Room Temperature Phosphorescence and Circularly Polarized Luminescence Characteristics of Phosphine Oxides and Phosphoramides

Abstract

Room temperature phosphorescence (RTP) materials have attracted much attention in recent years owing to their potential applications in organic light-emitting diodes (OLEDs), security writing, time-gated bio-imaging, sensing, etc.1 The spin-forbidden phosphorescence phenomenon is rarely observed for organic molecules as the spin-orbit coupling (SOC) facilitated spin-flipping Sn↔Tn process is weaker in these compounds than their inorganic counterparts. Nonetheless, a significant number of organic RTP materials have been developed by breaking the symmetry of Sn and Tn excited states by way of incorporating heteroatoms having at least one lone pair of electrons or empty p orbital (B, N, O, S, P, Se, etc.) (El-Sayed’s rule).2 Despite these developments, the fundamental understanding of the relationship between photoluminescence (PL) lifetime, RTP quantum yield, and specificity of the orbital configurations of the excited state is still elusive in literature. Thus, this thesis aims to unravel the RTP characteristics of fluorophores, mainly phosphorus-containing compounds such as arylphosphine oxides and phosphoramides. The geometric and electronic features of the tricoordinate phosphorus atom in Ar3P favour n → π* transition and promote the spin-forbidden S1 → Tn intersystem crossing to populate triplet excited states. However, how different the P-centered lone pair electrons (LPE) in Ar3P compared to O-centered LPE in Ar3P=O in the production of triplet excitons through n(O) → π* transitions is not known in the literature. To understand the difference between Ar3P and Ar3P=O based n → π* transitions in the production of triplet excitons, we set to investigate the RTP characteristics of a series of simple triarylphosphines (3·1, 3·2, 3·3, and 3·4) and corresponding triarylphosphine oxides (3·5, 3·6, 3·7, and 3·8). By combining theoretical and experimental investigations, we have established that n → π* transitions in Ar3P=O stabilize the triplet state better than Ar3P; consequently, the former shows ultralong/persistent room temperature phosphorescence (ULRTP/pRTP) with a lifetime exceeding 100 ms. This study also established that intermolecular interactions are essential for stabilizing the triplet states. Unfortunately, the PL quantum yield of Ar3P=O is poor, limiting its practical applications.3 To widen the scope of these investigations, we aimed to study PL characteristics of Ar2P=O(H) 4·1, 4·2, and 4·3. We envisioned that replacing one of the aryl moieties in Ar3P=O with a hydrogen atom would make more room for intermolecular interactions and consequently improve the PLQY by stabilizing the triplet state better. The Ar2P=O(H) showed pRTP with improved PL quantum yield than Ar3P=O. Furthermore, we established that the n(O)→σ*(P-C) transitions in Ar2P=O(H) stabilize the triplet state better than n(P)→σ*(P-C) in Ar3P.4 To understand the effect of resonance interaction between N-centered LPE and P=X (X= O, S, Se) bond on the RTP characteristics of phosphoramides, we developed a series of donor-acceptor (D-A) systems comprising phenothiazine donor and P=X acceptor (5·1-5·3, 6·1-6·6).5-6 The donor-acceptor electronic interactions in these phosphoramides are judiciously fine-tuned by varying the number of phenothiazine units, and the Lewis acidity of the phosphorus center is controlled by the nature of heteroatoms (O, S, and Se) and the number of phenyl moieties (C6H5) attached to it. All compounds exhibit phosphorescence in the solid state under ambient conditions with an afterglow lifetime in the millisecond range (~ 8 – 40 ms). Through this study, we established that the steric crowding and the number of C6H5 moieties around the P=X unit play a crucial role in controlling the electronic coupling between the donor and acceptor moieties in phosphoramides containing non-planar donor moieties.5-6 Furthermore, for the first time, we demonstrated the chiroptical and magnetic chiroptical properties of phosphoramides. For the first time, we demonstrated the utility of cyclotriphosphazene as an optically innocent dendritic core for developing novel chiroptical materials.7 We have also developed a few donor-acceptor (D-A) architecture-based multifunctional organic luminophores for differential imaging of hypoxia/normoxia cancer cells.8 All these intriguing results are discussed in detail in this thesis.