Ligand macrocycle effects on the photophysical properties of rhodium(III) complexes: a detailed investigation of cis- and trans-dicyano(1,4,8,11-tetraazacyclotetradecane)rhodium(III) and related species

Abstract

Described are photophysical Properties for the dicyano tetravunacrocycle Rh(II1) complexes tram-and cis-Rh ([ 14]aneN4)(CN),+ and tram-Rh( [ 1 5]aneN4)(CN),+ (I, IV,and 11, respectively,[ 14]aneN4 = 1,4,8,1l-tetraazacyclotetradecane, [ 1S]aneN4 = 1,4,8,12-tetraazacyclopentadecane) plus related compounds. The electronic absorption spectra of the complexes are characterized by broad bands with A-in the near-ultraviolet region (e < 300 M-' cm-') assigned as spin-allowed ligand field (LF) transitions. Emission spectra are characterized by broad, unstructured, visible range bands assigned as spin-forbidden transitions from the lowest energy 'LF states. A Franck-Condon analysis of the emission spectrum based on a two acceptor mode model was performed, and the results were consistent with average bond displacements in the excited state of ca. 0.14 A for a FUI -N bond and 0.02 A for the N-H bonds. Both I and I1 luminesce strongly in ambient temperature aqueous solutions, but the measured lifetime (T,,,) of the former proved to be an order of magnitude longer. The r, values were measured over the range 77-300 K, and lifetimes of both I and I1 exhibited temperature-independent (at lower T) and temperatureactivated regions. Notably, the activation energy for nonradiative decay (k,) of the 3LF state of I is significantly larger than that observed for 11. At 77 K, values of T, for the various complexes were found to correlate with the estimated voo of the emission bands, and the isotope effect on the nonradiative rate resulting from D/H exchanging the amine protons was shown to increase with the number of N-H bonds in the complex. Molecular mechanics calculations were also camed out to probe the potential effects of ligand field excitation on the total strain energy of these tetraazamacrocycle complexes resulting from the connectivity between the amine nitrogens. These photophysical properties are discussed in terms of competitive weak and strong coupling mechanisms for nonradiative deactivation.

Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices

“…Although that state might be expected to be susceptible to ligand labilization in the equatorial plane, this pathway is effectively blocked by the macrocyclic constraint. The phosphorescence lifetime between 77 K and 300 K clearly shows both temperature independent behavior and a region (>160 K) that demonstrates an Arrhenius dependence with an E a of 6.4 kcal mol −1 (in MeOH/H 2 O) [122]. The phosphorescence lifetime in room temperature aqueous solution is 10 s, three orders of magnitude longer than that observed for most other Rh(III) acidoam(m)ine complexes.…”

“…Moreover, k • should be sensitive to deuterium labeling of ligands such as am(m)ines [121]. In this context, ES deactivation for several trans-Rh(N 4 )(CN) 2 + ions (where N 4 = a macrocyclic tetraamine ligand) has been shown to display two distinct temperature regimes [122]. Since these complexes display no unimolecular reactivity, the temperature effects on the lifetimes can be attributed to the competing weak and strong coupling mechanisms for nonradiative deactivation.…”

“…An interesting exception is trans-Rh(cyclam)(CN) 2 + , which is photoinert to ligand substitution [122,124,125]. Here the 3 T 1g (O h ) splits into 3 E g and 3 A 2g (D 4h ) levels, with the 3 A 2g being the lower energy (see also Section 4.3).…”

“…Molecular mechanics calculations for trans-Rh(cyclam)(CN) 2 + suggest an alternative explanation, namely, that the macrocyclic cyclam restricts the equatorial Rh-N expansion expected for the equilibrated triplet level [122]. For the lowest energy triplet excited state, this would result in a higher 0-0 energy, a steeper potential surface, and a relatively small nuclear displacement.…”

“…For the lowest energy triplet excited state, this would result in a higher 0-0 energy, a steeper potential surface, and a relatively small nuclear displacement. These latter features would lead to a higher activation energy for surface crossing to the ground state, thus disfavoring the strong coupling mechanism for nonradiative decay [18,122].…”