Photochromic spiro[7H-furo(3,2-f)-(2H-1)-benzopyran-7,2′-indolines]:: Experimental and computational evidence for the elusive intermediate of the photoinitiated ring opening reaction of spiropyrans

The photochromic behavior, absorption and luminescence spectra of a series of 1',3',3'-trimethyl-1,2-tetramethylenespiro[7H-furo(3,2-f)-(2H-1)-benzopyran-7,2'-indolines] 1 have been studied. In the case of 6-(tert-butyl) derivative 1d, the formation of an acoplanar cis-cisoid intermediate, the lifetime of which at 77K is long enough to observe its absorption and fluorescence spectra, has been detected. The occurrence of the intermediate on the reaction paths for the thermal and photoinitiated ring-opening processes has been also shown by DFT calculations.