Title: Electrochemical switching between second harmonic generation and fluorescence in bimetallic Ru complexes

Abstract:

The design and development of transition-metal containing chromophores where changing the oxidation state of the metal(s) alters their optical property are desirable for a variety of applications in chemistry and biology. If one of the optical properties is one-photon fluorescence and the other is second harmonic (two-photon) generation, a desirable change would be alternation between these two optical properties by tuning the oxidation state of the metal center(s) by external means. This will, first of all, make such a chromophore less dependent on the medium or environment which is unlikely to show similar alternating optical response based on the external stimuli. Here we report two Ru based bimetallic complexes, [NC-Ru-(bpy)2-CN-Ru(bpy)2-CN](PF6) (1) and [Ru(bpy)2-bptz-Ru(bpy)2](PF6)2(2) and investigated their second harmonic generation intensity and fluorescence quantum yields after oxidation/reduction of the Ru center(s) electrochemically.  We have determined from cyclic voltammetry (CV) that the first oxidation of 10 to 1+1 occurs at +0.75 V and the second oxidation of 1+1 to 1+2 at +1.34 V.  Similarly the first oxidation of 20 to 2+1 happens at a much higher potential of + 1.95 V and the second oxidation of 2+1 to 2+2 at +2.2V. The spectro-electrochemical studies suggest that the mixed-valent (MV) state of both the complexes exhibit a high energy MLCT band around ~530 nm and an inter-valence charge transfer (IVCT) band in the infrared around ~1200 nm. The MLCT band is attributed to metal d-orbital to ligand π* orbital charge transfer and the IVCT band is assigned to the metal-metal charge transfer from Ru(II) to Ru(III) states. In-situ second harmonic light scattering (SHLS) experiments were carried out in a customized electrochemical cell, where the SH intensity was measured with respect to a particular oxidation state of the metal. We find that the mixed-valent bimetallic complex produces more intense SH light compared to the other oxidation states. In addition, we monitored the change in fluorescence intensity of the bimetallic complexes in different oxidation states.

Biography:

Karthika C is a senior research fellow pursuing her Doctorate in physical chemistry at Inorganic and Physical Chemistry Department, Indain Institute Of Science, Bangalore, India. She obtained her Masters in chemistry in 2013 from University of Calicut, India. Currently she is working on the second harmonic quantification of organometallic complexes and manipulation of molecular properties using external stimuli. The research work involves synthesis, electrochemical measurements and second harmonic measurements. Her research interests are quantum mechanical calculations, spectro-electrochemistry and non-linear optics. She has presented abstract on redox switching of first hyperpolarizability of transition metal complexes at international conference on photonics, 2018 held in IIT Delhi.