Transient Absorption Dynamics of Sterically Congested Cu(I) MLCT Excited States

Abstract

Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark [Cu(dmp)2](+) (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as [Cu(dsbp)2](+) (dsbp =2,9-di(sec-butyl)-1,10-phenanthroline and [Cu(dsbtmp)2](+) (dsbtmp =2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced [Cu(dsbp)2](+) and [Cu(dsbtmp)2](+) species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including [Cu(dmp)2](+), consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: [Cu(dsbtmp)2](+) < [Cu(dsbp)2](+) < [Cu(dmp)2](+). The molecule with the most inhibited potential for distortion, [Cu(dsbtmp)2](+), possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn-Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.

TADF Material Design: Photophysical Background and Case Studies Focusing on CuIand AgIComplexes

“…(Figure ) Again, it is stressed that the prompt fluorescence could not be observed directly in our experiments conducted with ns time resolution. This agrees with measured ISC time of the order of several ps . The resulting energy level diagram and the relevant rates are summarized in Figure c.…”

“…An MLCT transition in Cu I complexes often leads to flattening distortions of the molecule in the excited state relative to the ground state geometry . Such distortions are usually connected with an increase of non‐radiative deactivations or even quenching of the emission due to a strong increase of the Franck‐Condon factors of the low‐lying vibrational modes of the excited state and the highly excited vibrational modes of the electronic ground state .…”

“…For completeness, it is remarked that the emission at ambient temperature frequently does not represent only TADF, but contains also some T 1 →S 0 (phosphorescence) contribution. (Prompt fluorescence is not observed due to fast ISC of the order of a few tens of ps . According to refs.…”

“…Then, depending on the class of molecules, prompt fluorescence and/or down‐ISC (as well as non‐radiative relaxation to the electronic ground state) can occur. For Cu I and Ag I complexes, being in the focus of this contribution, ISC from S 1 to the T 1 state is very effective, since fast intersystem crossing occurring in the time range of 3 to 30 ps has been observed . The individual value depends on molecular properties, for example, on the extent of SOC of higher lying singlets to the lowest triplet state, but also on the local environment, such as a fluid or a rigid matrix .…”

“… The emitter compounds should exhibit high photoluminescence quantum yields Φ PL . After an electronic excitation, however, Cu I and Ag I complexes experience distinct flattening distortions with respect to the ground state geometries . Usually, such geometry changes are related to an increase of non‐radiative deactivation or even result in total quenching of the luminescence by vibrational relaxation.…”