Controlled Photoinduced Generation of “Visual” Partially and Fully Charge Separated States in Viologen Analogues

Abstract

Charge-separated states with a lifetime scale of seconds or longer not only favor studies using various steady-state analysis techniques but are important for light-energy conversion and other applications. Through a steric-hindrance-induced method, unprecedented photoinduced generation of a partially charge separated (PCS) state with a lifetime of days has been detected in the "visual" mode during the decay of excited states to a commonly observed fully charge separated (FCS) state for viologen analogues. One pale yellow 4,4′bipyridine-based metalloviologen compound, with an interannular dihedral angle of 1.84°in 4,4′-bipyridine, directly decays to the purple FCS state after photoexcitation. The other pale yellow compound, with a similar coordination framework but a larger interannular dihedral angle (33.74°), changes first to a yellow PCS state and then relaxes slowly (in the dark in Ar, ca. 2 days; 70 °C in Ar, ca. 1 h) to the purple FCS state. The two-step coloration phenomenon is unprecedented for viologen compounds and their analogues and also rather rare for other photochromic species. EPR and Raman data reveal that photoinduced charge separation first generates univalent zinc and radicals and then the received electron in Zn(I) slowly distributes further to 4,4′-bipyridine. Reduction of πconjugation and a direct to indirect change in band gap account for the prolongation of the relaxation process and the capture of the PCS state. These findings help to understand and control decay processes of excited states and provide a potential design strategy for multicolor photochromism, light-energy conversion with high efficiency, or other applications.

Inverse photochromism in viologen–tetraarylborate ion-pair complexes: optical write/microwave erase switching in polymer matrices

“…Photochromism of viologen type molecule is the reversible transformation of the viologen specie between two forms, non-radical (V 2+ ) and radical (V •+ ). 1,2 The radical form has different uv-vis absorption spectra from non-radical species that the absorbance is only observed in the ultraviolet region. In this sense, the electronic conversion of non-radical to the radical species is possible by uv-absorption of electromagnetic radiation.…”

Quadruple Photoresponsive Functionality in a Crystalline Hybrid Material: Photochromism, Photomodulated Fluorescence, Magnetism and Nonlinear Optical Properties

“…[19][20][21][22][23][24][25][26][27][28][29][30] Besides the CHPMs based on photodeformable or photoresponsive ligands, the conjugated N-heterocycle moieties as potential electron acceptors (EAs) are also desirable units for yielding CHPMs. [41][42][43][44][45][46][47][48][49][50] The assembly of various EAs moieties, electron donors (EDs) species, and metal ions generates assorted CHPMs driven by photoinduced electron-transfer (ET) between EDs and EAs, [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] and the resulting CHPMs feature additional fascinating photoresponsive functionality like photomagnetism, photocurrent, sensor for X-ray, etc. [41][42][43][44][45][46][47][48][49][50]…”

“…[41][42][43][44][45][46][47][48][49][50] The assembly of various EAs moieties, electron donors (EDs) species, and metal ions generates assorted CHPMs driven by photoinduced electron-transfer (ET) between EDs and EAs, [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] and the resulting CHPMs feature additional fascinating photoresponsive functionality like photomagnetism, photocurrent, sensor for X-ray, etc. [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] Reviewing literature implies that the explored photoresponsive functionality in reported CHPMs mainly concentrate on dyad combination. For example, photochromism and photomagnetism was observable in 3d-4f hexacyanoferrate.…”

Unveiling electron transfer in a supramolecular aggregate for adaptive and autonomous photochromic response

“…Nevertheless, it is inevitable that significantly geometrical configuration change during the photo-induced color change process in these systems, which largely confine the dynamic photochromic behavior in solution or in the matrix, because the tight molecular packing in the crystalline or aggregated state restrains the isomerization of the active motifs (Kuroiwa et al, 2019;Ma et al, 2019;Chen et al, 2020). Although many strategies have been developed to overcome this issue (Torres-Pierna et al, 2020;Vá zquez-Mera, et al, 2013;Long et al, 2014), for example, Abe et al designed the imidazole-based molecular species with minimal conformation change (Kometani et al, 2020;Usui et al, 2020), while others provide flexible and soft host matrix to facilitate isomerization of the active motif in the confined space (Samanta et al, 2018;Williams et al, 2018), it is highly desirable to exploit a novel photochromic system that could fast respond to the light without altering the molecular framework and could be easily fabricated from readily available precursors, such as the extensively investigated redox-active viologen complexes (Liu et al, 2021;Chen et al, 2017;Li et al, 2019) and, more importantly, have the capability to sense and adapt to the external environment for autonomous function. Herein, we report a dynamically responsive and environment compatible photochromic system by taking advantage of the planar, aromatic p-conjugated structural feature of naphthalenediimide (NDI) and its inherent redox activity.…”