Counterion-Mediated Luminophore Dimerization

Abstract

Emissive organic salts have long been integral to the discovery of fluorescence phenomena and functional luminescent dyes. Typically, one component of the salt acts as the photoactive unit (luminophore) and its nonemissive counterion is selected to independently tune bulk physical properties, such as solubility. However, the impact of counterion choice on the aggregation and resulting emissive state of organic salts in solution has not been widely investigated. Here, we report that a single cationic luminophore gives rise to either monomer, dimer, excimer, or multichromatic emission under otherwise identical conditions by varying only its counterion. We employ N-methyl quininium (MeQn⁺) as a permanently charged cationic luminophore, which we pair with a series of monovalent anions. At low solution-state concentrations, all the salts give identical absorption and emission spectra that correlate with the MeQn⁺ monomer. However, at higher concentrations, the emission, excitation, and absorption data differ, revealing the presence of monomer, dimer, excimer, or all three, depending on the structure of the anion. Understanding and modulating the formation of dimeric or other well-defined aggregated species by specific ion effects could be exploited in the design of molecular probes for biological systems or emissive thin-film dispersions for optoelectronic devices.

Keywords: Aggregation; Dimers; Excimers; Fluorescence; Noncovalent interactions.

Figure 1

Schematic representation of this work, in which the counterion (X⁻) of a cationic luminophore (gray) dictates whether a) monomer (purple), b) excimer (blue), and/or c) dimer (orange) emission is observed in solution.

Figure 2

a) Structural formula of the MeQn·X salt series. b) Steady‐state absorption (solid lines), emission (dashed lines, λ _ex = 300 nm) and excitation (dotted lines, detected at 360 nm) spectra of MeQn·X salts in MeCN (20 µM).

Figure 3

a–d) Normalized photoluminescence emission spectra of MeCN solutions. The emission of 20 µM solutions (gray solid line, λ _ex = 330 nm) is contrasted with the emission observed for 5 mM solutions (black solid line, λ _ex = 350 nm; or black dashed line, λ _ex = 450 nm). e–h) Normalized excitation spectra of MeCN solutions. The excitation of 20 µM solutions (gray solid line, detected at 365 nm) is contrasted with the excitation of 5 mM solutions (black solid line, detected at 360 nm; or black dashed line, detected at 500 nm). (a, e) MeQn·I, (b, f) MeQn·BF₄, (c, g) MeQn·MeSO₄, and (d, h) MeQn·OTf. Note that the band‐edge absorption is overemphasized in the excitation spectra acquired at 5 mM due to the very high optical density of luminophore.

Figure 4

a) Partial ¹H DOSY NMR spectra (600 MHz, 298 K, and CD₃CN) of MeQn·MeSO₄ (c = 10 mM, black and c = 20 mM, orange) showing the reduced diffusion at higher concentration indicative of ground‐state dimerization. Comparison of b) partial ¹H NMR spectra (500 MHz, 298 K, CD₃CN, and c = 5 mM) and c) excitation spectra (detected at 530 nm, MeCN, and c = 5 mM) of MeQn·PF₆ titrated with KTFA (c = 0–50 mM).

Figure 5

X‐ray crystallographic characterization of MeQn·MeSO₄.^{[ 55 ]} a) Solid‐state structure of an ion pair. b) A chain of MeQn ⁺ ions linked by O─H⋯N hydrogen bonding. The middle ion is overlaid with its Hirshfeld surface. c) A section of the packing structure that shows a MeSO₄ ⁻ ion, overlaid with its Hirshfeld surface, taking part in C─H⋯O hydrogen bonding to bridge two MeQn ⁺ ions, which d) exhibit face‐to‐face contact of their quinoline ring systems.