Isomerization dynamics of photochromic compounds in solution and single crystals

Abstract

Photochromic compounds are excellent model systems for the study of photoisomerization dynamics with atomic resolution using ultrafast electron diffraction (UED), which promises to deepen the fundamental understanding and optimization of chemical processes. These molecules have multiple stable isomers with distinct absorption properties, which can be selectively converted via light irradiation. Many photochromics undergo isomerization even in the crystalline phase, which is beneficial for the use of time-resolved diffraction techniques. However, before the realization of UED experiments, the suitability of a specific photochromic molecule needs to be assessed and the dynamics characterized, which can be done by means of transient absorption spectroscopy (TAS). Within the scope of this thesis, representatives from two different classes of photochromics, spiropyrans and diarylethenes, have been investigated by means of TAS and preliminary UED experiments, in both the crystalline phase and solution, including the construction of dedicated optical setups. Spiropyrans when excited in the ultraviolet undergo a ring-opening and planarization reaction associated with a bathochromic shift of the absorption spectrum into the visible range. The derivative spironaphthopyran (SNP) was studied via TAS in solution and the ring opening found to occur in the S1 electronic state under formation of an open-ring intermediate with a time constant of 300 fs, followed by relaxation to the planar isomer with a lifetime of 1.2 ps. Based on a newly developed spectral analysis, subsequent vibrational cooling was characterized and associated with lifetimes between 10 and 13 ps. The TAS results for SNP in the crystalline phase exhibit nearly identical isomerization dynamics, with time constants of 0.3 and 1.3 ps for the formation of the intermediate and product species, respectively. However, population of the open-ring isomer was found to decay with a lifetime of 650 ps. Supported by complementary UED experiments, this process was assigned to restoring forces from the crystal lattice. In the presence of a strong acid, spiropyrans become protonated having two stable openring isomers (Z and E), switchable by light. This motivated TAS investigations into the dynamics of the switching process for two different derivatives (spirobenzopyran and nitro-spirobenzopyran). The forward reaction was found to entail multiple species in the ground state, formed from a rapidly decaying excited state with a lifetime of ≈200 fs. Subsequent reaction to the planar E-isomer follows a biexponential behavior in the ps time domain, indicating the involvement of two distinct intermediates. The reverse reaction was determined to occur via an excited-state intermediate with a lifetime of 3.5 ps as well as a ground-state intermediate with a 40 ps lifetime. The third project in this thesis deals with the photo-induced ring-closing dynamics of three different diarylethene derivatives, ethyl-dithienylethene, butyl-dithienylethene and propyl-difurylethene, which were investigated in a comparative TAS study in solution and single crystals. Varying in the length of the alkyl chains at the reactive carbon atoms between which the bond is formed, as well as exchange of the heteroatom in the aryl substituents was found to significantly affect the cyclization. The determined reaction time in propyl-difurylethene of 300-400 fs was faster than in the dithienylethene derivatives, for which two distinct ring-closing pathways were found. Thus, the dominant factor was the nature of the heteroatoms rather than the alkyl length, which points towards the reaction being governed by the electronic structure rather than steric hindrance. Overall, the results from the crystalline samples were less complex than in solution due to the absence of other conformers and cyclization rates generally increased. Furthermore, static electron diffraction tests strongly suggest the suitability of all three compounds for systematic UED studies, since a distinct difference in the diffraction between open- and closed-ring isomer was observed.