Modulation of Charge Transfer by N-Alkylation to Control Photoluminescence Energy and Quantum Yield

Abstract

Charge transfer in organic fluorophores is a fundamental photophysical process that can be either beneficial, e.g., facilitating thermally activated delayed fluorescence, or detrimetnal, e.g., mediating emission quenching. <i>N</i>-Alkylation is shown to provide straightforward synthetic control of the charge transfer, emission energy and quantum yield of amine chromophores. We demonstrate this concept using quinine as a model. <i>N</i>-Alkylation causes changes in its emission that mirror those caused by changes in pH (i.e., protonation). Unlike protonation, however, alkylation of quinine’s two N sites is performed in a stepwise manner to give kinetically stable species. This kinetic stability allows us to isolate and characterize an <i>N</i>-alkylated analog of an ‘unnatural’ protonation state that is quaternized selectively at the less basic site, which is inaccessible using acid. These materials expose (i) the through-space charge-transfer excited state of quinine and (ii) the associated loss pathway, while (iii) developing a simple salt that outperforms quinine sulfate as a quantum yield standard. This <i>N</i>-alkylation approach can be applied broadly in the discovery of emissive materials by tuning charge-transfer states.

Structural, DFT calculations and photophysical and photochemical characteristics of 1-((E)-2-phenylethenyl)-2-(4-(2-((E)-2-phenylethenyl) phenoxy) butoxy) benzene (PPPBB)

“…Quantum yield of fluorescence(𝜑 𝑓 ) was determined by comparing the integrated fluorescence intensity of the sample solution with that of anthracene solution dissolved in cyclohexane, as a spectral and fluorescence reference of 𝜑 𝑓 = 0.36 at λ ex = 313 nm [30,31]. The concentration of the anthracene solution was adjusted at a small concentration to avoid reabsorption effect as its absorbance value did not exceed 0.15 absorbance units.…”