Electronic excited states and luminescence properties of palladium(II)corrin complex

Highlights

• Photophysical mechanism comprises the fluorescence and phosphorescence processes

• Deformation of the corrin ring leads to effective ISC process

• The determined rate constants confirm the photophysical properties

• Emission occur from the lowest singlet and triplet states

Abstract

The DFT and TD-DFT calculations were performed for palladium(II)1,2,2,7,12,12-heptamethylo-15-cyanocorrin complex ([Pd(II)HM-CN-corr]⁺). Experimentally [Pd(II)HM-CN-corr]⁺ exhibits a weak fluorescence and strong phosphorescence. For the [Pd(II)HM-CN-corr]⁺ complex the electronic spectrum was calculated using the BP86 and B3LYP functionals. The results shows that BP86 gives electronic transition energies in good agreement with the experiment, while B3LYP overestimates the calculated transition energies, which can be related to the order and energy of molecular orbitals. Therefore, the BP86 functional was used in further calculations. The presence of the solvent was taken into account by the use of the COSMO model. The spin-orbit (SO) states were calculated along the PEC determined by N-Pd-N bending angle active coordinate in the S₁ using the ZORA method. The rate constants of fluorescence, phosphorescence and intersystem crossing processes were determined. Based on calculations the photophysical processes occurring in [Pd(II)HM-CN-corr]⁺ can be explained. After light absorption there is excitation to the S₁ or S₂ states and then the internal conversion to S₁ state from S₂. Comparison of the calculated rate constants of fluorescence (3.3·10⁷ s^-1) and ISC to the close lying T₂ state (8.5·10¹⁰ s^-1) in the S₁ minimum shows that the ISC is more probable. This is an explanation why fluorescence in [Pd(II)HM-CN-corr]⁺ is weak. The vibrationally resolved fluorescence and phosphorescence band shapes were determined using the Franck-Condon approximation. The obtained band profiles agree qualitatively with the experimental ones. The most important molecular vibrations contributing to vibrational bands were determined. The calculated rate constants values and the obtained picture of photophysical processes agree qualitatively with the experimental data.

Introduction

Complexes of transition metals with tetrapyrrole ligands are in great importance in many vital life reactions both in the human organism as in the animal and plant worlds. Two groups of such complexes, i. e. heme and cobalamin (Cbl) systems play essential role as cofactors in numerous enzymatic reactions [[1], [2], [3], [4], [5], [6], [7]]. Concurrently, structural modification of biological forms of tetrapyrrole complexes is viewed as direction to obtain compounds which may be used in drug delivery and its photochemical activation [[8], [9]], photodynamic therapy (PDT) and treatment of cancer [[10], [11], [12], [13]], as photo induced markers and agents in medical imaging [8,[14], [15], [16], [17], [18], [19], [20]]. Photoactivity of transition metal complexes with porphyrin and corrin is generally associated with light emission or photo-induced dissociation of ligands. In the biochemical and medical applications, luminescent properties of such compounds in certain cases are a desired property. Many porphyrin complexes have been synthesized with transition metals and were investigated form point of view a potential application in PDT [10,21]. In contrast to porphyrins the photochemistry of corrin complexes with metals other than cobalt were rather very rarely studied. Recent trends in the development of Cbl derivatives for medicinal and biochemical applications are more focused on compounds which structural modifications involve axial ligands or nucleotide loop attached to corrin ring [22]. These structural modifications have led to the design of an exciting variety of modified B₁₂ derivatives as mediating agents in the delivery of drugs [[23], [24], [25], [26], [27]], fluorescence markers [[14], [15],20,[28], [29], [30]], radiopharmaceuticals and diagnostic agents. [[16], [17], [18], [19],28,[31], [32], [33], [34]] In the context of structural modification of B₁₂ system, very interesting is the design and study of luminescent B₁₂-based probes for molecular imaging [[14], [15],35]. A number of synthetic Cbl derivatives have also been investigated, which exhibit antimetabolic activity in relation to vitamin B₁₂. Such complexes, called antivitamins [[36], [37]], can be used in pathogenesis studies of vitamin B₁₂ deficiency. In this group Cbls, the complexes in which the cobalt atom is exchanged for rhodium are known [[38], [39], [40], [41], [42]]. However, outside a very narrow group of synthetic B₁₂ derivatives, photochemical properties for analogs Cbl, with a changed cobalt atom to other metal were not investigated and are practically completely unknown.

In the 1970s last century, Gardiner and Thomson investigated luminescent properties a range of corrin complexes with transition metals, such as nickel, copper, rhodium, palladium, platinum and iron.[43] It was found that such complexes of first transition row metals are not luminescent while those of the second transition row exhibit phosphorescence. One of them, the palladium(II)1,2,2,7,12,12-heptamethylo-15-cyanocorrin complex ([Pd(II)HM-CN-corr]⁺, Fig. 1) is exceptional in displaying both phosphorescence and weak fluorescence. Nevertheless, over the next five decades, these luminescence properties of palladium-corrin complex didn't arouse much interest, so this area of photophysics of B₁₂ analogues with changed central atom remained practically completely unexplored from experimental and theoretical point of view. Taking into account poorly explored territory and recent trends in the development of chemistry of B₁₂ analogues, palladium-corrin complexes are worth attention as compounds with great practical application potential. Currently, in comparison to the relatively well-known photochemistry of Cbls, understanding the influence of the central atom on the luminescence properties of B₁₂ analogs requires new experimental measurements but also theoretical insight in electronic structure. Very commonly the understanding of experimental findings needs insight in mechanistic aspects on molecular level. In above mentioned work the luminescent properties were simply correlated with "d" electronic configuration of transition metal. Indeed, for Co(II)corrin complex the d⁷ ground state electronic configuration is a doublet, with one unpaired electron on d_z2 orbital and can be written as (d_x2-y2)²(d_xz, d_yz)⁴(d_z2)¹(π*)⁰(d_xy)⁰, where π* orbital is localized on corrin ligand. The configuration for first, lowest singlet excited state S₁, in this case is the result of d→d excitation from another occupied d orbitals to the singly occupied orbital d_z2 and it can be presented as follows (d_x2-y2)²(d_xz, d_yz)³(d_z2)²(π*)⁰(d_xy)⁰. These two electronic states S₀ (d_yz)²(d_z2)¹ and S₁(d_yz)¹(d_z2)² differ only in the d orbitals occupation, and are near degenerate in energy [44], what causes that lowest excited state easily undergoes quenching by vibrational motion. Therefore, Co(II)corrin complex remains non-fluorescent species and the quenching mechanism play very important role in B₁₂ photochemistry [45]. On the other hand, the electronic configuration d⁸ of ground state for Pd(II)corrin, has closed shell character with doubly occupied d_z2 orbital, (d_x2-y2)²(d_xz, d_yz)⁴(d_z2)²(π*)⁰(d_xy)⁰. Because the unoccupied d orbital is higher in energy than antibonding π* orbital of corrin, the lowest singlet excited state S₁ has MLCT d→π* character with (d_x2-y2)²(d_xz, d_yz)³(d_z2)²(π*)¹(d_xy)⁰ electronic configuration. That way the energy gap between the S₀ ground state and S₁ lowest singlet state is about five times greater than S₀/S₁ gap in the co(II)corrin system, based on results at DFT(BP86)/TD-DFT(BP86) level of theory. The larger energy difference between ground and first excited singlet states, efficiently reduces a probability of nonradiative quenching and contribute to increase the probability of radiation emission. Thus, the d electronic configuration of the central metal atom determines the lowest excited state energetics but it is not an absolutely decisive factor in the competition between internal conversion (IC) and emission processes. For fully understanding of the fluorescence and phosphorescence phenomena in such complexes, a more detailed description of the molecular mechanism of the photophysics is required, especially in relation to ISC (Inter System Crossing) S/T process efficiency and the way of radiative or nonradiative quenching of the triplet channel.

In this work for [Pd(II)HM-CN-corr]⁺ palladium complex, the molecular emission mechanism was theoretically investigated based on the DFT and TD-DFT approach. DFT (Density Functional Theory) and TD-DFT (Time Dependent - Density Functional Theory) are practically the only available methods, which can effectively described electronic structures of the ground and excited states for large molecular system. In this work, the characters of lowest vertical, singlet and triplet states are described. The simple test of influence of functionals on excited states character were performed. Hybrid B3LYP and gradient-corrected BP86 functionals were taken into account on the DFT and TD-DFT level of theory. Based on the ground state and relaxed excited state geometries the photophysics mechanism of luminescence is described on molecular level. The ISC process efficiency and rates of photophysical processes is estimated with the use of computational methods.

In the authors belief, the work may also be an inspiration for novel experimental studies in field of photochemistry of metal-changed B₁₂ analogs.