Correlation between the Molecular Structure and Photoresponse in Aliphatic Self-Assembled Monolayers with Azobenzene Tailgroups

Abstract

We have compared the structural and photoisomerization properties of self-assembled monolayers (SAMs) comprising either the trans or cis isomers of azobenzene terminated dithiolane with in-chain amide unit, viz., 4-(phenyldiazenyl)phenyl-4-(1,2-dithiolane-3-yl)-butylcarboxamide ( 1). These films were prepared on Au(111) from solutions of both isomers. Structure and composition of the SAMs were studied by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The photoresponse of the films was monitored in real time by ellipsometry. SAMs fabricated from the trans isomer were found to be densely packed and highly ordered. These films did not show any discernible photoresponse upon irradiation with UV light, which, under favorable conditions, triggers the trans- cis isomerization. In contrast, films prepared from solutions containing predominantly the cis isomer were loosely packed and mostly disordered but exhibited reversible photoreactivity. The results confirm that steric effects, i.e., available free volume, play a dominant role for the photoresponse of aliphatic SAMs bearing the photoactive azobenzene group. The crystal structure of 1 ( trans isomer) exhibits a row-like aggregation of neighboring molecules by weak hydrogen bonds and can be taken as a model for the arrangement of 1 in the monolayer films. Further, in addition to the surface coordination behavior, we have also mimicked the chemisorption of the 1,2-dithiolane moiety onto the gold substrate in molecular coordination chemistry in oxidative addition reactions with the zero-valent platinum complex [Pt(PPh 3) 4].

New ferrocene based dithiolate ligands

“…2. The observed coupling constant values are in close agreement with those reported for related square planar Pt(II) complexes with two cis-phosphine ligands trans to two ligands with different trans influences [7,21,23,24]. The formation of 6e9 was also followed by NMR spectroscopy.…”

“…The coordination geometry of platinum center is a slightly distorted square planar with two phosphines and one dithiolate ligand that coordinates in a bidentate fashion to yield a six-membered ring. The PteS (2.339(1)e2.3603(1) Å) and PteP bond lengths (2.287(1)e 2.306(1)) and SePteS (average of 91.73(5) ) and PePteP angles (97.50 (5) ) compare well with those reported for related complexes [7,21]. Notably, the eSeCHeCH 2 e bond angle is markedly increased in 4 (112.6(4) ) compared to that in 1 (107.4 (7) ), a consequence of the ring opening and consistent with the change in 1 H NMR chemical shifts.…”

“…More recently, Kraatz and co-workers have coupled lipoic acid to the amine unit in [Fe(C 5 H 4 CO 2 Me)(C 5 H 4 NH 2 )], and oxidatively added the SeS bond onto Au nanoparticles for the electrochemical detection of HIV-1 reverse transcriptase [6]. It is possible to model such metal/sulfur surface chemistry processes with molecular coordination chemistry by the reactions of dithiolanes with a zero-valent platinum complex [7]. One of the significant features of disulfide redox chemistry is the oxidative addition of the SeS bond to low-valent metal centers, which can play an important role in different areas such as medicinal chemistry and transition metal catalysis [8e10].…”

“…H NMR (400 MHz, CDCl 3 , 23 C) d ¼ 7.47 (t,3 J HH ¼ 7.6 Hz, 6H),7.41 (t, 3 J HH ¼ 7.6 Hz, 6H), 7.24 (m, 6H), 7.14 (t, 3 J HH ¼ 7.6 Hz, 6H), 7.09 (t, 3 J HH ¼ 7.6 Hz, 6H), 6.55 (s, 1H), 4.61 (s, 1H), 4.57 (s, 1H), 4.14 (s, 5H), 3.97 (s, 2H), 3.34 (m, 2H), 2.89 (m, 1H), 2.14 (m, 1H), 2.10 (t,3 J HH ¼ 7.8 Hz, 2H), 1.68 (m, 1H), 1.56 (m, 2H), 1.34 (m, 4H). 13 C{ 1 H} NMR (100.5 MHz, CDCl 3 ) d ¼ 171.6 (C(O)), 134.8 (CH), 130.0 (CH), 127.5 (CH), 94.8 (CN), 69.1 (Cp), 64.2 (CH), 61.3 (CH), 39.6 (CHS), 38.7 (CH 2 ), 37.4 (CH 2 S), 27.5 (CH 2 ), 26.0 (CH 2 ), 25.3 (CH 2 ), 24.5 (CH 2 ).…”

Photoresponsive thin films containing an azobenzene derivative intercalated with a layered double hydroxide

“…Figure 3 shows the UV-vis absorption spectra of the horizontal-film before and after irradiation with UV light for 5 min. The trans isomer exhibits a strong π-π* absorption peak at 365 nm, and a weak (forbidden) n-π* band at around 451 nm [25]. Upon UV irradiation, the band caused by the trans-isomer decreases and the absorption band attributed to the cis-isomer at ∼451 nm becomes stronger, indicating trans to cis isomerization of the intercalated AO5 anions.…”