Effects of Steric Constraint on Chromium(III) Complexes of Tetraazamacrocycles. 3. Insights into the Temperature-Dependent Radiationless Deactivation of the ²E_g (O_h) Excited State of trans-[Cr(N₄)(CN)₂]⁺ Complexes

Abstract

Macrocyclic complexes of the type trans-[Cr(N4)(CN)2]+, where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam demonstrate significant variation in their room-temperature excited-state behavior; namely, the lifetimes of the 2Eg (Oh) excited states are 335, 23, and 0.24 micros, respectively. The lifetimes of these complexes have been measured in acidified H2O/dimethyl sulfoxide over the temperature range between -30 and +95 degrees C. Arrhenius activation parameters were calculated from these data. There was very little variation in the values of the Arrhenius preexponential factor between these three complexes, whereas the value of Ea is 40.6 kJ/mol for the cyclam complex, 35.5 kJ/mol for the 1,11-C3-cyclam complex, and 22.3 kJ/mol for the 1,4-C2-cyclam complex. Thus, differences in the room-temperature excited-state lifetimes can be rationalized based on the competition between thermally independent nonradiative relaxation and a thermally activated channel. To test whether a photodissociation mechanism involving Cr-macrocyclic N bond cleavage is a plausible explanation for the thermally activated relaxation pathway, samples of the cyclam complex were photolyzed in acidified D(2)O. A marked increase in the lifetime after photolysis demonstrated the occurrence of photodeuteration and thus a likely photodissociation of a macrocyclic N.

Electronic Energy Transduction from {Ru(py)4} Chromophores to Cr(III) Luminophores

“…Chromium­(III) complexes are also a playground for photochemists. − The most studied Cr­(III)-based systems include polypyridines − and fragments such as {Cr­(CN) 6 } − and {Cr­(aza-macrocycle)}. − They have been exploited as sensitizers or luminophores, not only in solution but also in the solid state . They have been designed in search for hole transfer to semiconductor substrates and for their use in upconversion schemes. , Chromium­(III) complexes also play an important role as earth-abundant materials in photocatalysis, − due to the combination of high oxidation power and long lifetime of their excited states. ,,− In general, emissive metal-centered (ligand field) excited states, labeled Cr*, dominate the photophysics of Cr­(III) fragments.…”

Solvent Effects on the Spectroscopic and Photophysical Properties of thetrans-(1,4,8,11-Tetraazacyclotetradecane)diisothiocyanatochromium(III) Ion,trans-[Cr(cyclam)(NCS)2]+

“…The photochemical and photophysical behavior of Cr(III) complexes have intrigued researchers for several decades. − These systems show several characteristics that make them particularly attractive for investigating photochemical and photophysical kinetics. They are thermally inert in solution but possess appreciable quantum yields for photosubstitution upon visible irradiation.…”

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

“…Note that near 293 K (vertical dashed line), the rate constant for the cyclam complex is relatively temperature independent, whereas the decay rate constants for the other two complexes exhibit Arrhenius behavior[78].…”