1 Photophysics of a Ruthenium Complex with a Ï-Extended

Subscriber access provided by TUFTS UNIV

A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Photophysics of a Ruthenium Complex with a #-Extended Dipyridophenazine Ligand for DNA Quadruplex Labeling Julian Schindler, Philipp Traber, Linda Zedler, Ying Zhang, Jean-Francois Lefebvre, Stephan Kupfer, Stefanie Gräfe, Martine Demeunynck, Murielle Chavarot-Kerlidou, and Benjamin Dietzek J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05274 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Photophysics of a Ruthenium Complex with a π-Extended Dipyridophenazine Ligand for DNA Quadruplex Labeling Julian Schindler,a,b Philipp Traber,a Linda Zedler,b Ying Zhang,a,b Jean-François Lefebvre,c,d Stephan Kupfer,a Stefanie Gräfe,a Martine Demeunynck,c Murielle Chavarot-Kerlidou,d and Benjamin Dietzek*,a,b

a

b c

d

Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany. Phone: +49 3641 206332; Fax: +49 3641 206399; E-mail: [email protected]

Department Functional Interfaces, Leibniz Institute of Photonic Technology Jena (IPHT),

Albert-Einstein-Straße 9, 07745 Jena, Germany.

Univ. Grenoble Alpes, CNRS, DPM, 38000 Grenoble, France.

Laboratoire de Chimie et Biologie des Métaux, Univ. Grenoble Alpes, CNRS, CEA, 38000 Grenoble, France.

ABSTRACT: The light-switch mechanism of the complex [Ru(bpy) 2 (Br-dpqp)](PF 6 ) 2 (1, bpy = 2,2′-bipyridine, Br-dpqp = 12-bromo-14-ethoxy-dipyrido[3,2-a:2′,3′-c]quinolino[3,2-h]phenazine), i.e., a light-up

probe for the selective labelling of G-quadruplexes, is investigated by time-resolved transient

absorption and emission spectroscopy. We show that, in contrast to the prototypical light-switch

complex [Ru(bpy) 2 (dppz)](PF 6 ) 2 (2, dppz = dipyrido-[3,2-a:2′,3′-c]phenazine), a 3ππ* state localized on the π-extended ligand is the state determining the excited-state properties in both

protic and aprotic environments. In aprotic environments, emission originates from a bright 3MLCT phen

state, which is thermally accessible from the 3ππ* state at ambient temperature. In the

presence of water, i.e., in environments resembling in cellulo situations, the thermally accessible

3MLCT state is altered and becomes close in energy to the 3ππ* state which induces a rapid excited-

state deactivation of the 3ππ* state and a comparably weak emission.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

1. INTRODUCTION Guanine-rich single-stranded DNA and RNA sequences are able to fold into stable four-stranded motifs, so-called G-quadruplexes.1,2 There is compelling evidence that these secondary structures

play a pivotal role in a variety of biological processes such as genome integrity and gene expression.1,2 A selective detection and localization of quadruplexes in living cells is required to

assess their biological function in more detail and to understand their involvement in the

development of cancer and other diseases.3,4 Promising tools for the visualization of G-

quadruplexes are environmentally sensitive luminophores, i.e., small molecules whose

luminescence is either switched on or greatly enhanced (light-up luminophores) by their interaction with G-quadruplexes.1,2,5,6

One notable example for the recognition of duplex DNA is the well-known molecular ‘‘light-

switch’’ complex [Ru(bpy) 2 (dppz)](PF 6 ) 2 (2, bpy = 2,2′-bipyridine dppz = dipyrido-[3,2-a:2′,3′-

c]phenazine),7,8 which is almost non-emissive in aqueous solution, but becomes emissive in

aprotic solvents or when intercalated into DNA. The light-switch effect is attributed to two triplet

metal-to-ligand charge transfer (3MLCT) states which are close in energy and both localized on

the dppz ligand: an emissive 3MLCT phen state with excess charge on the phenanthrolin (phen) moiety, and a barely emissive 3MLCT phz state, which is associated with the phenazine (phz)

moiety7,9–11 (the phen and phz moiety are highlighted in Figure 1A). The lack of emission in

aqueous solution is caused by H-bonding of water molecules to the phz nitrogen atoms, which

switches emission from the bright 3MLCT phen state to the “dark” 3MLCT phz state.7,9,11 When the

dppz ligand is intercalated between the DNA base pairs, the phz nitrogen atoms are protected

from water and the emissive 3MLCT phen state becomes dominant.7,8,12 However, a selective

recognition of G-quadruplexes requires preferential binding of the light-switch complex for quadruplexes over duplex DNA. This was recently achieved by decorating 2 with peptide sequences13 or by extending the dppz ligand structure itself.14–16 The increased steric demand of

π-extended dppz ligands is considered to inhibit the intercalation into duplex DNA,16 while the

larger π-system increases the ligand's affinity towards G-quadruplex DNA.14 The

complex

[Ru(bpy) 2 (Br-dpqp)](PF 6 ) 2

(1,

Br-dpqp = 12-bromo-14-ethoxy-

dipyrido[3,2-a:2′,3′-c]quinolino[3,2-h]phenazine, Figure 1A) bearing a π-extended dppz ligand

with high affinity and selectivity for DNA and RNA G-quadruplexes has been published recently.17,18 The complex has been tested for biological applications (using Cl- instead of PF 6 - as

counterion) and was found to be a valuable light-up probe capable of discriminating Gquadruplexes from duplex DNA in vitro and in cellulo.18 The selectivity of 1 for G-quadruplexes is 2


Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in part attributed to the acridine moiety of the π-extended ligand, which allows for favorable π-

stacking interactions between the ligand and the G-quadruplexes.18–20

The photophysics of ruthenium complexes with π-extended dppz ligands is diverse and often found to be drastically altered compared to the parent dppz complex.7,21–27 Intraligand (IL) excited

states drop substantially in energy by π-extension and, depending on the specific ligand

architecture, may become lower in energy than the 3MLCT states.25 As a consequence, low-lying 3IL

states (often 3ππ* states) have been observed for ruthenium complexes with π-extended dppz

ligands,23–25 which have significantly longer lifetimes than the charge-separated states usually

found for ruthenium polypyridine complexes. If the energy difference between the lowest lying

3IL

and 3MLCT states is within a few kcal mol−1, one state is thermally accessible from the other

(at ambient temperature) and energy transfer processes between the two triplet states can occur.24–26,28,29 The H-donor capacity of the solvent and the solvent's dipole have a major impact

on the relative energetic position of the excited states.7 As a consequence, a change in solvent

environment can result in the population of another excited sate as well in a drastically altered

excited-state lifetime and luminescence quantum yield.25,26,30,31 The considerable change of

luminescence lifetime upon interaction of the complexes with nucleic acid forms the basis for their use in fluorescence lifetime imaging microscopy for in vitro investigation of G-quadruplexes.6 In a

wider scope, the photophysics of Ru(II) complexes with π-extended ligands is of interest for a

potential application of these compounds as DNA intercalating drugs for, e.g., photodynamic

therapy.32 Specifically, a longer excited-state lifetime promotes 1O 2 sensitization and the excited-state lifetime positively correlates with the cytotoxicity determined in cell culture

experiments.32,33 Additionally, the excited-state character, such as 3ππ* or 3MLCT, might

determine the specific interaction of the photoactive complexes with their biological target environment:23,34 The 3MLCT excited state of some DNA-intercalated dyes enables for oxidation of guanine,23 while a prolonged lifetime, e.g., induced by an equilibrium between the 3MLCT state

with a 3ππ* state, quite generally causes high 1O 2 sensitization efficiency and thus higher

cytotoxicity. To establish profound structure-dynamics-function correlations, an understanding of the excited-state processes in Ru(II) complexes with π-extended ligands is essential.

In this study, we investigate the photophysics of 1 by time-resolved transient absorption and emission spectroscopy. Our goal is to understand how the π-extension of the dppz ligand changes

the photophysics of 1 in comparison to the parent complex 2. The investigations were carried out in acetonitrile (ACN) and in a 1:1 mixture of ACN/water. While ACN models an aprotic

environment, the ACN/water mixture gives insights on the impact of the protic solvent

environment found in cellulo. The time-resolved investigations were complemented by UV/Vis3



spectroelectrochemistry, resonance Raman spectroscopy and (time-dependent) density

functional theory calculations. 2. METHODS

The complex [Ru(bpy) 2 (Br-dpqp)](PF 6 ) 2 (1) was prepared as previously described.17 The dipyrido[3,2-a:2’,3’-c]phenazine (dppz) ligand and the [Ru(bpy) 2 (dppz)](PF 6 ) 2 complex (2) was

synthesized according to a reported procedure.35 [Ru(bpy) 3 ](PF 6 ) 2 was obtained from Sigma-

Aldrich (97%). Unless noted, measurements were performed at room temperature in air-

equilibrated spectroscopic-grade solvents. A mixture of ACN and water (50 vol% water) was

prepared using distilled water.

The ground-state absorption spectra were recorded using a Jasco V-670 spectrophotometer. The

steady-state emission spectra were obtained in quartz cuvettes with 1 cm path length (optical

density 0.05 at 450 nm, air-equilibrated solutions) using a Fluorolog spectrofluorimeter from

Horiba. Quantum yields Ф were calculated using [Ru(bpy) 2 (dppz)](PF 6 ) 2 in ACN as reference (airequilibrated solutions, Ф R = 0.018 ± 0.002)36 according to a literature procedure. All emission spectra were corrected for the solvent background and for the spectral sensitivity of the set-up relying on the data supplied by the manufacturer.

Resonance Raman (RR) measurements were performed in a quartz cuvette with 1 mm path length

using a previously described setup.26 The RR spectra of 1 and 2 in ACN (sample concentration 0.1

mM) were obtained by exciting the sample at either 405 nm (TopMode-405-HP diode laser,

Toptica, Germany) or 473 nm (diode pumped solid state laser, HB-Laser, Germany). The obtained RR spectra were smooth with a Savitzky Golay filter and baseline corrected.

The electrochemical analysis was performed using a BioLogic SP300 potentiostat controlled via the EC-Lab® V10 software. Cyclic and differential pulse voltammetry experiments were recorded in a classical single-compartment three-electrode cell combining a glassy carbon or a platinum

working electrode, a platinum wire counter-electrode and a custom-made Ag/AgCl reference

electrode (separated from the solution by a Vycor frit). Typical measurements were carried out at room temperature using 3 mL of argon-purged dimethylformamide (DMF) solution (0.1 M n-

Bu 4 NPF 6 as supporting electrolyte) of 0.5 to 1 mM of the complex (see Figures S2-S4 for details).

DMF was preferred to ACN as well-resolved cyclic voltammograms could be recorded using this

solvent. No significant variation of the redox potential of the first reduction process was observed for complex 1, using DMF instead of ACN. Measurements were corrected for the ohmic drop. 4


Page 4 of 27

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ferrocene was finally added at the end of each measurement as an internal reference, allowing every measured value to be referenced versus the Fc+/Fc redox couple.

The UV/Vis-spectroelectrochemistry (UV/Vis-SEC) measurements of 1 were conducted in argon-

purged acetonitrile (distilled from calcium hydride under an argon atmosphere) containing 0.1 M tetrabutylammonium tetrafluoroborate (Sigma-Aldrich, ≥99.0%) as supporting electrolyte. The

concentration of 1 in ACN was 0.1 mM. The measurements were carried out in a thin-layer SEC

cell with a path length of 1 mm (Bioanalytical Systems, USA). The cell was equipped with a threeelectrode setup consisting of a platinum mesh working electrode, a platinum counter electrode

and an Ag/AgCl pseudo-reference electrode. Cyclic voltammetry and chronoamperometry

experiments were performed using a PC-controlled SP-150 potentiostat (BioLogic, France). Singly oxidized and singly reduced 1 was generated by controlled potential electrolysis while UV/Vis-

spectra were collected simultaneously in transmission mode using a double-beam Cary 5000 UV/Vis spectrometer (Varian, USA).

The femtosecond transient absorption (fs-TA) measurements of 1 in ACN were carried out in a

quartz cuvette with 1 mm path length (optical density 0.3 at 400 nm). The TA setup has been

described elsewhere.37 The 400-nm pump pulse was obtained through the second harmonic

generation of the laser fundamental using a BBO crystal. A white-light supercontinuum, generated

in a CaF 2 plate, was employed as probe light. For kinetic analysis, the obtained data was chirpcorrected and analyzed by a global fit using a sum of exponential functions (ΔA(𝑡𝑡, 𝜆𝜆) =

38 ∑𝑖𝑖=𝑛𝑛 𝑖𝑖=1 A𝑖𝑖 (𝜆𝜆) exp(− 𝑡𝑡⁄τ𝑖𝑖 )). A temporal window around time-zero (±300 fs), in which contributions

of the coherent artefact are expected,39 was excluded in the data fitting. The wavelength-

dependent pre-exponential factors A𝑖𝑖 (𝜆𝜆) correspond to the decay associated spectr (DAS)

correlated to the kinetic components.

The nanosecond time-resolved transient absorption (ns-TA) and emission (ns-Em) measurements were performed using a previously reported system from Pascher Instrument AB.40 Each sample was kept either in a regular quartz cuvette with 1 cm path length or for measurements of deaerated samples in a sealed quartz cell with 1 cm path length (optical density

0.5 at 450 nm). The deaerated sample was prepared by the freeze − pump − thaw method. The sample was excited by 10 ps pump pulses centered at 450 nm. The probe light was provided by a

75 W xenon arc lamp. TA kinetics were detected as single-wavelength kinetic between 300 and

860 nm in steps of 20 nm. Emission kinetics were recorded using the ns-TA system without probe light. To correct for the contribution of emission to the TA signal, the TA and emission kinetics 5



were detected one after another and the emission signal was subtracted from the TA signal. The ns-TA and ns-Em data were analyzed by a global fitting routine using a single exponential function. All calculations were performed using Gaussian 09 Rev. A.02.41 Ground-state properties, as are the equilibrium geometries, vibrational frequencies and normal coordinates, were calculated for the

non-reduced singlet and triplet states of 1 as well as for the singly oxidized and singly reduced 1

within doublet multiplicity by means of density functional theory (DFT) in combination with the

XC functional B3LYP.42,43 The 6-31G(d) double-ζ basis set was used for all main group elements.44 The 28-electron relativistic core potential MWB was applied with the corresponding basis set for the ruthenium atom.45 Excited-state properties such as vertical excitation energies, oscillator

strengths, and excited-state characters were obtained for the non-reduced singlet and triplet

states of 1 in their respective equilibrium geometry from time-dependent DFT (TDDFT)

calculations within the adiabatic approximation with the same XC functional, pseudopotential and

basis set. Absorption spectra were simulated by calculating the lowest 100 excited states of the respective ground-state multiplicity. Solvent effects (ACN) were treated by means of the integral equation formalism of the polarizable continuum model.46 3. RESULTS AND DISCUSSION 3.1. Characterization of the Franck-Condon Region The electronic transitions originating from the electronic ground state within the Franck-Condon region were characterized by steady-state absorption and resonance Raman spectroscopy in

combination with TDDFT simulations, see Figure 1 and 2. In the middle- to near-ultraviolet region, complex 1 features strong absorption peaks at 285 and 344 nm (Figure 1B), which are

characteristic for IL transitions centered on the bipyridine ligands47 and the π-extended ligand

(S 23 and S 26 , Figure 1D), respectively. In the visible region, 1 shows a significantly higher molar absorption coefficient compared to the parent complex 2 (1: ɛ = 28,000 M-1 cm-1 at 431 nm,17

2: ɛ = 15,400 M-1 cm-1 at 448 nm). Nonetheless, both complexes show a similar MLCT transition, which shifts electron density mainly to the bpy ligands (S 12 , Figure 1D). However, contrary to 2,

for which the MLCT transitions to the phz moiety are reported to have almost no oscillator

strength,48 TDDFT predicts MLCT transitions for 1 which involve both the phen and phz sphere (S 14 ) as well as the entire extended ligand (S 16 ). Thus, π-extension of the ligand changes the

available MLCT transitions as compared to the dppz-complex. Additionally, the π-extension leads

to an intense transition associated with S 4 : This transition shifts charge density from the quinoline

(qn) moiety (highlighted in Figure 1A) and the metal center towards the phz moiety and can be

described as an intraligand charge transfer (ILCT) transition from the qn moiety to the phz moiety with some contribution of an MLCT transition.

6


Page 6 of 27

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1: Structures (A) and absorption spectra of 1 and 2 in ACN (B). The vertical transitions of 1 are calculated by TDDFT and their respective oscillator strength is presented by red bars (C) and broadened with Gaussian functions (full-width at half-maximum of 0.28 eV). Selected transitions are visualized by the accompanying shift of charge density from blue to red (D).

RR spectroscopy was applied to experimentally validate the nature of the electronic transitions of 1 upon excitation at 405 and 473 nm (Figure 2). In RR spectroscopy, the excitation wavelength

coincides with an electronic transition of the sample, which increases the Raman scattering

intensity of the vibrational modes associated with the electronic transitions by several orders of

magnitude.49,50 Thus, RR spectroscopy allows for drawing conclusions regarding the character of the electronic transitions: The RR spectrum of 1 obtained upon excitation at 405 and 473 nm

(Figure 2) reveals bands which are characteristic for vibrations of the bpy ligand (indicated by α)

and the dppz moiety of the extended ligand (indicated by β). These peaks are assigned by comparing the RR spectra of 1 with the spectrum of 2, [Ru(dppz) 3 ]2+ and [Ru(bpy) 3 ]2+ (Figure

S1).51 Additional peaks (indicated by #) are observed in the RR spectra of 1 which are not present in the spectra of 2 and [Ru(bpy) 3 ](PF 6 ) 2 . These peaks are associated with the electronic

transitions involving the qn moiety of the π-extended ligand. The RR spectrum of 1 obtained upon

405-nm excitation (red line, Figure 2A) is similar to the one of the reference complex 2 (black line)

and dominated by peaks characteristic for bpy-centered vibrations at 1173, 1318, 1562 and 1604

cm-1. These peaks originate from MLCT transitions to the bpy ligands (state S 12 , Figure 1D) and 7



can be found in the reference spectrum obtained for [Ru(bpy) 3 ](PF 6 ) 2 ) (Figure S1). The peaks

characteristic for vibrations of the dppz moiety (at 1309, 1571 and 1597 cm-1) and of the qn

moiety (at 1173, 1315 and 1459 cm-1) are absent or weak compared to the bpy-centered peaks.

This indicates that both the MLCT transitions towards the dppz moiety (state S 14 and S 16 ) and the

ILCT transitions involving the π-extended ligand (state S 4 ) are less prevalent upon 405-nm excitation than the MLCT transitions towards the ancillary bpy ligands. Contrary to excitation at

405 nm, the RR spectrum obtained upon 473-nm excitation (red line, Figure 2B) is dominated by

peaks associated with the dppz and qn moiety of the π-extended ligand and strongly deviates from

the RR spectrum of the reference complex 2 (black line, Figure 2B). The intense qn peaks observed

at 1173, 1315 and 1459 cm-1 clearly indicate the population of the strongly absorbing ILCT state

S 4 upon 473-nm excitation. Thus, the combination of RR measurements and TDDFT calculations

reveals that 405-nm excitation mainly populates MLCT states on the bpy ligands (state S 12 ,

Figure 1D), while excitation at 473 nm gives rise to ILCT transitions centered on the π-extended

ligand (state S 4 ).

Figure 2: Resonance Raman (RR) spectra of 1 (red) and 2 (black) in ACN upon excitation at 405 (A) and 473 nm (B): Peaks characteristic for vibrations of the bpy ligand (α) as well as of dppz (β) and qn (#) moiety of the π-extended ligand are marked. Solvent peaks are indicated by asterisks. RR spectra are normalized to the solvent peak at 920 cm-1.

3.2. Electrochemistry

8


Page 8 of 27

Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrochemical measurements were performed in order to identify the orbitals involved in the first oxidation and reduction process. These orbitals are to a first approximation also the orbitals

which are involved in the lowest MLCT state,47 i.e., the excited state which is often found to be an

integral part in the photophysics and photochemistry of ruthenium polypyridine complexes. The

redox potentials of 1 were determined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (Table 1, for CV and DPV scans, see Figures S2–S4). The cathodic scan of the CV shows three quasi-reversible one-electron reductions at –1.37, –1.79 and –2.00 V vs. Fc+/Fc

and an irreversible one at –2.23 V vs. Fc+/Fc (Figure S2). The π-extension of the ligand leaves the

first one-electron reduction (–1.37 V vs. Fc+/Fc) basically unchanged compared to the parent

complex 2 (–1.36 V vs. Fc+/Fc)52. In particular, no electronic repulsion between the two nitrogen

atom lone pairs located on the concave part of the bent ligand could be electrochemically detected here, by contrast with a previously reported study on a related bent polyazaaromatic system.53

The first reduction of 1 is – in analogy to 254 –localized on the phz moiety of the π-extended ligand,

which is supported by DFT calculations. The calculated spin density in singly reduced 1 is mainly

localized on the phz moiety of the π-extended ligand (inset of Figure 3A), which is in line with a phz-centered reduction. By comparison with 2 and [Ru(bpy) 3 ](PF 6 ) 2 (Table 1), the second and

third reductions (–1.79 and –2.00 V vs. Fc+/Fc) are assigned to the sequential one-electron

reductions of the bpy co-ligands.54,55 The fourth (irreversible) reduction (–2.23 V vs. Fc+/Fc) is

significantly more positive than the potential necessary to add a second electron to a formerly

reduced bpy ligand (–2.78 V vs. Fc+/Fc for [Ru(bpy) 3 ]2+)55 and is thus centered on the formerly reduced π-extended ligand of 1.

The first one-electron oxidation of 1 (+0.85 V vs. Fc+/Fc, for CV see Figure S3) is assigned to a

metal-centered Ru(III)/Ru(II) oxidation. This is in line with the electrochemical data on the parent

complex 2 (Table 1) and on related ruthenium tris-diimine complexes.54 The calculated spin density in singly oxidized 1 (inset of Figure 3A) resides essentially at the ruthenium center, as

expected for a Ru(III)/Ru(II) based oxidation. However, due to spin polarization,56 the unpaired d electron of the Ru(III) ion places some spin density of opposite sign at the ligand donor atoms.

Thus, the π-extension of the ligand does not significantly affect the electrochemical properties of 1. In analogy to the parent complex 2, the first oxidation is metal-centered, while the first reduction is phz-centered. Consequently, the lowest-lying MLCT state of 1 in a relaxed excited

state geometry is expected to be the MLCT state with excess charge localized on the phz moiety of the π-extended ligand (MLCT phz ).

9



Table 1: Electrochemical potentials of 1 and the reference complexes 2 and [Ru(bpy) 3 ](PF 6 ) 2 (E 1/2 in V vs. Fc+/Fc).a oxidation reduction 1 0.85 –1.37 –1.79 –2.00 –2.23b 2c 0.84 –1.36 –1.79 –2.01 –2.49 c [Ru(bpy) 3 ](PF 6 ) 2 0.82 – –1.73 –1.91 ‒2.18 a determined by DPV measurements, in degassed dimethylformamide containing 0.1 M tetrabutylammonium hexafluorophosphate b irreversible c taken from reference 52

3.3. UV/Vis-Spectroelectrochemistry UV/Vis-SEC is utilized to identify spectral features associated with the one-electron reduction and oxidation as a basis for the interpretation of the TA data (vide infra).57,58 The absorption spectrum

of singly reduced 1 (Figure 3A) is characterized by a broad absorption feature above 500 nm,

which is similar to singly reduced 254 and considered to originate from intraligand π*→π* transitions of the reduced phenazine ligand.54 Additionally, the absorption peak of 1 at 344 nm –

associated with ILCT transitions from the qn moiety to the phz and phen moiety (S 23 and S 26 ,

Figure 1D) – (almost) vanishes upon the first reduction. These transitions are inhibited due to the

charge on the phz moiety added upon the first reduction. The absorption spectrum obtained upon Ru(II)/Ru(III) oxidation of 1 shows three distinct changes: (i) the absorption peak of 1 at 344 nm

slightly shifts bathochromically to 346 nm and its absorbance decreases, (ii) a new absorption

peak appears at 360 nm and (iii) the MLCT band centered at ca. 450 nm decreases. The difference absorption spectra ΔA presented in Figure 3B highlights the spectral changes upon reduction or

oxidation with respect to the parent species. Based on these UV/Vis-SEC results, the absorption of the lowest charge-separated state in 1, i.e., the MLCT phz state, is simulated by summing the ΔA

spectra for singly reduced and singly oxidized 1. The so-obtained ΔA(MLCT phz ) spectrum (dashed line, Figure 3B) simulates the spectral changes expected upon formation of the MLCT phz with respect to the ground state.

10


Page 10 of 27

Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3: UV/Vis-SEC of 1 (0.1 mM) in 0.1 M n-Bu 4 NPF 6 /ACN. Absorption spectrum of 1 (A, blue line), singly reduced 1 (A, red line; electrolysis: 0.5 min, -1.2 V) and singly oxidized 1 (A, green line; electrolysis: 4 min, +1.55 V) (for details see Figure S5, potentials vs. Ag/AgCl pseudo-reference electrode). Difference absorption spectra ΔA describe the spectral change upon one-electron reduction (B, red line) and oxidation (B, green line). The ΔA(MLCT phz ) spectrum (B, dashed line), ΔA(MLCT phz ) = ΔA(singly oxidized) + ΔA(singly reduced), simulates the spectral change upon formation of the MLCT phz state with respect to the ground state of 1. The inset in A shows the DFT calculated isodensity surface of the spin density (SD) distribution for singly oxidized and singly reduced 1.

3.4. Steady-State Emission The use of 1 as a spectroscopic probe for DNA quadruplexes is based on its environment-sensitive

luminescence.18 The effect of the solvent on the steady-state emission of 1 is investigated in the

aprotic solvents ACN (relative permittivity ε r = 35.9)59 and propylene carbonate (PC, ε r = 62.9)59

as well as in the protic solvent mixture 1:1 mixture of ACN/water (ε r = 47.1),60 the results are

summarized in Table 2. The broad and structureless emission spectrum in ACN (red line, Figure

4) and PC (green line) resembles the steady-state emission spectrum of 2 in ACN (black dotted line). From this, it is concluded that the emission originates from conceptually the same state as

in the case of 2, i.e., from the 3MLCT phen state with excess charge on the phen moiety.9,11 Increasing the solvent polarity shifts the emission maximum only slightly from 628 nm in ACN (ε r = 35.9)59

to 636 nm in PC (ε r = 62.9)59, which coincides with the relatively solvent-insensitive 3MLCT phen

emission of 2.61 In contrast to the effects of aprotic solvents, the emission maximum shifts

considerably in presence of water molecules from 628 nm in ACN to approximately 700 nm in the

1:1 mixture of ACN/water (ε r = 47.1).60 Additionally, the luminescence quantum yield decreases

further in ACN/water (

1 Photophysics of a Ruthenium Complex with a Ï-Extended

Recommend Documents

1 Photophysics of a Ruthenium Complex with a Ï-Extended