Abstract
Novel carbon-rich photochromic dihydroindolizine DHI derivatives and new spirocyclopropenes have been synthesized. Three alternative synthetic pathways for the synthesis of DHI
10 have been established. Different Sonogashira-mediated coupling reactions have been applied to optimize the reaction conditions and to obtain the best yields. Palladium-mediated Sonogashira coupling of DHIs with 4-(thioacetyl)iodobenzene
13 and iodobenzene
17 yielded coupling products, which have potential applications in molecular electronics. Irradiation of photochromic DHIs
10a–
f,
12a–
f,
14a–
f,
16a–
f and
18a–
f with polychromatic light leads to betaines
9a–
f,
13a–
f,
15a–
f,
17a–
f and
19a–
f. The coloured betaine forms are obvious in CH
2Cl
2 solution with concentration of 1×10
−5
mol/L at room temperature because of their slower 1,5-electrocyclization. All the absorption maxima of the coloured betaines were found to be in the visible region and lie between 524 (betaine
9a) and 639
nm (betaine
15f). The kinetics of the thermal 1,5-electrocyclization was studied using multichannel UV–vis spectrophotometry. The kinetic measurements showed that the half-lives of the coloured betaines are in the second domain and lie between 112 and 1379
s. A highly pronounced increase in the half-lives of betaines bearing dimethyl substituted pyridazine compared with non-substituted pyridazine betaines was monitored. A large increase in the photostability of both DHIs and betaines under investigation compared with the standard DHI was observed. The charged zwitterionic betaine structures were stabilized by increasing the solvent polarity due to the electrostatic interactions between them. The tuning of the absorption maxima and the kinetic properties by changing the substitution in the fluorene part (region A) and pyridazine part (region C) will help these compounds to find their applications.
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