Spectroscopy, photophysical properties, and x-ray crystal structure of platinum(II) complexes of quaterpyridine

Abstract

Two novel platinum quaterpyridine complexes have been synthesized and characterized by *H NMR spectroscopy and X-ray crystallography: [Pt(pQP)](C104)2 (pQP = 3",5,5/,5'"-tetramethyl-2,2':6/,2'/:6",2",-quaterpyridine), monoclinic C2/c, a = 14.276 (2) k,b = 12.970 (2) k,c= 14.257 (1) A, ß = 102.29 (1)°, V = 2579.2 (6) A3, Z = 4; [Pt(QP)](C104)2 (QP = 2,2,:6',2,,:6",2",-quaterpyridine), monoclinic P2U a = 5.904 (3) k,b= 12.943 (2)A, c = 14.300 (4) A, ß = 100.70 (4)°, V = 1073.7 (6) A3, Z = 2. The shortest intermolecular Pt-Pt distances in [Pt(pQP)](C104)2 and [Pt(QP)](ClÓ4)2 are 7.249 (2) and 5.904 A, respectively. Both complexes exhibit photoluminescence in the solid state at 700 nm. Only [Pt(pQP)]2+ shows room-temperature emission in acetonitrile

Self-quenching and Cross-quenching Reactions of Excited Platinum(II) Diimine Complexes

“…Complexes with a cyclometalating diimine ligand, such as Pt(dpp)(CH 3 CN) þ (dpp ¼ deprotonated 2,9-diphenyl-1,10-phenanthroline), also emit from a lowest 3 MLCT excited state with surprisingly long lifetimes (>1 ms). [8,9,51] Of the complexes with a lowest 3 MMLLCT state, those containing a toluenedithiolate (tdt 2À ) ligand exhibit the longest excited-state lifetimes, which is consistent with an emissive state that is strongly stabilized with respect to LF states. For these reasons, fluid solution emission studies have focused on complexes with cyanide, arylacetylide, and toluenedithiolate ligands such as those listed in Table 1.…”

“…Transient emission measurements indicate that the excimer luminescence reaches its maximum intensity approximately 100 ns after excitation with a short laser flash, confirming the notion that the excimer is not a ground-state aggregate. Since these earliest observations, reports by Che, [8,9] Gray, [10] and Nagle and Vogler [11] have noted similar self-quenching behavior. However, it was not until 1999 that Eisenberg and coworkers [12] established that selfquenching, sometimes accompanied by excimer emission, is a general phenomenon, apparently characteristic of all platinum(II) diimine complexes exhibiting long-lived solution emission.…”

“…Using a Stern-Volmer analysis, the reciprocal of I was plotted against quencher concentration according to Eq. (8). For Pt(tmphen)(tdt), k i (5.23 Â 10 5 s À1 ) and k sq (4.2 Â 10 9 M À1 s À1 ) have previously been determined.…”

“…However, there is mounting evidence to suggest that halogenated solvents decrease the self-quenching rate slightly more than expected from viscosity considerations for a diffusion-controlled process. [52,53] It also should be noted that charged complexes exhibit selfquenching rates that are $5-10 times slower than observed for neutral complexes, [8,9,51] as expected from coulombic repulsions. However, the ionic strength dependence of these reactions has not yet been investigated.…”

“…The Pt(tmphen)(tdt) emission intensity was found to depend linearly on quencher concentration (0-1.8 Â 10 À3 M) according to Eq. (8). The resulting SternVolmer plots are shown in Figure 11 and the cross-quenching rates, ranging from 2.3-8.3 Â 10 9 M À1 s À1 , are given in Table 3.…”