Blue shifted phosphorescence (3LE) as compared to charge transfer singlet emission (1CT) in fluorenone-amine dyads under ambient conditions

Abstract

In pure organics, room temperature phosphorescence (RTP) can be enhanced by effective inter system crossing (ISC, from S1→T1) and fast radiative decay of T1. According to El-Sayed rule, ISC is quite fast when a transition between singlet and triplet involves the change of molecular orbitals i.e. 1(π,π*)→3(n,π*) or 1(n,π*)→3(π,π*). Therefore, conjugated organics having non-bonding electrons from carbonyl group or heteroatoms (O, N, S, Se, etc.) are expected to facilitate strong spin-orbit coupling and faster ISC. In this work, we synthesized metal-free organics composed of fluorenone core-secondary amine (Flu-Ph-CBZ, Flu-PNA and Flu-DNA) for harvesting both singlet and triplet excitons. The carbonyl group in the rigid aromatic planar fluorene core provides non-bonding electrons and acts as an electron acceptor moiety. These donor-acceptor based materials showed blue shifted phosphorescence maxima (T1→S0) as compared to their fluorescence emission (1CT→So), a very uncommon phenomenon. From our photophysical studies, ΔE (1LE-3LE) for Flu-Ph-CBZ, Flu-PNA and Flu-DNA are found to be 0.52, 0.23, and 1.20 eV, respectively. Similarly, ΔE (1CT-3LE) for Flu-Ph-CBZ, Flu-PNA and Flu-DNA are found to be 0.24, 0.19 and 0.53 eV, respectively. Flu-Ph-CBZ shows TADF like emission in Me-THF and phosphorescence in PMMA doped film under ambient conditions. Powder samples of Flu-PNA and Flu-DNA are showing room temperature phosphorescence under air. Phosphorescence lifetimes in powder sample was found to be ∼15 and 56 µs, respectively for Flu-PNA and Flu-DNA.