Electronic Energy Self-Exchange with Macrocyclic Chromium(III) Complexes

Abstract

The luminescence lifetimes of N-deuterated Cr(III) complexes of macrocyclic tetraamine ligands, trans-CrN(4)X(2)(n)()(+), are substantially longer than those of their undeuterated counterparts in room temperature solution. Thus, excited-state emission quenching of the longer lived species by the shorter lived species may be studied by analyzing the decay profile following pulsed excitation. Flash photolysis experiments were carried out for three deuterated/undeuterated pairs of trans-CrN(4)X(2)(n)()(+) complexes (where X = CN-, NH(3), and F-). For the trans-Cr(cyclam)(CN)(2)(+) system in H(2)O, it was determined that energy transfer was occurring between the deuterated and undeuterated species. Although the rate constant of energy transfer was too fast to measure explicitly, it could be bracketed as k(et) >>7 x 10(6) M(-1) s(-1). For this reaction it was possible to measure an equilibrium constant which was very near 1.0. For trans-Cr(cyclam)(NH(3))(2)(3+) in DMSO, it was also established that energy transfer was occurring and rate constants of 2.4 x 10(6) M(-1) s(-1) (mu = 0.1) and 9.7 x 10(6) M(-1) s(-1) (mu = 1.0) were determined by a Stern-Volmer analysis. For trans-Cr(tet a)F(2+) in H(2)O, no energy transfer was observed, which implies that the rate constant is <<3 x 10(5) M(-1) s(-1). Because these energy-transfer reactions represent self-exchange energy transfer and are thus thermoneutral, we are able to analyze the results using Marcus theory and draw some conclusions about the relative importance of nuclear reorganization and electronic factors in the overall rate.

Synthesis of cis and trans Bis-alkynyl Complexes of Cr(III) and Rh(III) Supported by a Tetradentate Macrocyclic Amine: A Spectroscopic Investigation of the M(III)–Alkynyl Interaction

“…The prospect of inventing functional photochemical molecular devices (PMDs) , has prompted research on intramolecular energy transfer between transition metal centers bridged by ligands that facilitate such electronic communication . We have had a long-term interest in the preparation and photophysical characterization of trans -Cr(N 4 )(CN) 2 + (where N 4 = a tetraazamarocyclic ligand) complexes and in the study of the rates of intermolecular electronic energy transfer between them . Our interest in the alkynyl ligands stems from their promise as isoelectronic substitutes for the CN – ligand and the possibility of preparing photoactive complexes with bridging alkynyl ligands that might mediate intramolecular electron- and energy transfer.…”

Measurement of Both the Equilibrium Constant and Rate Constant for Electronic Energy Transfer by Control of the Limiting Kinetic Regimes

“…Electronic energy transfer between transition metal complexes often involves spin and symmetry forbidden transitions and thus proceeds through the exchange mechanism, prompting the application of Marcus−Hush theory in order to understand the reaction dynamics. , A number of studies of energy transfer have been analyzed to determine the relative importance of thermodynamics, electronic factors, and nuclear factors in these bimolecular reactions. − The majority of these studies have involved Cr(III) donor and/or acceptor molecules due to the large body of existing knowledge on Cr(III) photophysics and photochemistry , Our interest in this area is to discover systems for which energy transfer self-exchange can be studied so that cross-exchange and self-exchange dynamics can be compared in a manner similar to what has been done for electron transfer . Several investigations of reactions approximating energy transfer self-exchange − utilized a method first described by Maharaj and Winnik involving flash photolysis of mixtures of cleverly designed chromophores. The chomophores are designed such that slight structural modifications substantially affect the excited state lifetime of the molecule but leave the electronic energy levels relatively unchanged.…”

Effects of Steric Constraint on Chromium(III) Complexes of Tetraazamacrocycles. Chemistry and Excited-State Behavior of 1,4-C2-Cyclam Complexes

“…These complexes have also demonstrated a remarkable increase in their excited-state lifetimes upon N-deuteration. , Extensive studies of these types of complexes by Endicott and co-workers have been directed toward determining the effect that stereochemical alterations have on excited-state characteristics with the goal of being able to design ligand systems to produce desired excited-state behavior. , Several other groups have also taken advantage of the photobehavior of these types of complexes. Recently, we have shown that energy-transfer self-exchange rate constants can be measured or bracketed with the complexes trans -[Cr(cyclam)(NH 3 ) 2 ] 3+ , trans -[Cr(cyclam)(CN) 2 ] + , and trans -[Cr(tet a)F 2 ] + by monitoring exchange between the longer lived deuteriocomplex and the shorter lived protiocomplex . Additionally, Ford et al have shown that the nitrito complexes of Cr(III) cyclam undergo reversible NO photolabilization, a fact attributed to the suppressed amine labilization of cyclam …”