Mechanism of high photoluminescence quantum yield of melem.

To produce high-efficiency organic light-emitting diodes, materials that exhibit thermally activated delayed fluorescence (TADF) are attracting attention as alternatives to phosphorescent materials containing heavy metallic elements. Melem, a small molecule with a heptazine backbone composed only of nitrogen, carbon, and hydrogen, is known to emit light in the near-ultraviolet region and exhibit high photoluminescence (PL) quantum yield and delayed fluorescence. However, the mechanism underlying the high PL quantum yield remains unclear. This study aimed to elucidate the mechanism of the high PL quantum yield of melem by examining its optical properties in detail. When the amount of dissolved oxygen in the melem solution was increased by bubbling oxygen through it, the PL quantum yield and emission lifetime decreased significantly, indicating that the triplet state was involved in the light-emission mechanism. Furthermore, the temperature dependence of the PL intensity of melem was investigated; the PL intensity decreased with decreasing temperature, indicating that it increases thermally. The experimental results show that melem is a TADF material that produces an extremely high PL quantum yield by upconversion from the triplet to the singlet excited state.

Growth and characterization of melem hydrate crystals with a hydrogen-bonded heptazine framework.

In carbon nitride (CN) compounds, hydrogen bonds play a major role in cohesion, in addition to dispersion forces. The crystal structures of CN compounds produced via thermal polymerization are complex, but they possess unique and attractive features. Melem is a well-known building unit of CN compounds such as melon and g-C3N4, which have recently attracted attention as photocatalysts. Melem hydrate (Mh) forms hexagonal prismatic crystals that are sufficiently porous to accommodate small molecules. In this study, we grew and characterized single crystals of Mh and investigated their optical properties and hygroscopicity. By precisely adjusting the hydration conditions, we succeeded in growing a well-formed hexagonal prismatic single crystal of Mh (Mhr) with a length measuring several tens of micrometers. Furthermore, we discovered a parallelogram-shaped Mh single crystal (Mhp), which possessed a different crystal structure and optical properties from those of Mh and melem crystals. Although the crystal structure of Mh was greatly disrupted by dehydration, it exhibited hygroscopicity and could absorb moisture even in air, restoring the crystal structure of Mh. In addition, Mh demonstrated a high photoluminescence quantum yield and long lifetime delayed fluorescence, similar to melem crystal. The high quantum yield of Mh can be attributed to the effect of the strong anchoring of the melem molecule by several hydrogen bonds in the Mh crystal, since the strongly anchored molecule is less likely to undergo radiation-free deactivation due to the small displacement of atomic positions in the excited state after light absorption. The findings obtained in this study shed light not only on the application of CN compounds as photocatalysts, but also on a wider range of applications based on their optoelectronic functions.

Triplet-triplet annihilation photon upconversion from diphenylhexatriene and ring-substituted derivatives in solution.

We report that diphenylhexatriene (DPH) and its ring-substituted derivatives act as emitter molecules in triplet-triplet annihilation photon upconversion (TTA-UC). A palladium porphyrin derivative, meso-tetraphenyl-tetrabenzoporphine palladium complex (PdTPBP), which acts as a sensitiser in TTA-UC, and DPH derivatives were dissolved in tetrahydrofuran (THF). The solution showed blue-green to green UC emission under photoexcitation at 640 nm in a nitrogen atmosphere. The UC quantum efficiency (ηUC) values of the DPHs were estimated, with (E,E,E)-1,6-bis[4-(di-2-picolylamino)phenyl]hexa-1,3,5-triene (pico DPH) showing the highest. In addition, the quantum yields of triplet energy transfer (TET) and triplet-triplet annihilation (TTA), which are elementary processes in TTA-UC, were estimated, as well as the triplet lifetimes of each DPH derivative. The results indicate that the TTA process governs the value of ηUC.

Chain-Length-Dependent Photophysical Properties of α,ω-Di(4-pyridyl)polyenes: Effects of Solvent Polarity, Hydrogen Bond Formation, Protonation, and N-Alkylation.

In this study, we investigated the solvent effects on the photophysical properties of α,ω-di(4-pyridyl)polyenes 1-5 having 1-5 double bonds. The solution photoproperties depend strongly on conjugation chain length. The absorption maximum (λa) of dipyridylethylene 1 is observed at around 290 nm and only slightly redshifts as the solvent polarity increases, whereas its fluorescence maximum (λf) redshifts from 368 nm in hexane to 403 nm in acetonitrile. Although 1 is a centrosymmetric molecule, its fluorescence energy linearly correlates with the Onsager solvent polarity function f(ε) - f(n2) = (ε - 1)/(2ε + 1)-(n2 - 1)/(2n2 + 1), indicating that the emission originates from an intramolecular charge transfer (CT) excited state. Exceptionally, λf in methanol is largely blue-shifted to 341 nm from those in other aprotic solvents. The fluorescence solvatochromism of longer polyenes 2-5 is much less significant than that of 1. Upon protonation and N-alkylation, both the absorption and fluorescence spectra of all five compounds are red-shifted in methanol. The largest shifts in λa and λf on protonation are observed for pentaene 5 (73 nm) and diene 2 (57 nm), respectively.

Crystal Structures and Fluorescence Spectroscopic Properties of a Series of α,ω-Di(4-pyridyl)polyenes: Effect of Aggregation-Induced Emission.

Crystal structures and fluorescence spectroscopic properties were investigated for a series of all-(E) α,ω-di(4-pyridyl)polyenes (1-5) with different number of double bonds (n). Molecules 1 and 2 (n=1, 2) in crystals are arranged to form partially π-overlapped structures, whereas those of 3-5 (n=3-5) are stacked in a herringbone fashion. All these molecules, the shorter polyenes in particular, are almost nonfluorescent in solution. In the solid state, 1 and 2 are highly emissive as pure organic solids [fluorescence quantum yields (φf )=0.3-0.5], while 3 and 4 are only weakly fluorescent (φf <0.05). The strongly n-dependent fluorescence properties can be attributed to the largely different molecular arrangements in the crystals. Although 5 is nonfluorescent in the solid state, we observe a very clear structure-property relationship in 1-4. Compounds 1 and 2 become much more emissive in the solid state than in solution as a result of the aggregation-induced emission (AIE) effect.

Solid-state one-way photoisomerisation of Z,E,Z-1,6-(4,4'-diphenyl)hexa-1,3,5-triene dicarboxylate examined using higher-order derivative spectra and powder XRD patterns.

Higher order derivative spectra were applied at first to one-way ZEZ-to-EEE photoisomerisation of dimethyl ester (ZEZ-DPH1) of the titled compound in a methylcyclohexane solution. Many common crossing points emerged in UV-induced derivative-spectral changes to reveal the direct ZEZ-to-EEE photoisomerisation without the transient formation of an intermediate to suggest the bicycle-pedal mechanism. The solid-state photoisomerisation was subsequently monitored by tracing changes in the fourth-order derivatives of absorption spectra of a thin crystalline layer of ZEZ-DPH1 prepared by the drop-casting method, because the distortion of absorption spectra due to light scattering is cancelled. It was suggested that the solid-state photochemical event consists of three steps: fast ZEZ-to-EEE photoisomerisation, a subsequent slow ZEZ-to-EEE photoisomerisation and very slow disappearance of the EEE-isomer. Studies on powder XRD were also carried out for a drop-cast solid layer of ZEZ-DPH1 to disclose the coexistence of a crystal form other than the original one, and the former exhibited faster ZEZ-to-EEE photoisomerisation when compared with the original crystal form. The results revealed by XRD analysis are in line with those obtained by higher-order derivative spectra, confirming the solid-state one-way photoisomerisation to take place through the bicycle-pedal process.

Fluorescence properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatriene (n = 1, 2): effects of internal rotation.

The fluorescence spectroscopic properties of (E,E,E)-1,6-di(n-naphthyl)-1,3,5-hexatrienes (1, n = 1; 2, n = 2) have been investigated in solution and in the solid state. In solution, the absorption maxima (λ(a)) of the lowest-energy band (1, 374 nm; 2, 376 nm in methylcyclohexane) were similar for 1 and 2, whereas the fluorescence maxima (λ(f)) (1, 545 nm; 2, 453 nm) and quantum yields (φ(f)) (1, 0.046; 2, 0.68) were very different regardless of the solvent polarity. The fluorescence spectrum of 1 was independent of the excitation wavelength (λ(ex)), whereas the spectrum of 2 was weakly λ(ex)-dependent. In the solid state, the spectroscopic properties of 1 and 2 were similar (λ(a) = 437-438 nm, λ(f) = 496-505 nm, φ(f) = 0.04-0.07). The origins of emission are both considered to be mainly monomeric. With the help of single-crystal X-ray structure analysis and ab initio quantum chemical calculation, we conclude that the red-shifted and weak emission of 1 in solution originates from a planar excited state having small charge transfer character, reached from a twisted Franck-Condon state by the excited-state geometrical relaxation accompanied by the internal rotation around the naphthalene (Ar)-CH single bond. The similar fluorescence properties of 1 and 2 in the solid state can be attributed to the restriction of the geometrical relaxation. The effects of the Ar-CH rotational isomerism on the fluorescence properties in solution, for 2 in particular, are also discussed.

Solid-state [2+2] photodimerization and photopolymerization of α,ω-diarylpolyene monomers: effective utilization of noncovalent intermolecular interactions in crystals.

[2+2] Photocycloaddition of olefins is a very useful reaction in synthetic organic chemistry to obtain cyclobutane-containing molecules, which are almost inaccessible by other methods. The reaction, when performed in the crystalline state, occurs more efficiently and selectively than in homogeneous solution due to tight and regular molecular arrangement in the crystal state. Despite numerous examples for the solid-state [2+2] photodimerization of monoenes, however, it is still a challenge to prepare not only dimers but also higher oligomers and polymers from conjugated polyenes, which have multiple reactive double bonds in a molecule. In our recent studies of the solid-state photoreactions of α,ω-diarylpolyenes, noncovalent intermolecular interactions in crystals were effectively utilized to prealign molecules in stacking arrangements, suitable for the [2+2] reaction. With appropriate ring-substituents, [2+2] photodimerization and photopolymerization of the polyenes took place, although the degree of polymerization was relatively low. This review will describe the details of these reactions.

Fluorescence spectroscopic properties of nitro-substituted diphenylpolyenes: effects of intramolecular planarization and intermolecular interactions in crystals.

The steady-state absorption and fluorescence properties of (E,E,E)-1,6-diaryl-1,3,5-hexatrienes (2, aryl = 2-nitrophenyl; 3, aryl = 3-nitrophenyl; 4, aryl = 4-nitrophenyl) have been investigated in solution and in the crystalline state. The solid-state absorption spectra of 2-4 shifted to longer wavelengths than those in solution. A combination of theoretical calculations and single-crystal X-ray structure analyses shows considerable planarization of molecules in the solid state, which is mainly responsible for the spectral red shifts. The effects of intermolecular interactions on the absorption spectra appeared to be relatively small in these crystals. This is consistent with the monomeric origin of the solid-state emission. Molecule 2 was nonfluorescent in all solvents studied, probably due to the efficient nonradiative deactivation from ionic species produced by excited-state intramolecular proton transfer (ESIPT) along the C-H...O-type hydrogen bonds. The fluorescence of 3, observed only in medium polar solvents, originated from an intramolecular charge transfer (ICT*) state, while that of 4 derived from locally excited (LE*) and/or ICT* states depending on the solvent polarity. All three molecules exhibited LE* fluorescence in the solid state. No observation of ICT* emission in crystals strongly suggests the twisted geometries for ICT* (TICT) of 3 and 4 in solution. The measurable fluorescence from crystal 2 can be attributed to the restricted torsional motions in the solid excited state.

(Z,E,Z)-1,6-Di-1-naphthyl-hexa-1,3,5-triene.

The title compound, C(26)H(20), lies about an inversion centre. The naphthalene unit and the hexa-triene chain are each approximately planar (maximum deviations of 0.0143 and 0.0042 Å, respectively), and are inclined to one another at a dihedral angle of 49.20 (4)°. The dihedral angle between the two naphthalene ring systems of neighboring mol-ecules is 85.71 (4)°.

Fluorinated diphenylpolyenes: crystal structures and emission properties.

(E,E,E)-1,6-Diaryl(Ar)-1,3,5-hexatrienes (2, Ar = 4-fluorophenyl; 3, Ar = 2,4-difluorophenyl; 4, Ar = 2,4,6-trifluorophenyl; 5, Ar = perfluorophenyl) and (E,E,E)-1-perfluorophenyl-6-phenyl-1,3,5-hexatriene (6) were prepared. The absorption and fluorescence spectra in methylcyclohexane solution showed only a small dependence on the fluorine ring substituent, and were similar to those of the unsubstituted parent compound (1, Ar = phenyl). The solid-state absorption and fluorescence spectra shifted to red relative to those in solution and strongly depended on the substituent. The emission from crystals 1-5 originated mainly from monomeric species with the maximum wavelength (lambda f(max)) of 440-465 nm, which overlapped the emission from molecular aggregates (1-4) or excimeric species (5) in the red region. Crystal 6 exhibited red-shifted (lambda f(max) = 530 nm) and structureless emission due to excimers. The cocrystal of 1 and 5 (1/5) showed red-shifted (lambda f(max) = 558 nm) and distinctly structured emission, not from exciplexes but from the excited states of molecular aggregates in which molecules 1 and 5 strongly interact already in the ground state. These assignments were confirmed by the results of fluorescence lifetime and quantum yield measurements in the solid state. Single-crystal X-ray structure analyses showed that the molecules were basically planar in each crystal, whereas the crystal packing was strongly substituent-dependent. Weak pi-pi interactions in the herringbone (1 and 2) and in the pi-stacked but largely offset structures (3 and 4) account for their predominantly monomeric origin of emission. The observation of excimer fluorescence from 5 was rather unexpected, since the molecules in this crystal were arranged in an offset stacking fashion due to perfluorophenyl-perfluorophenyl (C6F5...C6F5) interaction. The structures of 6 and 1/5 considerably resembled each other, in which molecules were pi-stacked with more face-to-face geometries than those in 5, as a result of strongly attractive perfluorophenyl-phenyl (C6F5...C6H5) interaction. Nevertheless, the fluorescence origin was clearly different for 6 and 1/5. This can be ascribed to the difference in the strength of orbital-orbital interaction between molecular pi-planes in the ground and excited states in crystals.

Four (E,Z,E)-1-(4-alkoxyphenyl)-6-(4-nitrophenyl)hexa-1,3,5-trienes.

The crystal structures of the four E,Z,E isomers of 1-(4-alkoxyphenyl)-6-(4-nitrophenyl)hexa-1,3,5-triene, namely (E,Z,E)-1-(4-methoxyphenyl)-6-(4-nitrophenyl)hexa-1,3,5-triene, C(19)H(17)NO(3), (E,Z,E)-1-(4-ethoxyphenyl)-6-(4-nitrophenyl)hexa-1,3,5-triene, C(20)H(19)NO(3), (E,Z,E)-1-(4-nitrophenyl)-6-(4-n-propoxyphenyl)hexa-1,3,5-triene, C(21)H(21)NO(3), and (E,Z,E)-1-(4-n-butoxyphenyl)-6-(4-nitrophenyl)hexa-1,3,5-triene, C(22)H(23)NO(3), have been determined. Intermolecular N...O dipole interactions between the nitro groups are observed for the methoxy derivative, while for the ethoxy derivative, two adjacent molecules are linked at both ends through N...O dipole-dipole interactions between the N atom of the nitro group and the O atom of the ethoxy group to form a supramolecular ring-like structure. In the crystal structures of the n-propoxy and n-butoxy derivatives, the shortest intermolecular distances are those between the two O atoms of the alkoxy groups. Thus, the nearest two molecules form an S-shaped supramolecular dimer in these crystal structures.

Fluorescence spectroscopic properties and crystal structure of a series of donor-acceptor diphenylpolyenes.

A series of p-nitro-p'-alkoxy(OR)-substituted (E,E,E)-1,6-diphenyl-1,3,5-hexatrienes (1a, R = Me; 1b, R = Et; 1c, R = n-Pr; 1d, R = n-Bu) were prepared. The absorption and fluorescence spectra in solution were almost independent of the alkoxy chain length. The absorption maximum showed only a small dependence on the solvent polarity, whereas the fluorescence maximum red-shifted largely as the polarity increased. The solid-state absorption and fluorescence spectra were red-shifted relative to those in low polar solvents and were clearly dependent on the alkoxy chain length. The fluorescence maxima for the crystals of 1b and 1d were observed at 635-650 nm, which were red-shifted by 40-50 nm relative to those for 1a and 1c. The Stokes shifts were all relatively small (3000-3500 cm-1). For all four compounds, the fluorescence decay curves in the solid state were able to be analyzed by single-exponential fitting to give the lifetimes of 1.1-1.3 ns. This indicates that the emission of 1a-d is not originated from an excimer or molecular aggregates, but from only one emitting monomeric species. The fluorescence quantum yields of 1a-d were considerably high compared with the values for organic solids, which is consistent with their monomeric origin of emission. Single-crystal X-ray structure analyses of 1a, 1c, and 1d showed that the crystal packing was dependent on the alkoxy chain length. The crystals of 1a and 1c had herringbone structure, whereas that of 1d had pi-stacked structure. Strong pi-pi interaction in the crystal of 1d would be the cause of the spectral red shifts relative to those for 1a and 1c. No observation of excimer fluorescence from crystal 1d can be attributed to the limited overlap between the pi-planes of the molecules due to its "slipped-parallel" structure.

Crystalline-state Z,E-photoisomerization of a series of (Z,E,Z)-1,6-diphenylhexa-1,3,5-triene 4,4'-dicarboxylic acid dialkyl esters. Chain length effects on the crystal structure and photoreactivity.

[reaction: see text] Crystalline-state Z,E-photoisomerization of a series of (Z,E,Z)-1,6-diphenylhexa-1,3,5-triene 4,4'-dicarboxylic acid dialkyl (R) esters [(Z,E,Z)-1a, R = Me; (Z,E,Z)-1b, R = Et; (Z,E,Z)-1c, R = n-Pr; (Z,E,Z)-1d, R = n-Bu] was investigated. All Z,E,Z isomers underwent one-way isomerization to the corresponding E,E,E isomers. The reaction efficiency was strongly enhanced as the length of the alkyl chain increased. Single-crystal X-ray analyses of (Z,E,Z)-1a-d showed that the alkyl chain part of the crystals became larger as the chain length increased. The conformational flexibility of the alkyl chains made the large change in the triene geometry in the lattice possible, leading to the enhancement of the photoreactivity in the crystalline state.

(E,E,E)-1,6-Bis(2,4-dichlorophenyl)hexa-1,3,5-triene.

The title molecule, C(18)H(12)Cl(4), lies about an inversion centre and the hexatriene chain is planar. The torsion angle of the single bond between the planes of the chain and the benzene ring is -8.6 (3) degrees. The dihedral angle between the planes defined by the chains of adjacent molecules is 50.0 (2) degrees. The shortest intermolecular distance between the Cl atoms is 3.514 (1) A. The molecules are joined through pi-pi-stacking and strong attractive Cl.Cl interactions.