Benzo-annulated chromenes, i.e., naphthopyrans, are well-known photochromic molecules that undergo photochemical ring-opening reactions to form two colored open-ring isomers, the transoid-cis and transoid-trans forms, upon light irradiation. Though the transoid-cis form returns thermally to the uncolored closed form, the fading rate of the transoid-trans form is extremely slow because of its higher thermal stability. This slow fading behavior of the transoid-trans form is responsible for the persistence of residual color for several minutes to hours, and prevents the application of such molecules to fast photoswitching materials. We have found a new simple and versatile strategy to substantially reduce the amount of the undesirable long-lived colored transoid-trans form by introducing an alkoxy group at the 1-position of azino-fused chromenes, i.e., 8H-pyranoquinazolines. The alkoxy group effectively reduces the formation of the transoid-trans form due to C-H···O intramolecular hydrogen bonding in the transoid-cis form. Moreover, the introduction of a condensed aromatic ring at the 3-position was found to be effective to increase the photosensitivity of the ring-opening reaction. This strategy can also be applied for naphthopyran derivatives and is useful for the development of fast photoresponsive photochromic lenses and fast photoswitching applications such as dynamic holographic materials and molecular actuators.
minutes/hours to completely return to the CF. The residual color imparted by the TT form and the slow thermal back-reaction of the TC form are considered as problems to be solved. [9][10][11][12][13] The thermal backreaction speed of colored naphthopyrans can be modulated by the introduction of an electron-donating group at the para position of the 3-phenyl ring. [4] However, substitution at the 3-phenyl ring typically causes significant changes in the color of the colored species. Although the speed of the thermal back-reaction can be modulated by exploiting steric effects to destabilize the TC form and the long-lived TT form, the synthetic procedures required to yield the desired products are not simple and moderate control of the thermal backreaction speed for photochromic lenses is difficult to achieve. [14][15][16][17][18][19] In order to design appropriate naphthopyran derivatives showing on-demand photochromic properties, the independent control of the thermal back-reaction speed is an important challenge in the development of advanced photochromic materials. Within this context, we herein present a molecular design strategy that can be used to modulate the thermal fading speed without significant changes to other photochromic properties.In this study, we designed novel naphthopyran derivatives possessing alkylenedioxy moieties as a means to control the thermal fading speed of the TC form and reduce the generation of the long-lived TT form. The reactivity and optical properties of the 1,2-alkylenedioxy benzene significantly depend on the ring-size of the alkylenedioxy moiety owing to changes in electron-donating ability and π-conjugation length upon changing the conformation of the ring. [20][21][22] We introduced alkylenedioxy moieties to the 9-and 10-positions of 3H-naphthopyran (3HNP) and investigated the effects of ring size on the photochromic properties of the resultant molecules. Remarkably, the thermal back-reaction speed of the newly designed naphthopyran derivatives was found to be controllable independently of other photochromic properties, such as the color of the colored species, by changing the alkyl chain length of the alkylenedioxy moiety. In addition, intramolecular hydrogen bonding is very important for modulating the conformation and stability of the structural isomers. [23,24] The undesirable residual color attributable to the formation of the TT form upon cessation of light irradiation is effectively reduced owing to the hydrogen bonding between the oxygen atom at the 10-position and the olefinic proton at the 1-position in the TC form, resulting in fast photoswitchable Photofunctional compounds have emerged as critically important materials for both fundamental studies and industrial applications. Control of the thermal decoloration speed to within several seconds while sustaining satisfactory photochromic colorability is an important challenge for the application of such materials to photochromic lenses and smart windows. Photochromic naphthopyran derivatives are utilized for photoch...
The development of red or near-infrared light (NIR) switchable photochromic molecules is required for an efficient utilization of sunlight and regulation of biological activities. While the photosensitization of photochromic molecules to red or NIR light has been achieved by a two-photon absorption process, the development of a molecule itself having sensitivity to red or NIR light has been now a challenging study. Herein, we developed an efficient molecular design for realizing red or NIR-light-responsive negative photochromism based on binaphthyl-bridged imidazole dimers. The introduction of electron-donating substituents shows the red shift of the absorption band at the visible-light region because of the contribution of a charge-transfer transition. Especially, the introduction of a di(4-methoxyphenyl)amino group (TPAOMe) and a perylenyl group largely shifts the absorption edge of the stable colored form to 900 nm. In addition, because the absorption band of one of the derivatives substituted with TPAOMe covers the whole visible-light region, the colored form shows a neutral gray color. Upon red (660 nm) or NIR-light (790 nm) irradiation, we observed the negative photochromic reaction from the stable colored form to the metastable colorless form. Therefore, the substituted binaphthyl-bridged imidazole dimers constitute the attractive photoswitches within a biological window.
Non-linear photoresponses against excitation light intensity are important for the development of attractive photofunctional materials exhibiting high spatial selective photoswitching that is not affected by weak background light. Biphotochromic systems composed of two fast photochromic units have the potential to show a stepwise two-photon absorption process in which the optical properties can be non-linearly controlled by changing the excitation light conditions. Herein, we designed and synthesized novel bisnaphthopyran derivatives containing fast photoswitchable naphthopyran units. The bisnaphthopyran derivatives show a stepwise two-photon-induced photochromic reaction upon UV light irradiation accompanied by a drastic color change due to a large change in the molecular structure between the one-photon product and the two-photon product. Consequently, the color of the bisnaphthopyran derivatives can be non-linearly controlled by changing the excitation intensity. This characteristic photochromic property of the biphotochromic system provides important insight into advanced photoresponsive materials.
Photocontrol of mechanical motions of small objects has attracted much attention to develop mesoscopic remote actuators. For this purpose, photoinduced morphological changes of molecules, molecular aggregates, and crystals have been extensively studied in the field of chemistry and materials science. Here, we propose direct use of momenta of light (i.e. radiation force) to control the motion of small objects, through photochromic reactions of pyranoquinazoline (PQ) derivatives. PQ is colorless in visible wavelength region while it is in closed form and undergoes photochemical ring-opening reactions to form colored isomers upon UV light irradiation; the open-ring isomers return to the colorless closed isomers mainly through the thermal back reaction. In the experiment, individual polymer microparticles with diameters of 7 μm incorporating PQ were trapped by optical tweezers. When the trapped microparticle was irradiated with UV light, the microparticle was pushed along the axis of light propagation about a few micrometers by absorption force arising from PQ in colored form. In addition, we found that dynamics of trapped microparticles was regulated by the thermal back reaction of PQ. The present results demonstrate that diversity of photochromic reactions can be transcribed into mesoscopic motions through the momentum exchange between light and molecules.
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