Shedding Light on the Structure of a Photoinduced Transient Excimer by Time-Resolved Diffraction

Abstract

Time-resolved single-crystal diffraction performed with synchrotron radiation shows that the 53(1) micros phosphorescent state, generated in the crystalline phase of trimeric {[3,5-(CF3)(2)Pyrazolate]Cu}(3) molecules by exposure to 355 nm of light at 17 K, is due to the formation of an excimer rather than the shortening of the intramolecular Cu...Cu distances within the trimeric units, or the formation of a continuous chain of interacting molecules. One of the intermolecular Cu...Cu distances contracts by 0.56 Angstroms from 4.018(1) to 3.46(1) Angstroms;, whereas the interplanar spacing of the trimers is reduced by 0.65 Angstroms; from 3.952(1) to 3.33(1) Angstroms. Density-functional theory calculations support the formation of a Cu...Cu bond through the intermetallic transfer of a Cu 3d electron to a molecular orbital with a large 4p contribution on the reacting Cu atoms.

Confined Metallophilicity within a Coordination Prism

“…The above-mentioned geometrical optimization (Figure 3) Figure S5 and Table S5 in Section 5 of the Supporting Information for details). This result is experimentally supported by a recent study by Coppens and co-workers, [19] who developed the technique of time-resolved single-crystal X-ray diffraction , namely Cu4, is mainly responsible for this drastic distortion; the distance between Cu4 and Cu1 is reduced by 0.758 from 4.165 at S 0 to 3.407 at T 1 (both calculated). Such intense contraction operated by the enhanced Cu I À Cu I bonding at the excited state even overwhelms the Cu À N coordinative bonds.…”

Cupriphication of gold to sensitize d 10 –d 10 metal–metal bonds and near-unity phosphorescence quantum yields

“…This assignment gives rise to a genuine Stokes' shift of 10,700 cm −1 , suggesting a large excited-state distortion of a dodecanuclear tetramer-of-trimer unit whereby the aurophilic interactions undergo excimeric contraction of Au(I)⋯Au(I) distances from 3.488 Å to significantly shorter distances. If the excited-state distortion were due to the contraction of the polar-covalent Cu(I)-Au(I) bond instead, a much smaller Stokes' shift than the >1.3 eV value would be expected, because it is hard to envision such a bond shortening way beyond 2.8750(8) Å, as supported by Coppens and coworkers' studies (28).…”

“…Likewise, the Cu(I) precursor for 1-4 herein exhibits colossal shortening of Cu⋯Cu intertrimer and interplanar separations by ca. 0.56 Å and 0.65 Å, respectively, upon photoexcitation; nevertheless, the resulting separations remained too long to be deemed a metal-metal bond (28). Bona fide single bonds exist upon full oxidation to Au(II) to attain ligand-assisted separations of 2.4752(9) Å (21), whereas more rare ligand-unassisted versions with ca.…”

Intertrimer and Intratrimer Metallophilic and Excimeric Bonding in the Ground and Phosphorescent States of Trinuclear Coinage Metal Pyrazolates: A Computational Study

“…Although the latter can now be determined experimentally by time-resolved diffraction, 12 the limited range and accessibility of such experimental investigations render it essential that computational methods be utilized for such purposes, especially given the excellent agreement of the calculated ground-and excited-state structures herein with the experimental ones, even for such large dimer-of-trimer systems. Currently, experiments and computations are underway in our labs to further explore these implications for the design of improved light-emitting devices.…”

“…This is so not only for the ground-state metallophilic bonding but also for excited-state excimeric M-M bonding, which is now accessible experimentally by time-resolved singlecrystal X-ray diffraction for these systems, as demonstrated in a recent elegant study by Coppens and co-workers. 12 One important aspect of the present study is modeling the photophysics of coinage-metal pyrazolate trimers, which are known to exhibit bright, tunable phosphorescence. The fascinating luminescence properties of these systems found experimentally include drastic sensitivities of the phosphorescence colors and lifetimes to solvent, temperature, concentration, rigidity of medium, substituents on the pyrazolate ring, and identity of the coinage metal.…”