The triplet pair is the key functional unit in triplet–triplet annihilation photon upconversion. The same molecular properties that stabilize the triplet pair also allow dimers to form on the singl...

Annihilation Versus Excimer Formation by the Triplet Pair in Triplet–Triplet Annihilation Photon Upconversion

The soot nucleation process, defined as the transition from molecular precursors to condensed matter, is the less understood step in the whole soot formation process. The possibility that polycyclic aromatic hydrocarbon (PAH) dimers, especially those containing moderate-sized PAHs, can play a major role in soot nucleation is a very controversial issue. Although PAH dimers have often been considered as potential soot precursors, their formation is not thermodynamically favored at a typical flame temperature, their binding energies being considered too weak to allow them to survive in this environment. Hereby, we report experimental evidence supporting the existence of PAH dimers in the proximity of the soot nucleation region of a methane laminar diffusion flame that gives strong evidence for the nucleation process to be kinetically rather than thermodynamically controlled.

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Dimers of polycyclic aromatic hydrocarbons: the missing pieces in the soot formation process.

Understanding structure–property relationships in multichromophoric molecular architectures is a crucial step in establishing new design principles in organic electronics as well as to fully understand how nature exploits solar energy. Here, we study the excited state dynamics of three bichromophores consisting of two perylene chromophores linked to three different crown-ether backbones, using stationary and ultrafast electronic spectroscopy combined with molecular dynamics simulations. The conformational space available to the bichromophores depends on the structure and geometry of the crown-ether and can be significantly changed upon cation binding, strongly affecting the excited-state dynamics. We show that, depending on the conformational restrictions and the local environment, the nature of the excited state varies greatly, going from an excimer to a symmetry-broken charge separated state. These results can be rationalised in terms of a structure–property relationship that includes the effect of the local environment.

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Tuning symmetry breaking charge separation in perylene bichromophores by conformational control

A new series of acceptor–donor–acceptor (A–D–A) type quinoacridine derivatives (1–3) with aggregation-induced red emission properties were designed and synthesized. In these compounds, the electron-withdrawing 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile groups act as electron-accepting units, while the alkyl-substituted conjugated core acts as electron-donating units. The restriction of intramolecular rotation was responsible for the AIE behavior of compounds 1–3. All compounds were employed as building blocks to fabricate one-dimensional (1-D) organic luminescent nano- or microwires based on reprecipitation or slow evaporation approaches. Morphological transition from zero-dimensional (0-D) hollow nanospheres to 1-D nanotubes has been observed by recording SEM and TEM images of aggregated sates of compound 2 in THF/H2O mixtures at different aging time. It was demonstrated that the synthesized compounds with different lengths of alkyl chains displayed different wire formation properties. The single-cry...

Quinoacridine Derivatives with One-Dimensional Aggregation-Induced Red Emission Property

The immobilization of molecular species onto electrodes presents a direct route to modifying surface properties with molecular fidelity. Conventional methods include direct covalent attachment and physisorption of pyrene-appended molecular compounds to electrodes with aromatic character through π–π interactions. A recently reported hybrid approach extends the synthetic flexibility of the latter to a broader range of electrode materials. We report an application of this approach to immobilization of pyrene-appended ferrocene onto pyrene-functionalized indium tin oxide (ITO). The modified ITO surfaces were characterized using X-ray photoelectron spectroscopy, fluorescence spectroscopy, and electrochemical techniques. An electron-transfer rate constant (kapp) of 100 ± 8 s–1 was measured between the electrode and immobilized ferrocene using electrochemical methods. For comparison, a ferrocene-modified electrode using conventional covalent attachment of vinylferrocene was also prepared, and kapp was measured t...

Interfacial Electron Transfer of Ferrocene Immobilized onto Indium Tin Oxide through Covalent and Noncovalent Interactions.

By nonresonant excitation below the absorption edge of pyrene and perylene molecules in solution, efficient excimer luminescence is observed. For perylene in solution, there are two kinds of nonresonant excimer emission that closely resemble Y- and E-emissions in perylene crystal and perylene in polymer. The concentration dependence of the photon flux density of monomer and excimer emission is reasonably explained by a simple model with a few fitting parameters based on the ordinary excimer formation process, in which the excited-state monomer interacts with a ground-state monomer. Observed quantum efficiency of excimer formation is by orders of magnitude higher than that expected if a monomer is excited from thermally populated vibrational levels. The experimental evidence for direct resonant excitation of excimers in perylene nanocrystals is obtained from the absence of anti-Stokes emission. This is due to a symmetry-breaking effect close to crystal surface, suggesting that excimers are formed through a more efficient process in solution, such as resonant excitation of weakly interacting molecules with a symmetry-broken intermolecular configuration.

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Excimer luminescence from nonresonantly excited pyrene and perylene molecules in solution.

The effect of light scattering on the extinction spectra of organic nanocrystals was investigated by absorption and extinction measurement and by theoretical calculation for typical organic nanocrystals of α-perylene. In the extinction spectra, the exciton band shows a size-dependent redshift and tail toward longer wavelengths due to scattering as the crystal size is increased beyond ∼50 nm. In the absorption spectra, the exciton band blueshifts from that in the bulk crystal for crystal sizes less than ∼200 nm due to the anisotropic exciton structure in perylene. The calculation qualitatively supports these results. The effect of scattering always makes the extinction band redshift from the absorption band.

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Effect of light scattering on the transmission spectra of organic nanocrystals

https://www.rs.kagu.tus.ac.jp/eiji/CPL2014-Numata.pdf

Optical size effect of organic nanocrystals studied by absorption spectroscopy within an integrating sphere

Cross-shaped excimer (self-trapped exciton) luminescence from α- and β-perylene single crystals of 50–100 μm was found when they were excited at the center of the crystals with a continuous-wave (cw) laser resonant with the exciton absorption. The cross shape is formed by the two lines which intersect at the excited position and are perpendicular to the sides of the crystals of parallelogram shape. Luminescence is emitted from the excited spot and 4 side edges in the cross shape. The most striking feature is that the luminescence intensity at the edges was as high as or higher than at the excited spot. The possibility of the exciton propagation or the waveguide effect is rejected both experimentally and theoretically. This phenomenon can be reasonably explained only when the radiative transition probability of excimers is significantly enhanced at the crystals side edges than at the center due to the lower symmetry.

Kazuya Nakagawa

Papers

Excimer luminescence from nonresonantly excited pyrene and perylene molecules in solution.

Effect of light scattering on the transmission spectra of organic nanocrystals

Optical size effect of organic nanocrystals studied by absorption spectroscopy within an integrating sphere

Cross-shaped photoluminescence of excimers in perylene crystals