The first synthesis of an all-benzene carbon nanocage 1, which represents a junction unit of branched carbon nanotubes, has been achieved. A stepwise assembly of six L-shaped units [cis-di(p-bromophenyl)cyclohexane derivative] and two three-way units (1,3,5-triborylbenzene) by cross-coupling and homocoupling provided the unstrained cyclic precursor. Acid-mediated aromatization of cyclohexane moieties in the precursor resulted in the formation of carbon nanocage 1. Photophysical measurements and DFT studies revealed the unique properties of 1, such as D₃ symmetry with degenerate HOMO/HOMO − 1 and LUMO/LUMO + 1, high fluorescence quantum yield (Φ_F = 0.87), and a relatively large two-photon absorption cross section (500 GM at 590 nm), which are attractive for various applications.

The two-photon absorption (2PA) properties of a series of dyes containing donor and acceptor groups connected via a π-electronic bridge were studied by using a femtosecond Z-scan technique. It was found that the nature of the π-electronic bridge does not substantially affect the measured 2PA intensities. In the case of the substituted azobenzenes the decrease of the strength of the donor and/or acceptor groups leads to decrease of the 2PA intensity, the effect of the acceptor strength being greater.

Accumulated thermal effect (ATE), which can be an artifact in femtosecond closed-aperture Z-scan measurements, was investigated by varying the repetition rate of the incident pulse for a dye solution in ${\mathrm{CHCl}}_3.$ It was found that the measurement is affected by two-photon-absorption-induced ATE, even at a low repetition rate of 1 kHz. The relaxation time of the ATE was found to be tens of milliseconds by time-resolved thermal-lens experiments and simulations, which is consistent with the observed repetition-rate dependence of the Z-scan measurements. The simulations for various commonly used solvents also exhibited that the ATE can be prominent in hydrocarbon and halogenated hydrocarbon solvents and inconspicuous in alcohols and water.

Binary crystalline solids consisting of a sensitizer (Pt-octaethylporphyrin) and an emitter (9,10-diphenylanthracene (DPA) or its alkyl-strapped derivative (C7-sDPA)) were fabricated by solution casting under rapid drying conditions and found to show efficient triplet–triplet annihilation upconversion by the suppression of component segregation. Microspectroscopic studies of individual crystalline particles of the binary solids revealed threshold intensities as low as 5 mW cm⁻², close to solar levels, and UC quantum yields of 20% were possible when using C7-sDPA as an emitter, even in air.