Mechanistic Insights into Photochromic Behavior of a Ruthenium(II)–Pterin Complex

Abstract

The pterin-coordinated ruthenium complex, [Ru(II) (dmdmp)(tpa)](+) (1) (Hdmdmp=N,N-dimethyl-6,7-dimethylpterin, tpa=tris(2-pyridylmethyl)amine), undergoes photochromic isomerization efficiently. The isomeric complex (2) was fully characterized to reveal an apparent 180° pseudorotation of the pterin ligand. Photoirradiation to the solution of 1 in acetone with incident light at 460 nm resulted in dissociation of one pyridylmethyl arm of the tpa ligand from the Ru(II) center to give an intermediate complex, [Ru(dmdmp)(tpa)(acetone)](2+) (I), accompanied by structural change and the coordination of a solvent molecule to occupy the vacant site. The quantum yield (ϕ) of this photoreaction was determined to be 0.87 %. The subsequent thermal process from intermediate I affords an isomeric complex 2, as a result of the rotation of the dmdmp(2-) ligand and the recoordination of the pyridyl group through structural change. The thermal process obeyed first-order kinetics, and the rate constant at 298 K was determined to be 5.83×10(-5) s(-1). The activation parameters were determined to be ΔH(≠) =81.8 kJ mol(-1) and ΔS(≠) =-49.8 J mol(-1) K(-1). The negative ΔS(≠) value indicates that this reaction involves a seven-coordinate complex in the transition state (i.e., an interchange associative mechanism). The most unique point of this reaction is that the recoordination of the photodissociated pyridylmethyl group occurs only from the direction to give isomer 2, without going back to starting complex 1, and thus the reaction proceeds with 100 % conversion efficiency. Upon heating a solution of 2 in acetonitrile, isomer 2 turned back into starting complex 1. The backward reaction is highly dependent on the solvent: isomer 2 is quite stable and hard to return to 1 in acetone; however, 2 was converted to 1 smoothly by heating in acetonitrile. The activation parameters for the first-order process in acetonitrile were determined to be ΔH(≠) =59.2 kJ mol(-1) and ΔS(≠) =-147.4 kJ mol(-1) K(-1). The largely negative ΔS(≠) value suggests the involvement of a seven-coordinate species with the strongly coordinated acetonitrile molecule in the transition state. Thus, the strength of the coordination of the solvent molecule to the Ru(II) center is a determinant factor in the photoisomerization of the Ru(II)-pterin complex.

Reversible Switching of the Luminescence of a Photoresponsive Gadolinium(III) Complex

“…[2][3][4][5] Recent developments have been dominated by the metal complexes that connect well-established organic photochromic molecules such as diarylethenes, azobenzenes, and spiropyrans. [1] Recently, photochromic metal complexes have been appealing because of their diverse structures and functions.…”

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Photochromism is a light-induced reversible change of color, and the fundamental concept and its applications have always fascinated scientists. [3, 5] In our efforts to develop a new class of luminescent lanthanide complexes with a novel light-harvesting hexadentate ligand ({(PyrO) 3 tacn} 3À = {(CH 2 C 16 H 8 O) 3 C 6 H 12 N 3 } 3À ), [6] we have serendipitously found an unprecedented luminescence photochromism of a gadolinium(III) complex [{(PyrO) 3 tacn}Gd(THF)] (1) in THF at room temperature: Upon irradiation (l irr < 405 nm) [7] , blue emission of 1 changes to intense red emission that reverts to the original blue emission after O 2 exposure (Scheme 1, Figure 1, and Movie S1 in the Supporting Information). [2][3][4][5] Recent developments have been dominated by the metal complexes that connect well-established organic photochromic molecules such as diarylethenes, azobenzenes, and spiropyrans.

…”

“…The red color originates mainly in a new emission band from 600 to 850 nm (l em max = 625 nm, t = 430 ms, F = 0.12; Figure S5 in the Supporting Information). [14] Through these processes, 3 O 2 is taken away from the system, and the red phosphorescence of 1 at room temperature becomes visible. On the basis of these results, the photoinduced red emission can be assigned to the phosphorescence from an intraligand excited state.…”

“…[1] Recently, photochromic metal complexes have been appealing because of their diverse structures and functions. [2][3][4][5] Recent developments have been dominated by the metal complexes that connect well-established organic photochromic molecules such as diarylethenes, azobenzenes, and spiropyrans. [2, 4] However, the photochromic metal complexes based on their intrinsic light-induced behavior, which could open a new field of photochromic chemistry, are rare because of limited methodologies to construct the photochromic systems and compounds.…”

“…[2, 4] However, the photochromic metal complexes based on their intrinsic light-induced behavior, which could open a new field of photochromic chemistry, are rare because of limited methodologies to construct the photochromic systems and compounds. [3, 5] In our efforts to develop a new class of luminescent lanthanide complexes with a novel light-harvesting hexadentate ligand ({(PyrO) 3 tacn} 3À = {(CH 2 C 16 H 8 O) 3 C 6 H 12 N 3 } 3À ), [6] we have serendipitously found an unprecedented luminescence photochromism of a gadolinium(III) complex [{(PyrO) 3 tacn}Gd(THF)] (1) in THF at room temperature: Upon irradiation (l irr < 405 nm) [7] , blue emission of 1 changes to intense red emission that reverts to the original blue emission after O 2 exposure (Scheme 1, Figure 1, and Movie S1 in the Supporting Information). In this photo-chromic system, room-temperature phosphorescence in solution, which is one of the rare properties of luminescent compounds, [8,9] plays a key role.…”