Fluorescence spectra acquisition: Fluorescence spectra were acquired using an Acton SP300i spectrometer (Princeton Instruments, USA) with a grating of 300 grooves/mm, coupled to a thermoelectrically cooled CCD camera (PIXIS 100, Princeton Instruments, USA) at a detector temperature of -45 °C. Spectra acquisition was controlled by the Winspec software package (Princeton Instruments, USA).