Room temperature phosphorescence (RTP) has great potential to be used for various applications in sensing, data encryption, anti-counterfeiting, bioimaging and microcrack detection owing to their unique photophysical properties [1]. Typically, RTP emission is achieved in inorganic material-based systems, including rare earth metals, which are associated with high processing costs, poor biocompatibility, etc. Recently, the focus has shifted to organic RTP systems due to their tunability of emission wavelength, low cost and ease of processing.
For practical applications, RTPs must be efficient and have a long phosphorescence lifetime, which is in principle a challenging task, since promoting intersystem crossing for high-yield triplet generation usually shortens their lifetime [2], [3]. A necessary rigid environment for RTP can be ensured by the crystalline structure of the emitter, however, creating doped polymer RTP systems, as opposed to relying solely on crystal systems, offers a promising approach to enhance the practical applicability of the phenomenon [1]. Consequently, it is necessary to identify and understand the limiting factors of the processes involved in RTP to improve the photophysical properties and application potential of RTP systems.
In this work, dozens of new phenothiazine 5,5-dioxide based RTP emitters have been synthesized, and their RTP properties have been investigated in both their crystal form and doped polymer films. It has been demonstrated that phenothiazine derivatives with broader conjugation (with –Ph or -pCNPh substituents at 3rd and 7th positions) frequently exhibited longer phosphorescence lifetime and higher quantum yield than those with narrower conjugation. Thus, the presence of larger substituents is associated with a decrease in non-radiative decay. Furthermore, it was found that doping the more rigid polymer, polyacrylic acid (PAA), with phenothiazine derivatives can further enhance phosphorescence properties compared to the polymethylmethacrylate (PMMA) matrix. In PAA films, 40% photoluminescence (PL) efficiency was achieved, half of which was phosphorescence, while in PMMA films emission quantum yield was lower, at around 30%. In the polymer films, phosphorescence lifetimes up to 0.7 s were achieved.