Optical studies were performed in DMF for all compounds at 25 o C unless otherwise indicated. Closed-to-open photoisomerization reactions of compound 1c have been carried out under 525 nm green light (34 W.cm -2 ) in CHCl3/CH3CN (1:1). A 1 mL UV cuvette was kept under ice-cold conditions to minimalize the heating effect. The UV-Vis absorbance data were recorded subsequently with a Cary 60 UV-Vis spectrophotometer. The open-to-closed isomerization was conducted thermally at 70 o C. The open-to-closed isomerization under the thermal conditions was carried out in the spectrophotometer itself using a Peltier heating system accessory with an accuracy of ±1 o C. The emission properties of 1c and 1c .-were also studied by fluorescence spectroscopy before and after the isomerization in quartz fluorescence cuvettes (Starna). The photoisomerization conditions the same as described for the absorption studies. Fluorescence measurements were carried out with a Horiba Duetta (Xe-150 W, 200-1100 nm).