P‐Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical, Photophysical and Theoretical Study

Abstract

Two novel helical naphthopyrans have been synthesised. The helical scaffold has the interesting effect of increasing the thermal stability of the transoid-trans (TT) open isomer formed upon UV irradiation of the closed form (CF), which transforms these naphthopyrans from thermal to photochemical photochromes. The photochromic performance is excellent in both polar and apolar solvents and the conversion percentage from the CF to the TT form can be as high as 92.8 %. We propose a new method to determine the quantum yields of the photochemical processes that lead to transoid-cis (TC) and TT isomers, and their molar absorption coefficients. The thermal stability of the TT and TC isomers has been studied in different solvents. The quantum yields of fluorescence before and after irradiation, along with the decay lifetimes, have also been measured. TD-DFT calculations have been performed to determine the relative thermodynamic stability of the species involved in the photochromic mechanism and to rationalise their spectral properties.

Helicenes as Chiroptical Photoswitches

“…This structural feature is distinctly different from that of o6, where the 2,4-pentadienal moiety is integrated within the outer, and not the inner, rim of the helicene backbone. Although o7/c7 ratio in the PSS of this P-type system was only 27:73, it could be improved (up to 93:7) in a hexyl derivative of 7, as demonstrated [23] by Frigoli, Jacquemin, Ortica, and co-workers. Most recently, it was shown [24] that both forms of the hexyl derivative of 7 are stable against racemization at room temperature and that the switching of an enantioenriched sample proceeds stereospecifically and can be monitored by CD spectroscopy.…”

Chiral separation of helical chromenes with chloromethyl phenylcarbamate polysaccharide‐based stationary phases

“…Some of the authors of the present work have recently shown that the thermal persistence of the color generated upon UV irradiation can be strongly enhanced upon shifting the photoequilibrium to the TT isomer, thus increasing the P‐type character of the chromene molecule and the subsequent introduction of a helical scaffold into the molecular structure represents a further step in this direction . Moreover, the presence of the axial chirality may give rise to interesting potential applications for these helical chromenes, such as the possibility of obtaining photochromic chiroptical materials to be used as chiral photo‐switches for developing smart materials distinguishing right‐ from left‐circularly polarized UV‐visible radiation .…”

A helical naphthopyran dopant for photoresponsive cholesteric liquid crystals

“…15,16 However, photoisomerizable helicene derivatives are rather unexplored as photoresponsive dopants compared to photochemically reversible P-type molecules, such as diarylethenes and azobenzenes. [1][2][3][4][5][6] Recently, Frigoli et al reported on the novel photochromism of helical naphthopyrans overcoming fast thermal relaxation, 17,18 and we anticipated that its reversible conformational change could provide efficient superstructural manipulation of CLCs. There has been a report on a non-chiral naphthopyran derivative which induces liquid crystallinity in the nematic LC host by photoisomerization, 19 whereas, to the best of our knowledge, the formation of a cholesteric superstructure and its manipulation have never been exploited based on the photoswitching of naphthopyran derivatives introduced as chiral dopants.…”

“…Synthesis of a helical naphthopyran derivative, CHR-Hexyl (10-hexyl-3,3-diphenyl-[3H]-benzo [5,6]phenanthro[4,3-f ]chromene), is described elsewhere, 18 and its molecular structure and the photoisomerization process are described in Fig. 1.…”

“…The transoid-trans (TT) isomer is more thermodynamically stable than the transoid-cis (TC) one and is the major photoproduct (up to 92% conversion ratio from CF to TT) at the photostationary state upon continuous irradiation. 18 Coloured forms can reconvert to the starting material both thermally and upon visible irradiation. Detailed photochemical characteristics of CHR-Hexyl are described in ref.…”