The PLQYs of the compounds (Supplementary Table ) in solid state are much lower than in solution, especially the 0.1M H2SO4 solution. For example, where 7bx series has PLQYs of 63.3-70.6% in 0.1 M H2SO4 it has PLQYs of 8.1-8.5% in PVA film. In the 0.1 M H2SO4 solution environment both nitrogen atoms are easily protonated and so the quantum yields are much higher. However, in the solid state the increased rigidity and lack of freely moving protons mean that the nitrogen atoms may not be sufficiently protonated, so we see a ten-fold reduction in the quantum yield. The spectral profile of the doubly protonated species remains the same in film or in solution. Moving to the PMMA films (which are akin to the MeCN measurements in solution) they are very different to the PVA films solidifying the argument that protonation has a significant part to play in PLQYs of these compounds. The compounds in PMMA do not rise above 2% whereas in PVA the 7bx series reaches almost 10%. Similarly, to the solution PLQYs, 7bx series has the highest quantum yields showing that the addition of a methoxy group affects the compounds PLQYs in solid state as well as in solution. The PLQYs for 7cx series of compounds is lower than those in the 7bx series despite them having an additional methoxy group.