Steady-state absorption spectra were recorded on a Varian-Cary 50 Bio UV-vis spectrophotometer and steady-state fluorescence measurements were carried out on a Spex Fluorolog 3 spectrofluorometer (Horiba Jobin Yvon). The prompt fluorescence lifetimes of 4CzBN were measured using a time correlated single photon counting (TCSPC) setup using PicoQuant laser diodes (405 nm) and a PMT detector (10 000 counts, 4096 channels). Steadystate upconversion fluorescence measurements were performed on a home-built system using a continuous-wave 405 nm OBIS laser (Coherent) as the excitation source. The measured maximum power output was 87.3 mW and the laser beam diameter was 0.8 mm. A linear variable neutral density (ND) filter was used to vary the laser intensity, and data were recorded using home-built LabView software.