Photophysical dynamics of a binuclear Cu(i)-emitter on the fs to μs timescale, in solid phase and in solution

Abstract

Understanding subtle aspects of photophysical behavior is the key to design and synthesize new and improved luminescent materials. We contribute to this with an in-depth photophysical characterization of the binuclear copper complex Cu(i)-NHetPHOS-tris-m-tolylphosphine (1), a member of a recently established emitter class for ultra-efficient, printed organic light-emitting diodes (OLEDs). To this end we studied 1 in solution and in solid form, i.e. neat film and KBr-pellet, by means of femtosecond time-resolved transient absorption/reflectivity, time-correlated single photon counting (TCSPC), and nanosecond time-resolved step-scan FTIR spectroscopy. Using these methods, we explore the photoinduced dynamics from ultrafast Franck-Condon state deactivation until the decay of the luminescent states. Upon photoexcitation, we observed multiexponential dynamics in both solution (e.g. acetonitrile 0.8 ps, 59 ps, 3 ns, 11-13 ns) and in solid state (e.g. neat film 0.3 ps, 35 ps, 670 ps, 0.5-1 μs, 3.5-4.5 μs) with four to five time-constants that significantly depend on the type of sample. Quantum chemical calculations at the DFT level in combination with step-scan vibrational spectroscopy provided structural information about the electronic ground state S and the lowest lying excited state T, and show that the latter is populated within 1 μs after photoexcitation. We found thermally activated delayed fluorescence (TADF) for this complex, which has been suggested to be the cause for its high efficiency in printed OLED devices. The results suggest that non-radiative processes, lowering the luminescence quantum yield in solution, are active on the ns to μs timescale.

Luminescence and Light‐Driven Energy and Electron Transfer from an Exceptionally Long‐Lived Excited State of a Non‐Innocent Chromium(III) Complex

“…[49] Furthermore,t pe could be susceptible to ligand-based redox chemistry (ligand non-innocence) similar to t bpy,tpy or Me PDP 2À , [1,2] contrasting ddpd as aredox-innocent spectator ligand. [6] In this study,wee xploit the complex [Cr(tpe) 2 ] 3+ with an Earth-abundant metal ion as ap otential substitute for the classical, precious metal containing chromophore [Ru-(bpy) 3 ] 2+ in luminescence,a sw ell as in photoinduced energy and electron transfer reactions.S ingle crystal X-ray diffraction, [50][51][52][53][54] NIR luminescence quantum yields [55] and lifetimes, variable temperature luminescence and step-scan FT-IR spectroscopy, [56][57][58] electrochemistry and spectroelectrochemistry,S tern-Volmer analyses as well as quantum chemical calculations [59][60][61][62][63][64][65][66][67][68][69] confirm the proposed design guidelines.…”

“…Thei ncrease in emission intensity is compatible with the proposed diminished k nr (surface) at lower temperature of the "pseudo-Stokes shifted" 2 X g (D 3d ) state (Figure 3). [79] To gain more insight into the geometries of the long-lived excited states,w es ubjected KBr disks of [Cr(tpe) 2 ][BF 4 ] 3 to time-resolved step-scan FTIR spectroscopy [56][57][58]80] in the energy range of 1750 to 1200 cm À1 at 290 and 20 K ( Figure 5a). Thenegative bands in the difference spectra indicate depopulation of the ground state while positive bands belong to the electronically excited state(s).…”

Spin Crossover and Long‐Lived Excited States in a Reduced Molecular Ruby

“…Finally, to probe conceivable long‐lived excited states of [Cr(ddpd) 2 ] 2+ at low and high temperature, step‐scan FT‐IR spectra of KBr pellets of [Cr(ddpd) 2 ][BF 4 ] 2 were recorded at 290 K (ca. 50 % low‐spin) and at 20 K (>98 % low‐spin) between 0–750 ns after excitation with a 355 nm pulse (Figure , Supporting Information, Figure S16).…”

Structural Characterization and Lifetimes of Triple‐Stranded Helical Coinage Metal Complexes: Synthesis, Spectroscopy and Quantum Chemical Calculations

“…Nowadays, large heterometallic complexes still lack thorough spectroscopic investigation, in comparison to the large number of investigations on mononuclear [1,2] and polynuclear homometallic [3,[4][5][6] complexes. Especially, heterometallic transition metal complexes are extremely demanding regarding high-level spectroscopic experiments, which is one reason there are so few examples found in the literature.…”

“…Yielding data of lifetimes in the nanosecond (ns) and microsecond (ms) time-range, the (TR) step-scan FTIR method is especially suited to investigate photophysical and photochemical processes in transition metal complexes, [17] such as photo-activated reactions, [17,18] or excitation and relaxation processes in electronic states. [2,[4][5][6][19][20][21] In combination with high level quantum chemical calculations, structural changes between different electronic states or different reaction states can be unveiled.…”