On the Condensed Phase Ring-Closure of Vinylheptafulvalene and Ring-Opening of Gaseous Dihydroazulene

Abstract

Dihydroazulenes are interesting because of their photoswitching behavior. While the ring-opening to vinylheptafulvalene (VHF) is light induced, the back reaction is known to proceed thermally. In the present paper, we show the first gas phase study of the ring-opening reaction of 2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (Ph-DHA) by means of time-resolved photoelectron spectroscopy which permits us to follow the ring-opening process. Moreover, we investigated s-trans-Ph-VHF in a series of transient absorption experiments, supported by ab initio computations, to understand the origin of the absence of light-induced ring-closure. The transient absorption results show a biexponential decay governed by a hitherto unknown state. This state is accessed within 1-2 ps and return to the ground state is probably driven through a cis-trans isomerization about the exocyclic C1═C2 double bond. The rapid decrease in potential energy disfavors internal rotation to s-cis-Ph-VHF, the structure that would precede the ring-closure reaction.

Reversible Multicolor Photochromism of Dihydroazulene Crystals

“…Upon UV irradiation in solution these compounds undergo a ring‐opening reaction to their more strongly colored metastable form, vinylheptafulvene (VHF), which can exist in syn ‐ and anti ‐conformations (Figure ) . Based on computational analysis, it was proposed that the photochromic reaction occurs via a zwitterionic transition state . This reaction is attractive due to the anticipated large structural difference between DHA and anti ‐VHF, and the possibility for fine‐tuning of the switch by chemical substitution .…”

Computational and Experimental Evidence of Two Competing Thermal Electrocyclization Pathways for Vinylheptafulvene

“…IRC calculations revealed that the conrotatory and disrotatory TSs both connect to DHA and VHF, see the Supporting Information for details. Earlier computational work includes a single TS, which we identify as the Disrot‐TS . Our identification of these two TSs has resulted in inclusion of these in the theoretical work in a number of recent publications …”

“…Earlier computational work includes as ingle TS, which we identify as the Disrot-TS. [9,25,26] Our identification of these two TSs has resulted in inclusion of these in the theoretical work in an umber of recent publications. [22,[27][28][29] The Gibbs energy differences between the lowest energy VHF conformer and TS, DG VHF!TS ,a re shown in Figure 2f or each investigated DHA/VHF system in vacuum and the four solvents (benzene, tetrahydrofuran, acetone, and acetonitrile).…”

Synthetic protocols for the key functionalizations of the photochromic dihydroazulene scaffold

“…Functionalization of position 2 on the DHA skeleton has been most extensively explored. 5,6,[9][10][11][12][13][14][15][16][17][22][23][24][25][26][27][28] Aromatic groups are the substituents most widely incorporated, but few examples with methyl group on C-2 or cyclopentyl between C-2/C-3 have also been reported. 22 As described in the introduction (Scheme 3), the syntheses have followed either: (i) the [8+2] cycloaddition protocol, (ii) the route via nucleophilic addition of acetophenones to tropylium, followed by a Knoevenagel condensation with malononitrile, or (iii) an initial Knoevenagel condensation followed by reaction with tropylium species.…”

“…8,9 This is in line with a suggested mechanism that involves a zwitterionic transition state, with a positively charged seven-membered ring and a negatively charged malononitrile moiety. 10 The longest-wavelength absorption maxima of DHA 1 and VHF 2 are at 353 and 470 nm, respectively, in acetonitrile ( Figure 1). 9 In this solvent, the quantum yield of the ring-opening reaction from 1 to 2 is 0.55 8 , and the rate constant for the back-reaction is 5.36 x 10 -5 s -1 .…”