Vibronic Coupling and Nonradiative Relaxation in Cyanine Dimers on DNA Scaffolds

Controlling molecular exciton states is a critical goal for realizing biomimetic light harvesting structure as well as alternative logic and circuit platforms based on exciton propagation. DNA scaffolds provide a means of purposefully creating molecular aggregates with well-defined dye positioning. Recently it has been shown that cyanine dimers on DNA can exhibit strong vibronic coupling,¹ support long lived vibronic coherences,² and give rise to delocalized biexciton states.³ Such properties could be exploited for future quantum information and sensing applications. However, a consequence of achieving strong coupling in these systems has been the observed degradation of the fluorescence properties of the dimer as compared to the component dyes.^{1, 3, 4} Such a nonradiative pathway can act as an energy sink in excitonic transmission lines⁵ or otherwise limit the utility of molecular aggregates so that mitigation strategies are needed.<br/>Here, we examine the effect of strong vibronic coupling on a series of cyanine-based dye-dimers on DNA scaffolds. We observe strong coupling indicated by Davydov splitting of the absorption spectra and a redistribution of oscillator strength among vibronic states. Applying the coupled oscillator model reveals the coupling strength, dye orientation, and an estimate of the delocalization among vibronic states. Ultrafast spectroscopy reveals that the reduction in fluorescence quantum yield arises from a nonradiative pathway that is not dominant in the constituent dyes. By varying the buffer solution to increase its viscosity and restrict dye motion, the nonradiative recombination can be suppressed. Temperature dependent fluorescence measurements provide a means to estimate the activation energy of this nonradiative pathway, which dominates at room temperature but is eliminated a low temperature. The coupling strength is varied by dye separation on the DNA scaffold as well as through addition of substituents and structural alterations to the component dyes. Energy level detuning through the use of heterodimers is also explored. We discuss the impact of vibronic coupling strength, dye separation, and molecular design on the dominance of the nonradiative pathway and suggest design rules to reduce or eliminate it. This represents a step towards overcoming a hurdle to realizing molecular excitonic circuits organized on DNA scaffolds.<br/><br/>1. Cannon, et al., JPC A 122, 2086 (2018); Cunningham, et al., JPC B 122, 5020 (2018); Rolczynski, et al., JPC A,<i> in press</i>, DOI:10.1021/ acs.jpca.1c07205 (2021)<br/>2. Sohail, et al., Chemical Science 11, 8546 (2020)<br/>3. Cunningham et al., JPC B 127, 8042 (2020)<br/>4. Huff, et al., JPC Lett 10 2386 (2019); Mazuski, et al., JPC Lett 11, 4163 (2020); Huff et al., JPC B 125, 10240 (2021)<br/>5. Cunningham, et al., Adv. Opt. Mater. 2100884 (2021)