Insights from Theory and Experiment on the Photochromic spiro-Dihydropyrrolo–Pyridazine/Betaine System

We elucidated the photochromic spiro-4a,5-dihydropyrrolo­[1,2-b]­pyridazine/betaine (DPP/betaine) system by comparing state-of-the-art density functional theory calculations with nanosecond/millisecond UV–vis absorption spectroscopy, as well as steady-state absorption and cyclization kinetics. Time-dependent density functional theory calculations are employed to examine the transformations occurring after photoexcitation. This study shows that the photochromic spiro-4a,5-dihydropyrrolo­[1,2-b]­pyridazine and spiro-1,8a-dihydroindolizine (DHI) systems react according to similar pathways. However, notable differences exist. Although photoexcitation of the spiro-DPP system also leads to cis-betaines, which then isomerize to trans-betaines, we found two distinct classes of cis isomers (cis-betaine rotamer-1 and cis-betaine rotamer-2), which do not exist in spiro-1,8a-dihydroindolizine. Similar to our previous study on the spiro-DHI/betaine system, a complicated potential-energy landscape between cis and trans isomers exists in the spiro-DPP system, consisting of a network of transition states and intermediates. Because the spiro-DPP/betaine is even more complicated than the spiro-DHI/betaine system, (substituted) photochromic systems featuring a 4a,5-dihydropyrrolo­[1,2-b]­pyridazine functional unit will require thorough in silico design to function properly as logical gates or in devices for information storage.