Diffuse reflectance measurements were carried out on finely ground powdered samples, diluted to 10 wt% with BaSO 4 to remove effects of strong absorption, using a Varian Cary 50 UVvis spectrometer equipped with a diffuse reflectance accessory (DRA) probe (Barrelino, Harrick Scientific) measured over the range 350-1000 nm. The optical absorption measurements were performed using an integration sphere equipped PerkinElmer lambda 750 UV-vis-NIR commercial setup. The set-up includes light source of tungsten-halogen (for vis-NIR region) and deuterium (for UV region) lamps, a conventional photomultiplier (PMT) detector, a 10 cm integrating sphere module attachment, a monochromator, and a detector slit width of 10 nm. All measurements were performed under standard ambient conditions. Films were prepared by drop-casting the crystals, dissolved/suspended in the appropriate solvent (DMF), on the quartz/glass slide substrate. The transmitted light was corrected using a reference of an empty quartz coverslip of the same type and thickness as the substrate used for the sample. The absorption onsets were determined by extrapolation from a Tauc plot, assuming indirect band gaps. Further details of the optical characterisation can be found in the supplementary information.