Oligothiophene functionalized dimethyldihydropyrenes I: syntheses and photochromicity

The syntheses of 2,7-di-tert-butyldimethyldihydrobenzo[e]pyrenes with thienyl (6), terthienyl (7), and pentathienyl (14) side chains at the 4,5- positions, ter- and pentathienyl side chains at the 4-position with ter- (39) and pentathienylcarbonyl (40) side chains at the 10- and 11-positions, 2-naphthoyl-7-tert-butyldimethyldihydropyrenes with ter- (53), penta- (54), and septithienyl (55) side chains at the 4,9-positions are described. These compounds are all photochromic and open to the corresponding cyclophanedienes with long wavelength (>490 nm) light, and as such, the conjugative path could change considerably, making them suitable to investigate as potentially switchable conducting molecules. In this paper, the syntheses and the photochemical and thermal isomerizations are studied; in the accompanying paper, the electrochemical and conductive properties are studied. Here, a comparison of the relative opening rates to that of the benzo[e]pyrene 4 (with no thienyl substituents) is made, and all of the above photochromes show considerably enhanced photo-opening of the DHPs to the CPDs. As examples, 14, 40, and 54 were cycled between the open and closed forms, and no decomposition was observed; however, when 54 was irradiated for 40 h with 254 nm light, some radicals did form, which enhanced the thermal closing rate, and so extensive irradiation with short wave UV is better avoided. The thermal closing reactions were also studied, and all of the above compounds close faster than benzo-CPD 4', though for the highly photochromic ter- and pentathienyl benzo-CPDs 39' and 40', the rate was not too enhanced from that of 4' and so are probably the best compromise between fast photochromicity and slow thermal reversion.