Bidirectional “Ping-Pong” Energy Transfer and 3000-Fold Lifetime Enhancement in a Re(I) Charge Transfer Complex

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Abstract
The synthesis and photophysics of a new Re(I)-carbonyl diimine complex, Re(PNI-phen)(CO)3Cl, where the PNI-phen is N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide is reported. The metal-to-ligand charge transfer (MLCT) emission lifetime was increased approximately 3000-fold at room temperature with respect to that of the model complex [Re(phen)(CO)3Cl] as a result of thermal equilibrium between the emissive 3MLCT state and a long-lived triplet ligand-centered (3LC) state on the PNI chromophore. This represents the longest excited state lifetime (τ = 651 μs) that has ever been observed for a Re(I)-based CT photoluminescence at room temperature. The energy transfer processes and the associated rate constants leading to the establishment of the excited state equilibrium were elucidated by a powerful combination of three techniques (transient visible and infrared (IR) absorption and photoluminescence), each applied from ultrafast to the micro/milliseconds time scale. The MLCT excited state was monitored by transient IR using CO vibrations through time intervals where the corresponding signals obtained in conventional visible transient absorption were completely obscured by overlap with strong transients originating from the pendant PNI chromophore. Following initial excitation of the 1LC state on the PNI chromophore, energy is transferred to form the MLCT state with a time constant of 45 ps, a value confirmed in all three measurement domains within experimental error. Although transient spectroscopy confirms the production of the 3MLCT state on ultrafast time scales, Förster resonance energy transfer calculations using the spectral properties of the two chromophores support initial singlet transfer from 1PNI* to produce the 1MLCT state by the agreement with the experimentally observed energy transfer time constant and efficiency. Intersystem crossing from the 1MLCT to the 3MLCT excited state is believed to be extremely fast and was not resolved with the current experiments. Finally, triplet energy was transferred from the 3MLCT to the PNI-centered 3LC state in less than 15 ns, ultimately achieving equilibrium between the two excited states. Subsequent relaxation to the ground state occurred via emission resulting from thermal population of the 3MLCT state with a resultant lifetime of 651 μs. The title chromophore represents an interesting example of “ping-pong” energy transfer wherein photon excitation first migrates away from the initially prepared 1PNI* excited state and then ultimately returns to this moiety as a long-lived excited triplet which disposes of its energy by equilibrating with the photoluminescent Re(I) MLCT excited state.

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