Ultrafast Excited States Dynamics of Orthogonal Photoswitches and The Influence of the Environment

Abstract

Molecular photoswitches are widely used in material sciences, physics, chemistry, and biology. As needs grow more complex, materials have to react more than one‐dimensionally. The use of multiple photoswitches at once opens manifold opportunities for further improved and more complicated systems. However, this requires independent addressability, i.e., orthogonality, and reversible processes. Herein, the first study on ultrafast excited state dynamics of two orthogonal photoswitches, a push‐pull azobenzene and a donor‐acceptor Stenhouse adduct is reported. In order to gain detailed insight in their interactions and mutual influences on their photoswitching behavior, they are addressed individually and simultaneously via transient absorption spectroscopy supported by quantum chemical calculations. They show reversible photoswitchability and in addition, can be used in 4D printing to provide easy access to a plethora of functional devices. Furthermore, environmental influences on the excited state dynamics are examined using different solvents and thin films. Both compounds photoisomerize independently when addressed individually or simultaneously and only little impacts on the excited state dynamics are found. Especially the vibrational relaxation is affected by different surroundings changing the energy dissipation while hardly affecting the electronic states involved. The orthogonal and simultaneous addressability is thereby crucial for their usage in 4D printed microactuators.

Characteristics and long-term kinetics of an azobenzene derivative and a donor–acceptor Stenhouse adduct as orthogonal photoswitches

“…The shorter wavelength of the Z -to- E photoisomerisation compared to E -to- Z isomerisation is caused by the push–pull character of the azobenzene substituents. 34 In contrast, photoexcitation of DASA causes a linear-to-closed switching composed of a photochemically induced Z -to- E isomerisation of the linear Z -DASA (isomer A) followed by a thermally induced C–C bond rotation and electrocyclisation (isomer C) with subsequent thermally driven proton transfer to generate the zwitterionic end product (isomer C′, see Fig. 1b).…”

“…Quantum chemical calculations reveal a higher polar character of the excited state of Azo and a less polar character of the excited state of DASA compared to the respective ground states causing the observed solvatochromism. 34 Hence, the absorption wavelength can be fine-tuned by altering the environment.…”

“…17 The assignment of the absorption bands to the corresponding isomers has been validated by quantum chemical calculations. 34 The linear/closed switching processes of DASA can also be initiated in an equimolar mixture of DASA + Azo when E -Azo is present (Fig. S4c and d, ESI†) or when Azo undergoes simultaneous E / Z isomerisation (Fig.…”

“…30–33 In order to demonstrate the orthogonal addressability of the Azo + DASA mixture, which is important for the performance of such a multifunctional smart material, we recently studied the photophysical properties and especially the ultrafast excited states dynamics of the single compounds and the mixture by means of static UV/VIS spectroscopy and femtosecond time-resolved transient absorption spectroscopy. 34 Therein, we investigated the reversible E / Z isomerisation of Azo (Fig. 1a) and reversible Z / E isomerisation of DASA (Fig.…”

“…For the ultrafast dynamics, we found no relevant interactions diminishing the performance of the photoswitches. 34 However, since important steps of the switching processes are thermally activated and occur on longer time scales, different experimental methods are needed to elucidate the dynamics of these steps and complete the picture upon switching the orthogonally addressable mixture of the photoswitches Azo and DASA.…”