Comprehensive Understanding of Structure- Photosensitivity Relationships of Photochromic [2.2]Paracyclophane-Bridged Imidazole Dimers

Abstract

The photochromic [2.2]paracyclophane-bridged imidazole dimers show instantaneous coloration upon exposure to UV light and rapid fading in the dark. Experimental details for the enhancement of the photosensitivity and the unique photoisomerization of newly designed [2.2]paracyclophane-bridged imidazole dimers are demonstrated. We explored the structure-property relationships and demonstrated an efficient strategy for designing high-performance fast-photochromic molecules with increased photosensitivity to solar UVA radiation. The [2.2]paracyclophane-bridged imidazole dimer consists of two types of imidazole rings, Im1 and Im2. Im1 is characterized by a 6π electron system with an electron-donating characteristic, whereas Im2 is distinguished by a 4π electron system with an electron-withdrawing characteristic. The introduction of electron-donating substituents into the phenyl rings attached to the electron-withdrawing Im2 was proved to enhance the photosensitivity with the aid of the intramolecular charge transfer transitions. The unique photoisomerization resulting from the changes in the bonding manner between two imidazole rings was also investigated in detail.

Negatively photochromic organic compounds: Exploring the dark side

“…Scheme 31. Synthesis, structure and photoisomerisation of heterocyclic azo dyes 124 -1262.8 Imidazolyl DimersWhilst exploring the synthesis of further analogues of hexaarylbiimidazoles208 in which the two imidazole cores are tethered together Abe et al,[209][210][211] made the serendipitous discovery of a new biimidazole which exhibited negative photochromism. The synthetic strategy commences with lithium -halogen exchange of 127 to introduce the aldehyde functions into the 1,1′-binaphthyl unit and then classical construction of the imidazole nucleus, the oxidation of which proceeds anomalously to afford the orange species 128 which upon exposure to visible light bleaches (Scheme 32).…”

A real-time dynamic holographic material using a fast photochromic molecule

“…The dimers exhibit instantaneous colouration upon exposure to UV light and rapid fading in the dark141516. Upon UV light irradiation, the C–N bond between the two imidazole rings of the [2.2]PC-bridged imidazole dimer is homolytically cleaved to give a pair of imidazolyl radicals, and the colour of the solution changes from colourless to blue.…”

Fluorescence modulation by fast photochromism of a [2.2]paracyclophane-bridged imidazole dimer possessing a perylene bisimide moiety

“…Because of the introduction of the electron-donating methoxy group, the [2.2]PC-bridged imidazole dimer unit of 1 has the relatively large absorption band at 350 nm, which can be ascribed to the intramolecular charge transfer transition from the dimethoxy phenyl groups to the electronwithdrawing 4π electron imidazole ring and that from the electron-donating 6π electron imidazole ring to the carboxyl phenyl group. 38 Thus, all in the absorption spectrum of 1 is mainly the sum of those of fluorescent (PBI) and photochromic units (bridged imidazole dimer) as expected using the amide bond spacer. The fluorescence quantum yields estimated by using cresyl violet (Φ=0.54 in ethanol) 39 as a standard for 540 nm light excitation and quinine sulfate ((Φ=0.55 in 0.5 M H2SO4) 40 as a standard for 360 nm excitation are summarized in Table 1.…”

“…Moreover, the sensitivity of the imidazole dimer unit to the UVA region is enhanced by the introduction of methoxy groups to achieve the efficient photochromic reaction upon UV light irradiation. 38 Thus, 1 has the satisfactory properties for demonstrating the fluorescence modulation.…”