Phosphorescent Cationic Iridium(III)Com p lexes w ith 1,3,4-Oxadiazole Cyclom etalating Ligands:Solvent-Dependent Excited-State Dynam ics

Zhuorn KungXin WngZhen WngGuiying HeQinjin GuoLeiHeAndong Xi

a.Beijing National Laboratory for Molecular Sciences(BNLMS),Key Laboratory of Photochem istry, Institute ofChem istry,Chinese Academy ofSciences,Beijing 100190,China

b.University of Chinese Academy of Sciences,Beijing 100049,China

c.College ofChem istry and Chem ical Engineering,Central South University,Changsha 410083,China

Phosphorescent Cationic Iridium(III)Com p lexes w ith 1,3,4-Oxadiazole Cyclom etalating Ligands:Solvent-Dependent Excited-State Dynam ics

Zhuoran Kuanga,b,Xian Wanga,b,Zhen Wangc,Guiying Hea,b,Qianjin Guoa,LeiHec?,Andong Xiaa,b?

a.Beijing National Laboratory for Molecular Sciences(BNLMS),Key Laboratory of Photochem istry, Institute ofChem istry,Chinese Academy ofSciences,Beijing 100190,China

b.University of Chinese Academy of Sciences,Beijing 100049,China

c.College ofChem istry and Chem ical Engineering,Central South University,Changsha 410083,China

To elucidate the nature of low-lying trip let states and the effect of ligand modifications on the excited-state properties of functional cationic iridium com p lexes,the solventdependent excited-state dynam ics of two phosphorescent cationic iridium(III)com p lexes, namely[Ir(dph-oxd)2(bpy)]PF6(1)and[Ir(dph-oxd)2(pzpy)]PF6(2),were investigated by fem tosecond and nanosecond transient absorption spectroscopy.Upon photoexcitation to them etal-to-ligand charge-transfer(M LCT)states,the excited-state dynam ics showsa rapid process(τ=0.7?3 ps)for the formation of solvent stabilized3M LCT states,which significantly depends on the solvent polarity for both 1 and 2.Sequentially,a relatively slow process assigned to the vibrational cooling/geometrical relaxation and a long-lived phosphorescent em issive state is identified.Due to the diff erent excited-state electronic structures regulated by ancillary ligands,the solvation-induced stabilization of the3M LCT state in 1 is faster than that in 2.The present results provide a better sight of excited-state relaxation dynam ics of ligand-related iridium(III)com p lexes and solvation effects on trip letmanifolds.Key words:Iridium comp lex,Phosphorescence,Metal-to-ligand charge transfer,Transient absorption,Solvation

I.INTRODUCTION

In recent years,the study of phosphorescent cyclometalated iridium(III)com p lexes have attracted enormous attention from academ ic to industrial communities in the development of phosphorescent organic light-em itting diodes(PHOLEDs)due to their potentialapp lications in full-color disp lays and energy-saving solid-state lightings[1–3].In addition to their superior photolum inescence and electrolum inescence properties, cyclom etalated iridium(III)com p lexes have been found to act as long-lived trip let photosensitizers w ith outstanding performances,which extensively expand app lications in trip let-triplet annihilation(TTA)[4,5],dyesensitized solar cells(DSSCs)[6],sensitization of singlet oxygen[7],and photocatalytic water sp litting[8].

As a third-row transition metal cation w ith an electron configuration of 5d6,iridium(III)is capable of form ing cyclom etalated cationic com p lexes,referred as [Ir(C∧N)2(N∧N)]+A?,w ith the cyclometalating ligands (C∧N),the ancillary ligand(N∧N)and the counter an-ion(A?)[9,10].For this typeof iridium(III)com p lexes, the frontier m olecular orbitals are delocalized over the entire m olecule w ith rem arkab le charge-transfer characteristics.The highest occupied molecular orbitals (HOMOs)are usually delocalized on the d orbitals of iridium(III)andπorbitals of cyclom etalating ligands, while the lowest unoccupied molecu lar orbitals(LUMOs)have dom inant contributions fromπ?orbitals of ancillary ligands[11,12].The HOMO and LUMO energies can be m odified by respectively alternating the C∧N and N∧N ligands,and then the characteristics of excited-states and the photochem ical performances are artificially controlled[12].The photoexcitation mainly promotes three electronic transitions,which are characterized as metal-to-ligand charge-transfer(MLCT), ligand-to-ligand charge-transfer(LLCT)and ligandcentered(LC)transitions[12].Im portantly,the excited singlet states undergo very rapid intersystem crossing (ISC),resulting in the excited trip let manifold population,as a consequence of a high spin-orbit coup ling(SOC)constant(3909 cm?1)com pared to some well-known octahedral coordination com p lexes of Fe (431 cm?1),Ru(1042 cm?1),Os(3381 cm?1),which is in direct proportion to the biquadrate of the atom ic number[13,14].Thus,the strong SOC in iridium(III) com p lexes achieves the internal quantum effi ciency ap-proaching～100%[1,3,15–17].The kinetic of ISC process of some iridium(III)comp lexes has been investigated using fem tosecond photolum inescence spectroscopy,which reveals that the ISC has a tim e scale down to or even less than 100 fs[18,19].The rate of the ultrafast ISC is equivalent to that of internal conversion(IC),indicating that the spin-fl ipped transition is not a rate-determ ining process[20].Furthermore,the spin-forbidden nature of the transitions from the trip let manifold to theground statearemoderately removed by the strong SOC,inducing the intense room-tem perature phosphorescence[21].

For iridium(III)com p lexesw ith effi cient ISC,the lowest triplet state em itting phosphorescence,according to Kasha’s rule,iseither a3MLCT ora3LC state[22].The relaxation processes of excited-states,which are accompanied by the intram olecular charge-transfer(ICT),are significantly affected by solvation effects[23–25].A fter the redistribution of charge density on solutemolecules induced by the photoexcitation,the surrounding solvent electric dipoles rearrange around the excited solute molecules,which affects the excited-state relaxation processes[26,27].By investigating the solutesolvent interaction,especially the dynam ics response, the sequential evolution am ong the excited-statemanifold could be resolved clearly w ith the aid of ultrafast pum p-probe transient absorption techniques[28].So far,for a w ide variety of ligand-dependent functional transition m etal com p lexes w ith effi cient ISC,excitedstate deactivation processes from Franck-Condon excitations to em issive triplet states,which are accompanied w ith changes of electronic configuration,solvent response,geometric structure and thermal equilibrium, still remain a sub ject of debate and confusion regarding the photophysical nature and characteristic time scale [29–33].

Currently,much attention concentrates on the structure-property relationship of heavy metal comp lexes to design,modify and optim ize their photolum inescence and photosensitization properties,where ligands are the key factors to control the relative energies of frontier orbitals and excited-state relaxation dynamics[34].We recently reported the synthesis of[Ir(dphoxd)2(bpy)]PF6(1)and[Ir(dph-oxd)2(pzpy)]PF6(2) (their m olecular structures are shown in Scheme 1), w ith the cyclom etalating ligand of 2,5-diphenyl-1,3,4-oxadiazole(dph-oxd)and the ancillary ligands of 2,2′-bipyridine(bpy)and 2-(1H-pyrazol-1-yl)pyridine (pzpy),respectively[10].Due to the stabilization of HOMO levels induced by the electron-deficient oxadiazole heterocycle in dph-oxd,comp lexes 1 and 2 have w ideenergy gaps(～2.8?3.5 eV)and effi cient blue-green (480?580 nm)phosphorescent em issions[10,34].In addition,com plex 2 in which theelectron-rich pzpy substitutes the bpy,presents a largely enhanced LUMO level relative to com p lex 1.A lthough their basic photophysical and electrochem ical properties have been discussed, the excited-state dynam ic processesof com plexes 1 and 2 are unclear and need to be investigated.Above all, elucidating the nature of their low-lying trip let states is ofgreat significant to understand the origin of phosphorescence and further reveal the ligand-related excitedstate behaviours in iridium(III)com p lexes.

Schem e 1 M olecular structures of(a)[Ir(dph-oxd)2(bpy)] PF6(1)and(b)[Ir(dph-oxd)2(pzpy)]PF6(2).

In the present work,to reveal and understand the photolum inescence properties and solvent-dependent excited states dynam icsof iridium(III)com p lexes in the excited trip let m anifold,ultrafast transient absorption spectroscopy isem p loyed for comp lexes1 and 2 in solutions.Threeaprotic solvents(chloroform,tetrahydrofuran and acetonitrile)w ith diff erent polaritiesareused to monitor the excited-state relaxation dynam ics and the electronic state evolutions.Focusing on the solvation response induced by dipole-dipole interaction,the temporal electric dipole change of excited com p lexes could be identified.W ith steady-state spectralmeasurem ents and theoretical calculations,the ICT characteristic in excited states of both comp lexes have been intensively studied.The fem tosecond transient absorption spectroscopy upon theM LCT stateexcitation showsa rapid processattributed to thegeneration of solvent stabilized3M LCT state,which is relevant to the solvent polarity. In addition,the vibrational cooling/geom etrical relaxationsand long-lived phosphorescent em ission statesare also identified.

II.EXPERIM ENTS

A.M aterials

The synthesisof[Ir(dph-oxd)2(bpy)]PF6(com p lex 1) and[Ir(dph-oxd)2(pzpy)]PF6(com p lex 2)has been described in detail elsewhere[10].Briefl y,for the synthesis of com p lex 1,[Ir(dph-oxd)2Cl]2reacted w ith bpy in CH2Cl2/CH3OH.A fter evaporation,and the residual was dissolved in deionized water and KPF6was added to the solution to form[Ir(dph-oxd)2(bpy)]+.The synthesis of comp lex 2 was sim ilar to that for comp lex 1, except that pzpy substituted bpy.Purified comp lexes were dissolved in solvents(i.e.dichloromethane,acetonitrile,tetrahydrofuran)for spectroscopic m easurements.A ll solvents involved were spectroscopic gradeand used as received.

B.Quantum chem ical calculations

On the basis of density-functional theory(DFT), quantum-chem ical calculations were performed to obtain the optim ized geom etries and electronic structures for com plex 1 and com plex 2.The method using B3LYP w ith basis sets of 6-31G(d,p)for C,H,N and O,and LANL2DZ for Ir was em p loyed to fu lly optim ize the ground-state geom etries w ithout any symm etry constraints.The verticalexcitation energies of lowlying excited-states were calculated on the optim ized ground-state geom etriesw ith the tim e-dependent DFT (TD-DFT)approach.Considering the solvent eff ects, self-consistent reaction field(SCRF)theory adopting the polarized continuum model(PCM)wasapp lied.A ll calculationswere carried out for isolated m olecules and conducted w ith the Gaussian 09 software package[35].

C.Steady state and transient absorption spectral measurements

The steady-state ultraviolet-visible absorption and photolum inescence spectra werem easured on a U3010 (Hitachi)spectrometer and an F4600(Hitachi)fluorescence spectrometer,respectively.The nanosecond transient absorp tion measurementswere perform ed using a nanosecond flash photolysis spectrometer(LP920, Edinburgh Instruments),excited by a Nd:YAG laser (Quanta-Ray,Spectra-Physics)at 355 nm w ith the FWHM of 8 ns.The fem tosecond transient absorption spectra w ith～100 fs time-resolution were measured on a home-built fem tosecond broadband pum p-probe setup,which has been described elsewhere[36,37]. Briefl y,a pulsew ith 400 nm,50 fs,90 nJand 1 kHz from a regenerative amplified fem tosecond laser acts as the pum p beam focused(spot size is about 130μm)on the sam p le.A white light supercontinuum(420?780 nm) generated by a water cell acts as a probe beam after an opticaldelay up to 1 ns.The thicknessof flow ing samp le cells is 1mm for transientmeasurements.For isotropic m easurements,the angle of pum p and probe beam polarization was set to themagic angle(54.7?).Before the data analysis,the chirp correction has been conducted to calibrate the spectral chirp caused by group velocity delay dispersion of the supercontinuum probe beam in fem tosecond transient absorption spectra.The tim e evolution differentialabsorbance?A(t,λ)wasanalyzed by singular value decom position(SVD)and globalanalysisusing G lotaran and TIMP software package[38,39].?A(t,λ)is a superposition of several principal spectral com ponentsεi(λ)weighed by their concentrationsci(t) [38,40]:

FIG.1 Normalized steady-state absorp tion and PL spectra of(a)com p lex 1 and(b)com p lex 2.The PL spectra were measured w ith excitation at 400 nm.The PL spectra at 77 K werem easured in M eCN glass.

III.RESULTS AND DISCUSSION

A.Steady state spectra

The normalized absorp tion and lum inescence spectra of com p lexes 1 and 2 in acetonitrile(M eCN),tetrahydrofuran(THF)and chloroform(CHCl3)at 298 and 77 K(in MeCN glass)are shown in FIG.1.The profi les of the absorption spectra are sim ilar for both comp lexes 1 and 2 in diff erent polar solvents.The intense absorption bandsbelow 350 nm in ultraviolet region are assigned to the spin-allowed1π-π?transitionsof the cyclometalating ligands or ancillary ligands,form ing the intramolecular1LC states[41].These bands extending from 350 nm to 450 nm show relatively lower absorption features,which are ascribed to spin-allowed or forbidden1M LCT/3M LCT states,1LLCT/3LLCT states and weak3LC transitions[41].Here,the spin-forbidden transitions aremoderately released by the strong SOC endowed by the heavy atom eff ect,which brings a certain extent of absorption and effi cient phosphorescence em ission at room-temperature[14].

Both comp lexes 1 and 2 em it green phosphorescence at 298 and 77 K w ith structured em ission spectra.Detailed em ission characteristics are disp layed in Table I. At 298 K,their em ission peaks show slight red-shifts relative to those at 77 K.The structured phosphorescence em ission curves w ith weak solvatochrom ism and rigidochrom ism indicate that3π-π?character dominates the lowest em issive trip let states[12].The sim-ilar em ission curves of com p lexes 1 and 2 indicate that their em ission properties are dom inated by the cyclometalating ligands of dph-oxd,which bears an electron-w ithdraw ing group,form ing the em issive dphoxd-centered(3π-π?)states[10,42,43].

TABLE I Em ission characteristics of com p lexes 1 and 2 in solutions.εis the dielectric constant of the solvent[44].

FIG.2 TDDFT calculated MOs contour p lots of com p lex 1 in MeCN associated w ith1MLCT transitions.

For general iridium(III)complexes,on account of the spin-fl ip exchange energy,the energetic order of these transition is1LC>1M LCT>3M LCT>3LC[12].The lowest triplet states,which em it effi cient phosphorescence,have a hybrid feature of3MLCT and3LC.It is found that the lum inescence quantum yields of both com p lexes 1 and 2 are remarkably solvent-dependent, as shown in Table I.For complex 1,the quantum yields aremeasured to be～0.31 in MeCN(ε=35.94),～0.48 in THF(ε=7.58)and～0.68 in CHCl3(ε=4.81),which are sim ilar to those for com p lex 2.For com p lexes1 and 2,strong non-radiative transitions induced by solvation occur before reaching the lowest em issive state,leading to the quenching of lum inescence in highly polar solvents[45].In this case,the3M LCT states,which are closely associated w ith solvation processes,are located at higher energy than the em issive3LC states.

In our previouswork,the em ission spectrum of comp lex 1 exhibited an obvious red-shift and a structureloss when increasing its doping concentration in the poly(methylm ethacrylate)m atrix,whereas com p lex 2 always exhibited structured em ission spectra w ith negligible red-shifts.A lso,a reversible piezochrom ic behaviorwasobserved for com p lex 1[10].Theseexperimental results indicate that for com p lex 1,the LC3π-π?and3MLCT/3LLCT triplet states are close-lying in energy and account for the finalem itting states[10,46–50].In contrast,the lowest trip let states of com p lex 2 m aintain dom inant3π-π?character w ith less attributes of ICT character[42,51,52].Quantum chem ical calculations are expected to have amore accurate description of the excited-state electronic structures,which is helpful to understand the solvent-dependent excited-state relaxation processes.

B.Quantum chem ical calculations

FIG.3 TDDFT calculated MOs contour p lots of com p lex 2 in M eCN associated w ith1M LCT transitions.

On the basisof DFT and TD-DFT calculations,optim ized ground-state geometriesand the contour p lots of molecular orbitals for com p lexes 1 and 2 in M eCN are shown in FIG.2 and 3;their corresponding simulated energies,oscillator strengths and main orbital contributions of vertical transitions,including the1M LCT transitions,are depicted in Tables IIand III.

In M eCN as solvent,all frontier orbitalsexhibit localizationsand distributionson segmentsof themolecules, as shown in FIG.2 and 3.For both com p lexes1 and 2, the HOMO orbitalsare localized on d orbitalsof iridium ions andπorbitals of the dph-oxd ligands.The LUMO of com p lex 1 is delocalized over the bpy ligand,and the LUMO of com p lex 2 isdelocalized over the dph-oxd ligand.For com p lex 2,the localization of LUMO on dphoxd rather on pzpy should be caused by theelectron-rich characteristic(leading to higher energy levels of unoccupied m olecular orbitals)of the pzpy ligand.Accordingly,the absorption bands between 360 and 450 nm, as shown in FIG.1,are mainly attributed to the contributions from Ir→ligands(1MLCT,d→π?)and ligand-centered(1LC,π→π?)transitions.M ore specifically,the1M LCT/1LC transitions(Ir→dph-oxd/dphoxd-centered,S0→S2for 1 and S0→S1for 2)have major contribution to the 400 nm(Ephoton=3.10 eV) absorption for both com p lexes 1 and 2.For comp lex 1,the1M LCT/1LLCT transition(Ir→bpy/dphoxd→bpy,S0→S1),as shown in Table II,exists at a relatively low energy(2.87 eV)and has considerable contribution to the 400 nm absorption.Nevertheless,for com p lex 2,the1M LCT/1LLCT(Ir→pzpy/dphoxd→pzpy,S0→S2)transition,as shown in Table III, has an excessive excitation energy(3.31 eV)that is larger than 400 nm absorption.Therefore,for com p lex 2,the1MLCT/1LC transition dom inates the 400 nm absorption.Sim ilarly,in THF and CHCl3,the S0→S1transition of com p lex 1 exhibits the1M LCT/1LLCT (Ir→bpy/dph-oxd→bpy)character w ith relatively low excitation energies(～2.8 eV),whereas in comp lex 2,the1M LCT/1LLCT transitions show higher excitation energies(～3.2 eV)and the1MLCT/1LC(Ir→dphoxd/dph-oxd-centered)transitions show the lowest excitation energies.

TABLE II TDDFT calculated electronic transition p roperties of com p lex 1 in M eCN(Et:transition energy,f:oscillator strength).

TABLE III TDDFT calculated electronic transition p roperties of com p lex 2 in MeCN.

Furtherm ore,from our previous calculation results on trip let manifolds[10],for com p lex 1,the T 3 (3MLCT/3LLCT,Ir→bpy and dph-oxd→bpy)and T1(3LC/3MLCT,dph-oxd-centered and Ir→dph-oxd) states have close-lying energy levels(2.70vs.2.54 eV). For com p lex 2,the3M LCT/3LLCT state(T 7)lies much higher in energy than T1(3.11vs.2.55 eV)and the lower triplet states all have dom inant3LC/3MLCT character.

Therefore,both com p lexes 1 and 2 could be selectively excited to the1MLCT states upon the 400 nm excitation.Due to their sim ilar structures but diff erent ancillary ligands,the low-lying singlet and trip let states of com p lex 1 have M LCT/LLCT character w ith remarkable changes of electric dipole moments;however,for com plex 2,only the MLCT transition could be induced,form ing partial ICT states along w ith less changes of electric dipole mom ents.The noteworthy ICT diff erences between com plexes 1 and 2 could be further determ ined by the follow ing time-resolved transient absorp tion measurements,where the solventdependent excited-state dynam ics for both com p lexes 1 and 2 were explored.

C.Fem tosecond transient absorption spectra

The fem tosecond transient absorption spectra of comp lexes 1 and 2 in MeCN upon 400 nm excitation and the transient absorption kinetics curves are depicted in FIG.4.It is found that,the board excited-state absorption(ESA)bands are generated in initial several hundred fem toseconds.In a few picoseconds,the ESA bands decay gradually.Then,they become stable in the entire delay tim e down to 1 ns,indicating a longlived spectral species left.According to steady-state spectra of com p lexes 1 and 2,the ground-state absorptions aremainly below 420 nm.Thus,there is negligible spectral overlapping of ground-state bleaching w ith the ESA spectral range in the observed transient absorption spectra.Furthermore,due to the heavy atom effect,the strong SOC results in an ultrafast ISC w ithin the tim e scale of severalhund red fem tosecondsafter the Franck-Condon excitation,leading to an accumulation of excited electrons on the trip let manifolds[30,41]. According to the transition selection rules,the transitions between the trip let and the singlet excited-states are highly forbidden.Therefore,there is no stimulated em ission in the observed transient absorption spectra.

In order to exp lore the relaxation dynam ic processes of excited states,global analysiswas used to obtain the best fi ts of the time-resolved absorption spectra w ith a sequentialmodel,as shown in Scheme 2.For comp lex 1 in M eCN,three lifetim es are required to adequately fi t the evolution dynam ics data.As depicted in FIG.4(c),the rapid,slow and long-lived dynam ic processesw ith the time constants of 0.7 ps,20 ps,and>1 ns are obtained from the iterative calculations,respectively.The absolute am p litude of each dynam ic curve represents the time-dependent concentration evolution of three components,respectively[53].The fi rst rapid process about 0.7 ps is assigned to the ultrafast ISC and the inertial part of solvent response[54–58], leading to the formation of3MLCT state[41,58].Simultaneously,the energy of themolecular system dissipates to the solvent environm ent by the inertial part of solvent response,further lowering the energy of the3MLCT state[40].The slow component about 20 ps is attributed to the intramolecular vibrational relaxation (IVR)and internal conversion(IC)of the trip letm anifold,including thegeometrical relaxation of ligandsand aromatic nucleus[41,59],together w ith the diffusive part of solvation response[55],resulting in the form ation of the lowest trip let state.The slowest com ponent w ith a time constant of>1 ns is attributed to the relaxation from the lowest em issive trip let state to the ground stateby radiativeand non-radiative relaxations. Actually,the lifetim e should be around～1μs,which is further measured by the nanosecond transient absorption spectroscopy in the condition ofair-saturated solution.As shown in FIG.5,the nanosecond probe transient spectra taken at the delay of 10?800 ns for comp lex 2 in MeCN were p lotted w ith the phosphorescence em ission curve and the fem tosecond transient absorption spectrum taken at 1 ns.Considering the em ission curve,the 100 ns time scale transient absorption spectral profi les are spectrally identicalw ith those taken at 1 ns.The nanosecond transient absorption decay monoexponentially back to the ground state,and no intermediate state is observed.Thus,these spectral decayindicates the lifetim e of the lowest trip let state,while the fi tting results of comp lexes 1 and 2 were shown in Table IV.

FIG.4 Fem tosecond transient absorp tion spectra of(a)com p lex 1 and(b)com p lex 2 in MeCN follow ing the excitation at 400 nm.Transient absorption kinetics curves of(c)com p lex 1 and(d)com p lex 2 probed at 570 nm.The dotted,dashed and dashed-dotted lines obtained from the best fi tting,represent the rapid,slow,and the long-lived com ponents,respectively. The solid curves are the best-fi tted data as indicated for show ing the quality of global fi tting results.

Schem e 2 Relaxation pathway of cationic iridium(III) com p lexes 1 and 2.IC=internal conversion,ISC=intersystem crossing,GS=ground state,M LCT=m etal-to-ligand charge-transfer state,LC=ligand-centered state.

FIG.5 Fem tosecond and nanosecond transient absorp tion spectra of com p lex 2 in MeCN taken at the delay of 1 and 10,200,400,600,800 ns,along w ith the phosphorescence em ission spectral p rofi le.

For com p lex 2 in M eCN,as depicted in FIG.4(b) and(d),three lifetimes of 0.8 ps,7 ps,and>1 ns are obtained from the global fi tting.Sim ilarly,these components are assigned to the solvent stabilized3MLCT state,vibrational cooling and geom etrical relaxation w ithin the trip letmanifold,and long-lived3LC,respectively.

FIG.6 Fem tosecond transient absorption spectra of(a)com p lex 1 and(b)com p lex 2 in CHC l3follow ing the excitation at 400 nm.Transient absorption kinetics curves of(c)com p lex 1 and(d)com p lex 2 p robed at 570 nm,the dotted,dashed and dashed-dotted lines obtained from the best fi tting,rep resent the rapid,slow,and the long-lived com ponents,respectively; the solid curves are the best-fi tted data as indicated for show ing the quality of global fi tting dotted.

Generally,the excited state dynam ics,especially for moleculesw ith ICT character,are intensely aff ected by the nature of solvent[60].Besides the strong polar solvent M eCN(ε=35.94),we further investigated the excited-state relaxations of complexes 1 and 2 by femtosecond transient absorption spectroscopy in less polar solvent CHCl3(ε=4.81)and THF(ε=7.58)[44].As shown in FIG.6,sim ilarly to the spectral evolutions in MeCN,the broad ESA bands form rapidly after the optical excitations,followed by a decay in the time scale of several picoseconds till a long-lived species remained w ithin themeasuring time range.Through the global analysis,three components corresponding to the rapid, slow,and long-lived dynam ic processes,respectively,are also obtained.A ll fi tting results are listed in Table IV.

The solvation mainly affects the rapid decay component w ithin～1 ps,which includes the ISC and the generation of solvent stabilized ICT states[59,61].For com p lex 1,the solvent-dependent tim e of the rapid process increases from 0.7 ps in MeCN,and 1.0 ps in THF to 1.4 ps in CHCl3,along w ith the decrease of the solvent polarity.The sam e tendency is observed for comp lex 2 as well.In a polar solvent,such as M eCN,the3MLCT is highly stabilized by the fast solvation because of the strong dipole-dipole interaction,while the potential energy surface lowers and lies at an energy level close to the em issive3LC state[40].According to the energy gap law,such solvation-induced stabilization results in a decrease of the trip let-state lifetim e and the lum inescence quantum yield[60].In contrast, in less polar solvents,such as THF and CHCl3,the3M LCT states are destabilized w ith a higher energy to3LC statesbecauseofweak solvation,leading to longerlived em issive trip let states and higher quantum yields.

TABLE IV Solvent-dependent relaxation dynam ics parameters for com p lexes 1 and 2 after 400 nm excitation.

Furthermore,comp lexes 1 and 2 show a small discrepancy in solvent-dependent excited-state dynamics due to their different excited-state ICT features. As shown in the steady-state spectra and quantum chem ical calculations,the lowest excited-states of comp lexes 1 and 2 both contain dom inantπ-π?characteristics.However,considerable MLCT/LLCT transitionsof com plex 1 can be induced by the low-energy excitation at 400 nm.In fem tosecond transient absorption spectra,the lifetim es(τ1)for the form ation of solvent stabilized3M LCT states are relatively shorter for comp lex 1 than that for complex 2 in the same solvent as shown in Table IV.Because solvent molecules rearrange around the excited molecu les according to the charge-density redistribution[40],the solvation process is faster and more drastic for solutemolecules containing strong ICT character[60].As shown in Scheme 2, the S0→S1/S2(1M LCT/1LLCT)transitions of comp lex 1 can be effi ciently excited w ith 400 nm excitation (Ephoton=3.10 eV),which facilitates the population of T3(3M LCT/3LLCT)through the rapid ISC.During this process,the charge density of com p lex 1 is highly separated,and molecules on ICT states are solvated simultaneously till the charge recombination happens. For com p lex 2,after the S0→S1(1M LCT)excitation at 400 nm,excited m olecules undergo ISC quickly and accumulate on triplet manifolds.Due to the excessive energy levelof3LLCT/3MLCT(T7)state,excited comp lex 2 at1M LCT state has greater odds to straightly relax to lower-lying trip let states w ith weak ICT character,leading to weaker and slower solvation relative to that of com plex 1.The fi rst component in relaxation process is associated w ith the solute ICT property and solvent polarity.The second com ponent includes the IC from the3MLCT state to the lowest em issive3LC state, togetherw ith an electron-hole recombination in excited states along w ith a solvation process of an IVR of the solutemolecules about～10 ps.Specifically,the octahedral coordination complexesmay also havemultip le rotationaland vibrationaldegreesof freedom since comp lexes 1 and 2 have diff erent ancillary ligands,which can not be identified from observed results.

IV.CONCLUSION

In this work,we have investigated two phosphorescent cationic iridium(III)com plexes[Ir(dphoxd)2(bpy)]PF6(1)and[Ir(dph-oxd)2(pzpy)]PF6(2)to reveal their solvent-dependent photolum inescence propertiesand excited-state dynam ics.Based on the steadystate spectra and quantum chem ical calculations,both com plexes have remarkable MLCT character in their low-energy transitions,while the LLCT transition in com plex 1 is easier to be formed than that in comp lex 2 upon 400 nm excitation.Through the transient absorption spectroscopy measurements w ith the excitation of1M LCT state,we have figured out the formation(τ=0.7?3 ps)of solvent stabilized3MLCT via ISC,which is affected by the solvent polarity.In addition,the vibrational cooling/geometry relaxation (τ=5?20 ps)and long-lived phosphorescent em ission processes(τ=～1μs)were identified.

In summ ary,for spin-m ixed phosphorescent iridium(III)com p lexes,the solvation inducesa prom pt generation of solvent stabilized3MLCT states and lowers the potential energy surfaces,which indeed influence the phosphorescence quantum yield.The present work provides a better insight of solvent dependent em ission propertiesand excited state relaxation dynam icsof iridium(III)comp lexes.

V.ACKNOW LEDGM ENTS

This work is supported by the National Basic Research Program of China(No.2013CB834604),the National Natural Foundation of China(No.21673252, No.21333012,No.21373232,and No.51403240)and the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB12020200).

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ceived on March 29,2017;Accepted on May 16,2017)

?Authors to whom correspondence shou ld be add ressed.E-m ail: andong@iccas.ac.cn,helei06@csu.edu.cn