Tuning of switching properties and excited-state dynamics of fulgides by structural modifications

Abstract

The ultrafast photo-induced dynamics of the E-isomers of four selected photochromic fulgides with distinct structural motifs have been elucidated by femtosecond broadband transient absorption spectroscopy in n-hexane as solvent. E→C and E→Z isomerisations, respectively, with time constants of ∼0.12 ± 0.02 ps and ∼0.34 ± 0.03 ps taking place in parallel were found for derivatives with a methyl substituent at the central hexatriene (HT) unit. In contrast, fulgides with increased steric constraints by an iso-propyl substituent or by intramolecular bridging displayed virtually zero E→Z isomerisation, but instead a desired accelerated and more efficient ring closure in a reaction time of only ∼50 ± 10 fs. Both photoisomerisations appear to follow excited-state pathways with distinctive conical intersections. For the ring closure, direct barrierless pathways with steep downhill gradients are likely. Furthermore, the results indicate conformer-specific reactions, with ring closure exclusively by the E(α) conformer and E→Z isomerisation predominantly by the E(β) conformer, because the E(α)→Z channel is unfavoured by the faster and kinetically more competitive E(α)→C reaction. DFT calculations of the equilibrium structures showed that the sterically demanding groups at the HT unit shift the conformer equilibria towards the E(α) conformers. At the same time, they appear to cause a favourable pre-orientation of the furyl unit that accelerates the conrotatory ring closure in the E(α)→C reaction. Benzo-annulation of the furyl unit has little effect on the observed dynamics. Overall, the results demonstrate how the excited-state dynamics and thereby the photoswitching properties of fulgides can be successfully tuned and improved by structural modifications at the chromophores.

Photochromism of Rotation‐Hindered Furylfulgides Influenced by Steric Modifications

“…An explanation for these different photochemical properties caused by steric or electronic differences can be derived from femtosecond timeresolved transient absorption spectroscopy. [10] In particular, the time-resolved measurements revealed that the photo-induced ring-closure reactions are considerably faster in the cases of 13 and 14 (τ ≈ 50 fs) compared with 12 and 15 (τ ≈ 110 and 140 fs, respectively). Thus, the molecule-specific dynamics in the excited electronic states appear to play decisive roles.…”

“…The ratio between rotamers is approximately 6:1, which is in excellent agreement with the calculated fraction of the E α conformer of 82 % and 18 % of the E β conformer. [10] Based on the knowledge of rotational barriers in the furylfulgide, it is likely that the E α Ǟ E β conversion, which becomes visible in this furylfulgide analogue, is due to increased steric hindrance.…”

“…New results derived from femtosecond timeresolved measurements and DFT calculations show that, besides the (E) Ǟ (Z) isomerization, there are other deactivation processes that reduce the quantum efficiency. [10] In addition to the mentioned electronic substituent effects, steric modifications have great influence on the photochromism. [11] In 1988, Yokoyama et al investigated the effects of bulky substituents R at the cylohexatriene moiety on the photochromic reaction of a furylfulgide.…”

Fulgimides as Light‐Activated Tools in Biological Investigations

“…Furan [12,21] 333-364 472-519 96-98 % 0.18 Thiophene [21,22,23] 272-339 414-532 51-92 % 0.13 Pyrrole [24] 364-389 584-642 30-60 % 0.20 Benzofuran [25,26,27,28] 330-387 488-511 43 % 0.17 Benzothiophene [10,29,30] Irradiation wavelengths within the visible light range are desirable in various photopharmacological studies, as light in this range does not greatly interfere with cellular function. [32] A bathochromic shift in the absorption spectra of fulgimides was achieved by the incorporation of an electron-rich heteroaromatic moiety [33][34][35] or a dicyanomethylene-modified anhydride.…”

Theoretical investigation on the spectroscopic properties of furylfulgide with different substituents and design of novel bis‐furylfulgimide photochromes

“…Tsinberg and coworkers researched the structural and spectral properties of indolylfulgides with different substituents by applying DFT and TDDFT methods. Siewertsen et al also revealed the importance of electronic and steric effect to the ring‐opening process and excited‐state dynamics of several structurally modified furylfulgides. They considered that the study of electronic and structural modification may be used to improve the photoswitching dynamic of fulgides.…”