Visible‐Light‐Triggered Molecular Photoswitch Based on Reversible E/Z Isomerization of a 1,2‐Dicyanoethene Derivative

Abstract

Ad esigned bis(dithienyl) dicyanoethene-based, strictly E/Z photoswitch( 4TCE) operates through stateselective (E and Zi somer) photoactivation with visible light. The Ea nd Zi somers of 4TCE exhibit remarkably different spectroscopic characteristics,i ncluding al arge separation (70 nm) in their absorption maxima (l max )a nd a2 .5-fold increase in molar extinction coefficient from cis to trans.T he energetically stable trans form can be completely converted to the cis form within minutes when exposed to white light, whereas the reverse isomerization occurs readily upon irradiation by blue light (l < 480 nm) or completely by thermal conversion at elevated temperatures.T hese features together with excellent thermal stability and photostability of both isomers make this new E/Z photoswitchapromising building blockf or photoswitchable materials that operate without the need for UV light.Photochromic molecules have attracted substantial research interest because of their promising applications to biomolecules,e nergy management, information storage,a nd nanotechnology. [1] Molecule-based photochromism can be realized either by reversible E/Z photoisomerization or photocyclization reactions ( Figure 1). [2] Only one of these mechanisms should be operative for ap hotoswitch so as to avoid having more than the two essential switching states ("on" and "off") in agiven application.Photoswitching through E/Z photoisomerization is exemplified by azobenzene and its derivatives [3] (Figure 1, Pathway 1). Azo compounds undergo large-scale structural changes from their more stable trans form to their cis form upon irradiation with UV light;t his process can be reversed thermally or through irradiation with blue light. These compounds have been widely used as photoswitches for organic electronics, [4] energy conversion, [5] and photocontrol of biological systems, [6] yet their applications are limited somewhat by two intrinsic drawbacks:1 )possible UVinduced photodamage of other moieties attached to the azo group, [7] and 2) overlap between n-p*b ands of the two isomers leading to low conversion yield (F Z!E = 53 %a nd F E!Z = 24 %). [8] Introducing electron-donating groups at the ortho or para positions of the phenyl rings can effectively redshift the spectrum, while adjusting energy levels of norbitals by attaching proper groups at the ortho positions can increase splitting of n-p*b ands of both isomers. [2,7,9] However,t hese modifications usually require ac omplicated synthesis and in some cases cause ad istortion of the coplanar trans form, which decreases desirable p conjugation and solid-state packing. [2] These drawbacks could be avoided with "nonazo" E/Z photoswitches,yet surprisingly little effort has been devoted to developing such visible-light-triggered, highly photoconvertible switches that are readily synthesized using simple reactions.In principle,d iarylethenes hold promise as alternative, "non-azo" E/Z photoswitches. [10] Thef irst reported diarylethene that showed reversible photochromism was cis-1,2dicya...

New molecular switch architectures

“…Typically, these moieties consist of two five-membered heterocyclic aromatic rings (e.g., furan or thiophene) linked by ethene through their β-positions. By moving the linker to the α-position (6), Guo et al (26) prevented pericyclization, forcing E/Z photoisomerization around the central double bond. The thermodynamically stable E isomer could be photoisomerized into the Z isomer with high efficiency using unfiltered "white" light (PSS of 100% Z with Φ E → Z undetermined), which reduces the moiety's end-to-end distance by about 7 Å.…”

“…Historically prevalent switches, including interlocked molecules, will not be discussed as they have been recently reviewed elsewhere (13)(14)(15)(16)(17)(18). Our focus will be on systems that bring unique dimensions to the field, including interswitch communication (19), new molecular motor designs (20)(21)(22), and novel photoswitching capabilities and properties (23)(24)(25)(26)(27)(28)(29), among others. Where possible, we will also show that new molecular architectures enable the design and synthesis of new molecular machines [e.g., a molecular robotic arm (30)] that otherwise would have been difficult to attain.…”

Visible-light-driven two-way photoisomerization of 1-(1-pyrenyl)-2-(2-quinolyl)ethylene in neutral and protonated forms

“…9 Therefore, the possibility of switching the photochrome using visible light has attracted increasing attention. [10][11][12][13] There are several mechanisms of spectral tuning. In nature, the spectral sensitivity of the retinal photochrome is tuned over a broad wavelength range by environmental variation since the retinal absorption spectrum depends on interaction with the opsin protein, to which retinal is bound (on the polarity of the retinal binding pocket of rhodopsin, on proximity of the counter ion).…”

“…X. Guo et al 11 have noted that surprisingly little effort has been devoted to developing visible-light-triggered "non-azo" E/Z photoswitches. They have designed bis(dithienyl)dicyanoethene (4TCE) with absorption bands in the visible region, which is capable of isomerizing under visible light.…”

“…They have designed bis(dithienyl)dicyanoethene (4TCE) with absorption bands in the visible region, which is capable of isomerizing under visible light. 11,21 An interesting property of 4TCE is that E/Z photoswitching operates by multiplicity-exclusive pathways: trans-to-cis isomerization only occurs in the S 1 state, whereas cis-to-trans isomerization occurs in the T 1 state. Moreover, the photoisomerization quantum yields vary with excitation wavelength; for example, ϕ ct is negligible at 530 nm and increases to 0.05 at 420 nm.…”