A Negative Activation Energy for Luminescence Decay:  Specific Solvation Effects on the Emission Properties of Bis(2,2‘-bipyridine)(3,5-dicarboxy-2,2‘-bipyridine)ruthenium(II) Chloride

Abstract

A new mixed-ligand polypyridylruthenium(II) complex, [Ru(bpy) 2 L]Cl 2 , has been prepared where bpy ) 2,2′-bipyridine and L ) 3,5-dicarboxy-2,2′-bipyridine. The ligand L is a non-symmetrically-substituted 2,2′-bipyridine having two hydrophilic carboxylate groups located at the 3-and 5-positions of only one of its two pyridyl rings. In acetonitrile, the photophysical properties of the metal complex include a long-lived excited state (λ em ) 637 nm, τ) 846 ( 11 ns, φ ) 0.046 at 295 K) whose decay involves an activated crossing to higher energy ligand field states (E a ) 4170 ( 200 cm -1 ). This behavior is similar to that observed for other ruthenium tris(bipyridyl) compounds. In contrast, the title compound displays several unusual photophysical properties in aqueous solution. These include a strongly red-shifted emission (λ em ) 685 nm) having a short, pH-dependent lifetime which is quenched by an excited-state proton transfer from solvent. The completely deprotonated form of the molecule is the dominant emissive species. Surprisingly, under neutral conditions the excited-state lifetime increases with increasing temperature, from a value of τ ) 54 ( 1 ns (λ em ) 686 nm, φ em ) 0.0036) at 280 K to τ ) 75 ( 1 ns (λ em ) 675 nm, φ em ) 0.0053) at 360 K. The data are fit to the Arrhenius expression to give E a ) -270 ( 15 cm -1 in H 2 O and E a ) -178 ( 10 cm -1 in D 2 O. Thermochromic emission data and temperature-induced energy-gap law behavior indicate that the unique photophysical properties of this compound are due to specific interactions involving protic solvent.

A General Solution for the 2‐Pyridyl Problem

“…9–16 All of these methods, however, suffer from one or more important limitations, including lack of air-stability of the 2-pyridyl building blocks, 10,12–16 use of toxic metals, 9 inability to isolate the building blocks in chemically pure form, 11 and/or inefficient couplings with more challenging halide coupling partners such as deactivated aryl chlorides. 11,12,13b–17 Overcoming all of these limitations, we herein report the first general solution for the 2-pyridyl problem.…”

General Method for Synthesis of 2-Heterocyclic N-Methyliminodiacetic Acid Boronates

“…12a In preliminary studies, however, we were only able to achieve a low yield of 1a from 3 . 3e For example, the dropwise addition of a freshly prepared THF solution of 3 to a stirred suspension of MIDA in DMSO at 55 °C over 1 hour (Fig.…”

“…10 Many different types of surrogates for 2-heterocyclic boronic acids have been developed, including trifluoroborate salts, 11 trialkoxy or trihydroxyborate salts, 12 diethanolamine adducts, 13 sterically bulky boronic esters, 14 and boroxines. 15 Important advances with 2-heterocyclic silanolates have also recently been reported.…”

Versatile tools in the construction of substituted 2,2′-bipyridines—cross-coupling reactions with tin, zinc and boron compounds

“…126 Interestingly, the synthesis of the sensitive dimethyl 2-pyridylboronate 77 was not described in detail until five years later, followed by the application in a cross-coupling reaction with 78 giving the disubstituted 2,2 0 -bipyridine 79 (Scheme 21). 127 Until the turn of the millennium only few investigations into the synthesis of 2-pyridylboron compounds have been reported. Terashima et al reported synthetic investigations on dialkylpyridylboranes, including the diethyl(2-pyridyl)borane (80).…”