Figure 1
(a) Pyridine and phenoxide as well as their conjugated acids pyridinium and phenol, respectively. (b) Equilibrium between azulene (shown by two representative resonance structures) and the corresponding conjugated acid. (c) Proton-gated photochromism of azulene-based diarylethene ATE in its open (top) and closed (bottom) isomers and their conjugated acids.
Source publication
Proton‐responsive photochromic molecules are attractive for their ability to react on non‐invasive rapid optical stimuli and the importance of protonation/deprotonation processes in various fields. Conventionally, their acidic/basic sites are on hetero‐atoms, which are orthogonal to the photo‐active π‐center. Here, we incorporate azulene, an acid‐s...
Contexts in source publication
Context 1
... used moieties are heteroatom-based, for example, pyridine [23,[26][27][28][29] or phenol. [16,24,30,31] The basic sites of these functional groups are located on the lone pairs of the heteroatoms, which are orthogonal to the p-core (Figure 1 a), thereby limiting their influence on the photophysical behavior of the switch. In strong contrast, azulene, one of the most representative nonalternant aromatic hydrocarbons, can be directly protonated on its p-skeleton forming a vinyl substituted aromatic tropylium species [32,33] and thereby is converted from a 10p electron aromatic system into a (6 + 2)p electron one (Fig- ure 1 b). ...
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... The basic sites of these functional groups are located on the lone pairs of the heteroatoms, which are orthogonal to the p-core (Figure 1 a), thereby limiting their influence on the photophysical behavior of the switch. In strong contrast, azulene, one of the most representative nonalternant aromatic hydrocarbons, can be directly protonated on its p-skeleton forming a vinyl substituted aromatic tropylium species [32,33] and thereby is converted from a 10p electron aromatic system into a (6 + 2)p electron one (Fig- ure 1 b). Due to this drastic change on p-conjugation upon protonation, incorporation of an azulene moiety into a photoswitchable molecule should lead to an unprecedented protonresponse since photochromism itself is also inherently based on altering p-conjugation. ...
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... To unravel the full potential of the proton-response of azulene in the development of novel multi-stimuli responsive molecular photoswitches, here we describe the effect of incorporating azulene into the skeleton of a diarylethene-type (DAE) photoswitch on its photochromic behavior. In this work, we synthesized azulenylthienylethene (ATE) where one of the aryl groups originates from guaiazulene, a naturally occurring and commercially available terpenoid azulene derivative (Figure 1 c). In strong contrast to most DAE-type photoswitches, which exhibit positive photochromism because of an extended conjugated system in the closed form, photocyclization of the protonated open isomer, o-ATE-H + , leads to a 100 nm hypsochromic shift of the strongest visible light absorption band, and thus constitutes an example of negative photochromism. ...
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... constitutes a typical spectral feature of DAE-type photoswitches during photocyclization, [7,8] suggesting the formation of c-ATE. Upon prolonged irradiation, the spectra continue to follow this trend with a clean isosbestic point at 343 nm, however, even after irradiating for 70 min the photostationary state (PSS) is not reached (see Figure S1 b). Alternative irradiation of the S 0 ! ...
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... irradiation of the S 0 ! S 1 transition wavelength of azulene using a 660 nm LED only led to negligible spectral changes despite its long duration of 16 h (see Figure S1 a). Attempts to switch back the in situ formed ring-closed c-ATE by irradiation at 546 nm were not successful as well (see Figure S1 c). ...
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... 1 transition wavelength of azulene using a 660 nm LED only led to negligible spectral changes despite its long duration of 16 h (see Figure S1 a). Attempts to switch back the in situ formed ring-closed c-ATE by irradiation at 546 nm were not successful as well (see Figure S1 c). These results suggest very low quantum yields for both, ring-closure as well as ringopening of o-ATE and c-ATE, respectively. ...
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... deviation may be explained by considering the fact that direct photochemical ring-closure of o-ATE could lead to a mixture of two regioisomers, whereas photoreaction of o-ATE-H + is regioselective (see Figure S9). There are only negligible UV/Vis absorption spectral changes for a solution of c-ATE kept in the dark at room temperature for 7 h (see Figure S10). Thus, and unlike c-ATE-H + , the neutral form c-ATE is thermally stable. ...
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... and unlike c-ATE-H + , the neutral form c-ATE is thermally stable. However, photochemical ring-opening of c-ATE is still inefficient (see Figure S11). The assigned molecular structures of both pairs of switching states, that is, o-ATE, o-ATE-H + , c-ATE, and c-ATE-H + , are further supported by calculated UV/Vis absorption spectra (see Figure S12), which are in reasonable agreement with experimental spectra. ...
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... photochemical ring-opening of c-ATE is still inefficient (see Figure S11). The assigned molecular structures of both pairs of switching states, that is, o-ATE, o-ATE-H + , c-ATE, and c-ATE-H + , are further supported by calculated UV/Vis absorption spectra (see Figure S12), which are in reasonable agreement with experimental spectra. ...
