Compensation of Hybridization Defects in Phosphorescent Complexes with Pnictogen-Based Ligands—A Structural, Photophysical, and Theoretical Case-Study with Predictive Character

Abstract

In this work, for the first time, the comparative use of P-, As-, and Sb-based ligands in phosphorescent coordination compounds is reported toward new coordination chemical concepts in the design and realization of tailored triplet emitters with nonconventional elements. By means of spectroscopic, X-ray diffractometric, and quantum-chemical methods, we reconstructed the nature of the chemical bonds as well as the influence of the increasingly heavy elements on the photoexcited state properties, which were correlated with the hybridization and polarizability of the pnictogen atoms (Pn). In particular, we elucidated the structural and photophysical properties of a series of homologous Pt­(II) complexes with monodentate ancillary ligands based on group 15 elements, namely P, As, and Sb. Six coordination compounds bearing tridentate dianionic 2,6-bis­(1H-1,2,4-triazol-5-yl)­pyridine luminophoric pincer ligands bearing either CF 3 or t Bu moieties on the triazole rings along with triphenylpnictogens (PnPh 3 ) as monodentate ancillary ligands ([CF 3 /Pn] or [ t Bu/Pn], respectively) have been investigated. The electron donating or withdrawing effect of the peripheral substituent ( t Bu vs. CF 3 , respectively) and its influence on the bonding, crystal packing as well as the excited state energies and lifetimes was assessed in fluid solutions, frozen glassy matrices, amorphous solids, and crystalline phases. A progressively red-shifted phosphorescence was observed with increasing atomic number along with a growing compensation of hybridization defects upon coordination of the Pn atom to the Pt­(II) center. The change of molecular geometry of the PnPh 3 unit upon complexation was extrapolated to predict the structural and excited state characteristics of the Bi-based analogues, which according to DFT calculations should be stable species and are the subject of ongoing synthetic efforts. In general, we envisage the use of these ligands for the relativistic enhancement of radiative deactivation rate processes, especially if Bi-based s-orbitals participate on the bond with the metal center, paving the road toward novel coordination compounds using abundant elements with high spin–orbit coupling for sustainable electroluminescent devices.

Conjugated Pt(II) Complexes as Luminescence-Switch-On Reporters Addressing the Microenvironment of Bacterial Biofilms

“…Generally speaking, when the complexes are dissolved in dilute fluid solution, different conformations are thermally accessible with comparable excited-state character, yet different deactivation rates and therefore multiexponential decays are observed. This effect is also noticed in the amorphous solids, where different molecular microenvironments are sensed, yielding multiexponential decays as it was previously observed for comparable Pt­(II) complexes. ,, In biomacromolecular microenvironments, multiexponential decays are frequently observed for luminescent reporters, which justifies the use of amplitude-weighted average lifetimes to estimate variations in radiative and radiationless deactivation rates …”

“…The weaker bands between 350 and 500 nm can be assigned to transitions into singlet excited states described as an admixture of LC and MLCT character, based on previous reports on analogous complexes. 18,37,43 The red-shifted absorption bands peaking at around 475 nm are indicative of ground-state aggregation phenomena mediated by intermolecular interactions involving the maltohexaose, if comparing P/ t bu with M/E-t bu, as well as to the amide bridge, as evidenced by comparison of M/A-CF 3 and M/E-CF 3 .…”

“…This effect is also noticed in the amorphous solids, where different molecular microenvironments are sensed, yielding multiexponential decays as it was previously observed for comparable Pt(II) complexes. 16,37,43 In biomacromolecular microenvironments, multiexponential decays are frequently observed for luminescent reporters, which justifies the use of amplitude-weighted average lifetimes to estimate variations in radiative and radiationless deactivation rates. 59 In the absence of physical quenching by the aqueous microenvironment, the Φ L is boosted and the lifetimes are prolonged for the solid-state samples (Table 1).…”

“…We have recently demonstrated the realization of 3 O 2 -sensing nanoparticles acting as self-referenced reporters by tweaking the relative intensity of the sensitive monomer emission (green) and the shielded aggregate luminescence (red), depending on the loading with Pt­(II) and Pd­(II) complexes bearing tetradentate luminophoric ligands with variable degrees of fluorination . We have also lately shown that the variation of monodentate coligands with variable bulk, ,− degree of coplanarization, and atomic number can drastically influence the structural and photophysical properties of monomeric and aggregated molecular entities with Pt­(II) centers. Moreover, ancillary ligands with dendrimeric anchors enabled us to label planktonic bacteria with Pt­(II)-based phosphorescence, whereas the adsorption of Pt­(II) complexes with suitably substituted tridentate luminophores allowed us to decorate proteins and gel-forming nanoclays …”

Intermolecular Interactions and Self-Assembly in Pt(II) Complex–Nanoclay Hybrids as Luminescent Reporters for Spectrally Resolved PLIM

“…The characteristic low-energy absorption bands in the range between 380 and 480 nm are attributed to transitions into singlet metal-toligand charge transfer ( 1 MLCT) states. [32][33][34]36 In Figure 3, the comparative photoluminescence spectra of the complexes are shown in fluid DCM at RT, in frozen DCM:MeOH (1:1) glassy matrices at 77 K, and for the hybrids dispersed in H 2 O at RT (the data for the solid powders of the neat complexes at RT and at 77 K are presented in Figures S2 and S3). The hybrids in aqueous media exhibit a bathochromic shift as compared to corresponding complexes dissolved in DCM solutions at RT, a trend that is even more pronounced with respect to 77 K where no solvent reorientation is possible to stabilize the excited states in the frozen matrices.…”