Excited‐State Topology Modifications of the Dihydroazulene Photoswitch Through Aromaticity

Abstract

The gain and loss of aromaticity plays a key role in organic chemistry and in the prediction of rate-determining steps. Herein, we explore the concept of aromaticity in photoisomerization reactions. Benzannulated derivatives of the dihydroazulene-vinylheptafulvene (DHA-VHF) photoswitch were investigated using transient absorption spectroscopy and timedependent density functional theory to elucidate the effect of built-in aromaticity on the switching properties. We found that benzannulation hampered the switching ability by enhancing an already existing barrier on the excited state surface. This enhancement was found to arise from a significant loss of aromaticity in the DHA-to-VHF transition state on the excited state potential energy surface. The VHF was found to be highly aromatic on the excited state surface, showing a reversal of aromaticity compared to the ground state. The barrier was found to be dependent on the position of benzannulation, since one derivative was found to switch as fast as the nonbenzannulated molecule although with lower efficiency, whereas another derivative completely lost the ability to undergo reversible photoswitching. The findings herein provide novel principles for the design of molecular photoswitches, shedding new light on excited state aromaticity, as previous discussions have mainly considered excited state aromaticity to be beneficial to switching. Our findings show that this view must be reconsidered.[a] Mr.

Donor-Acceptor Substituted Benzo-, Naphtho- and Phenanthro-Fused Norbornadienes

“…Thin-layer chromatography (TLC) was carried out using aluminum sheets precoated with silica gel. NMR spectra were acquired using a Bruker 500 MHz instrument with a non-inverse cryo-probe or a Varian 400 MHz instrument using the residual solvent peak as the reference (CDCl 3 : 1 H 7.26 ppm (CHCl 3 ), 13 C 77.16 ppm. d8-Toluene: 1 H 2.090 ppm (d7-toluene)).…”

“…(dt, J = 7.9, 1.7 Hz, 1H), 2.45 (dt, J = 7.9, 1.7 Hz, 1H) ppm. 13 (dt, J = 7.9, 1.7 Hz, 1H), 2.41 (dt, J = 7.9, 1.7 Hz, 1H) ppm. 13 To a solution of ((4-methoxyphenyl)ethynyl)trimethylsilane (230 mg, 1.13 mmol) in THF and MeOH (20 mL, 1:1, v/v) was added K 2 CO 3 (594 mg, 4.30 mmol) and the mixture was stirred at rt for 1 h. The mixture containing 6 was poured through a plug of silica (40-63 µm, CH 2 Cl 2 ), almost concentrated in vacuo zand then THF (50 mL) was added and the mixture was concentrated to approx.…”

“…86 (s, 3H), 3.83 (s, 3H), 2.56 (dt, J = 7.9, 1.7 Hz, 1H), 2.38 (dt, J = 7.9, 1.7 Hz, 1H) ppm. 13 Bromo-3-chloro-1,4-dihydro-1,4-methanoanthracene (12). A solution of potassium tert-butoxide (4.53 g, 40.4 mmol) in anhydrous THF (180 mL) was cooled to −78 • C, before 11 (9.32 g, 48.5 mmol) was added followed by n-BuLi (16.16 mL, 2.5 M, 40.41 mmol).…”

“…1 H NMR (500 MHz, CDCl 3 ): δ = 8.73 (d, J = 8.1 Hz, 2H), 8.06 (dd, J = 8.0, 1.4 Hz, 2H), 7.64 (ddd, J = 8.0, 6.9, 1.3 Hz, 2H), 7.59 (ddd, J = 8.1, 6.9, 1.4 Hz, 2H), 7.03 (d, J = 1.7 Hz, 2H), 4.64 (m, 2H), 2.56 (dt, J = 6.6, 1.7 Hz, 1H), 2.47 (dt, J = 6.6, 1.7 Hz, 1H) ppm. 13 Bromo-3-chloro-1,4-dihydro-1,4-methanotriphenylene (20). To a solution of t-BuOK (1.93 g, 17.2 mmol) in THF (300 mL) at −78 • C was added 19 (5.13 g, 21.2 mmol).…”

“…For this photo-thermoswitch couple, benzannulation was also, in most cases, accompanied by the loss of the desired photoactivity of the DHA photochrome. A theoretical study showed that loss of aromaticity in the excited state reaction profile inhibits the DHA-to-VHF photoswitching [13]. In consequence, it seems that we need to seek a compromise by designing systems with less significant loss of aromatic character during photoswitching.…”