The Versatile Photo-Thermal Behaviour of a 2-Hydroxyazobenzene

Abstract

Photochromic compounds are employed in implementing neuron surrogates. They will boost the development of neuromorphic engineering in wetware. In this work, the photochromic behaviours of (E)-3,4,6-trichloro-2-(p-diazenil)-phenol (t-DZH) and its conjugated phenoxide base (t-DZ) have been investigated experimentally in three different media: (1) pure acetonitrile, (2) in water and acetonitrile mixed in a 1/1 volume ratio, and (3) in an aqueous micellar solution of 3-(N,N-Dimethylmyristylammonio)propanesulfonate (SB3-14). The analysis of the spectral and kinetic features of t-DZH and t-DZ has been supported by quantum-mechanical DFT calculations, the maximum entropy method, and the determination of their colourability (C). The versatility of t-DZH and t-DZ makes them promising molecular probes of micro-environments and potential ingredients of photochemical oscillators required for implementing pacemaker neurons capable of communicating through optical signals in wetware.

Keywords: azobenzene; molecular probe; molecular switch; photochemistry; photochromism.

Figure 1

Photochromism of DZH, its conjugated base DZ, and structure of the SB3-14 surfactant.

Figure 2

Absorption spectra of t-DZH in (A) and t-DZ in (B) recorded in acetonitrile (black trace), water/acetonitrile = 1/1 (red trace), and in the aqueous micellar solution of SB3-14 (blue trace). Graph (C) reports the relative percentage transmittance changes induced by the deprotonation in the three media: acetonitrile (black trace), water/acetonitrile = 1/1 (red trace), and in the aqueous micellar solution of SB3-14 (blue trace).

Figure 3

Schematic representation of the energy levels of the frontier molecular orbitals for t-DZH and t-DZ along with the isodensity surface plots of HOMO—HOMO−3 and LUMO—LUMO+3 having isodensity contour equal to 0.03 (H and L stand for HOMO and LUMO).

Figure 4

Spectral modifications (A) induced by a stationary UV irradiation at ${\lambda }_{irr}=\left(363\pm 40\right) \mathrm{nm}$ for [t-DZH] = $4.2\times {10}^{-5} \mathrm{M}$ in [SB3-14] = 0.1 M: the arrows indicate the trends of absorbance in the different spectral regions (the black and cyan traces are the initial and final spectra, recorded before irradiation and at the photo-stationary state, respectively). Graph (B) shows how the absorbance at 360 nm changes upon UV irradiation. Graph (C) reports the trend of the relative percentage transmittance change, defined in Equation (3), as the function of wavelength.

Figure 5

Graph (A): Kinetics of the thermal cis-to-trans isomerization for DZH in the aqueous micellar solution of SB3-14 0.1 M (blue trace); in H₂O/CH₃CN = 1/1 (red trace); in CH₃CN (black trace). Graph (B): lifetimes distributions of the c-DZH conformers, determined through the maximum entropy method.

Figure 6

Spectral modifications (A) induced by a stationary UV irradiation at ${\lambda }_{irr}=\left(363\pm 40\right) \mathrm{nm}$ for [t-DZ] = $4.8\times {10}^{-5} \mathrm{M}$ in [SB3-14] = 0.1 M: the arrows indicate the trends of absorbance in the different spectral regions (the black and cyan traces are the initial and final spectra, recorded before irradiation and at the photo-stationary state, respectively). Graph (B) shows how the absorbance at 328 nm changes upon UV irradiation. Graph (C) reports the trend of the relative percentage transmittance change, defined in Equation (3), as the function of wavelength.

Figure 7

Trends of the absorbance at one specific wavelength for DZ that photo-isomerizes from trans-to-cis in the portion of absorbance that decays and isomerizes thermally from cis-to-trans in the portion of absorbance that grows. Graphs (A–C) refer to DZ in CH₃CN, H₂O/CH₃CN = 1/1, and aqueous micellar solution of [SB3-14] = 0.1 M, respectively. The cyan curves are the functions fitting only the growth that corresponds to the thermal isomerization.

Figure 8

pH changes generated by [t-DZH] = $1\times {10}^{-4}\mathrm{M}$ dissolved in H₂O/CH₃CN = 1/1 (A) and in an aqueous micellar solution of SB3-14 0.1 M (B). The pH drops were photo-induced using the same irradiation source (see Figure S16 and the end of Section 3.3 for more details).

Figure 9

Schematic representation of the photochromic properties of t-DZH and its conjugated base, t-DZ. The colours shown next to each molecule refer to those determined in the aqueous micellar solution of SB3-14 with [t-DZH] $\approx 4\times {10}^{-5} \mathrm{M}$.