Excited-State Dynamics in fac-[Re(CO)3(Me4phen)(L)]

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2010-11, Vol.114 (46), p.12129-12137
• Main Authors: Patrocinio, Antonio Otavio T., Brennaman, M. Kyle, Meyer, Thomas J., Murakami Iha, Neyde Y.
• Format: Article
• Language: English
• Subjects: A: Dynamics, Clusters, Excited States
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp104692w

Excited-state dynamics in fac-[Re(CO)3(Me4phen)(cis-L)]+ (Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline, L = 4-styrylpyridine (stpy) or 1,2-bis(4-pyridyl)ethylene (bpe)) were investigated by steady-state and time-resolved techniques. A complex equilibrium among three closely lying excited states, 3IL cis-L, 3MLCTRe→Me4phen, and 3ILMe4phen, has been established. Under UV irradiation, cis-to-trans isomerization of coordinated cis-L is observed with a quantum yield of 0.15 in acetonitrile solutions. This photoreaction competes with radiative decay from 3MLCTRe→Me4phen and 3ILMe4phen excited states, leading to a decrease in the emission quantum yield relative to the nonisomerizable complex fac-[Re(CO)3(Me4phen)(bpa)]+ (bpa = 1,2-bis(4-pyridyl)ethane). From temperature-dependent time-resolved emission measurements in solution and in poly(methyl methacrylate) (PMMA) films, energy barriers (ΔE a) for interconversion between 3MLCTRe→Me4phen and 3ILMe4phen emitting states were determined. For L = cis-stpy, ΔE a = 11 (920 cm−1) and 15 kJ mol−1 (1254 cm−1) in 5:4 propionitrile/butyronitrile and PMMA, respectively. For L = cis-bpe, ΔE a = 13 kJ mol−1 (1087 cm−1) in 5:4 propionitrile/butyronitrile. These energy barriers are sufficient to decrease the rate constant for internal conversion from higher-lying 3ILMe4phen state to 3MLCTRe→Me4phen, k i ≅ 106 s−1. The decrease in rate allows for the observation of intraligand phosphorescence, even in fluid medium at room temperature. Our results provide additional insight into the role of energy gap and excited-state dynamics on the photochemical and photophysical properties of Re(I) polypyridyl complexes.