Mechanism for the Indirect Photo-Transformation of Molecular Switches using a Pyrrolo–Pyrrole Two-Photon Absorbing Antenna

Electron-rich symmetric pull-push-pull molecules can act as efficient two-photon absorbing units. When these chromophores are bonded directly to photo-switchable molecules they can function as antenna systems to indirectly induce photochemical transformations in isomerizable groups after energy transfer. Recently, we developed an antenna–molecular switch system based on a pyrrolo–pyrrole two-photon active chromophore. When this antenna section is functionalized with symmetrically situated azo-sections as N,N’-pyrrolic-substituents, these azo molecular switches can be efficiently transformed from their E to their Z isomers after non-linear light absorption by the antenna, followed by indirect excitation of the azo-sections. In this contribution we present a case-study of one of these systems through femtosecond-resolved fluorescence to observe the dynamics involved in the excitation and relaxation steps within the antenna section, as well as the energy transfer pathways. By time-resolving the emission signals we observed that the energy transfer can occur in parallel with the relaxation within the first singlet vibronic states localized at the pyrrolo–pyrrole antenna. In fact, the antenna-to-azo section energy transfer shows a biphasic nature. At early times, during the relaxation within the antenna, there is an initial population of azo-section excited molecules. In addition, after the system has evolved to the fully relaxed S₁ state at the antenna section, the energy transfer has components related to thermal fluctuations which increase the couplings with the receiver states at the azo-switches giving transfer rates of the order of 10¹⁰ s⁻¹ at room temperature. The characterization of these relaxation and energy transfer steps, as well as the role of the solvent in these processes, gives insights for the development of future molecules with indirect two-photon isomerization properties using this kind of pull-push-pull two-photon active antenna. Due to their two-photon reactive properties, these systems can have applications in schemes where highly localized photo-isomerization is required.