conformational model for polar side‐chain containing amino acid residues: A comprehensive analysis of RHF,DFT, and MP2 properties of HCO‐L‐SER‐NH2

Geometric and energetic properties of a diamide of serine, HCO‐NH‐L ‐CH(CH₂OH)CO‐NH₂, are investigated by standard methods of computational quantum chemistry. Similarly to other amino acid residues, conformational properties of HCO‐L ‐Ser‐NH₂ can be derived from the analysis of its E=E(ϕ,ψ;χ₁,χ₂) hypersurface. Reoptimization of 44 RHF/3‐21G conformers at the RHF/6‐311++G** level resulted in 36 minima. For all conformers, geometrical properties, including variation of H‐bond parameters and structural shifts in the torsional space, are thoroughly investigated. Results from further single‐point energy calculations at the RHF, DFT, and MP2 levels, performed on the entire conformational data set, form a database of 224 energy values, perhaps the largest set calculated so far for any single amino acid diamide. A comprehensive analysis of this database reveals significant correlation among energies obtained at six levels of ab initio theory. Regression parameters provide an opportunity for extrapolation in order to predict the energy of a conformer at a high level by doing explicit ab initio computations only for a few selected conformers. The computed conformational and relative energy data are compared with structural and occurrence results derived from a nonhomologous protein database incorporating 1135 proteins. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 626–655, 2000     

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

This paper reports on the gas‐phase radical–radical dynamics of the reaction of ground‐state atomic oxygen [O(³P), from the photodissociation of NO₂] with secondary isopropyl radicals [(CH₃)₂CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O(³P)+(CH₃)₂CH→C₃H₆ (propene)+OH, is examined by high‐resolution laser‐induced fluorescence spectroscopy in crossed‐beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X²Π) products with low‐ and high‐N′′ components in a ratio of 1.25:1. No significant spin–orbit or Λ‐doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS‐QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential‐energy surface: direct abstraction, giving the dominant low‐N′′ components, and formation of short‐lived addition complexes that result in hot rotational distributions, giving the high‐N′′ components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast‐flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O(³P) with primary and tertiary hydrocarbon radicals allows molecular‐level discussion of the reactivity and mechanism of the title reaction.     

Single‐Molecule Spectroscopy,Imaging, and Photocontrol: Foundations for Super‐Resolution Microscopy (Nobel Lecture)

The initial steps toward optical detection and spectroscopy of single molecules in condensed matter arose out of the study of inhomogeneously broadened optical absorption profiles of molecular impurities in solids at low temperatures. Spectral signatures relating to the fluctuations of the number of molecules in resonance led to the attainment of the single‐molecule limit in 1989 using frequency‐modulation laser spectroscopy. In the early 90s, many fascinating physical effects were observed for individual molecules, and the imaging of single molecules as well as observations of spectral diffusion, optical switching and the ability to select different single molecules in the same focal volume simply by tuning the pumping laser frequency provided important forerunners of the later super‐resolution microscopy with single molecules. In the room temperature regime, imaging of single copies of the green fluorescent protein also uncovered surprises, especially the blinking and photoinduced recovery of emitters, which stimulated further development of photoswitchable fluorescent protein labels. Because each single fluorophore acts a light source roughly 1 nm in size, microscopic observation and localization of individual fluorophores is a key ingredient to imaging beyond the optical diffraction limit. Combining this with active control of the number of emitting molecules in the pumped volume led to the super‐resolution imaging of Eric Betzig and others, a new frontier for optical microscopy beyond the diffraction limit. The background leading up to these observations is described and current developments are summarized.     

Interplay of Exciton Coupling and Large‐Amplitude Motions in the Vibrational Circular Dichroism Spectrum of Dehydroquinidine

A detailed analysis of the computed structure, energies, vibrational absorption (VA) and circular dichroism (VCD) spectra of 30 low‐energy conformers of dehydroquinidine reveals the existence of families of pseudo‐conformers, the structures of which differ mostly in the orientation of a single O?H bond. The pseudo‐conformers in a family are separated by very small energy barriers (i.e., 1.0 kcal mol^?1 or smaller) and have very different VCD spectra. First, we demonstrate the unreliable character of the Boltzmann factors predicted with DFT. Then, we show that the large differences observed between the VCD spectra of the pseudo‐conformers in a family are caused by large‐amplitude motions involving the O?H bond, which trigger the appearance/disappearance of strong VCD exciton‐coupling bands in the fingerprint region. This interplay between exciton coupling and large‐amplitude‐motion phenomena demonstrates that when dealing with flexible molecules with polar bonds, vibrational averaging of VCD spectra should not be neglected. In this regard, the dehydroquinidine molecule considered here is expected to be a typical example and not the exception to the rule.     

Polymorph‐Dependent Thermally Activated Delayed Fluorescence Emitters: Understanding TADF from a Perspective of Aggregation State

Current research on thermally activated fluorescence (TADF) emitters is mainly based on the molecular levels, while the aggregation states of TADF emitters are to be explored deeply. Now two multifunctional emitters are reported with simultaneous TADF, aggregation induced emission (AIE), and multicolor mechanochromic luminescence (MCL) features. Both emitters also show polymorph‐dependent TADF emission. Crystal structure analysis reveals that the polymorphism is ascribed to the mutable conformations in different aggregation states. This work brings new insight to TADF emitters from a perspective of aggregation states.     

Fluorophores as Optical Sensors for Local Forces

The main aim of this study is to investigate correlations between the impact of an external mechanical force on the molecular framework of fluorophores and the resultant changes in their fluorescence properties. Taking into account previous theoretical studies, we designed a suitable custom‐tailored oligoparaphenylenevinylene derivative (OPV5) with a twisted molecular backbone. Thin foils made of PVC doped with 100 nM OPV were prepared. By applying uniaxial force, the foils were stretched and three major optical effects were observed simultaneously. First, the fluorescence anisotropy increased, which indicates a reorientation of the fluorophores within the matrix. Second, the fluorescence lifetime decreased by approximately 2.5 % (25 ps). Finally, we observed an increase in the emission energy of about 0.2 % (corresponding to a blue‐shift of 1.2 nm). In addition, analogous measurements with Rhodamine 123 as an inert reference dye showed only minor effects, which can be attributed to matrix effects due to refractive index changes. To relate the observed spectroscopic changes to the underlying changes in molecular properties, quantum‐chemical calculations were also performed. Semiempirical methods had to be used because of the size of the OPV5 chromophore. Two conformers of OPV5 (C₂ and C_i symmetry) were considered and both gave very similar results. Both the observed blue‐shift of fluorescence and the reduced lifetime of OPV5 under tensile stress are consistent with the results of the semiempirical calculations. Our study proves the feasibility of fluorescence‐based local force probes for polymers under tension. Improved optical sensors of this type should in principle be able to monitor local mechanical stress in transparent samples down to the single‐molecule level, which harbors promising applications in polymer science and nanotechnology.     

A Structurally Variable Porous Organic Salt Based on a Multidirectional Supramolecular Cluster

A porous organic salt (POS) composed of 2‐sulfophenyl anthracene (2‐SPA) and triphenylmetylamine (TPMA) forms five types of porous crystals, POS‐a–e, by recognizing subtle differences in the molecular structure of incorporated guest molecules. This structurally variable POS was hierarchically designed on the basis of a supramolecular cluster with a directionally flexible linker formed by the organic salt. X‐ray crystallographic analysis reveals that the salt forms six conformers attributable to rocking and rotational motions of the phenylene group in 2‐SPA. The clusters form POS crystals through different porous networks according to the conformers. The POS crystals show a wide range of fluorescence spectra that are responsive to differences in the molecular and electronic structure of the guest molecule. This remarkable behavior has potential application in sensitive chemical sensors that are responsive to slight differences in molecular structures.     

Structure and cooperativity of the hydrogen bonds in sodium dihydrogen triacetate

Geometry and energetics of low energy conformers of sodium dihydrogen triacetate (SDHTA) and its anion are studied using density functional theory (DFT) at the Becke, Lee‐Yang‐Parr hybrid functional (BLYP) and Becke, three‐parameter, Lee‐Yang‐Parr hybrid functional (B3LYP) levels. For both cases, two structures of comparable energy are found, which have different symmetry with respect to the two hydrogen bonds (HBs). DFT‐based Born–Oppenheimer molecular dynamics simulations are performed for SDHTA, which show that both structures are visited at room temperature conditions. The trajectory analysis further reveals that the two HBs behave anticooperative, that is, on average elongation of one HB is accompanied by a compression of the other one. This is in accord with nuclear magnetic resonance (NMR) experimental studies for a similar counter ion–dihydrogen triacetate complex. © 2012 Wiley Periodicals, Inc.     

Discovery of a New Light–Molecule Interaction: Supracence Reveals What Is Missing in Fluorescence Imaging

The currently understood principles about light–molecule interactions are limited, and thus scientific scope beyond current theories is rarely harvested. Herein we demonstrate supracence phenomena, in which the emitted photons have more energy than the absorbed photons. The extra energy comes from couplings of the absorbed and emitted photon to molecular phonons, whose potentials are constantly exchanging with molecular quantum energy and the environment. Thus, supracence is a linear optical process rather than a nonlinear optical process, such as second harmonic generation. Because supracence results in cooled molecular phonons and thus cooled molecules, behavior opposite to that of hot fluorescing emitters is expected. This report reveals certain supracence principles while contrasting fluorescence with supracence in high‐resolution imaging.     

Walking Down the Chalcogenic Group of the Periodic Table: From Singlet to Triplet Organic Emitters

The synthesis, X‐ray crystal structures, ground‐ and excited‐state UV/Vis absorption spectra, and luminescence properties of chalcogen‐doped organic emitters equipped on both extremities with benzoxa‐, benzothia‐, benzoselena‐ and benzotellurazole ( 1_X and 2_X ) moieties have been reported for the first time. The insertion of the four different chalcogen atoms within the same molecular skeleton enables the investigation of only the chalcogenic effect on the organisation and photophysical properties of the material. Detailed crystal‐structure analyses provide evidence of similar packing for 2_O – 2_Se , in which the benzoazoles are engaged in π–π stacking and, for the heavier atoms, in secondary X???X and X???N bonding interactions. Detailed computational analysis shows that the arrangement is essentially governed by the interplay of van der Waals and secondary bonding interactions. Progressive quenching of the fluorescence and concomitant onset of phosphorescence features with gradually shorter lifetimes are detected as the atomic weight of the chalcogen heteroatom increases, with the tellurium‐doped derivatives exhibiting only emission from the lowest triplet excited state. Notably, the phosphorescence spectra of the selenium and tellurium derivatives can be recorded even at room temperature; this is a very rare finding for fully organic emitters.     

Two 1,2,4‐Triazole‐based Zinc(II) Complexes with Aromatic Acid as Coligand: Synthesis,Structure and Fluorescence Properties

Two two‐dimensional (2‐D) trz‐based coordination polymers, {[Zn(trz)(mb)]·H₂O}_n ( 1 ) and {[Zn(trz)(ca)]·H₂O}_n ( 2 ) (Htrz = 1,2,4‐triazole, Hmb = 4‐methylbenzoic acid, and Hca = trans‐cinnamic acid), have been synthesized by diffusion method and fully structural characterized by elemental analysis, FT‐IR, single‐crystal X‐ray crystallography, TG and fluorescence spectra. Structural analysis reveals that both complexes exhibit the analogous 2‐D Zn^II‐trz layer motif with hydrophobic aromatic rings attached on both sides despite their different crystal system and space group (orthorhombic, Pbca for 1 and monoclinic, P2₁/c for 2 ). Interestingly, the discrete water‐dimer and infinite 1‐D water‐chain were observed to be entrapped in the 2‐D layer of 1 and 2 , respectively, resulted from the different orientation of lattice water molecules as well as the patterns of hydrogen bonds involved. In addition, their similiar thermal behaviors and fluorescence emissions originated from intraligand electronic transfer were also investigated and compared.     

Cerium‐Based M4L4 Tetrahedra as Molecular Flasks for Selective Reaction Prompting and Luminescent Reaction Tracing

The application of metal–organic polyhedra as “molecular flasks” has precipitated a surge of interest in the reactivity and property of molecules within well‐defined spaces. Inspired by the structures of the natural enzymatic pockets, three metal–organic neutral molecular tetrahedral, Ce‐TTS, Ce‐TNS and Ce‐TBS (H₆TTS: N′,N′′,N′′′‐nitrilotris‐4,4′,4′′‐(2‐hydroxybenzylidene)‐benzohydrazide; H₆TNS: N′,N′′,N′′′‐nitrilotris‐6,6′,6′′‐(2‐hydroxybenzylidene)‐2‐naphthohydrazide; H₆TBS: 1,3,5‐ phenyltris ‐4,4′,4′′‐(2‐hydroxybenzylidene)benzohydrazide), which exhibit different size of the edges and cavities, were achieved through self‐assembly by incorporating robust amide‐containing tridentate chelating sites into the fragments of the ligands. They acted as molecular flasks to prompt the cyanosilylation of aldehydes with excellent selectivity towards the substrates size. The amide groups worked as trigger sites and catalytic driven forces to achieve efficient guest interactions, enforcing the substrates proximity within the cavity. Experiments on catalysts with the different cavity radii and substrates with the different molecular size demonstrated that the catalytic performance exhibited enzymatical catalytic mechanism and occurred in the molecular flask. These amides were also able to amplify guest‐bonding events into the measurable outputs for the detection of concentration variations of the substrates, providing the possibility for metal–organic hosts to work as smart molecular flasks for the luminescent tracing of catalytic reactions.     

Freezing single molecule dynamics on interfaces and in polymers

Heterogeneous line broadening and spectral diffusion of the fluorescence emission spectra of perylene diimide molecules have been investigated by means of time dependent single molecule spectroscopy. The influence of temperature and environment has been studied and reveals strong correlation to spectral diffusion processes. We followed the freezing of the molecular mobility of quasi free molecules on the surface upon temperature lowering and by embedding into a poly(methyl methacrylate) (PMMA) polymer. Thereby changes of optical transition energies as a result of both intramolecular changes of conformation and external induced dynamics by the surrounding polymer matrix could be observed. Simulations of spectral fluctuations within a two-level system (TLS) model showed good agreement with the experimental findings.     

Ab Initio MP2 and Density Functional Theory Computational Study of AcAlaNH2 Peptide Hydration: A Bottom‐Up Approach

AcAlaNH₂?n H₂O (n=1–13) complexes have been proposed as models to account for water solvent effects on the molecular properties of N‐acetyl‐L ‐alanine amide. Ab initio computations are planned to evaluate peptide–water interactions and to provide a means for approximating relative effects of the short‐range many‐body interactions arising in real solution without introducing any external parameters intended to quantify empirical or semiempirical potential‐energy functions. The present bottom‐up approach reveals the formation of compact ring clusters of water molecules strongly bonded to peptidic polar groups throughout hydrogen bonds. The explicit coordination of water molecules around the peptide renders the fully extended (FE) and polyproline II (PPII) conformers more stable with respect to the 3₁₀ helix or γ turn. The alternance of donor and acceptor groups on both sides of the FE and PPII conformers allows for synergy and extensive H‐bonding.     

Polymorphs of the antiviral drug ganciclovir

Ganciclovir (GCV; systematic name: 2‐amino‐9‐{[(1,3‐dihydroxypropan‐2‐yl)oxy]methyl}‐6,9‐dihydro‐1H‐purin‐6‐one), C₉H₁₃N₅O₄, an antiviral drug for treating cytomegalovirus infections, has two known polymorphs (Forms I and II), but only the structure of the metastable Form II has been reported [Kawamura & Hirayama (2009). X‐ray Struct. Anal. Online , 25 , 51–52]. We describe a successful preparation of GCV Form I and its crystal structure. GCV is an achiral molecule in the sense that its individual conformers, which are generally chiral objects, undergo fast interconversion in the liquid state and cannot be isolated. In the crystalline state, GCV exists as two inversion‐related conformers in Form I and as a single chiral conformer in Form II. This situation is similar to that observed for glycine, also an achiral molecule, whose α‐polymorph contains two inversion‐related conformers, while the γ‐polymorph contains a single conformer that is chiral. The hydrogen bonds are exclusively intermolecular in Form I, but both inter‐ and intramolecular in Form II, which accounts for the different molecular conformations in the two polymorphs.     

Populations of Ethanol Conformers in Liquid CCl4 and CS2 by Raman Spectra in OH Stretching Region (cited:2)

Combining Raman spectroscopy with density functional theory, the populations of the trans- and gauche- ethanol conformers are investigated in carbon tetrachloride (CCl₄) and carbon disulfide (CS₂). The spectral contributions of two ethanol conformers are identified in OH stretching region. The energy difference between both conformers is estimated with the aid of the calculated Raman cross sections. It can be seen that the trans- ethanol is more stable in CCl₄ and CS₂ solutions. The spectra are also obtained at different temperatures, and it is found the van't Hoff analysis is invalid in these solutions. By taking accounts of the Boltzmann distribution and theoretical Raman cross section, the energy difference is found to be increased with temperature, which shows the weak intermolecular interactions can enhance the population of _trans- ethanol.     

Versatile Benzimidazole/Triphenylamine Hybrids: Efficient Nondoped Deep‐Blue Electroluminescence and Good Host Materials for Phosphorescent Emitters

Two new bipolar compounds, N,N,N′,N′‐tetraphenyl‐5′‐(1‐phenyl‐1H‐benzimidazol‐2‐yl)‐1,1′:3′,1′′‐terphenyl‐4,4′′‐diamine ( 1 ) and N,N,N′,N′‐tetraphenyl‐5′‐(1‐phenyl‐1H‐benzimidazol‐2‐yl)‐1,1′:3′,1′′‐terphenyl‐3,3′′‐diamine ( 2 ), were synthesized and characterized, and their thermal, photophysical, and electrochemical properties were investigated. Compounds 1 and 2 possess good thermal stability with high glass‐transition temperatures of 109–129 °C and thermal decomposition temperatures of 501–531 °C. The fluorescence quantum yield of 1 (0.52) is higher than that of 2 (0.16), which could be attributed to greater π conjugation between the donor and acceptor moieties. A nondoped deep‐blue fluorescent organic light‐emitting diode (OLED) using 1 as the blue emitter displays high performance, with a maximum current efficiency of 2.2 cd A⁻¹ and a maximum external efficiency of 2.9 % at the CIE coordinates of (0.17, 0.07) that are very close to the National Television System Committee’s blue standard (0.15, 0.07). Electrophosphorescent devices using the two compounds as host materials for green and red phosphor emitters show high efficiencies. The best performance of a green phosphorescent device was achieved using 2 as the host, with a maximum current efficiency of 64.3 cd A⁻¹ and a maximum power efficiency of 68.3 lm W⁻¹; whereas the best performance of a red phosphorescent device was achieved using 1 as the host, with a maximum current efficiency of 11.5 cd A⁻¹, and a maximum power efficiency of 9.8 lm W⁻¹. The relationship between the molecular structures and optoelectronic properties are discussed.