NIR Photocleavage of the Si–C Bond in Axial Si-Phthalocyanines

Abstract

The use of light-triggered photolysis provides a powerful tool for unique syntheses and for applications that require remote operation such as drug delivery or molecular switches. Here, we describe the photochemistry of a recently developed alkylsilicon phthalocyanine Pc 227, which undergoes an exchange of the alkyl ligand for a ligand derived from the solvent when the axial Si-C bond is photolyzed in a solvent with low-energy visible light. In this work with methanol as the solvent, we investigate the formation of the methoxy analogue of the therapeutic drug Pc 4, (termed Pc 233) upon irradiation. Using steady-state spectroscopy and characterization of the photoproducts, the competing pathways between direct ligand exchange on the central silicon atom and delocalization of the radical produced by homolysis on the phthalocyanine ring is observed. The delocalized radical intermediate is quite long-lived. At long times this intermediate decomposes without significant formation of Pc 233. The results of this investigation provide insights into recent work utilizing Pc 227 for drug delivery applications and for future work on the use of phthalocyanines as long-wavelength phototriggers.

Defining the conditional basis of silicon phthalocyanine near-IR ligand exchange

“…An alternative pathway to Si-O bond cleavage might entail energy transfer from the excited Pc macrocycle, in either the singlet or triplet state, to the Si-O bond and bond cleavage. 25 As the radical anion hydrolysis pathway is a putatively a light independent process, we sought to generate this species through ground state chemistry and then assess the axial ligand exchange reaction. To generate the radical anion intermediate independently, we examined the impact of stronger reductants without irradiation in the presence and absence of O 2 .…”

“…Although the Si-OH products have not been isolated in our studies, such species are known to be insoluble and prone to aggregation. 27 - 28 Direct Si-O bond homolysis, observed with the Si-C bond of SiPcs 25 is not feasible due to the significantly higher bond strength of the Si-O bond (~80 kcal vs ~40 kcal for Si-C bond). 29 More specifically, we determined the energy barriers that must be overcome to replace OPh and OMe with OH, i.e., energetics for the competing reactions of neutral SiPc(OMe)(OPh)--H 2 O to form the products Si(OH)(OPh)--HOMe vs. Si(OMe)(OH)--HOPh, and analogously for the radical anion species.…”

Theoretical and Experimental Studies on the Near‐Infrared Photoreaction Mechanism of a Silicon Phthalocyanine Photoimmunotherapy Dye: Photoinduced Hydrolysis by Radical Anion Generation

“…. It has been proposed that photochemical Si−C bond cleavage proceeds by a dissociation channel through a charge‐transfer excited state for a related SiPc dye with an alkyl ligand . Figure (A) shows the ground state and excited state potential energy curves of neutral compound 2 calculated with long‐range correction (LC) for density functional theory (DFT) and time‐dependent DFT (TDDFT), which are established to potentiate quantitative analyses of charge‐transfer processes (see Section S−I in the Supporting Information for computational details).…”

Silicon phthalocyanines: synthesis and resurgent applications

“…The research groups of Kenney and Burda performed theoretical and experimental measurements to identify the preferred mechanistic pathway of the light triggered Si-C bond cleavage process. 22,104,105 Characterization of the photoproducts using various techniques such as steady state spectroscopy, electrospray ionization mass spectrometry, liquid chromatography mass spectrometry, nuclear magnetic resonance and electron paramagnetic resonance hinted towards formation of a relatively long-lived SiPc radical as the key intermediate which consequently undergoes solvolysis. 104 Computational studies additionally confirmed homolytic bond cleavage process involving radical intermediates.…”