ENGINEERING NEW OPTICAL PROPERTIES THROUGH STRONGLY CONJUGATED AND WEAKLY CONJUGATED HYDROPORPHYRIN ARRAYS

Abstract

Many classes of organic fluorophores have been developed and rigorously studied over the last century (coumarins, cyanines, porphyrins, etc.), each with their own strengths and weaknesses. When single chromophores are inadequate or underperforming for a given application, strongly conjugated or weakly conjugated multichromophore arrays can be prepared. Strongly conjugated arrays feature two chromophores linked via sp or sp2 hybridized atoms, allowing delocalization of ? electrons over the entire system. This causes broadening and red shifting of lowest energy absorption and emission bands, increased absorptivity, the rise of new absorption features and increased non-linear optical properties (two-photon absorption cross-section, hyperpolarizability), all of which are all sought after for solar light harvesting. In weakly conjugated arrays, chromophores are held in close proximity without sharing ? electrons, allowing them to retain individual absorption and emission properties while gaining the ability to interact through excited state energy transfer. With careful molecular design, one can exploit energy transfer to create a variety of systems suitable for fluorescence sensing or in vivo imaging applications. Here, we developed several series of both strongly and weakly conjugated arrays. All designs feature hydroporphyrins (chlorins and bacteriochlorins) due to their unique photophysical properties, including three absorption bands in three spectral regions (UV, green, red-NIR), narrow emission in the red-NIR window and large fluorescence quantum yield. First, we developed long wavelength absorbing and emitting chlorin monomers and dimers, in order to understand the relationship between linker type and electronic communication. Next, two chlorins were bridged by an endiynyl linker capable of photoinduced isomerization, to afford an array with both through bond and through space electronic communication. The remaining work was broken into three phases focused on the design and preparation of weakly conjugated arrays for in vivo imaging and fluorescence guided surgery. In the first phase we developed bright BODIPY-chlorin arrays with a large pseudo-Stokes' shift and high energy transfer efficiency. Second, amphiphilic bacteriochlorin-BODIPY arrays were prepared to facilitate solubility in aqueous media. Finally, a series of BODIPYs with tunable absorption were conjugated to a common bacteriochlorin, to enable multiplexed imaging with variable excitation and common emission.