Exceptional three- to six-photon absorption at organometallic dendrimers

Abstract

The light-intensity dependence of multi-photon absorption (MPA) affords outstanding spatial control. Furthermore, compared to the higher-energy photons needed for analogous linear absorption, the lower-energy photons involved in MPA often correspond to important wavelengths, such as those of the biological and telecommunications "windows". It is therefore of crucial importance to develop molecules that exhibit outstanding MPA cross-sections. However, although progress has been made with two-photon absorption, there is currently a dearth of efficient instantaneous n-photon absorbers (n > 2), a key reason being the scarcity of structure-property studies required to understand higher-order MPA. We herein report systematically-varied metallodendrimers up to third-generation in size, together with their nonlinear absorptive responses over the spectral range 600-2520 nm. We show that the dendrimers exhibit exceptional instantaneous three- to six-photon absorption cross-sections, with maximal values increasing with dendrimer generation and installation of solubilizing group, and we report that changing the groups at the dendrimer periphery can shift the wavelengths of the nPA maxima. We also describe time-dependent DFT studies that have facilitated assignment of the key linear and nonlinear transitions and disclosed the crucial role of the metal in the outstanding MPA performance.

Fig. 1. Organoruthenium dendrimers in this study. [Ru]: trans-[Ru(κ²-dppe)₂] (dppe = 1,2-bis(diphenylphosphino)ethane). Wavy lines: equivalent dendrons at dendrimer cores. Names follow the format: (i) nG (n-th generation dendrimer), (ii) xy (x and y phenyleneethynylene units between the branching points and the ligated Ru at each generation level, commencing at the core), (iii) a comma “,” separating the “xy” numbers of OPE units at each generation level, and in some cases (iv) –s (installation of solubilizing Et groups at the central phenylene of the OPE unit between the first- and second-generation branching points).

Fig. 2. Computational models of the key dendrimer building blocks 1-M-n (n = 1–4), 2-M-n (n = 2, 3), and 3-M-3.

Fig. 3. Wavelength dependence of the nonlinear absorption of 3G_22,03,02,01–NO₂. Plots of σ₂ (blue scatter), σ₃ (red scatter), and σ₄ (green scatter), overlaid on the UV/Vis spectrum (black line), and including plots of the UV/Vis spectrum as a function of twice (blue line), three times (red line), and four times (green line) the wavelength.

Fig. 4. Wavelength dependence of the nonlinear absorption of 3G_22,03,02,01. Plots of σ₂ (blue scatter), σ₃ (red scatter), σ₄ (green scatter), σ₅ (purple scatter), and σ₆ (orange scatter) overlaid on the UV/Vis spectrum (black line), and including plots of the UV/Vis spectrum as a function of twice (blue line), three times (red line), four times (green line), five times (purple line), and six times (orange line) the wavelength.

Fig. 5. Calculated 2PA cross-sections of 1-M-n (n = 1–3) (red traces), experimental 2PA cross-sections of 1-M-n(dppe) (n = 1–3) (red circles), linear absorption spectra of 1-M-n(dppe) (n = 1–3) (black traces), and plots of the UV/Vis spectrum as a function of twice the wavelength (light gray traces). Insets: the metal-to-ligand charge transfer (MLCT) corresponding to the calculated low-energy 2PA maxima.