Two-photon isomerization properties of donor-acceptor Stenhouse adducts

Abstract

Donor-acceptor Stenhouse adducts (DASAs) are important photo-responsive molecules that undergo electrocyclic reactions after light absorption. From these properties, DASAs have received extensive attention as photo-switches with negative photochromism. Meanwhile, several photochemical applications require isomerization events to take place in highly localized volumes at variable depths. Such focused photoreactions can be achieved if the electronic excitation is induced through a non-linear optical process. In this contribution we describe DASAs substituted with extended donor groups which provide them with significant two-photon absorption properties. We characterized the photo-induced transformation of these DASAs from the open polymethinic form to their cyclopentenic isomer with the use of 800 nm femtosecond pulses. These studies verified that the biphotonic excitation produces equivalent photoreactions as linear absorbance. We also determined these DASAs' two-photon absorption cross sections from measurements of their photoconverted yield after biphotonic excitation. As we show, specific donor sections provide these systems with important biphotonic cross-sections as high as 615 GM units. Such properties make these DASAs among the most non-linearly active photo-switchable molecules. Calculations at the TDDFT level with the optimally tuned range-separated functional OT-CAM-B3LYP, together with quadratic response methods indicate that the non-linear photochemical properties in these molecules involve higher lying electronic states above the first excited singlet. This result is consistent with the observed relation between their two-photon chemistry and the onset of their short wavelength absorption features around 400 nm. This is the first report of the non-linear photochemistry of DASAs. The two-photon isomerization properties of DASAs extend their applications to 3D-photocontrol, non-linear lithography, variable depth birefringence, and localized drug delivery schemes.

Scheme 1. (a) Structures of DASAs studied in this paper and (b) photoisomerization of DASAs.

Scheme 2. Synthesis of DASAs 2–6.

Fig. 1. Absorption spectra of DASAs 1–6 highlighting the structural-dependence in toluene. The donor groups R are color coded with the respective spectrum. The inset shows the details of the region from 375 to 425 nm, together with the spectral intensity profile for the two-photon spectrum of the 800 nm excitation pulses (black line, two photon equivalent maximum wavelength: 400 nm).

Fig. 2. Comparison of two-photon induced spectral changes for DASAs 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), and 6 (f) in toluene. The black lines indicate the absorbance before 800 nm irradiation, and the red symbols indicate the spectra after 5 min irradiation with a 1 kHz pulse train with pulsed intensity of 3.7 GW cm⁻². The sample volume was 3 ml. These partial conversions were made to directly compare the two-photon reactivity of the different compounds.

Fig. 3. Absorbance changes for 3 (a) and 4 (b) in toluene upon 800 nm irradiation with different pulsed intensities. Inset: linear fits of the change in absorbance vs. intensity in log–log plots. The slope for 3 was: 2.1, R²:0.99. For 4 the log–log slope was 1.8, R² = 0.97. For each intensity, a fresh solution sample was used.

Fig. 4. (a) Biphotonic 800 nm conversion of 5 at 3.70 GW cm⁻² showing the sample absorbances at different pulsed irradiation times. Inset: zoom of the region around the isosbestic point (325 nm). (b) Biphotonic bleaching of 5 measured at 632 nm and the respective thermal return in chlorobenzene at 50 °C. The return to the equilibrium absorbance level was fitted to an exponential model giving an observed return rate constant of k = 0.076 min⁻¹, (R² = 0.99). The yellow zone corresponds to the irradiation period. The inset shows several cycles of photo-transformation and thermal returns.

Fig. 5. Equilibrium geometries corresponding to the global minima of molecular systems 1–6.

Fig. 6. Isosurface (0.04) of the molecular orbitals involved in the description of the low-energy singlet electronic states for systems 2 (a), 5 (b) and 6 (c). The respective surfaces for molecules 1, 3 and 4 are included in the ESI.