Overview of the workfunction (obtained from the secondary cutoff observed with UPS/PEEM), optical bandgaps (obtained from UV-Vis absorption spectra) and electronic bandgaps (measured by photoemission spectroscopies UPS and IPES), together with the DFT calculated optical and HOMO–LUMO gaps. Note: the curcumin calculations are performed on ‘trans’ molecules (i.e. differing from the model if Fig. 5 by the placement of the central H atom), however this does not play an important role in the E_g calculation. Note also that the UPS-IPES bandgap uncertainty is an optimistic estimate.

	Empirical				Calculated
	Workfunction [eV]	E g optical [eV]		E g electronic [eV]	E g optical [eV] (single molecule)	E g [eV] (HOMO–LUMO)
		Evaporated	In solution	UPS/IPES
Curcumin	3.2 ± 0.1	2.33 ± 0.02	2.64 ± 0.01	2.3 ± 0.1	3.08	3.60
Bixin	3.3 ± 0.1	2.5 ± 0.1	2.33 ± 0.02	2.8 ± 0.1	2.62	2.22
Indigo	1.7 ± 0.1	1.71 ± 0.01	1.92 ± 0.01	1.9 ± 0.1	1.81	2.05