Steric substituent effects of new photochromic tetrahydroindolizines leading to tunable photophysical behavior of the colored betaines

Various substituted photochromic tetrahydroindolizine (THIs) derivatives 4a–r have been synthesized by nucleophilic addition of 1-phenyl-3,4-dihydroisoquinolines 2 to spirocyclopropene 1. After irradiation of the THIs 4a–r with UV light, a blue to green colored zwitterionic betaines 3a–r was formed which undergoes a thermal electrocyclization leading to colorless THIs 4a–r. Both the half-life of the thermal back reaction and the absorption spectra of the colored forms are found to be strongly dependent on the position and size of the substituent in the 10′b-phenyl ring. A strong pronounced effect was observed by comparison of each ortho-substituents to the corresponding para-substituents which showed increase in the half-life by a factor of 4 for o-, p-fluoro-substitution 3h–j and a factor of 2000 for o-, p-iodo-substitution 3q,r. Meta- and para-substitution showed no pronounced influence on the half-life. The steric effect of different groups was well proved by CHARMm calculation. The computational results provided a new insight into the reaction pathway signifying the steric effect of substituents on THIs 4. The energies of the unsubstituted THI 4a, the ortho-substituted THIs 4b,e,k and the meta-derivatives 4c,f,l as well as their betaines for the cisoid 3″ and transoid 3‴ were studied. The structures and energies for 3b,e,k,c,f,l were monitored. In conclusion, the formation of the twisted open form 3′ is responsible for value of the rate constant of the thermal cyclization.