Theoretical Characterization of the Lowest Triplet Excited States of the Tris-(1,4,5,8-tetraazaphenanthrene) Ruthenium Dication Complex

Abstract

We present a theoretical study of the ground and the lowest triplet excited states of the tris-(1,4,5,8-tetraazaphenanthrene) ruthenium complex [Ru(tap)3]2+. Density functional theory (DFT) was used to obtain the relaxed geometries and emission energies (Delta-SCF), whereas time-dependent DFT (TD-DFT) was used to compute the absorption spectrum. Our calculations have revealed the presence of three low-lying excited-state minima, which may be relevant in the photophysical/photochemical properties of this complex. Two minima with similar energies correspond to the MLCT 3A2 and MLCT 3B metal-to-ligand charge-transfer states, the first one corresponding to a D3 structure, whereas the second is a slightly localized C2 species. The third and lowest one corresponds to the metal-centered MC 3A state and displays a pronounced C2 distortion. We have examined for the first time the localized character of the excitation in the computed MLCT states. In particular, we have evaluated the pseudorotation barrier between the Jahn-Teller C2 MLCT 3B minima in the moat around the D3 conical intersection. We have shown that the complex should be viewed as a delocalized [Ru3+(tap(-1/3))3]2+ complex in the lowest MLCT states, in agreement with subpicosecond interligand electron transfer observed by femtosecond transient absorption anisotropy study. Upper-bound estimates of the MLCT-->MC (3 kcal/mol) and MC-->MLCT (10 kcal/mol) activation energy barriers obtained from potential energy profiles in vacuum corroborate the high photoinstability of the MLCT states of the [Ru(tap)3]2+complex.

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“…The combination of TD-DFT and DFT calculations are especially helpful in determining the energy position and the electronic and molecular structures of 3 MC states. [37,96,97] As mentioned above, the thermal population of these states might be one of the possible non-radiative deactivation processes that could take place in Ir-iTMCs and, likewise ruthenium(II) metal complexes, [98] might cause complex instability and degradation both in solution and in LECs. 3 MC states formally involve the transfer of one electron from the HOMO dp orbitals (t 2g ) to the antibonding ds* orbitals (e g ) of the iridium ion.…”

The photochemistry of transition metal complexes using density functional theory

“…Consistently, DFT and TD-DFT show that the lowest triplets are of MLCT character and that dissociative 3 MC states are not easily accessible [101]. Alary et al [102,103] studied by DFT the excited state properties of the [Ru(tap) 3 ] 2+ (tap = tris- (1,4,5,8-tetraazaphenanthrene)) and [Ru(bpz) 3 ] 2+ (bpz = 2,2 -bipyrazine) complexes. These high redox potential complexes form stable photoadducts with amino acids and nucleobases that can be exploited for anti-cancer therapy [104,105].…”

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“…Kombinierte TD-DFT-und DFT-Rechnungen eignen sich besonders gut für die Bestimmung der Energien und der elektronischen Strukturen und Molekülstrukturen der 3 MCZustände. [37,96,97] Wie schon erwähnt, erçffnet die thermische Besetzung diese Zustände einen strahlungslosen Desaktivierungsweg, durch den Ir-iTMCs, und auch Ruthenium(II)-Komplexe, [98] sowohl in Lçsung als auch in LECs zersetzt werden kçnnten.…”