Tuning the photoreactivity of Z-hexatriene photoswitches by substituents – a non-adiabatic molecular dynamics study

Abstract

To understand how substituents can be used to increase the quantum yield of photochemical electrocyclic ring-closing of the Z-hexa-1,3,5-triene (HT) photoswitch forming cyclohexadiene (CHD), we investigate the S1 photo dynamics of HT and its derivatives 2,5-dimethyl-HT (DMHT), 2-isopropyl-5-methyl-HT (2,5-IMHT), 1-isopropyl-4-methyl-HT (1,4-IMHT), and 2,5-diisopropyl-HT (DIHT) using time-dependent density functional theory surface hopping dynamics. We report detailed photoproduct distributions, formation mechanisms, branching ratios, and wavelength-dependent product quantum yields. Most products have been confirmed experimentally and include all-trans HT derivatives, cyclopropanes, cyclobutenes, cyclopentene, cyclohexadienes, and bicyclic compounds. Regarding CHD formation, we find that for the 2,5-substituted derivatives DMHT, 2,5-IMHT, and DIHT, the branching ratios increase with increasing size of the substituents. In contrast the branching ratios of the E/Z-isomerization decrease with increasing size of the substituents. Due to steric interactions, increasing the size of the substituents increases the amount of gZg rotamers in the ground state, which are prone to CHD formation and have lower E/Z-isomerization probability. Furthermore, we find [1,4], [1,5], and [1,6]-sigmatropic hydrogen shift reactions occurring at large percentages (5% to 15%); for sterical reasons these reactions stem from tZg conformers. DIHT shows the lowest percentage of side product formation among the 2,5-substituted molecules and highest CHD branching ratio; its CHD quantum yield can be increased up to more than 64%, by excitation of DIHT on the red tail of its absorption spectrum, whereas the Z/E-isomerization is reduced below 5% and side reactions practically vanish. This makes DIHT the best candidate for applications in molecular switches.

Calculation of vibrationally resolved absorption spectra of acenes and pyrene

“…[5][6][7][8] This approach is particularly successful if the overall absorption spectrum of a compound is caused by different conformers present in the ground state. [9][10][11] However, this procedure is computationally demanding and important effects arising from the quantum nature of the nuclei are not taken into account implying that peaks of pure vibrational origin are missing. 4 A physically more appealing alternative for modeling the band broadening is to take vibronic effects into account by explicitly calculating the coupling between vibrational motion and electronic transitions.…”

Azologization and repurposing of a hetero-stilbene-based kinase inhibitor: towards the design of photoswitchable sirtuin inhibitors

“…ADC(2) and CC2 calculations make use of the resolution-of-identity approximation [73]. ADC(2) calculations were also done using the continuum solvent model COSMO as previously described [54–55 74–76]. A dielectric constant of 62.14 and a refractive index of 1.3379 were used, which corresponds to a solvent of a 6/4-mixture of methanol/water, as experimentally determined [77–78].…”

Azologization of a hetero-stilbene based c-RAF kinase inhibitor: Towards the design of photoswitchable sirtuin inhibitors

“…ADC (2) and CC2 calculations make use of the resolution-of-identity approximation [48]. ADC(2) calculations were also done using the continuum solvent model COSMO as previously described [33,34,[49][50][51]. A dielectric constant of 62.14 and a refractive index of 1.3379 were used, which corresponds to a solvent of a 6/4-mixture of methanol/water, as experimentally determined [52,53].…”