Tuning the photophysical properties of N N Pt(II) bisacetylide complexes with fluorene moiety and its applications for triplet-triplet-annihilation based upconversion

Fluorene-containing aryl acetylide ligands were used to prepare N boolean AND N Pt(II) bisacetylide complexes, where aryl substituents on the fluorene are phenyl (Pt-1), naphthal (Pt-2), anthranyl (Pt-3), pyrenyl (Pt-4), 4-diphenylaminophenyl (Pt-5) and 9,9-di-n-octylfluorene (Pt-6) (where N boolean AND N ligand = 4,4'-di-tertbutyl- 2,2'-bipyridine, dbbpy). All the complexes show room temperature (RT) phosphorescence. The emissive T-1 excited states of Pt-1, Pt-5 and Pt-6 were assigned as metal-to-ligand-charge-transfer state ((MLCT)-M-3), whereas for Pt-2, Pt-3 and Pt-4, the emissive T-1 excited states were identified as the intraligand state ((IL)-I-3), based on steady state emission spectra, the lifetime of the T-1 state, emission spectra at 77 K, spin density analysis and the time-resolved transient absorption spectroscopy. Exceptionally long lived T-1 excited state was observed for Pt-3 (tau - 66.7 mu s) and Pt-4 (tau - 54.7 mu s), compared to a model complex dbbpy Pt(II) Bisphenylacetylide (tau =1.25 mu s). RT phosphorescence of anthracene was observed at 780 nm with Pt-3. The critical role of the fluorene is to move the absorption of the complexes to the red-end of the spectra, but at the same time, without compromising the energy level of the T-1 state of the complexes. The advantage of this unique spectral tuning effect and the long-lived T-1 excited states of Pt-4 was demonstrated by the enhanced performance of the complexes as triplet sensitizers for triplet-triplet annihilation (TTA) based upconversion; an upconversion quantum yield (Phi(UC)) up to 22.4% was observed with Pt-4 as the sensitizer. Other complexes described herein show negligible upconversion. The high upconversion quantum yield of Pt-4 is attributed to its intense absorption of visible light and long-lived T-1 excited state. Based on the result of Pt-4, we propose that weakly phosphorescent, or non-phosphorescent transition metal complexes can be used as triplet sensitizers for TTA upconversion, compared to the phosphorescent triplet sensitizers currently used for TTA upconversion. Our results will be useful for the design of transition metal complexes to enhance the light-absorption and thereafter the cascade photophysical processes, without decreasing the T-1 excited state energy levels, which are important for the application of the complexes as triplet sensitizers in various photophysical processes.