Near-IRPhosphorescentRuthenium(II)andIridium(III)PeryleneBis

Phosphorescent Complexes DOI:10.1002/anie.201410437 Near-IR Phosphorescent Ruthenium(II)and Iridium(III)Perylene Bisimide Metal Complexes**

Marcus Schulze,Andreas Steffen,*and Frank Würthner*

Abstract:The phosphorescence emission of perylene bisimide derivatives has been rarely reported.Two novel ruthenium(II) and iridium(III)complexes of an azabenz-annulated perylene bisimide(ab-PBI),[Ru(bpy)2(ab-PBI)][PF6]21and[Cp*Ir-(ab-PBI)Cl]PF62are now presented that both show NIR phosphorescence between750–1000nm in solution at room temperature.For an NIR emitter,the ruthenium complex 1displays an unusually high quantum yield(F p)of11%with a lifetime(t p)of4.2m s,while iridium complex2exhibits F p< 1%and t p=33m s.1and2are the first PBI-metal complexes in which the spin–orbit coupling is strong enough to facilitate not only the S n!T n intersystem crossing of the PBI dye,but also the radiative T1!S0transition,that is,phosphorescence.

P erylene bisimide(PBI)dyes are well-known for their high tinctorial strength,intense fluorescence,excellent(photo)-stability,and their ability to form stable radical anions.[1]With this unique combination of optical and electronic properties they have entered many research fields,including organic electronics,[2]optical sensing,[3]single-molecule spectrosco-py[4]and supramolecular photochemistry.[5]Many of these applications make use of the energetically lowest singlet excited state(S1)of PBI.However,there are only few reports of PBI derivatives of which the triplet excited state T1can easily be accessed and its triplet properties be used.One example has been described by Janssen and co-workers,who investigated a cofacially stacked PBI dimer and could demonstrate triplet state formation by singlet oxygen sensi-tization experiments.[6]Photoexcitation of this dimer stack leads to a highly polarized charge transfer(CT)state,which accelerates the intersystem-crossing(ISC)process.Studies by Flamigni and co-workers on perylene tris-dicarboximides revealed the rare case of triplet excited state formation in monomeric perylene compounds.[7]However,the phosphor-escence spectrum could only be obtained in a77K glass matrix by an external heavy atom effect that is mediated by ethyl iodide.Despite the achieved progress in studying the triplet state of PBI systems by indirect methods,the most obvious access to the T1state is by an internal heavy atom effect,which can be accomplished by introduction of a late-transition-metal center with strong spin–orbit coupling(SOC) directly in the molecule itself to facilitate ISC.Castellano and co-workers contributed to this approach several PBI-Pt conjugates with acetylene bridges,and they were able to populate the PBI intraligand triplet(3IL)state with up to 55%quantum yield.[8]Other examples of the Wasielewski group showed that attachment of either an Ir complex to the imide position of PBI or a Ru fragment to the PBI bay area leads to very fast charge separation,which is a competitive pathway due to the strong photo-oxidation properties of the dye.[9]Rybtchinski and co-workers delivered an example of a PBI with a Pd complex directly attached at the bay position, which counterintuitively has negligible influence on the PBI 1IL state properties exhibiting a remarkable high fluorescence quantum yield(F f)of65%.[10]Although access to the PBI3IL state has been achieved,phosphorescence originating from that state has apparently not been observed for PBI-based transition metal complexes.[11]Obviously,the SOC of the metal atoms used in the reported systems is not of sufficient extent for the radiative transition from T1to the ground state S0,that is,phosphorescence.

Herein,we report the first examples of PBI-metal complexes[Ru(bpy)2(ab-PBI)][PF6]21and[Cp*Ir(ab-PBI)Cl]PF62,which show emission from their PBI3IL state even at room temperature in solution.The azabenzannulated perylene bisimides(ab-PBIs)were prepared by an optimized method[12]in three steps,starting from perylene bisanhydride, in overall yield of30%(for details see the Supporting Information).Afterwards,the bipyridine-like ab-PBI ligand 3a(R1=2,2-diisopropylphenyl)was reacted with[RuCl2-(bpy)2]and an excess of AgClO4,and subsequent anion exchange with NH4PF6afforded the complex[Ru(bpy)2(ab-PBI)][PF6]21in65%yield(Scheme1).The iridium complex [Cp*Ir(ab-PBI)Cl]PF62was prepared by the reaction of ab-PBI3b(R2=3-pentyl)with half an equivalent of[Cp*IrCl2]2 and subsequent anion exchange with NH4PF6in67%yield. Both complexes,1and2,were characterized by elemental analysis,NMR spectroscopy(Supporting Information,Figur-es S4–S7),MALDI-TOF and high-resolution electrospray ionization(ESI)mass spectrometry(Supporting Information, Figures S12–S15).

The molecular structure of2was confirmed by single-crystal X-ray diffraction studies(Figure1).The iridium(III)

[*]M.Schulze,Prof.Dr.F.Würthner

Universit?t Würzburg,Institut für Organische Chemie and

Center for Nanosystems Chemistry

Am Hubland,97074Würzburg(Germany)

E-mail:wuerthner@chemie.uni-wuerzburg.de

Dr.A.Steffen

Universit?t Würzburg,Institut für Anorganische Chemie

Am Hubland,97074Würzburg(Germany)

E-mail:andreas.steffen@uni-wuerzburg.de

[**]This work was supported by the Bavarian Research Program“Solar Technologies Go Hybrid”.M.S.thanks the Fonds der Chemischen Industrie for a Kekul?fellowship.A.S.is grateful to Prof.T.B.

Marder for his generous support.We thank Dr.Christian Burschka for the single crystal X-ray analysis,and Waldemar Waigel and Lisa

Otter for the synthesis of PBI

compounds.

Supporting information for this article is available on the WWW under 2b1f984583c4bb4cf6ecd12d/10.1002/anie.201410437.

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center in 2is coordinated in a piano-stool conformation in which the Cp*ligand occupies three of the six possible coordination sites.All bond lengths and angles of 2are in agreement with the corresponding reference compound [Cp*Ir(bpy)Cl]Cl,[13]except the iridium(III)–nitrogen bond Ir àN5(2.138(5) )and the Ir àCp*distance (1.803(8) ).For both,elongations compared to the reference (d (Ir àbpy)=2.076(8)/2.090(9)and d (Ir àCp*)=1.786 )are observed and attributed to the electron-withdrawing character of the PBI moiety.Furthermore,the pyridyl ring is twisted out of the flat PBI plane (f (C4-C5-C8-C9)=3.808)by 18.38owing to the steric constraint between the C62and C23hydrogen atoms.

We have investigated the electrochemical properties of PBI complexes 1and 2by cyclic voltammetry.These studies revealed a shift of the two reversible PBI reductions PBI à/0and PBI 2à/àof the free ab-PBI ligands (both ligands have identical redox potentials;see the Supporting Information,Table S1)from à1.06V and à1.34V vs Fc +/0,respectively,to à0.85V and à1.23V for 1and to à0.82V and à1.22V for 2upon complexation (Figure 2a).The easier reduction of ab-PBI upon coordination to the Lewis acidic metal ion centers is accompanied by a more difficult oxidation of the metal centers in these complexes.The reversible Ru 3+/2+redox

couple appears at +1.02V in [Ru(bpy)2(ab-PBI)][PF 6]21(+0.88V in [Ru(bpy)3]2+)[15]and the irreversible Ir 4+/3+oxidation in [Cp*Ir(ab-PBI)Cl]PF 62at +1.46V (+1.40V in [Cp*Ir-(bpy)Cl]+).[16]This redox behavior of 1and 2is related to the electron-withdrawing nature of the electron-poor PBI unit,which destabilizes the higher oxidized state of the metal

centers.

Scheme 1.Synthesis of complexes [Ru(bpy)2(ab-PBI)][PF 6]21and [Cp*Ir(ab-PBI)Cl]PF 62.Reaction conditions:a)[RuCl 2(bpy)2],AgClO 4,LiCl,NEt 3,CHCl 3/EtOH 3:1,658C,48h,Ar;NH 4PF 6,65%;b)[Cp*IrCl 2]2,CHCl 3,408C,16h,Ar;NH 4PF 6,

67%.

Figure 1.Molecular structure of [Cp*Ir(ab-PBI)Cl]PF 62in the solid state (ellipsoids set at 50%probability;hydrogen atoms and counter-ion omitted for

clarity).

Figure 2.a)Cyclic voltammograms of ab-PBI (only 3a shown),1,and 2.The measurements were performed in dry dichloromethane at room temperature at a concentration of 2.5·10à4mol L à1(electrolyte:

0.1mol L à1n Bu 4NPF 6).The values were corrected versus ferrocene as an internal standard.b)UV/Vis spectra of ab-PBI (only 3a shown),1,and 2(along with these of corresponding references [Ru(bpy)3][PF 6]2and [Cp*Ir(bpy)Cl]Cl,[14]dashed lines)in dichloromethane at a concen-tration of 1·10à5molL à1at room temperature.

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The optical properties of the new PBI-based complexes 1and2were studied by UV/Vis absorption and luminescence spectroscopy.The absorption features of the PBI ligand(3a and3b)are fairly recognizable with its strong p–p*band at about470nm(e%65000L molà1cmà1)and the corresponding vibronic progressions(Figure2b).In compound1and2,these characteristics are not changed fundamentally,except for a small5nm hypsochromic shift and a broadening of the fine structure.However,compound1shows a new additional broad absorption band at around520nm,which is assigned to a bathochromically shifted metal-to-ligand charge transfer (MLCT)transition,which relates to the MLCT state of [Ru(bpy)3]2+at450nm.[17]The iridium compound2exhibits several new bands between370–420nm,which are apparently arising from MLCT transitions,and thus leading to a nearly constant absorptivity from250to470nm with an extinction coefficient of about30000L molà1cmà1.Both electrochem-ical and absorption spectroscopic data reveal that attachment of metal fragments close to the PBI core by an azabenzannu-lated PBI does not lead to a simple superposition of the separate ground state properties of the inpidual compounds (for example,PBI absorption),but creates hybrid states with new electronic features(for example,MLCT absorption at 520nm in1).

The electrochemical and absorption spectroscopic results are rationalized by DFT and TD-DFT studies(BHandHL YP-D3BJ/def2-TZVP,gas phase;for details,see the Supporting Information),which suggest that the HOMO and HOMOà1 of1and2being PBI-based p-orbitals,while the metal d-orbitals are found to contribute to HOMOà2to HOMOà4 and further low lying occupied MOs.The LUMO is in the case of the iridium complex2mainly located at the imide moieties of the PBI ligand,whereas in[Ru(bpy)2(ab-PBI)][PF6]21the LUMO shows some additional contributions of one of the metal coordinating bipyridine ligands.The HOMO–LUMO gaps are found to be about2.39and2.34eV for1and2, respectively.The lowest energy band of1at520nm contains indeed some MLCT character,although it is mainly intrali-gand charge transfer(ILCT)in nature,and most of the higher-lying Franck-Condon(FC)singlet states are pure ILCT states(Figure3a).In contrast,the S1state of2is a pure ILCT state,while the higher energy excitations observed in

the range between370–420nm are of MLCT character.The experimental and theoretical findings show well a higher preference of Ru II to undergo low-energy MLCT in compar-ison to Ir III.A similar analysis can also be applied to the FC triplet excited state T1(Figure3b),which contains significant 3MLCT contribution for1but is a pure3ILCT for2,giving rise to the experimental differences of these two types of excited states(see below),that is,a broad emission band for the ruthenium complex1and a well-resolved vibronic fine structure of the emission originating from the iridium compound2.

The emission spectra of both complexes show an almost quantitative quenching of the ab-PBI fluorescence at484nm (Figure4a;Supporting Information,Table S2).Instead, a long-lived emission in the NIR regime at750–1000nm appears with peak maxima at780nm for1and at745nm for2 in degassed dichloromethane at room temperature(Fig-ure4b).Furthermore,the vibronic fine structures of the

broad emission of1and the resolved band in2have energetic separations of about1400cmà1,which is typical for PBI aromatic vibrations.[18]In line with our theoretical studies,we attribute the phosphorescence in1to originate from a mixed

3ML(ab-PBI)/3ILCT state,whereas emission in2is3IL(ab-

PBI)in nature.The emission maxima of the complexes correspond to a3IL(PBI)energy of about1.6eV,which is significantly higher than the estimated value for bay-unsub-stituted PBIs(1.2eV).[19]A similar triplet excited-state energy of1.68eV has been found by Flamigni upon attach-

ment of electron withdrawing functional groups at the perylene moiety.[7]Introduction of the electron-withdrawing

aza-nitrogen in ab-PBI and the Lewis acidic metal fragments

in1and2apparently stabilize the bonding PBI molecular orbitals to a greater extent than the anti-bonding MOs.As

a consequence,a hypsochromic shift of the

transitions

Figure3.Calculated transition densities for a)FC-S1and b)FC-T1of

1(left)and2(right),respectively,at the BHandHLYP-D3BJ/def2-TZVP

level of

theory.

Figure4.a)Emission(solid)and excitation spectra(dashed)of ab-PBI

(only3a shown).b)Emission(solid)and excitation spectra(dashed)

of1and2.All of the spectra were measured in degassed dichloro-

methane at298K.

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originating from these frontier orbitals is observed in comparison to the parent PBI,which counts for the singlet excited states but even more for the triplet excited states(see triplet energies above).The accompanying smaller distance between the PBI S1and T1state and the presence of the intermediary MLCT state at520nm in1facilitate the T1 population(for further details,see the Supporting Informa-tion,Figure S18).

The emission lifetime of the ruthenium1and iridium2 complexes at room temperature in degassed dichloromethane were found to be 4.2m s and33m s,respectively.Typical phosphorescence lifetimes of purely organic PBIs are around 100m s[19]and thus the shorter lifetime of1indicates a strong spin–orbit coupling contribution of the metal center. Although iridium is the heavier5d metal,this effect seems to be more pronounced for1than for2,which can be attributed to the higher degree of MLCT in the emissive state of the Ru complex(see above).Furthermore,[Ru(bpy)2(ab-PBI)][PF6]21displays a higher phosphorescence quantum yield(F p=11%)than2(F p<1%),and furthermore shows a more efficient phosphorescence than found for other ruthenium polypyridyl complexes,[20]exceeding even the6% efficiency of[Ru(bpy)3]2+itself(Supporting Information, Table S2).

In conclusion,we have reported the first example of a PBI-based transition-metal complex in which the metal fragment not only triggers the population of the triplet state in the PBI by ISC,but also shows a remarkable intense NIR phosphorescence with F P=11%for[Ru(bpy)2(ab-PBI)]-[PF6]21.Photophysical and theoretical studies of the PBI-based Ru1and the Ir2complex underline the significance of metal orbital interactions with the PBI ligand for the spin-forbidden transitions S1!T n and T1!S0.The fact that quantum yields in the NIR spectral region are intrinsically decreased due to the“energy-gap law”problem[21]makes the complex[Ru(bpy)2(ab-PBI)][PF6]21with its high light absorptivity an interesting NIR-phosphorescence material. Received:October24,2014

Published online:&&&&,&&&&

.Keywords:dyes/pigments·iridium·perylene bisimides·phosphorescence·ruthenium

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Communications

Phosphorescent Complexes

M.Schulze,A.Steffen,*

F.Würthner*&&&—&&&

Near-IR Phosphorescent Ruthenium(II)

and Iridium(III)Perylene Bisimide Metal

Complexes

The attachment of a ruthenium(II)or

iridium(III)metal complex to the perylene

core of an azabenzannulated perylene

bisimide(ab-PBI)leads to strong phos-

phorescence out of the PBI triplet state

after visible-light absorption.The near-IR

phosphorescence of the ruthenium com-

plex has a quantum yield(F p)of11%,

which is remarkably high in comparison

to other NIR emitters.

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