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Compounds with multiple photoswitching units are appealing for complex photochemical control of molecular materials and nanostructures. Herein, we synthesized novel meta‐ and para‐ connected (related to the nitrogen of the indoline) azobenzene‐spiropyran dyads, in which the central benzene unit is shared by both switches. We investigated their photochemistry using static and time‐resolved transient absorption spectroscopy as well as quantum chemical calculations. In the meta‐compound, the individual components are photochemically decoupled due to the meta‐pattern. In the para‐compound the spiro‐connectivity leads to a bifunctional photoswitchable system with a red‐shifted absorption. The azobenzene and the spiropyran can thus be addressed and switched independently by light of appropriate wavelength. Through the different connectivity patterns two different orthogonally photoswitchable systems have been obtained which are promising candidates for complex applications of light control. [ABSTRACT FROM AUTHOR]

Abstract: Nonphotochemical quenching (NPQ) is a fundamental mechanism in photosynthesis by which plants protect themselves against excess excitation energy and which is thus of crucial importance for plant survival and fitness. Recently, carotenoid radical cation (Car•+) formation has been discovered to be a key step in the feedback deexcitation quenching component (qE) of NPQ, whose molecular mechanism and location remains elusive. A recent model for qE suggests that the replacement of violaxanthin (Vio) by zeaxanthin (Zea) in photosynthetic pigment binding pockets can in principle result in qE via the so-called “gear-shift” or electron transfer quenching mechanisms. We performed pump-probe measurements on individual antenna complexes of photosystem II (CP24, CP26 and CP29) upon excitation of the chlorophylls (Chl) into their first excited Qy state at 660 nm when either Vio or Zea was bound to those complexes. The Chl lifetime was then probed by measuring the decay kinetics of the Chl excited state absorption (ESA) at 900 nm. The charge-transfer quenching mechanism, which is characterized by a spectral signature of the transiently formed Zea radical cation (Zea•+) in the near-IR, has also been addressed, both in solution and in light-harvesting complexes of photosystem II (LHC-II). Applying resonant two-photon two-color ionization (R2P2CI) spectroscopy we show here the formation of β-Car•+ in solution, which occurs on a femtosecond time-scale by direct electron transfer to the solvent. The β-Car•+ maxima strongly depend on the solvent polarity. Moreover, our two-color two-photon spectroscopy on CP29 reveals the spectral position of Zea•+ in the near-IR region at 980 nm. [Copyright &y& Elsevier]