Solution Phase and Surface Photoisomerization of a Hydrazone Switch with a Long Thermal Half-Life

Abstract

Photoswitches can be employed for various purposes, with the half-life being a crucial parameter to optimize for the desired application. The switching of a photochromic hydrazone functionalized with a C6 alkyl thiolate spacer (C6 HAT) was characterized on a number of metal surfaces. C6 HAT exhibits a half-life of 789 years in solution. Tip-enhanced Raman spectroscopy (TERS) was used to study the photoisomerization of the C6 HAT self-assembled monolayers (SAM) on Au, Ag and Cu surfaces. The unique spectroscopic signature of the E isomer at 1580 and 1730 cm-1 in TER spectra allowed for its discrimination from the Z isomer. It was found that C6 HAT switches on Au and Cu surfaces when irradiated with 415 nm, however it cannot isomerize on Ag surfaces, unless higher energy light is used. Based on this finding, and supported by density functional theory calculations, we propose a substrate-mediated photoisomerization mechanism to explain the behavior of C6 HAT on these different metal surfaces. This insight into the hydrazone's switching mechanism on metal surfaces will contribute to the further exploitation of this new family photochromic compounds on metal surfaces. Finally, although we found that the thermal isomerization rate of C6 HAT drastically increases on metal surfaces, the thermal half-life is still 6.9 days on gold, which is longer than the majority of azobenzene-based systems. Figure 1. A. Light induced Z/E isomerization of C6 HAT; B. UV-vis spectra of C6 HAT in toluene (1x 10-5 M) before (solid line) and after irradiation with 410 nm light (dashed line), followed by irradiation with 340 nm light (dotted line).

Molecular photoswitches in aqueous environments

“…288 However, if a suitable H-bond acceptor, such as a pyridine (3.12), or a carbonyl group (3.13), or both (3.14), are stabilizing the metastable isomer through H-bonding to the hydrazone hydrogen, 306,307 bi-stable (photo)switches with very long thermal lifetimes (up to a few thousand years) 308 in various solvents and the solid state can be obtained. [308][309][310][311] Moreover, hydrazone-based photoswitches Fig. 18 Thermally stable Z and metastable E isomer of Iminothioindoxyl (ITI).…”

Hydrazones as New Molecular Tools

“…In general, the newly developed switches have very high photoswitching efficiencies, i.e., high PSSs and quantum yields and can be switched in various media-from fetal bovine serum buffer to the solid-state and even on metal surfaces. 55 Here, we want to focus on the unique properties of these switches, especially in the context of configurational switches, namely their bistability and emission toggling.…”

Structural and Spectroscopic Properties of Benzoylpyridine‐Based Hydrazones

“…Therefore, the scrutiny of the changes in the IR and Raman spectra induced by the isomerization is important to identify isomerization markers that could be used to experimentally probe the conversion between the two forms. To carry out our analysis, we correlate experimental and calculated spectra, considering that in the literature there are a few papers related to this aspect [28] . We report in Figure 6 the experimental and calculated Raman spectra of the three derivatives and in Figure 7 the IR ones in the fingerprint region with highlighted the bands of interest.…”

“…It also offers the possibility of a non‐destructive readout of memory devices based on organic photochromic compounds [24–26] . Moreover, advanced vibrational spectroscopy techniques like surface‐enhanced or tip‐enhanced Raman spectroscopy (SERS; TERS) were already successfully used to study the photoisomerization of azobenzene and hydrazone photoswitch self‐assembled monolayers on metal surfaces [27,28] . However deeper vibrational characterization of photochromic hydrazones has not been carried out yet.…”