Paper Introduction Amrutha A.S. 24/10/2014

Photoisomerisation and ligand-controlled reversible aggregation of azobenzene-functionalised gold nanoparticles Functionalized nanoparticles: size dependent optical, electronic, magnetic and chemical properties. Applications: medical diagnostics drug delivery cancer therapy nanoelectronics information storage etc. This group shows interest in the nanoparticles with surface bound molecular switches which can be controlled by UV or Visible light. Mechanisms hindering the outcome of photoswitching properties of molecules on surface: Dense packing of the surface-bound molecules less space for transformation to proceed Energy transfer Change in the properties of NPs upon isomerisation of the switches in the ligand shell leads to aggregation of the NPs.

Experimental section Functionalised AuNPs with AB-derivatised alkylthiols as ligands and normal alkyl thiols as coligands in mixed monolayers. The alkyl lengths of the thiols were systematically varied. Obtained NPs were characterised using TEM, NMR and UV-Vis spectroscopy. UV-Vis spectra of the AB-functionalised NPs showed, Local surface plasmon resonance (LSPR) band at 515nm ππ* band of AB ligands in their transform at 350nm Weaker nπ* AB band at 455nm (barely visible) (365nm for trans cis, 455nm for cis trans)

At UV PSS, NPs with shortest azoligand (A) show significant residual trans AB With longer azoligands, the trans to cis switching efficiency increased, while the cis to trans efficiency decreased. For the ligand-coligand combination with the longest alkyl spacer (F) at UV PSS 95% cis 5%trans and at Vis PSS 25%cis and 75% trans was seen. Greater differences were seen in the shape of the LSPR bands when the longer azoligands are combined with short alkylthiolates (C,E), where as little change is observed in all systems with decanthiolate coligands (B,D,F) after irradiation. Strong broadening and decrease in intensityof the LSPR band together with a sizeable red shift are found after isomerisation of samples carrying ABs with long alkyl spacers (C7,C11) in combination with pentanethiolate coligand (C,E). Aggregation of NPs after switching of the attached ABs to their cis form.

Reasons for aggregation Dipole-dipole interactions between the cis-ABs The dipole moment of cis-AB (3.2D) causes weaker solvent stabilization of the NPs in the nonpolar toluene such that they aggregate. Longer alkylthiolate coligands better shield the cis-AB and lead to better solvation and there by prevent the association. Aggregation potential increases with the increased AB surface coverage. Aggregation occurs only when the length of the coligand is shorter than the length of azoligand. Shorter chain length of coligand closer contact between the cis-switched NPs stronger interaction of the dipoles. Cis switched NPs with short coligands less well stabilised by the solvent Solubility of the NPs is not the main driving force for the aggregation where as dipole-dipole forces are.

Conclusions AuNPs with mixed azothiolate-alkylthiolate monolayers were studied. Samples could be photoswitched between the two PSSs without visible photodegradation. Observed differences in the switching efficiency indicate steric or electronic hindrance of the isomerisation when the alkyl spacer of the azoligand becomes too short. Fully reversible aggregation of NPs switched to the cis state when a long azoligand was combined with a coligand posessing shorter alkyl chain. Tendency for aggregation depends on the cis-AB surface concentration, and dipole-dipole attraction is the main driving force for the observed aggregation. Azothiolate AuNPs without alkylthiolates were practically insoluble. The approach using mixed monolayers offers control of the photoaggregation potential of functionalised AuNPs by optimising the the chain length of the coligand and the surface concentration of the azoligand.