Pigment Yellow 101: A showcase for photo-initiated processes in medium-sized molecules

Pigment Yellow 101 (P.Y.101) is a fluorescent yellow pigment which exhibits a surprisingly rich photochemistry of several competing reaction pathways as revealed by recent time-resolved femtosecond experiments. Our elaborate quantum chemical investigations employing density functional theory (DFT) and time-dependent DFT (TDDFT) show that the observed fluorescence competes with excited state intramolecular proton transfer and trans–cis isomerization processes. Moreover, the experimentally observed spectral features of the complicated excited state dynamics can be assigned to stable trans-diol, trans-keto and cis-diol, cis-keto isomers on the ground and excited state surfaces. Still, due to its molecular size P.Y.101 poses a challenge to electronic structure theory and many problems occur in particular with respect to the excited state calculations. Thus, P.Y.101 serves also as an educative example for which TDDFT yields a reasonable vertical electronic spectrum, but fails in the prediction of excited state structures, when standard GGA or hybrid functionals with low fractions of Hartree–Fock exchange are employed. This failure is attributed to the charge-transfer failure of TDDFT.     

5,12‐Diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxalines: Solid‐State Fluorescence,AIE Properties,and Orbital Switching by Substituent Effect

The compound 5,12‐diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxaline 1 a and its derivatives were prepared, and their solid‐ and solution‐state spectroscopic properties were studied; 1 a shows stronger fluorescence in solution than in the solid state due to aggregation caused by self‐quenching. Phenyl‐ or alkoxy‐substituted derivatives 1 b – d show solid‐state fluorescence with moderate quantum yields of about Φ=0.12–0.15, although the corresponding values are 0.01–0.07 in solution. The spectroscopic properties of alkoxy‐substituted derivatives were hardly changed compared to 1 a and 1 b , although 1 a and 1 b have similar absorption and fluorescence maxima in solution and in the solid state. DFT calculations indicate that orbital switching occurs between HOMO and HOMO‐1 and HOMO‐2 due to orbital interactions with introduced substituents. Crystal structure analysis revealed that the molecules have bent structures around tertiary nitrogen atoms and form a characteristic dimeric structure.     

How a Small Structural Difference Can Turn Optical Properties of π‐Extended Coumarins Upside Down: The Role of Non‐Innocent Saturated Rings

The fluorescence properties of two new families of heterocycles possessing either a seven‐ or five‐membered ring attached at the core molecule are entirely different in solution and in the solid state. Crystallization has the effect of inhibiting non‐radiative excited‐state deactivation pathways, operative in solution for the seven‐membered ring compounds, thus leading to significant fluorescence efficiency in the solid state, with quantum yields ranging from 0.10 to 0.36. Conversely, the five‐membered ring derivatives, which display notable emission properties in solution, are almost non‐emissive in the crystalline state, characterized by a long‐range π‐stacked arrangement. When embedded in polymeric films, both series show fluorescence features similar to the solution case, with remarkable fluorescence quantum yields ranging from 0.09 to 0.41. According to quantum chemical calculations, 3H‐chromeno[3,4‐c]pyridine‐4,5‐diones show the specific mechanism of fluorescence quenching. The derivatives bearing the seven‐membered ring undergo, in solution, a significant structural deformation in the excited state, resulting in a large decrease of the energy gap between S₁ and S₀ and hence to a substantial contribution of the internal conversion in the relaxation process. The fluorescence quenching of the five‐membered ring derivatives is in turn related to the intermolecular interaction between adjacent molecules prevailing to a greater extent in the crystal lattice.     

Protein‐like Enwrapped Perylene Bisimide Chromophore as a Bright Microcrystalline Emitter Material

Strongly emissive solid‐state materials are mandatory components for many emerging optoelectronic technologies, but fluorescence is often quenched in the solid state owing to strong intermolecular interactions. The design of new organic pigments, which retain their optical properties despite their high tendency to crystallize, could overcome such limitations. Herein, we show a new material with monomer‐like absorption and emission profiles as well as fluorescence quantum yields over 90 % in its crystalline solid state. The material was synthesized by attaching two bulky tris(4‐tert‐butylphenyl)phenoxy substituents at the perylene bisimide bay positions. These substituents direct a packing arrangement with full enwrapping of the chromophore and unidirectional chromophore alignment within the crystal lattice to afford optical properties that resemble those of their natural pigment counterparts, in which chromophores are rigidly embedded in protein environments.     

LONG-LIVED AFTERGLOW OF PHOTOSYNTHETIC PIGMENTS IN SOLUTION: PHOTOCHEMILUMINESCENCE

Abstract —Our recent research on photochemiluminescence (PCL) of pigments in solutions is reviewed. PCL was observed in the course of photooxidation by oxygen of chlorophyll a , bacteriochlorophyll, protochlorophyll, their analogs, synthetic dyes and aromatic hydrocarbons. The PCL of chlorophyll was studied in detail. It depends on oxygen concentration, intensity of exciting light, pH, nature of pigments, solvents etc. The thermochemiluminescence was observed after illumination of liquid and solid pigment solutions at low temperature (down to - 170C). The excitation spectra of PCL coincide with the pigment absorption spectra. The PCL emission spectra in most cases differ from those of pigment fluorescence. Electron acceptors, electron donors, radical inhibitors and β-carotene quench PCL. The quenching efficiency of electron acceptors is similar to their action on the chlorophyll triplet state. The quenching effect of radical inhibitors and β-carotene correlates with their activity in reaction with singlet oxygen. The effect of quenchers on the chlorophyll fluorescence, photobleaching and pigment sensitized oxygenation was studied. Analysis of experimental data allowed the assumption that chemiluminescence accompanies the decomposition of labile pigment peroxides. The accumulation of peroxides is probably due to the reaction in the complex of pigment and singlet oxygen, formed as a result of energy transfer from photoexcited (triplet) pigment molecules to oxygen. The terminal chemiluminescence emission proceeds from the singlet excited states of molecules of pigments and products of their oxidation.     

Time resolved spectroscopy on Pigment Yellow 101 in solid state

The photochemistry of the fluorescent Pigment Yellow 101 (P.Y.101) in the crystalline phase is investigated combining time resolved vibrational and electronic spectroscopy. The experiments reveal a complex reaction dynamics spanning several orders of magnitude in time and including excited state intramolecular proton transfer (ESIPT) from the initial trans-diol conformer. Following photoexcitation and the subsequent wavepacket motion out of the Franck–Condon region two tautomers, an excited trans-diol and trans-keto state are formed. The cis–trans isomerization of the keto form, which was experimentally observed and theoretically confirmed in DFT calculations in studies on P.Y.101 in solution is obstructed in the solid state, consequently the formation of the cis conformer is not directly observed. In addition, a long lived photoproduct with red shifted vibrational frequencies is identified. The life time of this intermediate is determined to be 50 μs, although an unambiguous assignment to a specific molecular configuration cannot be given at present.     

Polyfunctional Lewis Acids: Intriguing Solid‐State Structure and Selective Detection and Discrimination of Nitroaromatic Explosives

Synthesis and crystal structures of three porphyrin‐based polyfunctional Lewis acids 1 – 3 are reported. Intermolecular HgCl ??? HgCl (linear and μ‐type) interactions in the solid state of the peripherally ArHgCl‐decorated compound 3 lead to a fascinating 3D supramolecular architecture. Compound 3 shows a selective fluorescence quenching response to picric acid and discriminates other nitroaromatic‐based explosives. For the first time, an electron‐deficient polyfunctional Lewis acid is shown to be useful for the selective detection and discrimination of nitroaromatic explosives. The Stern–Volmer quenching constant and detection limits of compound 3 for picric acid are the best among the reported small‐molecular receptors for nitroaromatic explosives. The electronic structure, Lewis acidity, and selective sensing characteristics of 3 are well corroborated by DFT calculations.     

Formation of One‐Dimensional Helical Columns and Excimerlike Excited States by Racemic Quinoxaline‐Fused [7]Carbohelicenes in the Crystal

A series of quinoxaline‐fused [7]carbohelicenes (HeQu derivatives) was designed and synthesized to evaluate their structural and photophysical properties in the crystal state. The quinoxaline units were expected to enhance the light‐emitting properties and to control the packing structures in the crystal. The electrochemical and spectroscopic properties and excited‐state dynamics of these compounds were investigated in detail. The first oxidation potentials of HeQu derivatives are approximately the same as that of unsubstituted reference [7]carbohelicene (Heli), whereas their first reduction potentials are shifted to the positive by about 0.7 V. The steady‐state absorption, fluorescence, and circular dichroism spectra also became redshifted compared to those of Heli. The molecular orbitals and energy levels of the HOMO and LUMO states, calculated by DFT methods, support these trends. Moreover, the absolute fluorescence quantum yields of HeQu derivatives are about four times larger than that of Heli. The structural properties of the aggregated states were analyzed by single‐crystal analysis. Introduction of appropriate substituents (i.e., 4‐methoxyphenyl) in the HeQu unit enabled the construction of one‐dimensional helical columns of racemic HeQu derivatives in the crystal state. Helix formation is based on intracolumn π‐stacking between two neighboring [7]carbohelicenes and intercolumn CH ??? N interaction between a nitrogen atom of a quinoxaline unit and a hydrogen atom of a helicene unit. The time‐resolved fluorescence spectra of single crystals clearly showed an excimerlike delocalized excited state owing to the short distance between neighboring [7]carbohelicene units.     

State-crossing from a Locally Excited to an Electron Transfer State(SLEET) Model Rationalizing the Aggregation-induced Emission Mechanism of (Bi)piperidylanthracenes

Luminogens with aggregation-induced emission(AIE)characteristics(or AIEgens)have been widely used in various applications due to their excellent luminescent properties in molecular aggregates and the solid state.A deep understanding of the AIE mechanism is critical for the rational development of AIEgens.In this work,the“state-crossing from a locally excited to an electron transfer state”(SLEET)model is employed to rationalize the AIE phenomenon of two(bi)piperidylanthracenes.According to the SLEET model,an electron transfer(ET)state is formed along with the rotation of the piperidyl group in the excited state of(bi)piperidylan-thracene monomers,leading to fluorescence quenching.In contrast,a bright state exists in the crystal and molecular aggregates of these compounds,as the intermolecular interactions restrict the formation of the dark ET state.This mechanistic understanding could inspire the deployment of the SLEET model in the rational designs of various functional AIEgens.     

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.     

Effect of inorganic cores on dye properties of inorganic-organic hybrid pigments yellow 12

Some inorganic-organic hybrid pigments were fabricated by depositing pigment yellow 12(P.Y.12) on the surface of three inorganic cores with different particle size (white carbon black,microsilica,palygorskite). Effect of inorganic cores on the morphology and structure of the hybrid pigments were systematically investigated by nanoparticle analyzer, Fourier transforms infrared spectroscopy, and X-ray diffraction. Results showed that all three inorganic cores were encapsulated by the organic pigment. The particle size of hybrid pigments was all smaller and had narrower diameter scatter than the original pigment. The water dispersion and flow ability of these fabricated pigments were slightly improved. Thermal and UV-Vis analyses showed that the hybrid pigments had better thermo- and photo-stabilities. Additionally, the properties of the hybrid pigments including color strength, lightness, and yellow hue index were also improved and the modified pigment with white carbon black had the best coloring performance and better heat resistance than original P.Y.12.     

Colorimetric and Fluorescent Chemosensors for the Detection of 2,4,6‐Trinitrophenol and Investigation of their Co‐Crystal Structures

A full account of our studies of 2,4,6‐trinitrophenol (TNP) sensing is provided. A series of chemosensors 2 , 3 , 4 , 5 with a variety of aromatic chromophores for specific recognition of TNP has been designed and then realized through the fluorescence “on/off” mechanism. These chemosensors demonstrated highly selective, sensitive, and fluorescent quenching of TNP with remarkable visual changes through the intramolecular charge‐transfer (ICT) process. Their host–guest interactions were investigated by ¹H NMR spectroscopic titrations and their corresponding co‐crystal structures, which showed that the 1:1 host–guest complexes were formed by multiple hydrogen‐bond interactions in solution or in the solid state. The origins of the significant affinity demonstrated during the fluorescence recognition process were further disclosed through DFT calculations of corresponding compounds.     

The Effects of Heteroatoms Si and S on Tuning the Optical Properties of Rhodamine‐ and Fluorescein‐Based Fluorescence Probes: A Theoretical Analysis

The effects of the incorporated heteroatoms Si and S on tuning the optical properties of rhodamine‐ and fluorescein‐based fluorescence probes is investigated using DFT and time‐dependent DFT with four different functionals. As previously proposed, the large redshift (90 nm) produced by a Si atom in both the absorption and emission spectra can be attributed to the σ*–π* conjugation between the σ* orbital of the Si atom and the π* orbital of the adjacent carbon atoms. However, the presence of a Si atom does not alter the fluorescence quenching mechanism of the nonfluorescent forms of the investigated compounds. For the first time, these theoretical results indicate that the n orbital of the S atom plays an important role in determining the optical properties of the nonfluorescent form of rhodamine‐based fluorescence probes. It alters the fluorescence quenching mechanism by lowering the energy of the dark nπ* state, which is due to breakage of the C10?S52 bond upon photoexcitation.     

Thermally reversible fluorescent polymorphs of alkoxy-cyano-substituted diphenylbutadienes: role of crystal packing in solid state fluorescence

Correlation of fluorescence and crystal packing in thermally interconvertible polymorphic states of octyloxy-cyano-substituted diphenylbutadiene possessing visually distinguishable fluorescence reveals that solid state fluorescence of this class of derivatives depends on their monomer-J-aggregate ratio, controlled by variations in their molecular packing.     

具有聚集诱导发光性能的乙烯基衍生物的合成及其对DNT的检测

采用Wittig-Horner反应合成了两种具有聚集诱导发光(AIE)性能的乙烯基衍生物.紫外及荧光光谱显示,两种化合物在聚集态时分子间的π-π相互作用很弱,二者在聚集态时都展示了很强的荧光增强发光性能.由于二者所具有的扭曲的分子构型提供了2,4-二硝基甲苯(DNT)气态分子扩散所需的分子通道,因此对DNT蒸气的检测均显示了较高的荧光猝灭效率,且其荧光猝灭具有较好的可逆性.     

Shedding Light on the Origin of Solid-State Luminescence Enhancement in Butterfly Molecules

Different molecular strategies have been carefully evaluated to produce solid-state luminescence enhancement (SLE) in compounds that show dark states in solution. A set of α-phenylstyrylarene derivatives with a butterfly shape have been designed and synthesised, for the first time, with the aim of improving the solid-state fluorescence emission of their parent styrylarene compounds. Although these butterfly molecules are not fluorescent in solution, one of them (1,2,4,5-tetra(α-phenylstyryl)benzene) exhibits a fluorescence quantum yield as high as 68 % in a drop-cast sample and 31 % in its crystalline form. In contrast, 1,3,5-tris(α-phenylstyryl)benzene and 4,6-bis(α-phenylstyryl)pyrimidine do not show SLE. A range of fluorescence spectroscopy experiments and DFT calculations were carried out to unravel the origin of different photophysical behaviour of these compounds in the solid state. The results indicate that a rational strategy to control the SLE effect in luminogens depends on a delicate balance between molecular properties and inter-/intramolecular interactions in the solid state.     

Systematic Investigation of Molecular Arrangements and Solid‐State Fluorescence Properties on Salts of Anthracene‐2,6‐disulfonic Acid with Aliphatic Primary Amines

Organic salts of anthracene‐2,6‐disulfonic acid (ADS) with a wide variety of primary amines have been fabricated, and their arrangements of anthracene molecules and solid‐state fluorescence properties investigated. Single‐crystal X‐ray studies reveal that the salts show seven types of crystal forms and corresponding molecular arrangements of anthracene moieties depending on the amine, while anthracene shows only one form and arrangement in the solid state. Depending on the molecular arrangements, the ADS salts exhibit various solid‐state fluorescence properties: spectral shift (30 nm) and suppression and enhancement of the fluorescence intensity. Especially the ADS salt with n‐heptylamine (nHepA), which shows discrete anthracene moieties in the crystal, exhibits the highest quantum yield (Φ_F=46.1±0.2 %) in the series of ADS salts, which exceeds that of anthracene crystal (Φ_F=42.9±0.2 %). From these systematic investigations on the arrangements and the solid‐state properties, the following factors are essential for high fluorescence quantum yield in the solid state: prevention of contact between π planes of anthracene moieties and immobilization of anthracene rings. In addition, such organic salts have potential as a system for modulating the molecular arrangements of fluorophores and the concomitant solid‐state properties. Thus, systematic investigation of this system constructs a library of arrangements and properties, and the library leads to remarkable strategies for the development of organic solid materials.     

Heterocyclic effect for optical properties of naphthoquinone-based pigment: 2-methyl-3-heteroarylthio-1,4-naphthalenedione

Three novel naphthoquinone-based heterocyclic pigments, 2-methyl-3-[(1-methyl-1H-imidazol-2-yl)thio-1,4-naphthalenedione, (4-methyl-4H-1,2,4-triazol-3-yl)thio-1,4-naphthalenedione, and (1-methyl-1H-tetrazol-5-yl)thio]-1,4-naphthalenedione, are synthesized, and their optical properties in both solution and solid states are investigated. Depending on the heteroarylthio ring in the pigment, variation in optical properties is observed, e.g. characteristic colours for each pigment in the solution and solid states. The achiral pigment containing the 1-methyl-1H-tetrazol-5-yl ring exhibits a chiral space group and a CD signal in the solid state.     

Molecular design of novel non-planar heteropolycyclic fluorophores with bulky substituents: convenient synthesis and solid-state fluorescence characterization

Novel indeno[1,2-b]benzo[4,5-e]pyran-11-one-type fluorophores exhibiting intense solid-state fluorescence were conveniently synthesized and the relation between their solid-state photophysical properties and the X-ray crystal structures were investigated, which demonstrates that non-planar structures with sterical hindered substituents prevent the fluorophores from forming short pi-pi contacts causing fluorescence quenching in the solid state.