Tuning the Triplet Excited State of Bis(dipyrrin) Zinc(II) Complexes: Symmetry Breaking Charge Transfer Architecture with Exceptionally Long Lived Triplet State for Upconversion

Abstract

Zinc(II) bis(dipyrrin) complexes, which feature intense visible absorption and efficient symmetry breaking charge transfer (SBCT) are outstanding candidates for photovoltaics but their short lived triplet states limit applications in several areas. Herein we demonstrate that triplet excited state dynamics of bis(dipyrrin) complexes can be efficiently tuned by attaching electron donating aryl moieties at the 5,5'-position of the complexes.F or the first time, al ong lived triplet excited state (t T = 296 ms) along with efficient ISC ability (F D = 71 %) was observed for zinc(II) bis(dipyrrin) complexes, formed via SBCT.T he results revealed that molecular geometry and energy gap between the charget ransfer (CT) state and triplet energy levels strongly control the triplet excited state properties of the complexes.A ne fficient triplet-triplet annihilation upconversion system wasd evised for the first time using aS BCT architecture as triplet photosensitizer,r eaching ah igh upconversion quantumy ield of 6.2 %. Our findings provide ab lueprint for the development of triplet photosensitizers based on earth abundant metal complexes with long lived triplet state for revolutionaryphotochemical applications.

Metal Coordination Effects on the Photophysics of Dipyrrinato Photosensitizers

“…The solvent effect was also observed by Mahmood et al [167]. The triplet lifetimes of the compounds were higher in glycerol triacetate (τ <,𝐤 = 743 µs and τ <,𝐣 = 1244 µs), a slightly viscous solvent, which stabilizes the triplet state, likely due to TTA inhibition.…”

“…Pd-1 displayed a slightly smaller absorption wavelength (477 nm vs. 485 nm), a larger molar absorption coefficient (1.78•10 5 M -1 cm -1 vs. 1.15•10 5 M -1 cm -1 ), similar fluorescence quantum yield (0.008 vs. 0.006), and a larger Stokes shift (2565 cm -1 vs. 660 cm -1 ) (Table 1 and Table 3, appendix). Interestingly, the singlet oxygen quantum yield of Pd-1 (0.17 in MeCN) is slightly larger than both Zn(II) complexes Zn-4b (R 2 = H) and Zn-4b (R 2 = I) (0.10 in DCM and 0.10 in MeCN, respectively) [79,167,178]. These properties, especially the higher quantum yield without the presence of iodosubstituents, indicate that similar bis(dipyrrinato)Pd(II) complexes may also show promise for application in PDT.…”

“…Zn-ions are much cheaper, more abundant, and biologically relevant than most of the TM-ions discussed before, which makes these complexes suitable as alternative PSs for PDT. Several Zn(II) dipyrrinato complexes have been reported (Figure 16, Table 1 and 2) [8,79,110,120,[161][162][163][164][165][166][167][168][169][170].…”

“…Concerning the (dipyrrinato)Zn(II) complexes, attention has been risen towards the symmetry breaking charge transfer (SBCT) and the formation of ligand-ligand charge transfer states for complexes in solution ( 1 LLCT), which are also called Intramolecular Charge Transfer (ICT) states) [164][165][166][167][168]. Triplet generation can be enhanced by tuning this process.…”

“…Mahmood et al studied the series of bis(dipyrrinato)Zn(II) complexes Zn-4b,i,j,k (R 2 = H) and demonstrated that their triplet excited state dynamics could be effectively tuned using structural modifications [167]. The anthracene and pyrene substituted complexes Zn-4j (R 2 = H) and Zn-4k (R 2 = H) were reported to have significantly longer triplet lifetimes (295 µs and 146 µs in DCM, resp.)…”