Influences of Both N,N,N- and N,N,C-Coordination Modes of Tolyl-terpyridine on the Photophysical Properties of Cyclometalated Ru(II) Complexes: Combined Experimental and Theoretical Investigations on Acid/Base-Dependent Reversible Cyclometalation

With the goal of developing a new strategy for the synthesisofluminescent Ru(II) complexes, we have prepared herein a new set ofbis-tridentate complexes of the type [(py-bpy-Ph-X)Ru(tpy-PhCH3)]ClO4 (X = -CH3, -CH2Br, and -CHO) incorporating both non-cyclometalatedand cyclometalated coordination motifs of two isomeric forms of methylphenyl-terpyridine(tpy-PhCH3). Thorough characterization of the synthesizedcomplexes is carried out using standard analytical tools and singlecrystal X-ray diffraction. Detailed investigations on their photophysicaland electrochemical behaviors are carried out in MeCN. The presenceof a carbanionic center in the cyclometalating unit increases theabsorption spectral window of the complexes into a longer-wavelengthregion. The complexes also exhibit room-temperature luminescence inthe NIR domain with enhanced excited-state lifetimes (up to 20.1 ns)compared to their non-cyclometalated counterpart, [Ru(tpy-PhCH3)(2)](2+). In the presence of acid, thenon-coordinated nitrogen atom in the secondary coordination sphereof the complexes allows fine-tuning of the absorption and emissionspectral properties. Excess acid induces de-coordination of the Ru-Cbond, which is signaled by a remarkable alteration of their spectralprofiles. Cleavage of the Ru-C bond is also possible upon treatingthe acidified solution of the complexes with visible light. Restorationof the Ru-C bond is again feasible upon treating the solutionwith base at an elevated temperature (& SIM;70 & DEG;C). In essence,on-off and off-on switchingof emission is facilitated upon alternating treatment of the Ru(II)complexes with acid, base, and temperature. DFT and TD-DFT calculationsare also performed for assignments of the spectral bands as well asto understand structural changes associated with the switching behaviorsof the complexes. Anew array of luminescent cyclometalated Ru(II) terpyridinecomplexes having both N-N-N and N-N-Ccoordination motifs are designed. The non-coordinated nitrogen atomin the outer coordination sphere allows fine modulation of the spectralproperties by acid-base equilibria. Excess acid induces de-coordinationof the Ru-C bond, which again restores upon treatment withbase at elevated temperatures. Thus, on-offand off-on emission switching is feasible upontreatment with acid, base, and temperature in an appropriate sequence.

Automated summary

In this study, a new strategy for the synthesis of luminescent Ru(II) complexes was developed. The synthesis involved the preparation of a new set of bis-tridentate complexes incorporating both non-cyclometalated and cyclometalated coordination motifs. The complexes exhibited enhanced absorption in a longer-wavelength region and room-temperature luminescence in the NIR domain. The spectral properties of the complexes could be fine-tuned by acid-base equilibria, with the Ru-C bond being de-coordinated in the presence of excess acid and then restored upon treatment with base at elevated temperatures.