Frederichs, B.

Staerk, H.

English

MPG.PuRe

Chemical Physics Letters 460 (2008) 116–118Contents lists available at ScienceDirectChemical Physics Lettersjournal homepage: www.elsevier .com/locate /cplet tEnergy splitting between triplet and singlet exciplex states determined withE-type delayed fluorescenceBernd Frederichs, Hubert Staerk *Max-Planck-Institut für biophysikalische Chemie, Abt. Spektroskopie und photochemische Kinetik, 37070 Göttingen, Germanya r t i c l e i n f o a b s t r a c tArticle history:Received 29 April 2008In final form 26 May 2008Available online 17 June 20080009-2614/$ - see front matter  2008 Elsevier B.V. Adoi:10.1016/j.cplett.2008.05.086* Corresponding author. Fax: +49 551 201 1501.E-mail address: hstaerk@gwdg.de (H. Staerk).In the redox system with N,N-diethylaniline as electron donor and 1,4-dicyanobenzene as electron accep-tor the singlet and triplet exciplex states in the nonpolar solvent methylcyclohexane are almost degen-erate. The lowest excited state is the triplet exciplex state from which the singlet exciplex state ispopulated in a fast endothermic reaction. The energy splitting has been determined experimentally asDE  0.1 eV via E-type delayed fluorescence. 2008 Elsevier B.V. All rights reserved.1. IntroductionThe first experimental evidence of molecular complex forma-tion in the excited state between two different components of a re-dox system leading to a so-called hetero excimer was given alreadyin 1961 [1,2]. Since then a lot of research work has been done onnumerous examples of complex formation in the lowest electron-ically excited singlet state. Studies of complex formation in thelowest excited triplet state, however, have been very scarce. Thisis because generally phosphorescence can be observed in rigidmedia only where diffusion is extremely slow. Hence exciplexphosphorescence was studied only with complexes which are sta-ble in the ground state [3]. Later, Beens and Weller [4,5] attempteda classification of charge transfer (CT) systems and arrived at thegeneral rule for necessary conditions of exciplex formation onthe grounds of the redox properties of electron donor (D) andacceptor (A), ionization potential (IP) and electron affinity (EA):IPA  IPD > 0 and EAA  EAD > 0 ð1ÞThis requires thatIPA þ EAA > IPD þ EAD ð2Þi.e., the sum (IP + EA) of a molecular pair determines the direction ofthe electron transfer. From the redox data and singlet and tripletenergy data of the system N,N-diethylaniline (DEA = D) and 1,4-di-cyanobenzene (DCNB = A) it could be shown that, in the inert sol-vent methylcyclohexane (MCH), the triplet CT complex is formedfrom the triplet excited state of the donor, 3D*, and the acceptor A(at low concentration) in its ground state, according to the reaction3D þ A !kccA 3ðDþAÞ ð3Þll rights reserved.Furthermore, it was assumed [4,5] that the observed CT emissioncould only arise from the CT singlet state which is populated dueto the endothermic reaction3ðDþAÞ ! 1ðDþAÞ ð4Þobservable via E-type delayed fluorescence [6,7].The purpose of the present work was to extend the measure-ments of this system in order to verify the proposed [4,5] reactionmechanism and to obtain parameters which allow the determina-tion of the energy splitting DE between the exciplex triplet andexciplex singlet state.2. ExperimentalN,N-Diethylaniline (DEA), purissinum Fluka, was redestilled un-der nitrogen atmosphere in the dark. 1,4-Dicianobenzene (DCNB)was from Fluka. The water-free methylcyclohexane (MCH, throughmolecular sieve 4 Å, 2 mm) solutions were kept under nitrogenatmosphere during sample preparation and measurements. Sixfreeze–pump–thaw cycles of a vacuum degassing process wereapplied up to a final stationary pressure of 5  106 Torr beforesealing the quartz cuvettes. The versatile spectrometer equippedwith a cryostat has been described elsewhere [8]. The excitationlight (313 nm from an OSRAM 150Watt mercury high pressurelamp) was chopped with 50 rpm for the quasi photostationaryintensity measurements and with 3000 rpm for the lifetime mea-surements. The signals from the photomultiplier were measuredwith a PAR/HR-8 lock-in amplifier and PAR/TDH-9 multichannelanalyzer, respectively. It turned out to be of utmost importancefor this study around and below zero degrees centigrade that sam-ples are free of traces of water. Otherwise stray light effects duringspectroscopic measurements would lead to erroneous results.Fig. 3. Overall reaction scheme for the formation of the triplet exciplex 3(AD+) andits subsequent deactivation by the mechanism of E-type delayed emission. Primedletters pertain to reaction parameters which originate from a complex state.B. Frederichs, H. Staerk / Chemical Physics Letters 460 (2008) 116–118 1173. Results and discussionEmission spectra of deaerated solutions are shown in Fig. 1. Thedonor (D) is primarily excited. The intensity I of its fluorescencewith maximum at 338 nm remains unaltered with varying accep-tor concentration, while the intensity I0 of the blue-green emissionwith maximum at 489 nm increases with increasing acceptor con-centration cA. This emission can be strongly quenched by oxygenand by biphenyl whose triplet energy at 2.92 eV is below that ofdiethylaniline with 3.23 eV. Lifetime measurements of the blue-green emission give values of about 105 s at room temperature,which shows the delayed character of this fluorescence. For cA-con-centrations 2  105 M and 5  106 M exponential decay times ofthe delayed luminescence as a function of temperature are shownin Fig. 2. These observations corroberate a reaction mechanism asshown in the scheme in Fig. 3.The lifetime of the prompt fluorescence from 1D*,sF = (kf + kd + kisc)1  4 ns, is independent of the presence of theacceptor at the low concentrations used. The triplet lifetime ofthe donor (3D*) at zero acceptor concentration is sT = (kp + kn)1Fig. 2. Temperature dependence of decay times s0T(T) of delayed emission of theDEA/DCNB exciplex in MCH at 489 nm; from two samples with cA-concentrations2  105 M (open circles, decreasing temperatures; closed circles, increasingtemperature) and 5  106 M (triangles, decreasing temperatures). A concentrationdependence was not detectable.Fig. 1. Emission spectra (corrected) of deaerated solutions of 2  104 M N,N-diethylaniline (DEA = donor) and different conscentrations cA of 1.4-dicyanoben-zene (DCNB = acceptor) in methylcyclohexane (MCH) at room temperature. Exci-tation 313 nm (1D*).which has been determined in a separate absorption experimentas 9.3  106 s at room temperature.With this result and I0(cA) at 489 nm from Fig. 1 the complemen-tary Stern–Volmer equation1I0ðcAÞ¼ 1I0ð1Þ ð1þ ch=cAÞ ð5Þwith the half-value concentrationch ¼ ðkcsTÞ1 ð6Þcan be evaluated. By plotting 1/I0(cA) versus 1/cA one obtains I0(1)the (extrapolated) intensity at cA =1 and ch = 6  106 M. Finally,Eq. (6) gives the bimolecular rate constant for the diffusion con-trolled triplet exciplex formation (cf. Fig. 3) kc = 18  109 M1 s1.As a proof for the assumption that, with the chosen CT-pair, thesinglet and triplet exciplex levels are almost degenerate, a rathersimple relation is derived in the following, which can be evaluatedgraphically.From the reaction scheme in Fig. 3 the overall quantum effi-ciency u0 is obtained for the emission from the 1(AD+) state asthe product of all consecutive quantum efficienciesu0 ¼ uisc uc u0csi u0f ð7Þwithuisc ¼ kiscsf ð8Þuc ¼kcsTcA1þ kcsTcA ð9Þu0csi ¼k0csik0p þ k0n þ k0csið10Þu0f ¼kfk0f þ k0d þ k0icsð11ÞAt the limit of infinite acceptor concentration (cA?1), uc ap-proaches the value one and with the exciplex triplet lifetimes0T ¼ ðk0p þ k0n þ k0csiÞ1 we obtainu01 ¼ uiscu0fk0csis0T / I01 ð12Þk0csi may be treated in analogy to the classical example of E-type de-layed fluorescence [6,7]. This process corresponds to a thermal acti-vation into a higher vibrational level of the triplet CT state, followedby intersystem crossing into the singlet level of the CT complex. Onecan writek0csiðTÞ ¼k0isc3exp DERT ð13ÞFig. 4. Delayed exciplex emission intensity I0 plotted as a function of temperaturefor three acceptor (DCNB) concentrations cA. Donor concentrationcD = 2.0  104 M.Fig. 5. Eq. (14) plotted versus 1/T. DE  0.1 eV is obtained from the slope at lowertemperature.118 B. Frederichs, H. Staerk / Chemical Physics Letters 460 (2008) 116–118where k0isc is the rate constant of the radiationless process3(AD+) 1(AD+) and DE is the corresponding singlet–triplet en-ergy splitting. The multiplicity difference is taken into account bythe factor 1/3. Finally, we obtain with Eq. (12) the following tem-perature dependent relation:I01ðTÞs0T/ uiscu0fk0isc3exp DERT : ð14ÞI01(T) is obtained from the set of experimental data shown inFig. 4 where the delayed exciplex emission intensity, I0, is plottedas a function of temperature for three acceptor (DCNB) concentra-tions (cA); cf. Eq. (3).Thus, a semilogarithmic plot of Eq. (14), i.e., log I01 /s0T versus 1/T, with s0T from Fig. 2, yields the curve shown in Fig. 5. Its shape isdetermined by DE and the temperature dependence of loguiscu0f .However, whereas at high temperature it turns out thato loguiscu0f=o1=T > 0; ð15Þapproaching zero with decreasing temperature, one obtains at low-er temperature from the slope in Fig. 5 a lower limiting valueDE > 2.3 kcal/Mol  0.1 eV (4 kT at room temperature), for the en-ergy difference between the singlet and triplet exciplex levels ofthe system DEA/DCNB in MCH.The occurrence of E-type delayed emission from this CT com-plex made it possible to study CT complex formation in the tripletstate by emission spectroscopy.The same system, DEA + DCNB, however, in a polar solvent, e.g.acetonitrile, would be particularly suitable to further extend thestudies, with interesting aspects towards the generation of radicalion pairs in the overall triplet and singlet spin state – 3,1(2A + 2D+)are then the lowest states above the ground state – and the obser-vation of magnetic field effects of the type described earlier ([9]and references therein).The lifetime in acetonitrile of the primarily excited electron do-nor DEA is only a few nanoseconds. Fluorescence quencher concen-trations of the order of 105 M or lower would therefore notproduce radical ion pairs in the overall singlet spin state in a signif-icant amount but primarily and predominantly in the triplet state.The change of spin multiplicity of the initial triplet radical ion pairthat originates from the hyperfine coupling between electron spinsand nuclear spins within each radical ion leads to singlet radicalion pairs, with possible consecutive association to a radiative sin-glet exciplex (as we have shown with other systems [9]). The yieldof singlet products generated from radical ion pairs in the tripletspin state in a geminate recombination is expected to decrease(due to the Zeeman effect) to about 30–50% by an external mag-netic field of only 10–50 mT, compared to its zero field value. Sucheffects are particularly pronounced in systems where donor andacceptor are linked by a long aliphatic chain (e.g. (CH2)16–23) wherehomogeneous recombination reactions of the radical ion pair spe-cies are absent [9].References[1] H. Leonhardt, A. Weller, Z. Phys. Chem. N.F. 29 (1961) 277.[2] H. Leonhardt, A. Weller, Ber. Bunsenges. Physik. Chem. 63 (1963) 791.[3] S. Iwata, J. Tanaka, S. Nagakura, J. Chem. Phys. 47 (1967) 2203.[4] H. Beens, A. Weller, in: E.C. Lim (Ed.), Molecular Luminescence, Benjamin, NewYork, 1969, p. 203.[5] H. Beens, A. Weller, Acta Phys. Pol. 34 (1968) 539.[6] C.A. Parker, C.G. Hatchard, Trans. Faraday Soc. 57 (1961) 1894.[7] C.A. Parker, C.G. Hatchard, Trans. Faraday Soc. 59 (1963) 284.[8] H. Staerk, J. Lumin. 11 (1976) 413.[9] H. Staerk, H.-G. Busmann, W. Kühnle, R. Treichel, J. Phys. Chem. 95 (1991) 1906.

Energy splitting between triplet and singlet exciplex states determined with E-type delayed fluorescence.

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Energy splitting between triplet and singlet exciplex states determined with E-type delayed fluorescence.

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