Is a High Photoluminescence Quantum Yield Good Enough for OLEDs? Can Luminescence Rigidochromism Be Manifest in the Solid State? an Optoelectronic Device Screening Case Study for Diphosphine/Pyrazolate Copper(I) Complexes

ABSTRACT

This paper provides a 4th manifestation of a new tradition by which the editors of Comments on Inorganic Chemistry wish to lead by example, whereby we start publishing original research content that, nonetheless, preserves the Journal’s identity as a niche for critical discussion of contemporary literature in inorganic chemistry. (For the 1st, 2nd and 3rd manifestations, see: a) Otten, B. M.; Melancon, K. M.; Omary, M. A. “All That Glitters is Not Gold: A Computational Study of Covalent vs Metallophilic Bonding in Bimetallic Complexes of d¹⁰ Metal Centers – A Tribute to Al Cotton on the 10th Anniversary of His Passing,” Comments Inorg. Chem. 2018, 38, 1–35; b) Yaseen, W. K.; Sanders, S. F.; Almotawa, R. M.; Otten, B. M.; Bhat, S.; Alamo, D. C.; Marpu, S. B.; Golden, T. D.; Omary, M. A. “Are Metal Complexes “Organic”, “Inorganic”, “Organometallic”, or “Metal-Organic” Materials? A Case Study for the Use of Trinuclear Coinage Metal Complexes as “Metal-Organic Coatings” for Corrosion Suppression on Aluminum Substrates”, Comments Inorg. Chem. 2019, 39, 1–26; and c) Smith, J. B.; Otten, B. M.; Derry, P. J.; Browning, C.; Bodenstedt, K. W.; Sandridge, J. H.; Satumtira, N. T.; Zilaie, M.; Payne, J.; Nuti, R.; Omary, M. A.; Smucker, B. W. “Luminescent, Redox-Active (Dithiolato)Bis(Imine)Platinum(II) Divergent Complexes with Exchangeable Imine Ligands: An Experimental/Computational Study versus Their (Diimine)(Dithiolato)Platinum(II) Convergent Congeners”, Comments Inorg. Chem. 2019, 39, 188–215.) Herein, the dinuclear complexes {Cu[3,5-(CF₃)₂Pz](µ-dppm)}₂ and {Cu[3,5-(CF₃)₂Pz](µ-dppm)}₂•3THF were studied structurally, spectroscopically and via density functional theory (DFT). They were synthesized by reacting bis(diphenylphosphino)methane (dppm) with the cyclic trinuclear complex {μ-[3,5-(CF₃)₂Pz]Cu}₃ to effect nuclearity reduction. Two forms of crystalline solids, {Cu[3,5-(CF₃)₂Pz](µ-dppm)}₂ and {Cu[3,5-(CF₃)₂Pz](µ-dppm)}₂•3THF have been obtained using different recrystallization conditions. The {Cu[3,5-(CF₃)₂Pz](µ-dppm)}₂ complex was found through DFT computations to undergo a distortion from a Y-shaped coordination sphere in the S₀ ground state toward a T-shape in the T₁ photoexcited, lowest-lying, phosphorescent state. The distortion also causes the copper-copper bond length to contract and form an excimer bond (d_Cu-Cu = 2.577 Å). Experimentally, the presence of THF in the crystal was found to cause a blue shift, effecting a change in emission color from teal to blue to the naked eye, with a near-unity quantum yields (93%), rendering the latter solid suitable for inorganic LED applications but not OLEDs, as thin films exhibit a reduced quantum yield. Crystallographic evidence suggests that THF leads to a more compact lattice that makes the complexes more rigid and thus hinder the excited state distortions vs unsolvated crystals. Greater distortion leads to a lower energy radiative emission and thus a red shift in the emission color. Films were also studied and found to undergo further red shifting as a result of less rigidity in the media and more surface molecules susceptible to distortion, hence manifesting the luminescence rigidochromism optical phenomenon in the solid state as opposed to the traditional manifestation in frozen vs fluid solution. Photobleaching was studied in both the film and powder to assess photostability, which was superior in neat vs doped solids, which is also favorable for LED applications.

Graphical Abstract

KEYWORDS:

• Copper pyrazolates
• luminescence
• OLED

Acknowledgments

This research has been supported by the Welch Foundation (Grant Y-1289 to H.V.R.D. and Grant B-1542 to M.A.O.), and the U.S. National Science Foundation (Grant CHE-1413641 to M.A.O. for research support and Departmental Grants CHE-1531468 and CHE-1726652 for equipment support).

Conflicts of Interest

There are no conflicts of interest to report

Supplementary data

Supplemental data for this article can be accessed publisher’s website.

Additional information

Funding

This work was supported by the National Science Foundation [CHE-1413641]; Welch Foundation [B-1542, Y-1289].