Electronic excited states and luminescence properties of palladium(II)corrin complex
Graphical abstract
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 dz2 orbital and can be written as (dx2-y2)2(dxz, dyz)4(dz2)1(π*)0(dxy)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 dz2 and it can be presented as follows (dx2-y2)2(dxz, dyz)3(dz2)2(π*)0(dxy)0. These two electronic states S0 (dyz)2(dz2)1 and S1(dyz)1(dz2)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 dz2 orbital, (dx2-y2)2(dxz, dyz)4(dz2)2(π*)0(dxy)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 (dx2-y2)2(dxz, dyz)3(dz2)2(π*)1(dxy)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.
Section snippets
Computational details
In the calculations the DFT and TD-DFT [46] methods witch the BP86 [[47], [48]] gradient functional and B3LYP [[49], [50]] hybrid functional were applied. TZVPP basis set was used for carbon, palladium and nitrogen and TZVP for hydrogen atoms [51]. The calculations witch full geometry optimization were done for the ground state (GS, S0) of [Pd(II)HM-CN-corr]+. Based on the ground state geometry the TD-DFT calculations were performed for vertical singlet and triplet excited states. The geometry
Results and discussion
The molecular structure of the synthetic [Pd(II)HM-CN-corr]+ complex [43] is shown in Fig. 1. [Pd(II)HM-CN-corr]+ is a four-coordinate complex, in which the central Pd(II) ion is coordinated by four nitrogen atoms of the corrin ring. The palladium coordination sphere is roughly planar. The total charge of the complex is +1 and the palladium cation is of low spin d8 electronic configuration.
Summary and conclusions
In this work calculations were performed to determine the photophysical properties of the palladium(II)1,2,2,7,12,12-heptamethylo-15-cyano-corrin complex ([Pd(II)HM-CN-corr]+) using the DFT and TD-DFT methods with BP86 and B3LYP functionals. The results show that BP86 gives electronic transition energies in good agreement with the experiment, while B3LYP overestimates the calculated transition energies. The two lowest electronic transitions are very close, the differ only about 0.2 eV in
Declaration of Competing Interest
There are no conflicts to declare
Acknowledgments
The TURBOMOLE, ADF and Gaussian 2016 calculations were carried out in the Wrocław Centre for Networking and Supercomputing, WCSS, Wrocław, Poland, http://www.wcss.wroc.pl, under computational Grant No.18.
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