Combining photochromism and nonlinear optical (NLO) properties of molecular switches-functionalized self-assembled monolayers (SAMs) represents a promising concept toward novel photonic and optoelectronic devices. Using second harmonic generation, density functional theory, and correlated wave function methods, we studied the switching abilities as well as the NLO contrasts between different molecular states of various fulgimidecontaining SAMs on Si(111). Controlled variations of the linker systems as well as of the fulgimides enabled us to demonstrate very efficient reversible photoinduced ring-opening/closure reactions between the open and closed forms of the fulgimides. Thus, effective cross sections on the order of 10 −18 cm −2 are observed. Moreover, the reversible switching is accompanied by pronounced NLO contrasts up to 32%. Further molecular engineering of the photochromic switches and the linker systems may even increase the NLO contrast upon switching.
The photoinduced dynamics of two DTE-BODIPY conjugates A, B with carboxylic acid anchoring groups coupled to the surface of TiO2 were studied by ultrafast transient absorption spectroscopy. For compound A, with an orthogonal orientation of the BODIPY chromophore and the photoswitchable DTE unit, a charge separated state could not be reliably detected. Nevertheless, besides the energy transfer from the BODIPY to the closed DTE, indications for an electron transfer reaction were found by analyzing fluorescence quenching on TiO2 in steady state fluorescence measurements. For compound B with a parallel orientation of chromophore and photoswitch, a charge separated state was conclusively identified via the observation of a positive absorption signal (at λpr > 610 nm) at later delay times. An electron transfer rate of 7 x 1010 s-1 can be extracted, indicating slower processes in the dyads in comparison to previously published electron transfer reactions of DTE compounds coupled to TiO2.
The photoisomerization characteristic parameters [photostationary states (PSSs), cross section σ, and quantum yield η] of indolyl fulgimide compounds dispersed in solution are quantitatively compared to those of the same compounds covalently immobilized as a monolayer on functionalized Si(111) surfaces (see Adv. Mater. 2013, 25, 416). The photoisomerization kinetics of fulgimide compounds in solution is monitored using UV–vis spectroscopy and that of monolayers is monitored by in situ calibrated Fourier transform infrared spectroscopy. Although the isomeric composition at PSSs is very similar in solution and at surfaces, a strong influence of the incident light polarization is found on photoisomerization cross section σ (σs ≪ σp < σsol). Accounting for the local excitation electromagnetic field indicates that the fulgimide groups adopt a preferential tilt angle with respect to the surface normal and their transition dipole is almost parallel to the surface plane for either C or E/Z isomers. Density functional theory indicates that this orientation corresponds to photochromic groups that are lying in contact with the underlying monolayer. This interaction plausibly explains why the quantum yield of photoisomerization is found nearly twice lower at the surface than in the solution.
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