Physical and Theoretical Chemistry

Biography

Biography: Youngmin You

Abstract

Spin statics of excitons is the key factor that determines the efficiency for interconversion between photon and electron. For example, internal quantum yields of electrofluorescence devices are limited by 25% in the absence of processes that aid intersystem crossing. Charge collection efficiencies in photovoltaic devices are also intimately associated with spin distributions. Efforts have, thus, been paid to develop the materials that overcome the spin selection rule. Notable examples include organometallic complexes of Ir(III) or Pt(II) which exhibit strong spin-orbit coupling. Of recent interest are dipolar organic molecules and coordination compounds of Cu(I). These compounds possess charge-separated excited states with small exchange energies. This electronic structure allows for thermally activated reverse intersystem crossing, leading to exciton-harvested fluorescence emission. Our group was intrigued by the key role of the excitonic spin states in electroluminescence devices. We investigated the fluorescence properties of chromophores bearing n-p* transitions. Although n-p* molecules can serve as electroluminescent materials because of the harvesting of singlet and triplet excitons through El-Sayed-rule-allowed reverse intersystem crossing, the weak fluorescence emissions of such molecules have prevented applications into devices. To enable systematic studies, we prepared a series of electron-deficient coumarin compounds having aryl substituents with different band gap energies. We observed two orders of magnitude improvement in fluorescence quantum yields upon facilitating intra- and intermolecular electron transfer to the coumarins. Special focus has been paid to understand the electron-transfer processes and the molecular factors that controlled the kinetic steps. The mechanistic studies revealed that judicious control over excited-state potentials was crucial to achieving efficient fluorescence.