Steady-state PL spectra were measured by a CCD camera (Andor, Newton 920P BEX2-DD) equipped with a polychromator (Andor, Kymera 193i-B2). The light source for optical excitation was a Ti:sapphire regenerative amplifier (Spectra-Physics, Spitfire Ace, pulse duration = 120 fs, repetition rate = 1 kHz, pulse energy = 4 mJ/pulse, central wavelength = 800 nm). The fourth harmonic (360 nm) of the signal light (1440 nm) from an optical parametric amplifier (OPA, Light Conversion, TOPAS) pumped by the regenerative amplifier was used for sample excitation. The concentration of the solutions was 0.1 mM. TR-PL spectroscopy TR-PL spectra were measured by a polychromator and a streak camera system (Hamamatsu C4780, time resolution: < 30 ps). The sample excitation pulse was the same as the steady-state PL spectroscopy (360 nm). The polarizations of the excitation and detection lights was set to the magic angle configuration (~54.7°). The concentration of the sample solutions was prepared to be 0.1 mM. All measurements were conducted after one-hour bubbling using N2 gas.