Highly Fluorescent Pyridinium Betaines for Light Harvesting: An Experimental and Computational Investigation

We report the findings of our experimental and theoretical investigation into the properties of pyridinium enolates and their potential utility in light harvesting applications such as luminescent solar concentrators (LSCs). The synthesis, structures, photophysical characterization and quantum-chemical study of the five cyclobetaines as well as their suitability and performance in as the emissive component in LSCs is presented. Molecules and materials with strong absorption and intense emission profiles find utility in a wide range of applications including light harvesting for energy related applications and as optical tags for imaging in biology. Consequently, the design and synthesis of new chromophores and fluorophores, as well as an understanding regarding the origin of their molecular spectral properties are required. Betaine dyes are an important class of chromophore characterized by their dipolar electronic ground state, that can’t be represented by a neutral mesomeric form. One of the most intensely studied groups of molecules in this class are the pyridinium N-phenolates with Riechardt’s dye being the most wellknown. Intramolecular charge transfer on excitation of the π – π* transition from the phenolate donor to the pyridinium acceptor results in the perichromism observed for these dyes making them important polarity sensors. Unfortunately, the pyridinium Nphenolates are typically not fluorescent in solution due to rapid non-radiative relaxation processes such as large-amplitude intramolecular rearrangement, intramolecular electron transfer and back-electron transfer to the S0 state. There are however limited reports of emissive behavior at low temperature and in thin polymer films although this research area remains relatively unexplored. Encouragingly, coumarin pyridinium cyclobetaines recently developed by Hell et al. display a marked increase in quantum efficiency on conjugation to antibodies and subsequent protein binding while maintaining their large Stokes shifts. This ultimately led to their utility as the large Stokes shift fluorophore in stimulated emission depletion (STED) microscopy. The spectral properties of fluorophores for use as the large Stokes shift component in STED microscopy are similar to that of dyes for use as the light harvesting component in LSCs. A typical LSC device consists of a transparent planar waveguide integrated with a fluorescent/phosphorescent compound dispersed in a polymer matrix. The incorporated chromophore harvests light through absorption, meanwhile subsequent emission is focused to the edges of the waveguide by total internal reflection, where a photovoltaic cell is coupled to the LSC to generate electricity. This form of device construct has led to technology aimed at harvesting solar energy from a myriad of surfaces as an alternative to direct capture and conversion by solar panels. Two key photophysical properties of the chromophore affect the performance of LSC devices – photoluminescence quantum yield (PLQY) and Stokes shift. PLQY near unity and large Stokes shifts with negligible overlap of the absorption and emission profiles are ideal parameters for LSC dyes. While chromophores with such properties have been reported in the literature, they are relatively rare especially at higher dye concentrations that are required for light harvesting. Therefore, the design and facile preparation of Figure 1. Structures of 1a-1e (top) and general mechanism of pyridinium enolate formation (bottom). [a] J. Xu, B. Zhang, M. Jansen, Dr L. Goerigk, Dr W. W. H. Wong, Dr C. Ritchie School of Chemistry, The University of Melbourne Bio21 Institute, 30 Flemington Road, Parkville, Victoria 3010, Australia E-mail: critchie@unimelb.edu.au [b] M. Jansen Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, 48149 Münster, Germany [c] Dr. W. W. H. Wong ARC Centre of Excellence in Exciton Science School of Chemistry, The University of Melbourne Parkville, Victoria 3010, Australia Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))