Emission studies were conducted under the same conditions (10 M in DMF). Typically, the DHP → CPD isomerization leads to a reduction in intensity of the emission bands at >620 nm (arising from the S1→S0 transition in the DHP core). Surprisingly, in this case, upon exposure of the sample (10 μM in DMF) to 370 nm light, the weak emission of 1c at 658 nm enhanced in intensity drastically, suggesting the absence of ring opening isomerization of the DHP unit (Figure ). Interestingly, instead of 370 nm light, further exposure to 525 nm light caused a reduction in all DHP absorption bands and also a change in colour, typical for the formation of the ring opening isomer 1o. The changes in the UV-vis bands supported the formation of the 1o form (Figure ).