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A: Spectroscopy, Photochemistry, and Excited States
Elucidating the Solution-Phase Structure and Behavior of 8-Hydroxyquinoline Zinc in DMSO Kyle A. Grice, Graham Bailey Griffin, Phoebus Sun Cao, Cesar Saucedo, Aeshah H Niyazi, Fatimah Aldakheel, George E Sterbinsky, and Robert J LeSuer J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b12632 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018
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The Journal of Physical Chemistry
Elucidating the Solution-Phase Structure and Behavior of 8-Hydroxyquinoline Zinc in DMSO Authors: Kyle A. Grice*a, Graham B. Griffin*a, Phoebus Sun Caoa, Cesar Saucedoa, Aeshah H. Niyazia, Fatimah Alkadheela, George E. Sterbinskyb, Robert J. LeSuerc a
Department of Chemistry and Biochemistry, DePaul University, 1110 West Belden Ave, Chicago, IL 60614, USA b Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA c Department of Chemistry and Biochemistry, The College at Brockport, State University of New York, Brockport, NY, 14420 USA * email for corresponding authors:
[email protected],
[email protected] Abstract The solution-phase structure and electronic relaxation dynamics of zinc bis-8-hydroxyquinoline [Zn(8HQ)2] in dimethylsulfoxide (DMSO) were examined using a broad array of spectroscopic techniques, complimented by ab initio calculations of molecular structure. The ground-state structure was determined using Extended X-Ray Absorption Fine Structure (EXAFS) data collected on the Zn K-edge and DOSY NMR spectroscopy. The complex was found to be monomeric and octahedral, with two bidentate 8-hydroxyquinolate ligands and two DMSO molecules coordinated to the zinc through oxygen atoms. Electronic relaxation dynamics were examined with ultrafast transient absorption spectroscopy and complementary density functional calculations. Electronic relaxation was observed to proceed through both singlet and triplet pathways. This solution-phase data provides a deeper physical understanding of the behavior of this molecule, which has a variety of uses such as sensing, OLEDs, and biological applications.
1.1 Introduction The study of photochemically active systems has been a growing area in recent years, with specific focuses on new compounds for organic light-emitting diodes (OLEDs),1-3 photocatalysts for valuable organic transformations,4-6 and light-absorbing species for solar cells.7-9 In all of these areas, photoactive compounds derived from inexpensive, abundant materials are highly desirable because they can be easily scaled up for larger applications. Many of the canonical, well-explored photo-active species are based on ruthenium and iridium with bipyridine, phenylpyridine, or related ligands.10-11 Related compounds based on the 8-hydroxquinoline scaffold are known for their biological importance12-14, their optical properties,15 and their metallosupramolecular chemistry.16 The bis(8-quinolinolato)zinc complex, Zn(8HQ)2, exhibits photoluminescence in both solution and solid state and can be used in OLEDs in place of the commonly used tris-(8-hydroxyquinolinato)aluminum, Al(8HQ)3 (figure 1).17 In the solid phase, Zn(8HQ)2 can exist in oligomers, clusters, and nanoparticles,17-20 which have been used for vapor detection and light-emitting devices. Given the relative abundance of zinc and its benign environmental impact as compared to metals such as Ru, Ir, and even Al, Zn(8HQ)2 may also be a promising platform for photochemical applications or other large-scale applications.
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Figure 1. The 8-hydroxyquinoline anion, Zn(8HQ)2, and Al(8HQ)3. Unlike the octahedral Al(8HQ)3, which is most stable in the mer geometry,21 the structure of Zn(8HQ)2 is somewhat ambiguous. The X-ray crystal structure of solid Zn(8HQ)2 has been reported as either tetrameric,17-18 with bridging oxygen atoms on the 8HQ ligands, or as an octahedral centrosymmetric complex with trans-ligated water molecules.22-23 The solution phase structure has not yet been determined. One published report shows that both tetrameric and monomeric species can be observed in solution using 1H NMR spectroscopy.24 Additionally, in the presence of suitable donor ligands, Zn(8HQ)2 is known to bind 1 or 2 ligands in the solid state, either in cis or trans configurations (Figure 2).25-26
Figure 2. Known structures based on Zn(8HQ)2 and related molecules, as confirmed by crystal structure analysis, showing a variety of possible geometries. The structures are shown as balland-stick models with hydrogens omitted for clarity. Red = O, blue = N, gray = C, and purple = Zn. L = dipyrido[3,2-a:2',3'-c]phenazine. Despite the many studies of Zn(8HQ)2 and related compounds mentioned above, the photochemistry and relaxation dynamics of these compounds are not fully understood, particularly in solution. Here we apply a variety of approaches to elucidate the structure of Zn(8HQ)2 in both ground and excited states, in DMSO solution. We probe ground state molecular structure with X-ray absorption spectroscopy (XAS) and Diffusion Ordered Spectroscopy (DOSY) NMR techniques. We examine the structure of the low-lying electronic excited states using density functional theory, and measure relaxation dynamics from these excited states via transient absorption (TA) spectroscopy. We find that Zn(8HQ)2 in DMSO is exclusively trans-Zn(8HQ)2(DMSO)2, and that in DMSO solution photo-excitation results in the formation of a substantial yield of relatively short-lived triplet excited states. 2. Experimental 2 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
2.1 General Methods Unless otherwise noted, all chemicals were purchased from commercial sources and used as supplied. Ferrocene was sublimed prior to use. Zn(8HQ)(Tp) (Tp = hydrotris(3-phenyl-5methylpyrazol-1-yl)) and Zn(8HQ)2 were synthesized according to literature methods and characterized by 1H NMR spectroscopy and UV-Vis spectroscopy, and the spectra matched literature values.27 NMR spectra were obtained on a Bruker 300 MHz Avance Spectrometer at room temperature and referenced versus the residual solvent peak. UV-Visible spectra were obtained on a Varian Cary 100 UV-Vis spectrometer and Fluorescence spectra were obtained on a Cary Eclipse Fluorescence Spectrophotometer. 2.2 Transient Absorption Spectroscopy Samples for transient absorption spectroscopy were prepared by dissolving dry powder Zn(8HQ)2 in DMSO and diluting to an appropriate concentration. All samples were prepared such that they had an absorbance value of ~0.2 through the 2 mm optical cuvettes used, a concentration of ~75 𝜇g/mL. The samples were stirred with a magnetic stir bar during the experiment. Samples were used as prepared, without degassing or freeze-pump-thaw cycling to remove O2. A HELIOS femtosecond transient absorption instrument from Ultrafast Systems was used to generate transient absorption spectra. Ultrafast laser pulses were generated using a Tsunami oscillator and a Spitfire Pro amplifier, both from Spectra-Physics. A TOPAS-C optical parametric amplifier, also from Spectra-Physics, was used to generate pump laser pulses at 410 nm. Pump pulse energy was attenuated with a variable neutral density filter to produce 300 𝜇W pump pulses, with a repetition rate of 2.5 kHz. Probe pulses spanning the 450 nm to 750 nm wavelength range were generated by focusing the 800 nm ultrafast laser system output in a sapphire plate to produce a broadband continuum. A time delay was introduced between the pump and probe pulses via a computer controlled mechanical delay stage, and absorption spectra were recorded as a function of the time delay between laser pulses over a 3 ns time span. The instrument time resolution was ~250 fs. The reported time constants are the result of multiexponential fitting with the Origin 9.1 software package. The reported uncertainties are standard errors for the fit parameters. Photodamage to the sample during transient absorbance experiments was estimated using absorbance spectroscopy to reduce the sample concentration by
