Molecular-Level Study of Photoorientation in Hydrogen-Bonded Azopolymer Complexes

Abstract

To optimize azobenzene-containing materials for applications such as rewritable waveguides and holographic data storage, it is imperative to understand the effect of the azobenzene structure on the photoresponse of the material. Supramolecular materials, in which a complexed photoactive azobenzene controls the motion or other properties of a passive polymer, are uniquely convenient for studying the impact of specific chemical modifications. Here, we use polarization modulation infrared structural absorbance spectroscopy (PM-IRSAS) to hydrogen-bonded supramolecular azobenzene complexes using poly(4-vinylpyridine) (P4VP) as a model polymer. We show that changing the tail group from hydrogen (AH) to cyano (ACN) induces greater angular redistribution of the chromophores and, remarkably, provokes P4VP pyridine ring orientation. Increasing the degree of complexation decreases the saturated orientation of both AH and ACN, whereas for P4VP/ACN it increases the pyridine orientation as well as the orientation stability of both components. These results explain the contrasting photoinduced birefringence behavior previously observed for these complexes and identify azo-azo intermolecular interactions as the main reason. To our knowledge, this is the first molecular-level spectroscopic analysis of the contrasting contributions of azobenzenes to the photo-orientation of supramolecular azopolymer complexes and the first report of the large impact of small molecular changes on the capacity of azo dyes to transfer light-induced orientation to a photopassive polymer. 29 bonds. In many cases, the passive polymer host employed is poly(4-vinylpyridine) (P4VP). [16][17][18][19][20][21][23][24][25][26][27][28][29] Various studies have revealed that the chemical structure and the content of the supramolecularly attached azo have significant impact on the measured PIB [23][24][25]30 and on SRG formation efficiency. [20][21][22] For example, the saturated PIB per azobenzene unit of hydrogen-bonded P4VP-azo complexes, where the azos used differ only by the nature of its tail group (Scheme 1), increases by more than a factor of 2 when increasing azo content for the cyano-tailed azo (CN-tailed or ACN), whereas it decreases by up to a factor of 2 for the hydrogen-tailed azo (H-tailed or AH). 25

Photoresponsive polymers with multi-azobenzene groups

“…In addition, besides photoisomerization, 28,29 azobenzene-containing polymers (azopolymers) show other interesting photoresponsive properties such as photoinduced orientation ( Fig. 2b and c), [30][31][32][33][34][35][36][37][38][39][40][41][42][43] formation of surface relief gratings (SRGs) (Fig. 2d), [44][45][46][47][48] photoactuation ( Fig.…”

Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes

“…116 An infrared technique called polarization-modulation FTIR structural absorbance spectroscopy (PM-IRSAS), 127 which can decouple the orientational effects originating from different chemical groups of the material system, was used to follow the molecular-level kinetics of photo-orientation induced by linearly polarized light and its subsequent thermal relaxation in the complexes of 2 and 4 with P4VP. 117 This study showed that, while the saturated orientation per azo of both 2 and 4 decreases with increasing azo content, 2 orients more than 4 and only 2 also drives photo-induced orientation of the passive H-bonded pyridine groups of P4VP. Furthermore, it was concluded that 4 undergoes mainly angular hole burning resulting from angular-dependent trans-cis isomerization, whereas for 2 there is a significant additional contribution from angular redistribution.…”

Smart Supramolecular Self-Assembled Nanosystem: Stimulus-Responsive Hydrogen-Bonded Liquid Crystals

“…Supramolecular side-chain LCPs are usually formed by H-bonds between hydroxyl and cyano groups [ 90 , 99 , 100 , 101 , 102 ]. Wang et al employed 4-hydroxy-4′-cyanoazobenzene and poly(4vinylpyridine) with 4-phenylazophenol (P4VP) to construct H-bonded supramolecular azobenzene complexes (see Figure 3 ) [ 103 ]. Along with an increasing molar ratio of azobenzene groups to the pyridine ring of P4VP repeat units (F A ), H-bonding complexation, as well as the band intensity in infrared spectra, were enhanced.…”

“… Supramolecular complexes of poly(4vinylpyridine) (P4VP) with 4-phenylazophenol (A H , R = H) and 4-hydroxy-4′-cyanoazobenzene (A CN , R = CN), and photoinduced orientation as a function of time during orientation (laser on) of P4VP/A CN (100%) and P4VP/A H (100%). Reprinted with permission from reference [ 103 ]. Copyright (2018) American Chemical Society.…”