Intramolecular Hydrogen Bond in Pyridine Schiff Bases as Ancillary Ligands of Re(I) Complexes Is a Switcher between Visible and NIR Emissions: A Relativistic Quantum Chemistry Study

Rhenium(I) tricarbonyl complexes have been described as suitable fluorophores, particularly for biological applications. fac-[Re(CO)₃(N,N)L]^(0 or 1+) complexes, where N,N is a substituted dinitrogenated ligand (bipyridine or derivatives with relatively small substituents) and L the ancillary ligand [a pyridine Schiff base (PSB) harboring an intramolecular hydrogen bond (IHB)], have presented promissory results concerning their use as fluorophores, especially for walled cells (i.e., bacteria and fungi). In this work, we present a relativistic theoretical analysis of two series of fac-[Re(CO)₃(N,N)PSB]¹⁺ complexes to predict the role of the IHB in the ancillary ligand concerning their photophysical behavior. N,N corresponds to 2,2'-bipyridine (bpy) (series A) or 4,4'-bis(ethoxycarbonyl)-2,2'-bipyridine (deeb) (series B). We found that all the complexes present absorption in the visible light range. In addition, complexes presenting a PSB with an IHB exhibit luminescent emission suitable for biological purposes: large Stokes shift, emission in the range of 600-700 nm, and τ in the order of 10^-2 to 10^-3 s. By contrast, complexes with PSB lacking the IHB show a predicted emission with the lowest triplet excited-state energy entering the NIR region. These results suggest a role of the IHB as an important switcher between visible and NIR emissions in this kind of complexes. Since the PSB can be substituted to modulate the properties of the whole Re(I) complex, it will be interesting to explore whether other substitutions can also affect the photophysical properties, mainly the emission range.

Phenyl-pyta-tricarbonylrhenium(I) complexes: combining a simplified structure and steric hindrance to modulate the photoluminescence properties

Strongly luminescent tricarbonylrhenium(I) complexes are promising candidates in the field of optical materials. In this study, three new complexes bearing a 3-(2-pyridyl)-1,2,4-triazole (pyta) bidentate ligand with an appended phenyl group were obtained in very good yields owing to an optimized synthetic procedure. The first member of this series, i.e. complex 1, was compared with the previously studied complex RePBO to understand the influence of the fluorescent benzoxazole unit grafted on the phenyl ring. Then, to gauge the effect of steric hindrance on the luminescence properties, the phenyl group of complex 1 was substituted in the para position by a bulky tert-butyl group or an adamantyl moiety, affording complexes 2 and 3, respectively. The results of theoretical calculations indicated that these complexes were quite similar from an electronic point of view, as evidenced by the electrochemical study. In dichloromethane solution, under excitation in the UV range, all the complexes emitted weak phosphorescence in the red region. In the solid state, they could be excited in the blue region of the visible spectrum and they emitted strong yellow light. The photoluminescence quantum yield was markedly increased with raising the size of the substituent, passing from 0.42 for 1 to 0.59 for 3. The latter complex also exhibited clear waveguiding properties, unprecedented for rhenium complexes. From this point of view, these easy-synthesized and spectroscopically attractive complexes constitute a new generation of emitters for use in imaging applications and functional materials. However, the comparison with RePBO showed that the presence of the benzoxazole group leads to unsurpassed mechanoresponsive luminescence (MRL) properties, due to the involvement of a unique photophysical mechanism that takes place only in this type of complex.

Photoluminescence enhancement of Re(i) carbonyl complexes bearing D-A and D-π-A ligands

Three Re(i) carbonyl complexes [ReCl(CO)3(Ln)] bearing 2,2'-bipyridine, 2,2':6',2''-terpyridine, and 1,10-phenanthroline functionalized with diphenylamine/or triphenylamine units (L1-L3) were synthesized to explore the impact of highly electron donating units appended to the imine ligand on the thermal and optoelectronic properties of Re(i) systems. Additionally, for comparison, the ligands L1-3 and parent complexes [ReCl(CO)3(bipy)], [ReCl(CO)3(phen)] and [ReCl(CO)3(terpy-κ2N)] were investigated. The thermal stability was evaluated by differential scanning calorimetry. The ground- and excited-state electronic properties of the Re(i) complexes were studied by cyclic voltammetry and differential pulse voltammetry, absorption and emission spectroscopy, as well as using density-functional theory (DFT). The majority of the compounds form amorphous molecular materials with high glass transition temperatures above 100 °C. Compared to the unsubstituted complexes [ReCl(CO)3(bipy)], [ReCl(CO)3(phen)] and [ReCl(CO)3(terpy-κ2N)], the HOMO-LUMO gap of the corresponding Re(i) systems bearing modified imine ligands is reduced, and the decrease in the value of the ΔEH-L is mainly caused by the increase in HOMO energy level. In relation to the parent complexes, all designed Re(i) carbonyls were found to show enhanced photoluminescence, both in solution and in solid state. The investigated ligands and complexes were also preliminarily tested as luminophores in light emitting diodes with the structures ITO/PEDOT:PSS/compound/Al and ITO/PEDOT:PSS/PVK:PBD:compound/Al. The pronounced effect of the ligand chemical structure on electroluminescence ability was clearly visible.

Optimization of aggregation-induced phosphorescence enhancement in mononuclear tricarbonyl rhenium(i) complexes: the influence of steric hindrance and isomerism

In order to improve the remarkable performance of a mononuclear tricarbonyl rhenium(i) complex (ReL1) that exhibits rare aggregation-induced phosphorescence enhancement (AIPE) behavior, two new complexes (ReL3 and ReL4) were prepared and investigated. They incorporate a 2-pyridyl-1,2,4-triazole (pyta) ligand connected to a 2-phenylbenzoxazole (PBO) moiety. Complex ReL3 differs from ReL1 by the presence of a bulky tert-butyl substituent, and ReL4 is an isomer where the PBO group is linked to the pyta ligand by its phenyl group. Theoretical calculations were in congruence with electrochemical and spectroscopic properties in solutions. Both new compounds exhibited strong AIPE and much better solid-state emission efficiency than ReL1, with photoluminescence quantum yields up to 55% for ReL4. Crystallographic data indicate that this increase in emission efficiency is due to optimum packing that prevents quenching. This work shows that minor structural changes may have major effects upon the solid-state spectroscopic properties and it provides a rational basis for accessing AIPE-active strongly emissive rhenium(i) complexes.

Synthesis and Electrochemical and Spectroscopic Characterization of 4,7-diamino-1,10-phenanthrolines and Their Precursors

New approaches to the synthesis of 4,7-dichloro-1,10-phenanthrolines and their corresponding 9H-carbazol-9-yl-, 10H-phenothiazin-10-yl- and pyrrolidin-1-yl derivatives were developed. Their properties have been characterized by a combination of several techniques: MS, HRMS, GC-MS, electronic absorption spectroscopy and multinuclear NMR in both solution and solid state including ¹⁵N CP/MAS NMR. The structures of 5-fluoro-2,9-dimethyl-4,7-di(pyrrolidin-1-yl)-1,10-phenanthroline (5d), 4,7-di(9H-carbazol-9-yl)-9-oxo-9,10-dihydro-1,10-phenanthroline-5-carbonitrile (6a) and 4,7-di(10H-phenothiazin-10-yl)-1,10-phenanthroline-5-carbonitrile (6b) were determined by single-crystal X-ray diffraction measurements. The nucleophilic substitutions of hydrogen followed by oxidation produced compounds 6a and 6b. The electrochemical properties of selected 1,10-phenanthrolines were investigated using cyclic voltammetry and compared with commercially available reference 1,10-phenanthrolin-5-amine (5l). The spatial distribution of frontier molecular orbitals of the selected compounds has been calculated by density functional theory (DFT). It was shown that potentials of reduction and oxidation were in consistence with the level of HOMO and LUMO energies.

The unsuspected influence of the pyridyl-triazole ligand isomerism upon the electronic properties of tricarbonyl rhenium complexes: an experimental and theoretical insight

How can the intimate nature of the triazole moiety govern the geometry and luminescence properties of a rhenium complex?