Effective Reversible Photoinduced Switching of Self‐Assembled Monolayers of Functional Imines on Gold Nanoparticles

Abstract

In the past decade, surface-functionalized gold nanoparticles have received increasing attention from scientists due to their potential application in areas such as biology, electronics, catalysis, and materials science. [1][2][3][4] Usually, self-assembled monolayers (SAMs) of different thiol compounds modify bare gold colloids by strong Au-S interaction to form particles of reduced polydispersity with controlled size. [5,6] When capped by azobenzene [7][8][9][10][11][12] or stilbene [8,13] compounds, some of these functional nanoparticles can reversibly change the degree of solvation by light stimuli, an effect resulting from photoisomerization of these groups.The molecular structures of azobenzene and stilbene derivatives are susceptible to cis-trans isomerization by irradiation. In solution, illumination with UV light effectively results in isomerization from the trans-to the cis-isomer, while back isomerization can be done either by irradiation with visible light or thermal relaxation.[14] In 1998, Evans et al. [7] detected reversible photoswitching of azobenzene between the cis and trans states in the mixed monolayers on colloidal gold, but it was not observed in single-component monolayers. Three years later, Whitesell et al.[8] studied the influence of chain length upon the photoisomerization quantum yields of azobenzenederived, alkanethiol-capped gold nanoparticles. It was found that the quantum yields of isomerization markedly increased with increased linker length, but were still much lower than for the corresponding thiol in solution. These earlier reports conclude that more free volume for every chromophore unit is necessary to achieve photoswitching on the surfaces. Further, optimized systems with mixed SAMs, [11] SAMs of asymmetric disulfides, [9] or suprarmolecular complexes with cyclodextrin (CD) [10] were designed and exhibited improved photoswitching.Besides azobenzene and stilbene, the imine functional group is also photoisomerizable. The imine group undergoes photoinduced trans!cis isomerization with half the energy barrier of the corresponding azobenzene. Moreover, the activation energy in the thermal relaxation process is about 16-17 kcal mol À1 , which is appreciably lower than for azobenzene (23 kcal mol À1) and stilbene (42 kcal mol À1 ), and results in a far shorter half-life of the cis isomer of only 1 second at 25 8C.[15]Low energy barriers allow the extremely rapid, reversible geometrical change under light stimuli, which is the key feature for light-driven devices. [16][17][18] Furthermore, various conjugated imine compounds can be modularly synthesized by one-step condensation reactions of different aldehydes with a particular surface-mounted substrate amine. This provides the chance to prepare a series of imine derivatives for functionalizing gold surfaces. As a result, imine compounds could be better candidates for use in light-responsive systems. Since gold nanoparticles stabilized by SAMs containing C=N bonds have hitherto rarely been investigated, [19,20] we decided to react b...

Light‐Triggered Self‐Assembly of Gold Nanoparticles Based on Photoisomerization of Spirothiopyran

“…Self-assembly of AuNPs is one powerful method for the size control, a simple and low-cost method to produce ensembles of AuNPs in a controllable manner. [10][11][12] Lighttriggered methods have been studied extensively [13][14][15][16][17][18][19][20][21][22][23][24] because light is non-invasive and can be delivered instantaneously to a precise location. All of the early reported methods employ AuNPs modified with covalently bound photoresponsive molecules.…”

“…These AuNPs, when left in the dark, are well dispersed in solvents because of the electrostatic repulsion between AuNPs. Photoactivation of the surface molecules promotes several reactions, such as isomerization of azobenzenes, [13][14][15][16][17][18][19][20][21] dimerization of thymine, [22,23] and coupling of aldehyde with amine moieties. [24] These reactions change the surface polarity, surface electronic charge, or particle geometry, promoting aggregation of the AuNPs.…”

Unusually high fluorescence quantum yield of a homopolyfluorenylazomethine – towards a universal fluorophore

“…While high degrees of polymerization do not necessarily afford increased degrees of conjugation, they introduce structural constraints into the polymer. 67 This precludes tracking the photoisomerization spectroscopically. The contrasting F fl difference between the low and high DP n samples of 3 clearly demonstrates the impact of structural elements on the fluorescence yield.…”

“…66 The energy barrier for Z -E conversion is understood to be sufficiently low that the isomerization can thermally occur even at low temperatures. 67 This precludes tracking the photoisomerization spectroscopically. The collective solvent data nonetheless illustrate the importance of structural elements and the orientation of the azomethine in the polymer for achieving high fluorescence with polyazomethines.…”

Amino-substituted spirothiopyran as an initiator for self-assembly of gold nanoparticles

“…[10][11][12] Several methods have been proposed based on temperature, 13,14 pH, 15,16 and ionic strength 17,18 as external stimuli. In particular, light-triggered methods have been studied extensively [19][20][21][22][23][24][25][26][27][28][29][30] because light is noninvasive and can be delivered instantaneously to a precise location. All of these light-triggered methods need AuNPs modied with covalently-bound photoresponsive molecules.…”