Invited feature articlePhotochromism of Dihydroindolizines. Part XXI: Multiaddressable Photochromic Performances based on Pyrrolo[1,2-b]pyridazine photochromes: Kinetics, Substituent Effect and Solvatochromism

•For the first time, photochromism of DHIs were studied in various organic solvents polarities and temperatures.•Compensation effect revealed between activation parameters.•Two-step mechanism for 1,5-electrocyclization correspond to the transoid-cisoid isomerization.•Empirical Kamlet-Taft and Catalán relationship showed the polarity is the major factor for solvent dependence.

In the present study, the thermal 1,5-electrocyclization back reaction from betaines (colored forms) to dihydroindolizines (DHIs, colorless) was studied in seven organic solvents with different polarities at various temperatures. The spectroscopic, thermodynamic and photochromic properties of such photo- and solvatochromic material based on betaines/DHIs is investigated. It is obvious that the rate of the thermal back reaction depend on solvent polarity. Exploration of the activation parameters unravel an underlying compensation between ΔH≠ and ΔS≠. The 1,5-electrocylization decoloration reaction is found to follow two-step mechanism, the first step corresponds to the transoid-cisoid isomerization and the second one is the ring closure from the cisoid intermediate and formation of the carbon-carbon bond to form the initial spiro isomer. The special solvent effects were discussed based on the linear free energy relationship. The solvatochromism of the dihydroindolizines (DHIs) was studied and analyzed by means of the empirical Kamlet-Taft and Catalán solvent parameter set. The obtained results suggest that the change of polarizability within the solvents is the major factor for the observed solvent dependence.

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