Oligophosphine-thiocyanate Copper(I) and Silver(I) Complexes and Their Borane Derivatives Showing Delayed Fluorescence.

Gomathy Chakkaradhari,Toni Eskelinen,Alexey S Melnikov,Pipsa Hirva,Igor O Koshevoy,Andrey Belyaev,Cecilia Degbe,Elena V Grachova,Sergey P Tunik

doi:10.1021/acs.inorgchem.8b03166

Abstract

The series of chelating phosphine ligands, which contain bidentate P2 (bis[(2-diphenylphosphino)phenyl] ether, DPEphos; 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, Xantphos; 1,2-bis(diphenylphosphino)benzene, dppb), tridentate P3 (bis(2-diphenylphosphinophenyl)phenylphosphine), and tetradentate P4 (tris(2-diphenylphosphino)phenylphosphine) ligands, was used for the preparation of the corresponding dinuclear [M(μ2-SCN)P2]2 (M = Cu, 1, 3, 5; M = Ag, 2, 4, 6) and mononuclear [CuNCS(P3/P4)] (7, 9) and [AgSCN(P3/P4)] (8, 10) complexes. The reactions of P4 with silver salts in a 1:2 molar ratio produce tetranuclear clusters [Ag2(μ3-SCN)(t-SCN)(P4)]2 (11) and [Ag2(μ3-SCN)(P4)]22+ (12). Complexes 7–11 bearing terminally coordinated SCN ligands were efficiently converted into derivatives 13–17 with the weakly coordinating –SCN:B(C6F5)3 isothiocyanatoborate ligand. Compounds 1 and 5–17 exhibit thermally activated delayed fluorescence (TADF) behavior in the solid state. The excited states of thiocyanate species are dominated by the ligand to ligand SCN → π(phosphine) charge transfer transitions mixed with a variable contribution of MLCT. The boronation of SCN groups changes the nature of both the S1 and T1 states to (L + M)LCT d,p(M, P) → π(phosphine). The localization of the excited states on the aromatic systems of the phosphine ligands determines a wide range of luminescence energies achieved for the title complexes (λem varies from 448 nm for 1 to 630 nm for 10c). The emission of compounds 10 and 15, based on the P4 ligand, strongly depends on the solid-state packing (λem = 505 and 625 nm for two crystalline forms of 15), which affects structural reorganizations accompanying the formation of electronically excited states.

Highlights

The excited states of thiocyanate species are dominated by the ligand to ligand SCN → π(phosphine) charge transfer transitions mixed with a variable contribution of MLCT
The chemistry of luminescent copper(I) complexes has been significantly revitalized by the phenomenon of thermally activated delayed fluorescence (TADF), which was found to be an intrinsic molecular property of a gradually increasing number of these compounds.[1−5] The given photophysical mechanism implies a small energy gap between the lowest lying singlet and triplet excited states (ΔE(S1−T1) is preferably less than 1000 cm−1) that makes possible the fast population of the T1 state and its efficient thermal equilibration with emissive S1 state by means of reverse ISC (T1 → S1)
Modulation of the photophysical properties of Cu(I) TADF emitters is typically achieved by varying the electronic characteristics of the ligand contributing to LUMO that primarily affects the energy of the excited state

Summary

■ INTRODUCTION

The chemistry of luminescent copper(I) complexes has been significantly revitalized by the phenomenon of thermally activated delayed fluorescence (TADF), which was found to be an intrinsic molecular property of a gradually increasing number of these compounds.[1−5] The given photophysical mechanism implies a small energy gap between the lowest lying singlet and triplet excited states (ΔE(S1−T1) is preferably less than 1000 cm−1) that makes possible the fast population of the T1 state (via S1 → T1 intersystem crossing, ISC) and its efficient thermal equilibration with emissive S1 state by means of reverse ISC (T1 → S1). The substantially red-shifted emission of complex 10, observed in a nonrigid fluid medium (degassed tetrahydrofuran solution, λem = 690 nm at 298 K, Φem = 2.1%, Figure S15), correlates with this hypothesis It has been noted for other Cu and Ag compounds that their photophysical behavior can be altered by quite subtle structural changes,[21] and even two independent molecules in the unit cell are capable of showing different emissions.[63] The low quantum yield of 10 in solution is not surprising as large structural distortions occurring in the excited state typically facilitate its nonradiative deactivation.[21,64] In addition, mechanical grinding converts all modifications of 10 into essentially the same type of amorphous solid with yellow-orange luminescence.

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES