Unexpected Photochemical Reactivity of Ruthenium(II) Polypyridine Complexes Induced by a Bis(bidentate) Phosphine

Unprecedented conformational modulation of the efficiency of luminescence in Ru(II) bipyridyl complexes containing a bis(bidentate) phosphine

“…In 1-4, all mean values of the Ru-N bond lengths and N-Ru-N angles are identical within statistical significance. Since the X-ray structures of 1-4 are the only known structures of pure diastereoisomers of RuP 2 N 4 phosphine complexes [1], this is an extraordinary proof that differences in the ground state energies of the RuN 4 chromophore moieties cannot exert the dominant influence on the photophysical properties of 1-4. However, the pronounced folding of the cyclobutane rings of 155.3°in 3 and 148.0°in 4 leads to a reduction of the P-Ru-P chelate angles of 86.54(5)°in 1 (mean value) and 86.50(4)°in 2 [1] to 84.76(3)°in 3 and 84.85(5)°in 4.…”

“…Furthermore, two different diastereoisomers occur for 3 and 4: Fig. 1 shows the K/MPPP form for 3 and its K/MMMP counterpart for 4 [1,5], where of course also their mirror images are present and a detailed discussion will be reported elsewhere. It is important to note that 3 and 4 consist of pure diastereoisomers leading to unequivocal photophysical properties.…”

“…The observed luminescent lifetimes are very long: 910(20) ns in CD 3 CN and 794(20) ns in CH 3 CN at 298 K. The bimetallic structure of rac-(DD/KK)-[Ru 2 (dppcb)(bpy) 4 ](PF 6 ) 4 (1) and meso-(DK/KD)-[Ru 2 (dppcb)(bpy) 4 ](PF 6 ) 4 (2), where dppcb is cis, trans, cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane, is responsible for this dramatic effect. An enhanced folding of the cyclobutane rings drastically reduces the efficiency of luminescence as observed in the new monometallic complexes [Ru(dppcb)(bpy) 2 ](PF 6 ) 2 (3) and [Ru(dppcbO 2 )(bpy) 2 ](PF 6 ) 2 (4), where dppcbO 2 is cis, trans, cis-1,2-bis(diphenylphosphinoyl)-3,4-bis(diphenylphosphino)cyclobutane.The completely unexpected photochemical reactivity of the ruthenium(II) polypyridine complexes rac-(DD/KK)-[Ru 2 (dppcb)(bpy) 4 ](PF 6 ) 4 (1) and meso-(DK/KD)[Ru 2 (dppcb)(bpy) 4 ](PF 6 ) 4 (2) (see Chart 1), where dppcb is cis, trans, cis-1, 2,3,4-tetrakis(diphenylphosphino)cyclobutane, has prompted us to study their photophysical properties [1]. It was recognized early that the p acidity of phosphine ligands destabilizes the lowest MLCT (metal-to-ligand charge-transfer) state to a far greater extent than the d-d state, when combined with [Ru(bpy) 2 ] 2+ , leading to a complete lack of luminescent properties at room temperature [2].…”

“…The completely unexpected photochemical reactivity of the ruthenium(II) polypyridine complexes rac-(DD/KK)- [Ru 2 (dppcb)(bpy) 4 ](PF 6 ) 4 (1) and meso-(DK/KD) [Ru 2 (dppcb)(bpy) 4 ](PF 6 ) 4 (2) (see Chart 1), where dppcb is cis, trans, cis-1, 2,3,4-tetrakis(diphenylphosphino)cyclobutane, has prompted us to study their photophysical properties [1]. It was recognized early that the p acidity of phosphine ligands destabilizes the lowest MLCT (metal-to-ligand charge-transfer) state to a far greater extent than the d-d state, when combined with [Ru(bpy) 2 ] 2+ , leading to a complete lack of luminescent properties at room temperature [2].…”

“…Furthermore, the emission quantum yield (U r ) of 6.4 · 10 À4 for 1/2 in CH 3 CN at 298 K is reduced by nearly a factor of 100 compared to the quantum efficiency of [Ru (bpy) 3 ] 2+ , which is 0.062 [9]. Very long excited state lifetimes allow some interesting excited state reactivity, since the reactivity properties of comparable complexes have turned out to be quite extraordinary based on the accessibility of long-lived excited states [1,10].…”