J. C.S. Faraday I, 1980, 76,162-1 73Inorganic Photophysics in SolutionPart 4.-Deactivation Mechanisms of the 2Ea State of Crxsl Complexes from LifetimeStudies1BY STEPHEN R. ALLSOPP, ALAN Cox, TERENCE J. Urn* AND W. JOHN REEDDepartment of Chemistry and Molecular Sciences, University of Warwick,Coventry CV4 7AL, West MidlandsANDSILVANA SOSTERO AND OUZIO TRAVERSO1st it u to Chimico dell’ Univer siti di Fer rar a,Via L. Borsari 46, 44100 Ferrara, ItalyReceived 26th February, 1979The temperature dependences of the luminescence lifetimes, 7lWn, of several CrIII complexes ofparticular photochemical interest have been determined over temperature ranges 77-370 K in avariety of media, especially 9 mol dmm3 LiCl+ HzO and cellulose acetate film. In most cases, kl,,fits an expression k ~ 1 + A m exp( -AEzR/RT) where the subscripts refer to temperature-independentand (temperature-dependent) non-radiative terms. The strong solvent-dependence of AE&, whichis always only a fraction of the “TzB.;Eg splitting, suggests a chemical pathway for deactivation ofthe 2E’ state rather than a purely physical route such as back-intersystem crossing. For complexeswhich have been examined in a wide variety of media, values of In ANR correlate fairly well withthose of AEzR, implying a common overall mechanism.~One of the persisting themes of inorganic photochemistry for over a decade hasbeen the identification of the photoreactive state of octahedral CrlI1 complexes.2 Byanalogy with the situation established for polyacenes, initial discussion centred 2 *around the weakly absorbing (and in many instances luminiscent) 2Eg state which wasknown (i) to be of longer lifetime than the lowest quartet state, 4T2g, and (ii) to bepopulated by intersystem crossing from the latter (fig.1). Later approaches 2 *noted that additives capable of quenching phosphorescence from the 2Eg level ofcertain CrII’ complexes did not always quench their photochemical activity, whilstdirect population of 2Eg states either by energy transfer 2 b s or by laser photolysis6did not result in the same quantitative photochemistry realised by direct irradiation intothe quartet states and opinion moved generally in favour of the 4T2g state as the levelresponsible for Cr”’ photochemistry.2 More recent work has re-established aphotochemical role for 2Eg states, although it remains to be resolved whether this ismerely as a source, through back-intersystem crossing, of (reactive) 4T2g states or as agenuine photoreactive state per se.Thus Maestri et al. 8a suggest that 97 % of thephotoaquation of [Cr(bi~y)~]~+ (bipy = 2,2’-bipyridine) proceeds directly through the2Eg state, whilst Sandrini et aLSb propose that the photoaquation of trans-[Cr(en),(NCS),]+ involves two routes, i.e. 20% directly through 4T2g and 80% byback-intersystem crossing into 4T2g from the 2Eg level. Another recent view is thatof regarding the 4T2g and 2Eg states as separate minima on a continuous potentialsurface. b* 8 b The Jablonski diagram for CrIII is further complicated by the existenceof several cases of CrlllJluorescence, e.g.in [Cr(urea),13+ ion, which exhibit very largeStokes shifts, typically of the order of several thousand cm-l, with emission maxima16ALLSOPB, COX, KEMP, REED, SOSTERO, TRAVERSO 163of considerably longer wavelengths than the phosphorescence, indicating the emittingstate of the 4T2g level to be greatly distorted from its initial Frank4ondon geometryand to be at a lower energy than the 2Eg level. (The appearance of fluorescence wasassociated later with a small 4T2e-2EcI splitting).9d This situation prompted thedesignation of " thermally relaxed excited " or " thexi " state for the fluorescent4T2g level.2e-photoreactionFIG. l.-Jablonski dia,gram for a CrIn complex of octahedral symmetry.ISC refers fo irrtprsysterncrossing ; BISC to back-intersystem crossing.Photophysical investigations of CrlI1 complexes have reflected the principalphotochemical problem, i.e. is luminescence from the 2Eg state quenched by (i) aphotochemical mechanism, (ii) by back-intersystem crossing, O (iii) by direct energytransfer into vibrational modes of the ligands or the solvent l1 or (iv) by some com-bination of (i)-(iii) which varies from complex to complex ? Methods of investigationof this problem have included measurements both of 4p and 7p and of their temperatureactivation (particularly of crystalline,1° but also of glassy and fluid sarnples),l2 ofjudicious changes in the ligand and solvent environments of the complex,ll* ofintersystem crossing rates and efficiencies1 and of lifetime dependence on excitation~avelength.'~ The general picture appears to be one where 2Eg states lose theirenergy by (a) a temperature-independent radiative pathway (possibly coupled with atemperature-independent non-radiative pathway) and (b) a temperatursdependentnon-radiative pathway .In this account we present data for 2Eg state lifetimes for several key CrlI1 com-plexes in (i) fluid and glassy aqueous solutions (9 mol dm-3 LiCl) over a wider tem-perature range than investigated hitherto,l (ii) organic solvents covering a wide rangeof polarity, (iii) polymer film.We have also analysed the data of Pfeil l6 into tern-perature-independent (TI) and temperature-dependent non-radiative (NR) compon-ents and have compared the results for AE& (in the total expression164 INORGANIC PHOTOPHYSICS I N SOLUTIONwith those given for CrXI1 complexes in crystalline and glassy alkanolic xnedia,loa*enabling a discussion of the character of the non-radiative processes.EXPERIMENTALLIFETIME MEASUREMENTSThese were determined by monitoring emission from solutions or glasses of the Cr"'complexes following delivery of a 25 ns pulse of 347 nm radiation.The means of tempera-ture control, measurement and processing of individual kinetic runs and of iterated fittingto eqn (1) have been given bef0re.l' A new frequency doubling crystal of rubidium dihydro-gen arsenate was employed.MATERIALSWater was multiply distilled from alkaline K[Mn04]. Organic solvents were of spectro-scopic grade.CrlI1 complexes were prepared by conventional routes. In the case of[Cr(phen),][ClO,J,, the very long room-temperature lifetime in deaerated aqueous solution,2 7 0 ~ s ~ which compares with a figure of 2 7 0 ~ s due to Serpone and Bolletta,12g could beachieved only by repeated recrystallisation from water combined with chloroform extractionof organic impurities.ABBREVIATIONSThese are as follows : bipy = 2,2'-bipyridine, phen = 1,lO-phenanthroline, acac = acetyl-acetone, en = 1 ,Zdiaminoethane, glycol = 1 ,2-dihydroxyethaneY terpy = 2,2',2"-ter-pyridine, DMF = dimethylformamide, tn = 1 ,=l-diaminopropane (often called trismethylene-diamine), CA = cellulose acetate.RESULTS AND DISCUSSIONFORM OF TEMPERATURE DEPENDENCEFor all complexes we have examined, irrespective of the solvent medium, plots ofIn klum (the luminescence decay constant) against T-l display two distinct regions,illustrated by fig.2(a)-(c). klum barely increases in the temperature range 77-200 K,4 6 8 10 12103 KITFIG. ALLSOPP, COX, KEMP, REED, SOSTERO, TRAVERSO 165t-121806lo3 KITlo3 KITFIG. 2.-Temperature dependence of luminescence lifetime of CrIII complexes. x , experimentalpoints ; full line, computer fit to eqn (1) (for individual parameters see table 1). (a) [Cr(en)J3+ in9 mol dm-3 LiCl + H20 ; (b) [Cr(phen)J3+ in cellulose acetate film ; (c) [Cr(ter~y)~]~+ in 9 mol dm-3LiCl+ H20.but at higher temperatures normal Arrhenius behaviour is found leading to an activa-tion energy, AE&, and a corresponding frequency factor, ANR.In some cases alldata points could be iterated by computer to an expression of the form described byeqn (l), yielding values for kTI, AE& and AN& In a few cases the iteration was un-successful : in some of these this is probably due to the presence of a phase transitionwithin the .matrix, but it was nonetheless possible to discern two principal terms of thetype described in eqn (1). Magnitudes of the three parameters are collated in table 1,together with those given elsewhere in the literature for the complexes we havTABLE 1 .-ACTIVATION PARAMETBRS FOR THERMAL DEACTIVATION OF LUMINESCENCE OF C r I n COMPLEXESLINE HOSTScomplex mediuM1 o-2kn/s-' 10-lodpq&-(1.09&0.88)x lo2- --(1.1 10 & 0.002) x lo2(9.18k0.09)~ 10'i1.87+ 0.04--1.05 x2 .9 ~ 103(1.3$0.7JX ---1.05 X(1.41+1.1)X2.1 x 1-0.36O.lS+0.05-ALLSOPP, COX, KEMP, REED, SOSTERO, TRAVERSO 167Y". wv,t 2Xn0.4O N rl- 0 0Xn I I3> 3 2 e B en3 UL4TABLE 1 .-continuedcomplex10-lOAm/~-medium 10-2k+-1MeOH+ H20+ glycol 2.62+ 0.10[Al(acac)31 (2.30k0.01) x lo1crystalline (7.82k 0.76) x lo150 % in [Al(a~ac)~] (4.27k0.40) x lo110 % in [Al(acac)J (2.2320.06)~ lo10.1 % in [Al(a~ac)~] (2.34k0.05) x lo1EtOH (2.1740.09) x lo1n-PrOH (2.04+ 0.24) x lo1n-BuOH (2.1040.08) x lo1n-C5H90H (2.07k0.13) x lo1n-C6H1 1OH (2.04k0.15)~ lo1n-C7H130H (2.28k0.06) x lo1n-C8H150H (2.1720.09)~ lo1isopentane + 3-methylpentane (2.31 k0.05) x lo1poly(methy1 methacrylate) (2.17k0.02) x 10'MeOH + H20 + glycolformamidef MeOH[for many other solvents, see ref.12(e)][Cr(terpy> 213+ HzO+ LiCl (1.7 +O.l)x lo1CA film (2.0520.09) x 10'5 % in A1Cl3.6D20 (2.36k0.62)~ lo1Cr3+ 2 % in NaMgAI(C20&.9H20 (1.16_+0.04)~ lo15 % in K3[Co(CN)6] (8.2010.35)~ether+isopentane+ alcohol -glycerin+ H2O - 2.88k0.8- [Cr(CN) 61 3-(8.26+0.90)(2.54k0.47)(1.54+ 3.33)(1.39k0.99)~(1.36k0.97)(7.70+ 1.43)(4.1 k 4 . 8(8.1 k 6.8)(8.2 & 13.9)(4.2 4 8 . 6(9.2 k 4 . 2(4.8 k 0 . 9(3.16+ 0.76)14.1 f1.4(2.1 + 1 . 1(7.8 2 8 . 8(3.0 k0.8)(1.7650.97)~(1.69k0.63)~(8.88k2.82)(4.03 k 4.74)(7.372 1.99)Adapted from ref.12(c) except for [Cr(CN)$'-, which was calculated from data of ref 12(e). b GraphicaALLSOPP, COX, KEMP, REED, SOSTERO, TRAVERSO 169examined (or their close analogues). We have also extracted activation parametersfrom the luminiscence lifetime data of Pfeil16 on a number of CrlI1 complexes insolution over fairly extended temperature ranges, which also reveal good fitting toeqn (1). Finally, we have fitted the data of Forster et aLIOa* on [Cr(acac),] lumines-cence in crystalline hosts, alkanolic media and in polymer film to eqn (1) to yieldvalues for kTI, ANR and AE& (table 1). Forster et preferred a more complexexpression to fit the temperature dependence of their data, i.e.but we have found that these data can be reasonably well-fitted by iteration to eqn (1),although the comparatively few data points associated with each temperature study(typically 12 to 15) naturally introduces a considerable standard deviation, particularlyin ANR of eqn (1).The lifetime of [Cr(ter~y)~]~+ is remarkably short at 300 K(z = 180 ns) compared with those of [Cr(bipy),I3+ (87.8 ps) and [Cr(phen)J2+(270 ps). This influence of the terpy ligand parallels that found for the analogousRu" complexes, thus 7298K for [Ru(bipy),12+ and [Ru(phen),12+ is 612 and 1280 ns,respectively, whilst [R~(terpy)~]~+ is essentially non-luminescent * (all data refer toaqueous 9 mol dm-, LiCl solution).MAGNITUDES OF ANR A N D AE&For a given CrIX1 complex, the values of Am and AE& can cover a wide range. Forexample, AE& (in kJ mol-l) for [Cr(phen),13+ varies from 17.962 1.2 in H20lZc and12.4kl.O in MeOH to 68.8f2.1 in cellulose acetate film.However, the latter verylarge energy requirement is compensated by NN 1O6-fold larger A factor and the variousdata for [Cr(phen),I3+ fit a Barclay-Butler plot [fig. 3(a)] reasonably well with a cor-relation coefficient of 0.994. A fair correlation (of Coefficient 0.927) between In ANRand AE$ is also exhibited by the activation data for [Cr(acac),] in 16 environments ofwidely differing character [fig. 3(b)]. Finally, the data of Wasgestian and coworkers1 2efor [Cr(CN)6]3- luminescence in a variety of organic solvents [fig. 3(c)] also show thissame correlation, but less exactly (correlation coefficient = 0.722). This type ofapproximate fit has been adduced l5 as evidence of a common mechanism for a seriesof reactions displaying a range of AE* values and has been used by Adamson 5a in adiscussion of the phosphorescence of [Cr(NH,), (NCS),]- in 12 solvents : in this casethe slope is 0.41 mol kJ-l, in good agreement with those of fig.3 (see legend).MECHANISM OF NON-RADIATIVE ENERGY-LOSS FROM 2Eg LEVELThe principal findings of this study are : (i) the (temperature-dependent) non-radiative energy-loss mechanism for each complex is adequately described by a singleenergy term and therefore may well refer to a single process ; (ii) the activation energyterm for this single process is extremely sensitive to environment for certain complexes,e.g. those of phen, acac and terpy ; (iii) despite (ij), good Barclay-Butler plots (for datacovering all media) are given in these particular cases, indicating the probability of acommon mechanism; (iv) (as noted before 12ai c* the magnitude of AE,f, is invari-ably only a fraction of the spectroscopic splitting of the 4T2g and 2Eg levels (table 1).Since the locations of the spectroscopic 4T2g and 2Eg levels for a given C Pcomplex are not noticeably solvent-sensitive the splitting of these levels is similarlyindependent of environment.Consequently it is to be expected that a mechanism of2Eg quenching by back-intersystem crossing, even though this may involve an activa-tion barrier (to a cross-over point) greater than the spectroscopic splitting energy(typically of magnitude 12c 5000-10 008 cm-1 or 50-100 kJ mol-I), should not sho170 INORGANIC PHOTOPHYSICS I N SOLUTIONCA filmL i C I - H0 20 40 60 80AE&/kJ mol-l7 "/013I I 1 I I I I I5 10 15 20 25 30 35 40A%*R/kJ mol-1FIG.ALLSOPP, COX, KEMP, REED, SOSTERO, TRAVERSO 1711012I I I 1 I20 25 30 35 40AE&/kJ mol-lFIG. 3.-Barclay-Butler plots for the Arrhenius parameter for the non-radiative decay term for CrIII(a) Data for [Cr(phen),]’* (slope = 0.32 k 0.02 mof kJ-’).(b) Data for [Cr(acac)J (slope = 0.48 k0.05 mol kJ-l). Key : I, 100 % crystal ; 2, 50 % infAl(acac)3]; 3, 10 % in [Al(acac)J; 4, 0.1 % in [AI(acac),]; 5, EtOH; 6, n-PrOH; 7, n-BuOH;8, n-pentanol ; 9, n-hexanor ; 10, n-heptanol ; 11, n-octanol ; 12, ether+isopentane+alcohol ; 13,isopentane+ 3-methylpentane ; 14, poly(methy1 methcrylate) ; 15, MeOH + H20 +glycol ; 16,[Al(aca~).~].(c) Data for [Cr(CN),I3- (slope = 0.52 0.2 mol kJ-l).Key : 1, DMF ; 2, MeCN ; 3, i-PrCN ;4, PhCN ; 5, i-PrOH ; 6, n-PrOH ; 7, EtOH ; 8, EtOD ; 9, MeOH ; 10, MeOD ; 11, methyrformamide ;12, fomamide.complexes in various media taken from table 1.much solvent dependence on AE*. This situation should also apply even if back-intersystem crossing occurs to a cross-over point 12a between the ”Eg level and a“ thermally relaxed ” 4T2g level, which would account for the s d l vdues of AE&.The large variation with solvent of AE& found in this work, therefore, points to theoperation of some other mechanism which embraces cmsjtderable partieiptkm bythe solvent atmosphere of the complex. One eminently suitable candidate must bea mechanism of ligand dissociation-return of the typ172 INORGANIC PHOTOPHYSICS I N SOLUTIONThis would accommodate those extremely large values for AE& sometimes obtainedwith cellulose acetate film as host, in addition to the low photodecomposition quantumyields found with some CrIrl complexes under conditions where they luminescestrongly. An alternative mechanism which would reflect solvent character is that ofcoupling of the electronic excitation energy to excited vibrational modes of the solvent :however, the magnitudes of solvent isotope effects (klum(NzO) /klUmcD, em too smallfor this to be a dominant pathway and it is not clear why such a 1 should haveactivation energies of the magnitude found.We thank the S.R.C.for support of S. R. A. throughapost-doctoralresearchassist-antship, W. J. R. through a post-graduate studentship and for a grant to purchase thelaser system. We acknowledge support from NATO for the Warwick-Ferraraexchange and from the Royal Society for a grant to purchase the RDA crystal.Finally, we thank Mr. R. H. Frowen for help with preliminary studies of the[Cr(bi~y)~]~+ system and Prof. V. Carassiti for valuable comments.Part 3. S . R.-Allsopp, A. Cox, T. J. Kemp, W. J. Reed, V. Carassiti and 0. Traverso, J.C.S.Faraday I, 1979,75,353.(a) G. B. Porter, in Concepts of Inorganic Photochemistry, ed. A. 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