Photoinduced Energy and Electron-Transfer Reactions by

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Photo-Induced Energy and Electron-Transfer Reactions by Polypyridine Ruthenium (II) Complexes Containing a Derivatized Perylene Diimide Edjane Rocha dos Santos, Joao Pina, Tiago Venâncio, Carlos Serpa, José M. Gaspar Martinho, and Rose M. Carlos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06693 • Publication Date (Web): 25 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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The Journal of Physical Chemistry

Photo-Induced

Energy

and

Electron-Transfer

Reactions

by

Polypyridine Ruthenium (II) Complexes Containing a Derivatized Perylene Diimide Edjane R. dos Santosa, João Pinab, Tiago Venâncioa, Carlos Serpab, José M. G. Martinho*,c Rose Maria Carlos*,a

a

Departamento de Química, Universidade Federal de São Carlos, CP 676, 13565-905,

São Carlos, SP, Brasil. b

CQC, Universidade de Coimbra, Departamento de Química, 3004-535, Coimbra,

Portugal c

Centro de Química-Física Molecular e IN- Instituto de Nanociência e Nanotecnologia,

Instituto Superior Técnico, Universidade de Lisboa, 1049-001, Lisboa, Portugal.

* To whom correspondence should be addressed: Rose Maria Carlos ([email protected]) Phone: +55 16 3351-8780; Fax: +55 16 3351-8350 José Manuel Gaspar Martinho ([email protected]) Phone: + 351 2184192500.

1

ACS Paragon Plus Environment


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ABSTRACT The [Ru(II)(phen)2(pPDIp)]2+ complex, where pPDIp is the symmetric bridging ligand phenanthroline-perylene-phenanthroline shows strong electronic absorption bands attributed to the pPDIp and {Ru(phen)2}2+ moieties in acetonitrile. The chargeseparated intermediate {Ru(III)(phen)2(pPDIp-•)} was detected by transient absorption spectroscopy upon electronic excitation either in the pPDIp or the complex moieties. The charge-separated intermediate species decays to generate the triplet state 3*pPDIpRu(II) (τP = 1.8 µs) that sensitizes the formation of singlet molecular oxygen with quantum yield φ∆=0.57. The dyad in deaerated acetonitrile solutions is reduced by triethylamine (NEt3) to the [Ru(II)(phen)2(pPDIp•-)] radical anion in the dark. The electron-transfer reaction is accelerated by light absorption. By photolysis of the radical anion, a second electron transfer reaction occurs to generate the [Ru(II)(phen)2(pPDIp2)] dianion. The changes of the color of solution indicate the redox states of complexes and offer a sensitive reporter of each stage of redox reaction from start to finish. The reduced complexes can be converted to the initial complex, using methyl viologen or molecular oxygen as electron-acceptor. The accumulation of electrons in two wellseparated steps open promising opportunities such as in catalysis.

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INTRODUCTION Perylene diimides (PDIs) are fluorophores with exceptional thermal and photochemical stability, strong absorption in the visible and high fluorescence quantum yields.1-4 Due to these properties, PDIs have been used in diverse applications such as light emitting diodes,5,6 field-effect transistors,7,8 sensing,9,10 and photovoltaic cells.11,12 However, dyads of PDIs with other moieties, namely transition metal complexes, enlarge the processes of electronic excitation and decay of PDI, broadening the scope of their potential applications.13-38 The long-lived triplet state of PDI can be populated via strong spin-orbit coupling induced by heavy metals, as observed in Pt(II) square planar complexes with PDI covalently linked to the metal center by an acetylide bond.14-19 However, in palladium complexes where two Pd centers are attached to PDI by metalcarbon σ bonds, the spin-orbit coupling is very small and PDI is highly fluorescent.20 PDI is a good electron-acceptor and therefore charge-transfer interactions are expected and indeed observed in dyads comprising a PDI linked by the N-imide position to a porphyrin, and by a diphenylethyne linker to a Ru-porphyrin,21,22 Ruphthalocyanine,22,23 and Ru-polypyridine24,25 complexes. However, depending on the complex, other competitive deactivation pathways of the electronic excited states, behind electron transfer, can occur. The binuclear Ru-tetraphenylporphyrin (TPP), DPyPBI[Ru(TPP)(CO)]2, complex shows a rich photophysics dependent on the excitation wavelenght. Upon electronic excitation of the DPyPBI moiety a fast photoinduced electron transfer (ET) occurs from the Ru-TPP to the perylene derivative (DPyPBI) with charge separation (CS) and charge recombination (CR) lifetimes of τCS= 5.6 ps, and τCR= 270 ps, respectively. Contrarily, upon excitation of the Ru-porphyrin moiety, an efficient triplet-triplet energy transfer process occurs with the population of 3



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the DPyPBI triplet state that decays to the ground-state with lifetime τP = 9.8 µs.22 On the other hand, upon photoexcitation of either chromophore of the homologue phthalocyanine (Pc) ruthenium complex, BPyPDI[Ru(CO)Pc]2, the [Ru(CO)Pc•+BPyPDI•--RuCOPc] radical ion pair with lifetime of 115 ns (∆GET(Ru(CO)Pc) = - 0.64 eV, and ∆GET(BPyPDI) = - 0.88 eV) was generated.24 Similarly, upon upon photoexcitation of either chromophore of the tetranuclear [PDIpy4{Ru(CO)Pc}4] complex with the PDI located in the center of the 4{Ru(CO)Pc} moieties, generates a short-lived radical ion pair (τ= 260 ps) that deactivates by charge recombination to yield the

3*

{RuCOPc} triplet state.23 Nevertheless, the electron-transfer quenching is

negligible for complexes of PDI-pyridine/terpyridine ligands with several metals such as Ru,26,27 Pt,13 Ir,28,29 Fe,30 Cu,31 and Zn32-37 is negligible and so they are highly fluorescent. Recently,

attention

has

been

focused

in

Ru(II)-polypyridine

complexes mostly because they give rise to 100% population of the emissive, longlived, redox-active triplet state, 3*MLCT, Ru, dπ → π*,α-diimine.39 The {Ru(PDI2-bpy)(tBuCN)2(Cl)2} complex containing two pendant PDIdicarboxydiimide chromophores conjugated to bipyridyl (bpy) produces interesting results. Upon light excitation on the PDI moiety, a fast electron transfer from the Ru(II) moiety to 1*PDI generates the radical anion Ru(III)PDI•- with charge separation (∆GCS = - 1.0 eV) and charge recombination (∆GCR = - 1.2 eV). The charge recombination reaction resulted in the long-lived

3*

PDI (τP= 39 µs) that competes with the PDI

fluorescence (τF = 4.55 ns).26 Subsequently, the incorporation in Ru(bpy) complexes of PDI derivatives, functionalized in the bay positions (1,6,7,12-positions of PDI) with different electron donor or acceptor show that the 3*PDI triplet is reached (τP= 63-57 µs) 4


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regardless of the electronic excited moiety, but the deactivation pathways were dependent on the properties of the PDI derivative.25 The bipyridine complexes of Ru(II) containing an azabenz-annulated perylene bisimide, [Ru(bpy)2(abBpy)]2+, also triggers the population of the

3*

PDI (φP=0.11) that

decays with a lifetime τP=4.2 µs. 38 These studies exemplify how the choice of the ligands around the Ru(II) coordination sphere can be used to modulate the the photophysial behavior of the complexes. These findings encouraged us to prepare the dyad cis-[(phen)2Ru(pPDIp)]2+, where pPDIp is a perylene pendant group functionalized with one of the coordinated phenanthroline ligands, Scheme 1. The dyad was designed to improve the electronic coupling between the PDI and the Ru(II)-polipyridine moiety so that the 3*MLCT is able to tune the population of the 3*

PDI triplet excited states, by a fast electron injection in the forward direction to give

the charge-separated excited state, {(phen)2Ru3+(pPDIp•-)}, that decays to populate the triplet state of PDI. Our strategy is to keep the tris-chelate structure around the Ru(II) metal center using the complex [Ru(phen)3]2+, to guarantee a strong absorption to the 1*MLCT state that rapidly decays to populate the

3*

MLCT. The rigid structure of the {Ru(phen)3}2+

with a strong ligand field coordination will provide the stability of the complex that hampers its photodissociation. We expected that optical excitation on the MLCT (Ru, dπ → pPDIp,π*) or ILCT (pPDIp,π → pPDIp,π*) absorption band of dyad could generate the charge-separated excited state {(phen)2Ru(III)(pPDIp•-)} that can decay by charge recombination to generate the 3*pPDIp triplet. 5



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The dyad performance benefits from the properties of the PDI and Ru(II) polypyridine complexes and expands their properties by: (1) Generation upon electronic excitation the charge-separated intermediate {(phen)2Ru(III)(pPDIp•-)} irrespective of the moiety being electronically excited; (2) Population of the long-lived triplet 3*PDI (τP = 1.8 µs) from charge-recombination of the intermediate that sensitizes the formation of the singlet molecular oxygen (1∆g), with high yield; (3) The dyad in the presence of triethylammine (NEt3) is reduced to yield the [(phen)2Ru(pPDIp•-)]+ radical anion, which generates upon light irradiation the very stable [(phen)2Ru(pPDIp2-)] dianion.

Scheme 1. Molecular structure of complex [(phen)2Ru(II)(pPDIp)](PF6)2

EXPERIMENTAL SECTION

General RuCl3xH2O, 1,10′-phenanthroline (phen), Perylene-3,4,9,10-tetracarboxylic dianhydride, lithium chloride, and tetrabutylammonium hexafluorophosphate were

6


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obtained from Aldrich;. HPLC grade acetone and DMF were distilled just before being used. The electron transfer reactions, electrochemical and spectroscopic experiments were conducted under nitrogen atmosphere. The cis-[Ru(phen)2Cl2]⋅2H2O was prepared according to the procedures described in the literature.40 The CHN elemental analysis of complexes was performed on an EA 1110 CHNS-O Carlo Erba Instrument. FTIR spectra were recorded on a Bomem-Michelson 102 spectrometer in CD3CN solutions and for solids using KBr pellets. The NMR experiments were acquired with a BRUKER DRX-600 spectrometer in CD3CN or DMSO using tetramethylsilane (TMS) as internal standard. 1H-1H gCOSY experiments were performed with a spectral width of 8,196.72 Hz, acquisition time of 0.25 s, relaxation delay of 1s, and 16 scans for each increment (256 increments). 1

H-13C g-HSQC experiment was performed using spectral widths of 8,196.72 Hz for the

1

H dimension and 36,057.691Hz for the

13

C dimension. Long-range 1H-13C g-HMBC

correlation maps were acquired with spectral widths of 8,196.72 Hz in the 1H dimension and 36,049.42 in the

13

C dimension. For both 2D heteronuclear correlation maps, the

acquisition time was 0.25 s, the relaxation delay was 1s, and 80 scans were performed for each increment (256 increments). TopSpin 3.0 software (Bruker BioSpin) was used for data acquisition and processing. Electrochemical measurements were recorded using a µAutolab Type III potentiostat. Generally, the concentration of the acetonitrile solutions was 10-3 mol L-1, the supporting electrolyte was TBAPF6 (0.1 mol L-1), and the scan rate was 100 mV/s. A standard three-electrode configuration composed of a platinum disk as the working electrode (d = 2 mm, diameter), auxiliary electrodes (d = 4 mm), and Ag/AgCl (KCl

7



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salt) wire as the reference electrode. This electrode was calibrated with the internal reference Fc+/ Fc, which was found at 0.54 V vs Ag/AgCl in CH3CN. The electronic absorption spectra were recorded on an Agilent 8453A UVvisible spectrophotometer or on a Jasco V-660 UV-vis spectrophotometer. Synthesis of cis-[Ru(phen)2(pPDIp)](PF6)2. cis-Ru(phen)2Cl2 (357 mg; 0.067 mmol) was dissolved in DMF (15 mL) at which pPDIp (75 mg; 0.1 mmol) was added. The solution was stirred under nitrogen atmosphere during 24 h under reflux. Following that the volume of the solution was reduced and a stoichiometric amount of NH4PF6 and deoxygenated diethyl ether were added to promote precipitation. The orange precipitate of cis-[(phen)2Ru(pPDIp)](PF6)2 was cooled at 0 °C for 30 min. washed with water and ethyl

ether,

dried

under

vacuum,

and

finally

recrystallized

(80%

yield).

C72H38F12N10O4P2Ru(1498.16): Calcd: C 57.72, H 2.55, N 9.35; found: C 57.04, H 2.64, N 9.11. Steady State Spectroscopy. The luminescence spectra were recorded on a Shimadzu RF-5301PC spectrofluorometer or on a Horiba Jobin Yvon Fluorolog 3-22 spectrofluorometer. The luminescence quantum yields were calculated by the ratio method using [Ru(bpy)3]2+ in acetonitrile (λexc= 436 nm, φem= 0.0629) as a standard.41 Photolysis. Photolysis of the complex (∼10-6 mol.L-1) solutions (3.5 mL) were performed in deaerated solutions by N2 bubbling under magnetic stirring. The solutions were irradiated at 420 nm with a 300 W Xenon lamp (model 6258 from Newport) or on a RMR-600 model Rayonet Photochemical reactor using RMR-4200 lamps. The reactions were followed by UV-vis absorption in square quartz cells of 1cm optical path length. Time-Resolved Photoluminescence. The fluorescence decay curves with picosecond resolution were obtained by the single photon timing technique using the following 8


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excitation light sources: (1) the fundamental and the second harmonic of a cavitydumped

DCM

dye

laser

(DCM

=

4-(Dicyanomethylene)-2-methyl-6-(4-

dimethylaminostyryl)-4H-pyran dye (λexc = 315-340 nm; 630-680 nm); (2) Rhodamine 6G dye (λexc=284-310 nm; 568-620 nm) synchronously pumped by a solid state Nd:YVO4 laser (Vanguard, Spectra Physics) delivering 2Wof 532 nm light at

a

repetition rate of 76 MHz with a pulse duration of ∼12 ps; (3) second harmonic of a tuneable wavelength (700–1000 nm) Ti:Sapphire laser (Tsunami, Spectra Physics, Mountain View, CA) pumped by the second harmonic of a Nd:YVO4 laser (Milennia Xs, Spectra Physics, Mountain View, CA), delivering 100 fs pulses at a repetition rate of 80 MHz. Intensity decay measurements were performed by alternate collection the instrument response function and decay curves, using an emission polarizer set at the magic angle. The instrument response function was recorded at the excitation wavelength with a scattering suspension. For the decays, a cut-off filter was used to remove all excitation light. The emission signal passed through a depolarizer, a Jobin Yvon HR-320 monochromator with a grating of 100 lines/mm, and it was detected with a Hamamatsu 2809U-01 microchannel plate photomultiplier (MCP-PT). The instrument response had effective FWHM of 35 ps. Fluorescence intensity decay curves were obtained by excitation light at 280, 340, and 532 nm using the DCM and Rhodamine 6G dye lasers, with emission collected at 570, 600, and 620 nm, respectively. The decay curves were analyzed using a home-made, non-linear, least-square reconvolution software based on the Marquard algorithm.42 The quality of the fit was evaluated by the reduced χ2, the weight residuals, and the autocorrelation of the residuals. Femtosecond

Transient

Absorption.

The

experimental

setup

for

ultrafast

spectroscopic and kinetic measurements consists of a broadband (350-1600 nm) HELIOS pump-probe femtosecond transient absorption spectrometer from Ultrafast 9



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Systems, equipped with an amplified femtosecond Spectra-Physics Solstice-100F laser (displaying a pulse width of 128 fs and 1 kHz repetition rate), coupled with a SpectraPhysics TOPAS Prime F optical parametric amplifier (195-22000 nm) for pulse pump generation. Probe light in the Vis range was generated by passing a small portion of the 795 nm light from the Solstice-100F laser through a computerized optical delay stage (with time window up to 8 ns) and focusing on a sapphire plate to generate white-light continuum in the 450-800 nm range. All measurements were obtained in a 2 mm optical path length quartz cuvette, with absorptions in the range 0.3-0.5 at the pump excitation wavelength. To avoid multiphoton absorption, the laser pump power was kept at ≤0.5 µJ. Transient absorption data were analyzed using the Surface Xplorer PRO program from Ultrafast Systems. Nanosecond

Transient

Absorption.

The

nanosecond

transient

absorption

measurements were recorded in an Applied Photophysics LKS.60 laser flash photolysis using a Nd:YAG laser (Spectra Physics Quanta-Ray GCR-130) and a Tektronix TDS3052B Oscilloscope (5GS/s). A pulsed 150 W Xe lamp was used to analyze the transient absorption that was detected by a R928 photomultiplier at a right angle with the excitation beam. The signal from the photomultiplier was fed into the digital analyzer and transferred to an IBM RISC computer and processed with the software furnished by Applied Photophysics. The transient spectra were obtained by monitoring the optical density (OD) change at intervals of 5-10 nm over the 300-800 nm range, and averaging at least five decays at each wavelength. The samples were irradiated either with the third-harmonic pulse (355 nm, 8 ns fwhm) or with the second-harmonic pulse (532 nm, 8 ns fwhm) of the laser. Singlet Oxygen Activation. Room-temperature singlet oxygen phosphorescence was detected at 1270 nm using a Hamamatsu R5509-42 photomultiplier cooled to 193 K in a 10


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liquid nitrogen chamber (products for research, model PC176TSCE-005). The excitation light of 355 nm or 532 nm comes from the Nd:YAG laser (Spectra Physics Quanta-Ray GCR-130). A 600-grove diffraction grating was used to extend spectral response to the infrared. A Scotch RG1000 filter was placed in the detection path to eliminate the first harmonic contributions from the infrared sensitizer emission. 1Hphenalen-1-one (perinaphthenone) in toluene (λexc= 355 nm),43 φ∆= 0.93, rose Bengal in methanol (λexc= 532 nm)44, φ∆= 0.76 and [Ru(phen)32+] (λexc= 355 nm)45, φ∆ = 0.71 were used as standards. Chemical and Photo-induced Electron-Transfer Reactions. For this purpose, a specially designed cuvette was built. It consists of a round-bottomed vessel (1 mL) connected to a quartz cell (1 cm optical length) that it is fixed to a gas/vacuum manifold by a hose. The CH3CN solution of complex was placed in the cuvette together with NEt3 and the MV2+ was introduced in the round-bottom vessel. After that, the system was deaerated for 20 min using super pure N2 atm. The absorption spectrum of the acetonitrile solution of the complex-NEt3 mixture was measured and photolyzed with 420 nm or 520 nm light. When the irradiation was stopped, the MV2+ was added and the subsequent reaction in the dark was followed by changes in the UV-vis absorption spectrum.

RESULTS AND DISCUSSION Synthesis and Characterization The pPDIp ligand was synthesized by the reaction of perylene-3,4,9,10teracarboxylic acid anhydride (PDA), with 5-amino-phenanthroline (1:3 molar ratio) in quinoline using zinc-acetate as catalyst, following the procedure described in the literature.46 The molecular structure of the ligand was confirmed by. 1H-NMR (Figure 11



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S1 and Table S1). This was further supported by the FTIR CO stretching band that was shifted from 1780 cm-1 in the unreacted PDA to 1700 cm-1 inpPDIp, Figure S2. The cis-[Ru(phen)2(pPDIp)](PF6)2 complex was synthesized by reacting cis[Ru(phen)2Cl2]47 with 1.5 eq. of pPDIp in DMF. The complex was isolated as a hexafluorophosphate salt, whose composition and structure were verified by CHN analysis and NMR, respectively. The 1H signals of the perylene unit appear in the chemical shifts range, 8.7 < δ < 9.0 ppm, while those of phenanthroline appear 7.5 < δ < 8.7 ppm (Figure S3, Table S1). As expected, the phenanthroline protons are composed of eight peaks instead of four of the free ligand, because the 1H protons of the ligands in the complex are not equivalent.48. Electrochemistry. The cyclic voltammogram of complex in CH3CN using a Pt electrode is shown in Figure 1, Table 1. The voltammograms were independent of number of scans and sweep rate, Figure S4. Using these data and the first order derivative of the voltammetric curves it was possible to identify the redox couples at 0.235/-0.314 V, (E1/2 = -0.27) and -0.397/-0.702 (E1/2 = -0.55) vs Ag+/AgCl which were ascribed to radical anion (pPDIp/pPDIp•-) and dianion (pPDIp•-/pPDIp-2) moieties of complex respectively, as reported for other PDI derivatives.49-56 The reductions at 1.17/-1.24 (E1/2 = -1.20) and -1.34/-1.57 (E1/2 = -1.45) were attributed to the phenanthroline ligands coordinated to the Ru(II) center. The redox couple at + 1.50/+1.43 V (E1/2 = +1.47) versus Ag+/AgCl was assigned to Ru(II/III) reversible couple39,40,57

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Figure 1. Cyclic voltammogram of cis-[Ru(phen)2(pPDIp)](PF6)2 complex in acetonitrile (0.1 M (nBu)4NPF6, platinum working electrode, (potential versus Ag/AgCl) and scan rate of 100 mV s-1. E1/2 for Fc+/Fc couple = +0.54 V measured under the same experimental conditions

The UV-vis spectroelectrochemistry experiments in the range of -1.57 to +1.57 V vs Ag/AgCl, are shown in Figure S5. The spectroelectrochemical oxidation of the Ru(II) complex at constant potential +1.57 V show the formation of

the

electrogenerated Ru(III) product with absorption bands at 440, 280 and 250 nm. In accord, the solution color changed from intense orange to yellow, Figure S5. The reduction of the corresponding Ru(III) complex was not reversible. The reductive electrolysis at -0.35 V, show the appearance of a weak band at 704 nm attributed to [Ru(phen)2(pPDIp•-)]+, (λmax = 704 nm). When the reduction was performed at -0.76 V, the successive formation of pPDIp dianion complex, [Ru(phen)2(pPDIp2-)] with absorption at λmax = 565 was observed, Figure S5. The changes in color of the solution from orange to violet are depicted in Figure S5. The oxidation of the corresponding anion radical and dianion pPDIp moieties of complex was not reversible. These results 13



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are in accordance with the reduction of pPDIp moiety of complex by one-electron and two-electron using light irradiation and NEt3 as sacrificial agent, described below.

Table 1. Redox potentials (E1/2) and energies of the electronic excited states (E0-0) in eV for pPDIp in DMF and [Ru(phen)]32+ and [(phen)2Ru(II)(pPDIp)](PF6)2 complexes in CH3CN. E1/2/eV Compound [Ru(phen)3]

2+

RuII/III

pPDIp0/•-

pPDIp•-/-2

phen

-0.43

-0.70

-1.05; -1.32

-0.27

-0.55

-1.20; -1.45

1*

pPDIp

1*

MLCT

3*

MLCT

+1.30

pPDIp [(phen)2Ru(pPDIp)]2+

E0-0/eV

+1.47

1.77

2.75

UV-visible Spectroscopy: Figure 2A and 2B shows the absorption spectrum the excitation and fluorescence spectra of the pPDIp free ligand in dilute solutions (1.1 × 10-6 mol L-1), where aggregation is negligible. The fluorescence spectrum of the pPDIp exhibited a mirror image of the absorption spectrum with a Stokes shift of ~360 cm-1. The fluorescence spectrum, quantum yield φ = 0.97, and lifetime τF = 3.5 ns are identical in aerated and deaerated solutions, and almost invariant with the excitation wavelength (355, 450, and 525 nm) and temperature (25 - 75 0C), which is in close agreement with other reports.1,2

14


2.1

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Figure 2. (A) Absorption spectrum of pPDIp in DMSO. (B) Fluorescence (λexc = 355 nm, red) and Excitation (λem = 590 nm, green) spectra of pPDIp in CH3CN (1.1 × 10-6 mol L-1).

The absorption spectrum of complex (Figure 3A) can be decomposed as the sum of the ligand pPDIp vibrational resolved absorption spectrum (Figure 2A), plus the MLCT broad absorption band of the [Ru(phen)3]2+ complex with maximum at ~450 nm. This indicates that the complex does not behave as a super molecule, but rather as a donor-acceptor dyad. Upon excitation at 413 nm, the complex exhibits a broad emission with maximum at 600 nm (Figure 3B); whereas by excitation at 450 nm (Figure 3C), the spectrum is composed of a broad emission at 600 nm and a shoulder at 525 nm, suggesting a different repartition of the excitation light between the {Ru(phen)3}2+ and the pPDIp moieties at each excitation wavelength. Consistent with these results, the excitation spectrum recorded at 600 nm shows two overlapping absorption bands characteristic of the {pPDIp} and {Ru(phen)3}2+ moieties.

15



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Figure 3. (A) UV-vis absorption spectrum (orange) of a 4.1 × 10-6 mol L-1 solution of [(phen)2Ru(II)(pPDIp)](PF6)2 in CH3CN and the contributions of the pPDIp (red) and {Ru(phen)3}2+ MLCT absorption (green) bands. Luminescence spectra (red) by excitation at λexc= 413 nm (B) and λexc= 450 nm (C) and excitation spectra (green) recorded at λem= 600 nm.

This was confirmed by the time-resolved luminescence decays of air equilibrated dilute solution in CH3CN. The electronic excitation at the {pPDIp} moiety of [(phen)2Ru(II)(pPDIp)](PF6)2 at 532 nm requires a biexponential fit (Figure 4) with lifetimes of 10 ps (90%) and 3.5 ns (10%). The long lifetime is identical to that of free ligand suggesting the presence of free ligand in the solution. The presence of the free ligand even in a very small amount can make a substantial contribution to the decay due to its very high quantum yield (approximately 100 times higher than that of the spinforbidden 3MLCT {Ru(phen)3}2+ transition to the ground state). The very short lifetime of ~10 ps is absent in the free ligand decay which suggests the presence of fast deactivation processes owing to the presence of the {Ru(phen)3}2+ moiety in the dyad.

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Figure 4. Luminescence decays (••••) of complex [(phen)2Ru(II)(pPDIp)](PF6)2 in CH3CN at 22 0C by excitation at λexc = 532 nm and emission recorded at λem = 570 nm. The Decay was fitted (red) with a sum of two exponentials by convolution with the instrumental response function (blue): The quality of the fit was judged by χ2 = 1.21 and the plot of the weighted residues.

By excitation at 450 nm, both the {Ru(phen)2}2+ and the {pPDIp} moieties are electronically excited. The decays recorded with a small time window (~10 ns) are identical to those obtained by electronic excitation at λexc = 532 nm. Using a large time window (~500 ns), it is possible to observe a long decay component with a very small pre-exponential factor and lifetime higher than 100 ns. This component of the decay was obtained with a large uncertainty by the single-photon timing (SPT) technique. These results indicate that at least upon excitation at 450 nm a long lived state in acetonitrile air-equilibrated solutions should be reached. Figure 5 shows the transient absorption spectra and the decays of the complex in acetonitrile by excitation at 525 nm, at which only the pPDIp moiety absorbs. The bleaching of the ground state absorption of the pPDIp at 530 nm is accompanied by the 17



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appearance of a broad band with maximum at 700 nm, which was previously attributed to the pPDIp•- radical anion (Figure 5A).24,26,49 This can be envisaged as the chargeseparated state *{(phen)2Ru(III)(pPDIp•-)} resulting from the transfer of one electron from Ru(II) to pPDIp. Later, this state decays (Figure 5B) and originates the triplet state {(phen)2Ru(II)(pPDIp3*)}, with transient absorption at 505 nm. The kinetics of growth and decay of the pPDIp•- radical anion at approximately 700 nm can be fitted with a sum of three exponentials (Figure 5C). A very fast decay component of 0.6 ps is followed by a growing component of 2.9 ps (negative pre-exponential) that is followed by a decay component of 28 ps. The short component of 0.6 ps probably occurs due to the cooling of the vibrational hot pPDIp•- radical anion after being created; the growing component comes from the singlet excited sate of 1*{(phen)2Ru (II)(pPDIp)}; and the decay component of 28 ps is related to the decay of the pPDIp•- radical anion to form the triplet state {(phen)2RuII(pPDIp3*)}. The growth of the 3*pPDIp triplet state (Figure 5D) recorded in the transient absorption band (475-775 nm)1,25 at 505 nm provides a rise-time component of 22 ps. This lifetime is inferior to the decay component of the pPDIp•- radical anion (28 ps) probably due to the interference of the ground state depletion of the 1*{(phen)2Ru(II)(pPDIp)} at approximately 530 nm. Indeed, the transient absorption trace recorded in a new transient absorption band of the triplet state (575 - 800 nm)1,25 of {(phen)2Ru(II)(pPDIp3*)} at 635 nm provides a lifetime component of 27 ps, which is in close agreement with the decay of the pPDIp•- radical anion of 28 ps. This species presents a long decay that cannot be recorded within the time window of the fs transient absorption equipment. These results support the population of the

3*

pPDIp by a charge-transfer mechanism instead of the intersystem

crossing pathway that can be envisaged due to the presence of the Ru heavy metal.

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Figure 5: Transient absorption spectra of the collected at 0.267 ps (blue), 0.587 ps (red) and 0.907 ps (black) (A) and 1.63 ps (pink), 29.5 ps (blue), 61.1ps (red line) to 213 ps (black) (B) after fs-pulse excitation at 525 nm. The Kinetics of growth and decay (••••) and fit (red) of the radical anion pPDIp•- at 700 nm (C) and 3pPDIp* at 505 nm (D) and the corresponding weighted residues.

Upon excitation at 450 nm, both the {pPDIp} and the {Ru(phen)3}2+ moieties absorb with a repartition of approximately 64% of the excitation energy being absorbed by the {Ru(phen)3}2+ component. Although most of the excitation radiation is absorbed by the {Ru(phen)3}2+ moiety we must consider the influence of the decaying processes that occur after excitation of the pPDIp moiety. Figure 6A and 6B shows the fs-transient absorption spectra upon excitation at 450 nm for several delay times after excitation of the complex. The bleaching of both the {pPDIp} moiety (530 nm) and {Ru(phen)3}2+ (450 nm) absorption bands are accompanied by the appearance of a strong absorption band at 696 nm due to the pPDIp•- radical anion. At longer delay times, we can clearly 19



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identify a transient absorption species at 505 nm, which is associated with the decay of the species at 696 nm and the recovery of the shoulder with a red shift to 580 nm. The difference absorption spectra versus time profiles at 700 nm and at 500 nm are shown in Figure 6C and 6D. The 700 nm traces exhibit a fast-growth rising component (τ = 8.3 ps) that decays with a lifetime (τ = 25 ps) that matches the rise time component of the species monitored at 500 nm (τ = 26.3 ps), and it is attributed to the population of 3*

pPDIp. These results show that the triplet state {(phen)2Ru(II)(pPDIp3*)} is reached

irrespective of the moiety that is electronically excited (pPDIp or the {Ru(phen)3}2+ moieties). By excitation of the {Ru(phen)3}2+ moiety in the 1*MLCT state, a very fast process (not detectable with the time-resolution of the equipment) occurs to generate the triplet state

3*

MLCT. From this state, a fast conversion to the pPDIp•- radical anion

occurs with a rise time of 8.3 ps that decays by two consecutive paths: a very fast one with lifetime decay of 1.2 ps due to a conformation relaxation process, and another with a lifetime decay of 28.1 ps. The latter decay corresponds to the rise-time component of the 3*pPDIp triplet state with maximum at 505 nm (23 ps) contaminated by the ground-state bleaching. Indeed, when observed in the low energy transient absorption band of 3*pPDIp triplet at 635 nm, it gives a value of 28 ps that is equal to the decay of the pPDIp•- radical anion. These results support a charge-transfer mechanism instead of a Resonance Energy Transfer (RET)58 from the

3*

MLCT to the

pPDIp in the ground state. RET processes can occur by coulombic (Förster mechanism) and/or exchange coupling (Dexter mechanism) mechanisms: both are inefficient because the Förster mechanism is spin-forbidden and the wave-function overlap required for an efficient exchange process is very small.

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Figure 6. Transient absorption spectra at 0.213 ps (gray), 0.320 ps (red) and 0.533 ps (blue) (A) and 0.867 ps (orange), 1.59 ps (red) and 30.2ps (yellow) (B) after fs-pulse excitation at 450 nm. The Kinetics of growth and decay (••••) and fit (red) of the radical anion pPDIp•- at 700 nm (C) and 3pPDIp* at 505 nm (D) and the corresponding weighted residues.

Upon excitation with ns-pulses of 355 nm light only a positive absorption due to the formation of 3*pPDIp is observed (Figure 7A). The decay at 505 nm gives a lifetime of 1.8 µs attributed to the phosphorescence of pPDIp in the absence of oxygen. This resembles the lifetime of 100 ns observed by the SPT technique in aerated solutions of the complex in acetonitrile.

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Figure 7. (A) Transient absorption spectra of the complex collected at 120 ns (red), 200 ns (green), 280 ns (blue), 400 ns (pink), 600 ns (orange), and 1000 ns (gray) by ns-pulse excitation at 355 nm. (B) The decay (blue) and a single-exponential fit (green) of transient absorption of {(phen)2Ru(II)(pPDIp3*)} at 500 nm for deaerated dilute solution (4.0 x 10-6 mol L-1) in CH3CN and the weighted residuals (χ2 = 1.01).

Qualitative Energy Level diagram of excited states of complexes The results presented herein demonstrate that the triplet state of 3*pPDIp is reached irrespective of excitation in the pPDIp or {Ru(phen)3}2+ moieties. In general, for Ru(II) polypyridine complexes such as [Ru(phen)3]2+,39,40 the population of the 1*

MLCT is followed by an efficient ISC to the

3*

MLCT. Nevertheless, the difference

absorption spectra and the fast kinetics indicate the involvement of the charge-separated intermediate state {(phen)2Ru(III)(pPDIp•-)}2+ regardless of the moiety of complex being electronically excited. The driving force (∆G◦ET) for the intramolecular electron transfer involved in both paths was estimated based on the Rehm-Weller equation59 using the electrochemical data and the lowest energy excited state (E0-0) values shown in Table 1. A diagram displaying the energy levels of the states involved in the processes upon 22


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electronic excitation of the dyad is shown in Scheme 2 (coulombic interactions were neglected). Upon excitation of the dyad to the 1*MLCT state by light of wavelength λ < 450 nm, a fast intersystem crossing occurs to generate the

3*

MLCT state in a non-

equilibrium conformation. After thermal equilibration, an electron is transferred from Ru(III) to the pPDIp (τ = 8.3 ps) to yield the charge-separated intermediate state {(phen)2Ru(III)pPDIp•-}2+ with charge separation energy of + 1.75 eV; (∆GCS = - 0.35 eV). This intermediate decays (τ = 23 ps) to generate the triplet state 3*pPDIp. Using excitation light with wavelength higher than 525 nm the unique species that absorbs is the pPDIp moiety of the dyad. The

1*

pPDIp singlet decays rapidly to

generate the pPDIp•- radical anion (τ = 2.9 ps) by an electron transfer reaction from Ru(II) to 1*pPDIp (∆GCS = - 0.61 eV). Similarly, the charge-separated state decay (τ=28 ps) to generate the

3*

pPDIp. The difference in the lifetime values is related with the

contamination of the decays as was discussed before. The phosphorescence lifetime of the 3*pPDIp was determined by ns-flash photolysis as τP= 1.8 µs in CH3CN deaerated by N2 bubbling solutions. In conclusion, the spectroscopic properties of the complex enable us to efficiently produce the triplet state of pPDIp in a large optical window (350-525 nm) by excitation either in the pPDIp or in the Ru(II) moieties. Furthermore, the chemical and photochemical stability and the photophysical properties of the dyad open the possibility for electron- and energy-transfer processes with potential namely in catalysis.

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Scheme 2: Scheme of the processes occurring after electronic excitation of the [(phen)2Ru(II)(pPDIp)]2+ complex.

Photoinduced energy transfer processes The long-lived 3*pPDIp moiety of complex was efficiently quenched by ground state molecular oxygen (3Σg) to form the corresponding singlet oxygen (1∆g) via an energy-transfer process between the

3*

pPDIp moiety and the triplet ground-state of

molecular oxygen. Figure 8 shows the luminescence decays collected at 500 nm by ns flash-photolysis in the presence of oxygen (air equilibrated solutions) and after degassing with N2. The decays can be well fitted with a mono-exponential with 280 ns (air equilibrated) and 1.8 µs (deaerated by N2 bubbling) solutions, respectively. The quantum yields for singlet oxygen sensitization were obtained by time-resolved measurements in aerated CH3CN solutions (upon excitation at 355 nm or 532 nm), monitoring the phosphorescence of the singlet oxygen at 1270 nm as a function of laser intensity, using phenalenone, [Ru(phen)3]2+ and rose bengal as reference sensitizers. The singlet oxygen decay is mono-exponential with lifetime of 70 µs. The 1O2 24


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phosphorescence intensity varies linearly with the laser intensity, thus discarding the occurrence of triplet-triplet annihilation (Figures S6 and S7). The quantum yield of singlet oxygen sensitization is φ∆ = 0.57. A similar value was found upon excitation in the pDPIp moiety at 532 nm. These values are comparable to those obtained for the singlet oxygen sensitization of PDI diimide derivatives.60-62

Figure 8. The decay of the transient signal at 500 nm of the dyad upon ns-laser excitation at 355 nm for air equilibrated (black) and deaerated solution (blue). The decays were fitted with a single exponential function for air-equilibrated (red) and deaerated (green) solutions. The quality of the fit was judged by the weighted residuals ( χ2 = 1.0, for both fits).

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Chemical and Photo-induced electron-transfer reactions In this section, the ability of the dyad to participate in intermolecular electrontransfer reactions using triethylammine (NEt3) as a chemical sacrificial electron-donor was studied. In these experiments, dilute solutions of the complex (~2.2 × 10-5 mol L-1) in acetonitrile were prepared to avoid aggregation, and the reactions were followed by UV-vis absorption. Typically, the procedure was performed in four steps: 1) in the dark, NEt3 was added to the solution of complex in the ratios of 1:1 or 1:2 under stirring, and oxygen was removed from the solution by N2 bubbling during 20 min; 2) the UV-vis spectral changes were followed in time until a final stable spectrum was reached; 3) the same solution was irradiated with continuous light at 420 nm or 520 nm; 4) the intermolecular electron-transfer reaction was followed in time and the changes in the absorption spectrum were recorded until a final stable spectrum was reached. Reaction in the dark: Figure 9(Top A and B) shows the spectroscopic changes over time seen when NEt3 was added to the solution of complex (1:1 NEt3 to complex ratio). The spectra show a progressive depletion of the absorption band of the pPDIp moiety of complex at 525, 489 and 476 nm concomitant with growth of absorption at 704, 796 and 953 nm assigned to the [Ru(phen)2(pPDIp)•-]+ radical anion. Isosbestic points at 545 nm were observed as the color of solution changed from orange to green. The plots of changes in the maximum absorption at 704 (consumption of green solution) and 565 nm (formation of violet solution) versus time were equivalent, suggesting a one-to-one correspondence. The chemical reduction of the pPDIp0 moiety by NEt3 in deaerated solutions is consistent with the electrochemical reduction potential values of NEt363,64 and complex. Thus the complex in its ground state can oxidize NEt3 to its aminium cation with the 26


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formation of [Ru(phen)2(pPDIp•-)]+, which is found to be very stable in the absence of oxygen.65 Photolysis: By irradiation of this solution with 420 nm light, the absorbance of [Ru(phen)2(pPDIp•-)]+ (green solution) increased until a plateau was reached. During prolonged continuous light irradiation, the green solution turned violet with the formation of [Ru(phen)2(pPDIp)2-]. The UV-vis spectroscopic changes of this solution are shown in Figure 9 (Bottom A and B). The plots of changes in the maximum absorption at 525 (consumption of orange solution) and 702 nm (formation of green solution) versus time were equivalent, suggesting a one-to-one correspondence. If irradiation is ceased after the pPDI•- and the pPDIp2- moieties of complex are formed, the absorption spectrum does not change for at least 6 hours in the absence of oxygen, showing the high stability of these species. However, in the presence of oxygen, the absorption spectra evolve to the initial spectrum of the complex-NEt3 solution, indicating that oxygen acts as an electron acceptor. The same behavior was observed when this experiment was repeated for the 2:1 complex:NEt3 ratio (Figure S8). At 520 nm irradiation, using the 2:1 complex:NEt3 ratio, only the green species is formed, indicating that the optical density of the radical anion at this wavelength is very small to reach its excited state, Figure S9. Based on these results, a possible pathway for the generation of the dianion [Ru(phen)2(pPDIp2-)] is through a second reduction of the electronic excited state of the radical anion [(phen)2Ru(pPDIp•-)]+. This process occurs only through the electronic excited state and, consequently, only by photolysis with 420 nm. The aminium cation (•NEt3+) is very reactive and, as suggested by Whitten66, can react with another molecule

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of NEt3 to produce the alkyl radical [CH3ĊHNEt2], which can be the reductant of the electronically excited anion.

[Ru(phen)2(pPDIp)]2+ + NEt3

[Ru(phen)2(pPDIp•-)]+ + CH3ĊHNEt2

dark

hν ν

[Ru(phen)2(pPDIp•-)]+ + •NEt3+

[Ru(phen)2(pPDIp2-)] + CH3CH=N+Et2

Figure 9. Variation of the absorption spectrum of [Ru(phen)2(pPDIp)]2+ complex and NEt3 (2:1, complex:NEt3 ratio) in CH3CN. Reaction in the dark (Top A) changes in the maximum absorbance of pPDIp → pPDIp•- moiety of complex over time (Top B).

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Photolysis with 420 nm light irradiation (Bottom A) changes in the maximum absorbance of pPDIp•- → pPDIp2- moiety of complex over time (Bottom B).

It is interesting to emphasize that the complex is a good oxidizing agent, whereas the {Ru(phen)2(pPDIp)2-} dianion is a good reducing agent (Ered = - 0.44 V vs Ag+/AgCl), which indicates that the reverse process can occur in the presence of a suitable electron-acceptor. To confirm this possibility, the procedure was repeated using the electron-acceptor N,N-Dimethyl-4,4’-bipyridinium chloride (MV2+, Ered = - 0.687 V vs Ag+/AgCl).67 In this experiment, the MV2+ was added to the solution of complex and NEt3 only after photolysis of the mixture, allowing evaluation of NEt3-complex interaction without interference of MV2+. Figure 10 shows the UV-vis absorption spectral changes in the dark after addition of MV2+ to the photolyzed solution containing NEt3 and complex. The broad absorption band with maximum at 605 and 394 nm appeared immediately after addition of MV2+ to the solution and increased over time until a stable spectrum was reached. This new absorption closely matches the methylviologen radical absorption spectrum68,69 (MV2+→MV•+, blue solution). The formation of MV•+ and the consequent consumption of the dianion were too fast to be observed with our equipment. However, the slow back reaction to MV2+ in dark was followed by UV-vis. The same behavior was observed when this experiment was repeated for the 1:1 complex:NEt3 ratio.

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Figure 10. Spectral changes during reaction in the dark observed after addition of MV2+ to the photolyzed solution of complex-NEt3 mixture.

The changes in solution color during the reductive reaction in the dark and under light irradiation of [Ru(phen)2(pPDIp)]2+ are depicted in Scheme 3.

30


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Scheme 3: Photographs showing the changes in the color of the complex during reductive and oxidative cycling processes.

Both the anion and dianion are very stable because the two terminal phenanthrolines stabilize the double negative charges on the PDI. The formation of stable perylene dianion derivatives by chemical and electrochemical processes was already observed.70,71 Indeed, to our knowledge, a perylene dianion derivative generated through a photo-induced electron-transfer reaction was first reported by Wasielewski72 for a D-A-D triad consisting of a PDI and two porphyrin moieties. Recently, it was shown that the PDI dianion catalyzes the reduction of stable aryl chlorides producing

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aryl radicals, which were trapped by hydrogen atom donors or used in carbon-carbon bond formation reactions.73,74 Herein, it was shown that, in the presence of NEt3, the reduction of the complex occurs to form the radical anion in the dark and that under light irradiation the reaction is faster. The photolysis of the radical anion generates the dianion in quantitative amounts in a controlled way. Moreover, the electrons resulting from the reduction reaction and the energy accumulated can be used to reduce MV2+ to MV•+ and recover the complex-NEt3 mixture in a cyclic process.

CONCLUSION. The dyad [Ru(II)(phen)2(pPDIp)]2+ expand the potential applications of PDIs and PDI derivatives complexes. Upon electronic excitation in both the absorption bands of the dyad the triplet state of pPDIp is reached through the recombination a chargeseparated state detected by fs transient absorption spectroscopy. The dyad photosensitizes the formation of singlet oxygen by triplet-triplet energy transfer with a quantum yield of 0.57. The complex radical anion is formed by electron transfer from NEt3. The very reactive aminium cation (•NEt3+) reacts with a new NEt3 molecule to generate an alkyl radical that can reduce the electronically excited anion to the dianion. The generation of these species opens the opportunity to use this complex as a catalyst in organic synthesis.

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ACKNOWLEDGMENTS: The authors would like to acknowledge FAPESP, (process nº. 2011/10882-5 and 2013/23943-8, 2014/12538-8), CNPq, and CAPES for the grants and fellowships received. The Fundação para a Ciência e a Tecnologia (FCT, Portugal), is

acknowledged

for

OE/QUI/UI0313/2014).

financial J.

Pina

support

(UID/NAN/50024/2013

acknowledges

a

post-doc

and

fellowship

PEst(ref.

SFRH/BPD/108469/2015) from FCT. The authors are indebted to Rafael Cavalieri Marchi, Isabele Aparecida Soares de Campos and Mariana Pigozzi Cali, for their help with the conduction of the electron-transfer experiments.

SUPPORTING INFORMATION This information is available free of charge via the Internet at http://pubs.acs.org Detailed experimental results, IR and 1H-NMR spectra and all the NMR data:1D (1H,

13

C) and NMR characterization of complex: 2D homonuclear (1H-1H COSY) and

heteronuclear (1H-13C-HSQC, 1H-13C-HMBC, 1H-15N-HMBC).

REFERENCES (1) Ford, W. E.; Kamat, P. V. Photochemistry of 3,4,9,10-Perylenetetracarboxylic Dianhydride Dyes. 3. Singlet and Triplet Excited-State Properties of the Bis(2,5-di-tert butylphenyl)imide Derivative. J. Phys. Chem. 1987, 91, 6373-6380. (2) Wurthner, F.; Saha-Moller, C. R.; Fimmel, B.; Ogi, S.; Leowanawat, P.; Schmidt, D. Perylene Bisimide Dye Assemblies as Archetype Functional Supramolecular Materials. Chem Rev. 2016, 116, 962-1052. (3) Kircher, T.; Löhmannsröben, H.-G. Photoinduced Charge Recombination Reactions of a Perylene Dye in Acetonitrile. Phys. Chem. Chem. Phys.1999, 1, 3987-3992.

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(4) Weil, T.; Vosch, T.; Hofkens, J.; Peneva, K.; Mullen, K. The Rylene Colorant Family Tailored Nanoemitters for Photonics Research and Applications. Angew. Chem. Int. Ed. 2010, 49, 9068-9093. (5) Kalinowski, J.; Di Marco, P.; Cocchi, M.; Fattori, V.; Camaioni, N.; Duff, J. Voltage-Tunable-Color Multilayer Organic Light Emitting Diode. Appl. Phys. Lett. 1996, 68, 2317-2319. (6) Kozma, E.; Mróz, W.; Galeotti, F. A Polystyrene bearing Perylene Diimide Pendants with Enhanced Solid State Emission for White Hybrid Light-Emitting Diodes. Dyes and Pigments. 2015, 114, 138-143. (7) Würthner, F. Plastic Transistors Reach Maturity for Mass Applications in Microelectronics. Angew. Chem. Int. Ed. 2001, 40, 1037-1039. (8) Yu, S. H.; Kang, B.; An, G.; Kim, B. S.; Lee, M. H.; Kang, M. S.; Kim, H.; Lee, J. H.; Lee, S.; Cho, K.; et al. pn-Heterojunction Effects of Perylene Tetracarboxylic Diimide Derivatives on Pentacene Field-Effect Transistor. Appl. Mater. Interfaces. 2015, 7, 2025-2031. (9) Ruan, Y.-B.; Li, A.-F.; Zhao, J.-S.; Shen, J.-S.; Jiang, Y.-B. Specific Hg2+-Mediated Perylene Bisimide Aggregation for Highly Sensitive Detection of Cysteine. Chem. Commun. 2010, 46, 4938-4940. (10) Huang, Y.; Zhang, W.; Zhai, H.; Li, C. Alkylsilane-functionalized Perylenediimide Derivatives with Differential Gas Sensing Properties. J. Mater. Chem. C. 2015, 3, 466472. (11) Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics. Science. 2001, 293, 1119-1122.

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Photoinduced Energy and Electron-Transfer Reactions by

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