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Photochemistry and Photophysics of Metal Complexes

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Inorganic Chemistry".

Deadline for manuscript submissions: closed (31 December 2022) | Viewed by 10625

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Dr. Andreas Winter

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Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany
Interests: metal complexes; metallo-supramolecular chemistry; opto-electronic devices; organic synthesis; photocatalysis
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Prof. Dr. Ulrich S. Schubert

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Laboratory of Organic and Macromolecular Chemistry (IOMC) Jena Center for Soft Matter (JCSM), Friedrich-Schiller-Universität Jena, Humboldtstr. 10, D-07743 Jena, Germany
Interests: automization; coordination chemistry; drug delivery; functional polymers; inkjet printing; metallo-supramolecular polymers; polymer batteries; polymer nanoparticles; self-healing materials
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Prof. Dr. Benjamin Dietzek-Ivanšić

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1. Friedrich Schiller University Jena, Institute for Physical Chemistry (IPC), Helmholtzweg 4, 07743 Jena, Germany
2. Leibniz Institute of Photonic Technology Jena, Research Department Functional Interfaces, Albert-Einstein-Strasse 9, 07745 Jena, Germany
Interests: artificial photosynthesis; photodynamic therapy; photocatalysis; charge-transfer dynamics at interfaces; chemistry and physics of electronically excited states; ultrafast spectroscopy

Special Issue Information

Dear Colleagues,

As the Guest Editors of this Special Issue of Molecules, "Photochemistry and Photophysics of Metal Complexes”, we would like to invite you to submit your research on this topic. Photoactive complexes based on main-group, transition, or rare-earth metals are widely used in a range of cutting-edge applications: (photo)catalysis, medicine, opto-electronics, photovoltaics, etc. The photochemical and photophysical properties of complexes mainly depend on the interplay between the organic ligands and the metal centers they are coordinated with. A fundamental understanding of such behavior can only be gained by bringing together synthesis, spectroscopy, and theory. Thus, today’s research on photoactive metal complexes represents an interdisciplinary field of research.

This Special Issue is not only devoted to fundamental studies on the photophysics and photochemistry of metal complexes, and the application of their light-induced reactivity in various fields of research will also be covered. We are looking forward to receiving contributions dealing with either of these fields of research. Articles may be full papers or a communication based on your own current research on metal complexes or may be a focused review article on some aspect of the subject. All submissions will be subject to peer review.

Dr. Andreas Winter
Prof. Dr. Ulrich S. Schubert
Prof. Dr. Benjamin Dietzek-Ivanšić
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

light-controlled reactivity
metal complexes
photochemistry
photoinduced processes
photoluminescence
photophysics
sensitizers

Published Papers (6 papers)

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Research

19 pages, 4761 KiB

Open AccessArticle

Red Light Absorption of [Re^I(CO)₃(α-diimine)Cl] Complexes through Extension of the 4,4′-Bipyrimidine Ligand’s π-System

by Nicolas Meitinger, Subrata Mandal, Dieter Sorsche, Andrea Pannwitz and Sven Rau

Molecules 2023, 28(4), 1905; https://doi.org/10.3390/molecules28041905 - 16 Feb 2023

Cited by 2 | Viewed by 1622

Abstract

Rhenium(I) complexes of type [Re(CO)₃(NN)Cl] (NN = α-diimine) with MLCT absorption in the orange-red region of the visible spectrum have been synthesized and fully characterized, including single crystal X-ray diffraction on two complexes. The strong bathochromic shift of MLCT absorption was achieved through extension of the π-system of the electron-poor bidiazine ligand 4,4′-bipyrimidine by the addition of fused phenyl rings, resulting in 4,4′-biquinazoline. Furthermore, upon anionic cyclization of the twisted bidiazine, a new 4N-doped perylene ligand, namely, 1,3,10,12-tetraazaperylene, was obtained. Electrochemical characterization revealed a significant stabilization of the LUMO in this series, with the first reduction of the azaperylene found at

E_{1 / 2}^{(0 / -)}

= −1.131 V vs. Fc⁺/Fc, which is the most anodic half-wave potential observed for N-doped perylene derivatives so far. The low LUMO energies were directly correlated to the photophysical properties of the respective complexes, resulting in a strongly red-shifted MLCT absorption band in chloroform with a λ_max = 586 nm and high extinction coefficients (ε_586nm > 5000 M⁻¹ cm⁻¹) ranging above 700 nm in the case of the tetraazaperylene complex. Such low-energy MLCT absorption is highly unusual for Re(I) α-diimine complexes, for which these bands are typically found in the near UV. The reported 1,3,10,12-tetraazaperylene complex displayed the [Re(CO)₃(α-diimine)Cl] complex with the strongest MLCT red shift ever reported. UV–Vis NIR spectroelectrochemical investigations gave further insights into the nature and stability of the reduced states. The electron-poor ligands explored herein open up a new path for designing metal complexes with strongly red-shifted absorption, thus enabling photocatalysis and photomedical applications with low-energy, tissue-penetrating red light in future. Full article

(This article belongs to the Special Issue Photochemistry and Photophysics of Metal Complexes)

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16 pages, 1915 KiB

Open AccessArticle

Photodynamics of the Molecular Ruby [Cr(ddpd)₂]³⁺

by J. Patrick Zobel, Hanna Radatz and Leticia González

Molecules 2023, 28(4), 1668; https://doi.org/10.3390/molecules28041668 - 09 Feb 2023

Cited by 2 | Viewed by 1803

Abstract

The introduction of strong-field ligands can enable luminescence in first-row transition-metal complexes. In this way, earth-abundant near-infrared emitters can be obtained using early 3d metals. A prime example is the molecular ruby [Cr(ddpd)₂]³⁺ (ddpd = N,N′-dimethyl-N,N′-dipyridin-2-ylpyridine-2,6-diamine) that can achieve high phosphorescence quantum yields at room temperature in aqueous solution. To understand these remarkable properties, here, we simulate its photodynamics in water using trajectory surface hopping on linear vibronic coupling potentials parametrized from multiconfigurational CASSCF/CASPT2 calculations. We find that after excitation to the second absorption band, a relaxation cascade through metal-centered states occurs. After an initial back-and-forth intersystem crossing with higher-lying doublet states, the complex relaxes through a manifold of quartet metal-centered states to the low-lying doublet metal-centered states which are responsible for the experimentally observed emission. These electronic processes are driven by an elongation of the Cr–ligand bond lengths as well as the twisting motion of the trans-coordinated pyridine units in the ddpd ligands. The low-lying doublet states are reached within 1–2 ps and are close in geometry to the doublet minima, thus explaining the high phosphorescence quantum yield of the molecular ruby [Cr(ddpd)₂]³⁺. Full article

(This article belongs to the Special Issue Photochemistry and Photophysics of Metal Complexes)

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12 pages, 3039 KiB

Open AccessArticle

Fluorescence vs. Phosphorescence: Which Scenario Is Preferable in Au(I) Complexes with Benzothiadiazoles?

by Radmir. M. Khisamov, Alexey A. Ryadun, Sergey N. Konchenko and Taisiya S. Sukhikh

Molecules 2022, 27(23), 8162; https://doi.org/10.3390/molecules27238162 - 23 Nov 2022

Cited by 2 | Viewed by 1170

Abstract

The photoluminescence of Au(I) complexes is generally characterized by long radiative lifetimes owing to the large spin-orbital coupling constant of the Au(I) ion. Herein, we report three brightly emissive Au(I) coordination compounds, 1, 2a, and 2b, that reveal unexpectedly short emission lifetimes of 10–20 ns. Polymorphs 2a and 2b exclusively exhibit fluorescence, which is quite rare for Au(I) compounds, while compound 1 reveals fluorescence as the major radiative pathway, and a minor contribution of a microsecond-scale component. The fluorescent behaviour for 1–2 is rationalized by means of quantum chemical (TD)-DFT calculations, which reveal the following: (1) S₀–S₁ and S₀–T₁ transitions mainly exhibit an intraligand nature. (2) The calculated spin-orbital coupling (SOC) between the states is small, which is a consequence of overall small metal contribution to the frontier orbitals. (3) The T₁ state features much lower energy than the S₁ state (by ca. 7000 cm⁻¹), which hinders the SOC between the states. Thus, the S₁ state decays in the form of fluorescence, rather than couples with T₁. In the specific case of complex 1, the potential energy surfaces for the S₁ and T₂ states intersect, while the vibrationally resolved S₁–S₀ and T₂–S₀ calculated radiative transitions show substantial overlap. Thus, the microsecond-scale component for complex 1 can stem from the coupling between the S₁ and T₂ states. Full article

(This article belongs to the Special Issue Photochemistry and Photophysics of Metal Complexes)

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22 pages, 5183 KiB

Open AccessArticle

Triplet Emitting C^N^C Cyclometalated Dibenzo[c,h]Acridine Pt(II) Complexes

by Joshua Friedel, Maren Krause, Rose Jordan, Iván Maisuls, Dana Brünink, Dominik Schwab, Nikos L. Doltsinis, Cristian A. Strassert and Axel Klein

Molecules 2022, 27(22), 8054; https://doi.org/10.3390/molecules27228054 - 19 Nov 2022

Cited by 2 | Viewed by 1497

Abstract

In a series of Pt(II) complexes [Pt(dba)(L)] containing the very rigid, dianionic, bis-cyclometalating, tridentate C^N^C²⁻ heterocyclic ligand dba^2– (H₂dba = dibenzo[c,h]acridine), the coligand (ancillary ligand) L = dmso, PPh₃, CNtBu and Me₂Imd (N,N’-dimethylimidazolydene) was varied in order to improve its luminescence properties. Beginning with the previously reported dmso complex, we synthesized the PPh₃, CNtBu and Me₂Imd derivatives and characterized them by elemental analysis, ¹H (and ³¹P) NMR spectroscopy and MS. Cyclic voltammetry showed partially reversible reduction waves ranging between −1.89 and −2.10 V and increasing along the series Me₂Imd < dmso ≈ PPh₃ < CNtBu. With irreversible oxidation waves ranging between 0.55 (L = Me₂Imd) and 1.00 V (dmso), the electrochemical gaps range between 2.65 and 2.91 eV while increasing along the series Me₂Imd < CNtBu < PPh₃ < dmso. All four complexes show in part vibrationally structured long-wavelength absorption bands peaking at around 530 nm. TD-DFT calculated spectra agree quite well with the experimental spectra, with only a slight redshift. The photoluminescence spectra of all four compounds are very similar. In fluid solution at 298 K, they show broad, only partially structured bands, with maxima at around 590 nm, while in frozen glassy matrices at 77 K, slightly blue-shifted (~580 nm) bands with clear vibronic progressions were found. The photoluminescence quantum yields Φ_L ranged between 0.04 and 0.24, at 298 K, and between 0.80 and 0.90 at 77 K. The lifetimes τ at 298 K ranged between 60 and 14040 ns in Ar-purged solutions and increased from 17 to 43 µs at 77 K. The TD-DFT calculated emission spectra are in excellent agreement with the experimental findings. In terms of high Φ_L and long τ, the dmso and PPh₃ complexes outperform the CNtBu and Me₂Imd derivatives. This is remarkable in view of the higher ligand strength of Me₂Imd, compared with all other coligands, as concluded from the electrochemical data. Full article

(This article belongs to the Special Issue Photochemistry and Photophysics of Metal Complexes)

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16 pages, 2389 KiB

Open AccessArticle

Photophysical Study on the Rigid Pt(II) Complex [Pt(naphen)(Cl)] (Hnaphen = Naphtho[1,2-b][1,10]Phenanthroline and Derivatives

by Maren Krause, Iván Maisuls, Stefan Buss, Cristian A. Strassert, Andreas Winter, Ulrich S. Schubert, Shruthi S. Nair, Benjamin Dietzek-Ivanšić and Axel Klein

Molecules 2022, 27(20), 7022; https://doi.org/10.3390/molecules27207022 - 18 Oct 2022

Cited by 2 | Viewed by 1431

Abstract

The electrochemistry and photophysics of the Pt(II) complexes [Pt(naphen)(X)] (Hnaphen = naphtho[1,2-b][1,10]phenanthroline, X = Cl or C≡CPh) containing the rigid tridentate C^N^N-coordinating pericyclic naphen ligand was studied alongside the complexes of the tetrahydro-derivative [Pt(thnaphen)(X)] (Hthnaphen = 5,6,8,9-tetrahydro-naphtho[1,2-b][1,10]phenanthroline) and the N^C^N-coordinated complex [Pt(bdq)(Cl)] (Hbdq = benzo[1,2-h:5,4-h’]diquinoline. The cyclic voltammetry showed reversible reductions for the C^N^N complexes, with markedly fewer negative potentials (around −1.6 V vs. ferrocene) for the complexes containing the naphen ligand compared with the thnaphen derivatives (around −1.9 V). With irreversible oxidations at around +0.3 V for all of the complexes, the naphen made a difference in the electrochemical gap of about 0.3 eV (1.9 vs. 2.2 eV) compared with thnaphen. The bdq complex was completely different, with an irreversible reduction at around −2 V caused by the N^C^N coordination pattern, which lacked a good electron acceptor such as the phenanthroline unit in the C^N^N ligand naphen. Long-wavelength UV-Vis absorption bands were found around 520 to 530 nm for the C^N^N complexes with the C≡CPh coligand and were red-shifted when compared with the Cl derivatives. The N^C^N-coordinated bdq complex was markedly blue-shifted (493 nm). The steady-state photoluminescence spectra showed poorly structured emission bands peaking at around 630 nm for the two naphen complexes and 570 nm for the thnaphen derivatives. The bdq complex showed a pronounced vibrational structure and an emission maximum at 586 nm. Assuming mixed ³LC/³MLCT excited states, the vibronic progression for the N^C^N bdq complex indicated a higher LC character than assumed for the C^N^N-coordinated naphen and thnaphen complexes. The blue-shift was a result of the different N^C^N vs. C^N^N coordination. The photoluminescence lifetimes and quantum yields Φ_L massively increased from solutions at 298 K (0.06 to 0.24) to glassy frozen matrices at 77 K (0.80 to 0.95). The nanosecond time-resolved study on [Pt(naphen)(Cl)] showed a phosphorescence emission signal originating from the mixed ³LC/³MLCT with an emission lifetime of around 3 µs. Full article

(This article belongs to the Special Issue Photochemistry and Photophysics of Metal Complexes)

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16 pages, 2895 KiB

Open AccessArticle

Noble Metal Complexes of a Bis-Caffeine Containing NHC Ligand

by Oliver Bysewski, Andreas Winter, Phil Liebing and Ulrich S. Schubert

Molecules 2022, 27(13), 4316; https://doi.org/10.3390/molecules27134316 - 05 Jul 2022

Cited by 4 | Viewed by 2078

Abstract

N-Heterocyclic carbenes (NHCs) have seen more and more use over the years. The go-to systems that are usually considered are derivatives of benzimidazole or imidazole. Caffeine possesses an imidazole unit and was already utilized as a carbene-type ligand; however, its use within a tridentate bis-NHC system has—to the best of our knowledge—not been reported so far. The synthesis of the ligand is straightforward and metal complexes are readily available via silver-salt metathesis. A platinum(II) and a palladium(II) complex were isolated and a crystal structure of the former was examined. For the Pt(II) complex, luminescence is observed in solid state as well as in solution. Full article

(This article belongs to the Special Issue Photochemistry and Photophysics of Metal Complexes)

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