- Email: [email protected]
Temperature dependence of singlet excitation energy migration in liquid benzene as revealed by picosecond laser photolysis
ELSEVIER
Jolrmal of Molecular
liquids.
65/M ( 19%) 39%3%
Temperature Dependencedf Singlet Excitation Energy Migration iu Liquid Benzene as Revealedby PicosecondLaser Photolysis Hiroshi Miyasakal and N&onr Mataga2 %eqxwtment df Polymer Scienceand Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 604, Japan 21nstitutefor Laser Technology, Utsubo-Hommachi l-8-4, Nishiku, Osaka 550, Japan Abstract Temperature dependenceof the excitation energy migration in liquid benzene was studied by monitoring the S,-S, annihilation processinduced by excitation with an intense picosecond (ps) 266 nm laser pulse. In the temperatureregion abovecu. 2O“C. the energy migration rate constant decreasedwith temperaturelowering ( which meansthat the energy migration is assisted by some thermally activated molecular motions), while, in the temperature region below ~0.20% down to the freezing point (5S*C), the rate constant increased with temperaturelowering. Such an increaseof the rate constantwith temperature lowering was not observedfor the cyclohexanesolution of benzene( 50 vol ‘%), where the usual Arrhenius type temperaturedependencewas demonstrated.These results suggestthat a kind of short rangeexcitonic state dependentupon the periodicity of liquid structure plays an important role in the energymigration in neat berzcne at low tcmpcrtture region. lutroduction
Several mechanisms have beenproposedfor the singlet excitation energy transfer processesin aromatic molecular liquids. Voltz predicted the i.lultipole-multipole interaction for the excitation,cnergytransfc: from the solvent toluene to the solute molecule’, although the equilibrium betweenthe excited monrmer and the excimer was neglectedin this theory. Contrary to this, Birks and Conte proposedthat the rapid formation and dissociation of the excimer provided the energy migration i,n aromatic molecular liquids such as benzene and toluenc”. Ohno and Kato suggested’ that the ex:rton drffusion, which was in narrower region than in the crystal, might take part in the energy migration processes in the excited liquid naphthalenederivatives. For the elucidation of the energy migmtion mechanism in aromatic liquids, the investigation for pure benzeneis meaningful since the liquid structure of neat benzene was reported to be very similar to that in the crystal 4 by X-ray diffraction and the electronic absorption band arising from the crystalline field in Sn*So transition was observeds. In this study, WC measured the concentrationand the temperature dcpcndenccs of the excitation energy migration rate in liquid bcnzencby directly observing the S,-S, annihilation process induced by the excitation with an intensepicosecond266nm laser pulse. On the basis of these results, we will discuss the energy migration p~xxss~ in liquid benzene from two viewpoints; the molecular translational motions and the cxciton diffusion dependent upon the periodic liquid structure in short range. 0167-73w95/so9.~ Q 1995 Elsevin S&enc.-e B.V. All rights reserved. SD1 0167-7322 (95) 009958
Experimental A microcomputer-controlled picosecond laser photolysis system with a repetitive mode-locked $Id)+:YAG laser was usedto measuretransient absorptionspectra.The details of this system have been reportedelsewhere6.The samples were excited with a single 266nm pulse with ca.2Opsfwhm and OS-1mJ output power. Cyclohexane(Wako Spectrosol) was used without further purification. Benzene(Merck Uvasol) was purified by passing through a column of activatedsilica gei (Wakogel200) and some sampleswere tirther dried by using activated molecular sieves (Wake 3A) in vacua so as to remove small trace of water. The experimentalrest&s were, however, unaffectedby this procedure.Samples were deaeratedby repeatedfreeze-pump-thaw cycles or by irrigating with nitrogen g8s stream. Temperature control system was constructedwith a refrigerated circulation unit (Neslab RTE-8), by which we controlled the temperatureof the samplefrom 5°C to 7oOCv.-ith the accuracyof t0.2”C. Results and Discussion Figure 1 shows transientabsorptionspectraof pure liquid benzeneexcited with a picosecond 266nm laser pulse at 22°C. Thesebroad spectrawith an absorption maximum around 505nm were ascribedto the excimer of benzene‘,*q9.As shown in this figure, this absorption signal decays in a few ns region, while the fluorescence lifetime of benzene excimer is 27ns’.Actually, no spectralevolution nor decreasein absorbancewas observedwhen excited with two-photon absorption of 355nm picosecondlaser pulse at this temperature 9. The decay profile is plotted in Fig.2, demonstratingthe linear relation between the reciprocal absorbanceand the time after excitation. Further, the slope was unaffected by the excitation intensity, indicating that this decay profile is due to the second-order kinetics. A simple calculation of the absorbanceof liquid benzeneat 266nm in the ground state gives the value of more than 100 in a lcm optical cell. Accordingly, the one-photon absorptionof an intense 266nm picosecondpulse producesdensepopulation of the excited moleculeson the surface or‘the samplecell, which induceseffective deactivation due to S,-S, annihilation. Figure 1. Transient absorption spectra of liquid benzene excited with a picosecond 266nm laser light.
400
500 600 700 Wavelength / nm
The S,-S, annihilation rate constantwas determinedin the following manner. First, the concentration of the cxcimer was obtained by dividing rhc observed absorbanceby its extinction coefficient and by the effective cell icngth where the S,*S, abso;bancc at 266nm was 1. In addition, in pure liquid benzene as well as in solution, there exists rapid .. . zqurhbtlum between monomer and excimer, of which time constants of association and dissociation are in the order of a few pslo. Hence, the sum of the monomer and excimer concentrations obtained by the equilibrium constant at each temperature”was used as the concentration of the excited singlet speciesfor the analysis.Although this assumption may affect to 5omeextent the accuracy of the obtained rate constant,the error of this estimation woild not dependupon the temperature. Figure 2. lwerse of transient absorbance vs. delay time relation of benzene excimer, excited at 266nm and observed at 505nm in neat liquid state. High excitation intensity for closed circles and low for open circles. i
Figure 3 shows the temperaturedependenceof the S/-S, annihilation rate constants for pure liquid benzene(a) and benzene(50 ~01%)in cyclohcxane(h). In both cases,the rtite constant decreasesconsistently with the temperature,but an increase at iow temperature region was clearly observedin the caseof pure liquid benzene.It is worth noting that the similar temperature dependenceas in Fig.3 (-) was also observed for the energy transfer process from excited benzeneto the solute biphenyl in benzenesolutionl*, where the rise of S,*S, absorptionof the guest molecule was monitored. Accordingly, it could be concluded that the rare constantobtained by the presentmethod correspondsto the energy migration processesin liquid benzene. Moreover, these results suggest that some intermolecular interactionsresponsiblefor the energy migration processespeculiar to the pure benzenearise as the temperaturedecreases.
,l~l, ! , L, ,-;I 3.2
3.4
3.6
3
3.2
3.4
3.6
Figure 3. Tempsrature !?ependenceof rate constant of S,-S, annihilation.(a) neat benzene and (b) benzene(56M) in cyclohexsne.
3%
The activationenergyestimatedfor the energymigration rateconstantin the normal Arrheniustype temperaturedependenceregion was 3.5-4.5 kcaJ/moI,which was in the order of the recttvationenergyfor the moleculardi-ffusion I3 and for the dissociationof the excimer into monomer excited state 14. From theseresults, it may be concluded that the energy migration processesassistedby some molecularmotions is dominant at higher temperature region: On the contrary, anothermechanismmay be responsible for the energy migration processin purebenzene at’thetemperatureregion below ca.20-25°C. As mentioned in the introductorysection, the liquid structureof benzeneat 25°C which was very similar to that in solid was demonstratedby means cf X-ray dift?action4. Moreover,~an exciton statein liquid ‘benzenewas predictedon the basisof the similarity of the S )-So absorptionspectrumin liquid phaseto that in solid crystals; K series on S,*Sa transifion inducedin the crystalline field was also observedin liquid benzene and this series becamemore apparentandsharperwith temperaturelowering. From theseresults,it can be deducedthat the structureof liquid benzenein QsmaNdomain is similar to that of the solid. integrating aboveargumentswith the presentresults on the temperatureand the concentration dependences of the St-St annihilationrate constants,it is suggestedthat the increase of the rateLonstantin pureliquid benzenebelow ca.20-25°C is attributableto an zxciton state arising from the periodicity of the local liquid structure.As the temperatureincreasesfrom the meltirg point to the rmrn temperature,the thermal motion of liquid lea&, to an decrease in the effective exciton size ( reactionradius ). The negativeslopes observed in Fig.3 (a) might be relatedwith the destructionof this periodicity. Hence the efficiency of this pathway decreaseswith increaseof temperature.In the high temperatureregion, other mechanisms, such as Birks mechanismand/orthe molecular diffusion, would play a dominant role in the energymigration processes.On the other hand,for the diluted solution, it is predicted that somemolecularmotions in the excitation migration process is dominantover the temperatureregion as examinedhere.More detailedstudies on theseproblems relating to the effective size of the domainwith crystal-like structureare now going on, results of which will be publishedshortly. References 1. R.Voltz, Rad.Res.Rev.,1(1968)301. 3 -. J.B.Birks and J.C.Conte,Proc.Roy.Soc.London,Scr.A,303 (1968385. T.Ohnoand S.Kato, Bull.Chem.Soc..Jpn.,54(1981)1517. 3. 4. A.H.Narten,J.Chem.Phys.,48(1968)1630. 5. T.Inagaki,J.Chem.Phys.,57(1972)2526. 6. H.Mcsuhara, N.Ikeda, H.Miyasaka, and N.Mataga,J. Spectrosc.Soc.Jpn.,31( 1982)19; H.Miyasaka,H.Masuhara,and N.Mataga,Laser Chem.l( 1983)357. 7. J.T.RichardsandJ.K.Thomas,Chem.Phys.Letters,5(1970)527. N.Nakashima,M.Sumitani, LOhmine, and K.Yoshihara,J.Chem.Phys.,72(1980)2226. 8. 9. H.Miyasaka,HMasuhara,and N.Mataga,J.Phys.Chem.,89(1985) 1631. 10. J.B.Birks, C.L.Braga,and M.D.Lumb, Proc.Roy.Soc.Ser.A.,283(1965)X3. 11. R.B.Cundalland D.A.Robinson,J.C.S.FaradayTrans. Ii, 68(1972)1133. 12. HMiyasaka, F.Ikejiri, and N.Mataga,to be published. 13. R.E.RathbunandA.L.Babb, J.Phys.Chcm.,65(1961)1072. 14. F.HirayamaandSLipsky, J.Chcm.Phys.,51(1969)1939.
Jolrmal of Molecular
liquids.
65/M ( 19%) 39%3%
Temperature Dependencedf Singlet Excitation Energy Migration iu Liquid Benzene as Revealedby PicosecondLaser Photolysis Hiroshi Miyasakal and N&onr Mataga2 %eqxwtment df Polymer Scienceand Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 604, Japan 21nstitutefor Laser Technology, Utsubo-Hommachi l-8-4, Nishiku, Osaka 550, Japan Abstract Temperature dependenceof the excitation energy migration in liquid benzene was studied by monitoring the S,-S, annihilation processinduced by excitation with an intense picosecond (ps) 266 nm laser pulse. In the temperatureregion abovecu. 2O“C. the energy migration rate constant decreasedwith temperaturelowering ( which meansthat the energy migration is assisted by some thermally activated molecular motions), while, in the temperature region below ~0.20% down to the freezing point (5S*C), the rate constant increased with temperaturelowering. Such an increaseof the rate constantwith temperature lowering was not observedfor the cyclohexanesolution of benzene( 50 vol ‘%), where the usual Arrhenius type temperaturedependencewas demonstrated.These results suggestthat a kind of short rangeexcitonic state dependentupon the periodicity of liquid structure plays an important role in the energymigration in neat berzcne at low tcmpcrtture region. lutroduction
Several mechanisms have beenproposedfor the singlet excitation energy transfer processesin aromatic molecular liquids. Voltz predicted the i.lultipole-multipole interaction for the excitation,cnergytransfc: from the solvent toluene to the solute molecule’, although the equilibrium betweenthe excited monrmer and the excimer was neglectedin this theory. Contrary to this, Birks and Conte proposedthat the rapid formation and dissociation of the excimer provided the energy migration i,n aromatic molecular liquids such as benzene and toluenc”. Ohno and Kato suggested’ that the ex:rton drffusion, which was in narrower region than in the crystal, might take part in the energy migration processes in the excited liquid naphthalenederivatives. For the elucidation of the energy migmtion mechanism in aromatic liquids, the investigation for pure benzeneis meaningful since the liquid structure of neat benzene was reported to be very similar to that in the crystal 4 by X-ray diffraction and the electronic absorption band arising from the crystalline field in Sn*So transition was observeds. In this study, WC measured the concentrationand the temperature dcpcndenccs of the excitation energy migration rate in liquid bcnzencby directly observing the S,-S, annihilation process induced by the excitation with an intensepicosecond266nm laser pulse. On the basis of these results, we will discuss the energy migration p~xxss~ in liquid benzene from two viewpoints; the molecular translational motions and the cxciton diffusion dependent upon the periodic liquid structure in short range. 0167-73w95/so9.~ Q 1995 Elsevin S&enc.-e B.V. All rights reserved. SD1 0167-7322 (95) 009958
Experimental A microcomputer-controlled picosecond laser photolysis system with a repetitive mode-locked $Id)+:YAG laser was usedto measuretransient absorptionspectra.The details of this system have been reportedelsewhere6.The samples were excited with a single 266nm pulse with ca.2Opsfwhm and OS-1mJ output power. Cyclohexane(Wako Spectrosol) was used without further purification. Benzene(Merck Uvasol) was purified by passing through a column of activatedsilica gei (Wakogel200) and some sampleswere tirther dried by using activated molecular sieves (Wake 3A) in vacua so as to remove small trace of water. The experimentalrest&s were, however, unaffectedby this procedure.Samples were deaeratedby repeatedfreeze-pump-thaw cycles or by irrigating with nitrogen g8s stream. Temperature control system was constructedwith a refrigerated circulation unit (Neslab RTE-8), by which we controlled the temperatureof the samplefrom 5°C to 7oOCv.-ith the accuracyof t0.2”C. Results and Discussion Figure 1 shows transientabsorptionspectraof pure liquid benzeneexcited with a picosecond 266nm laser pulse at 22°C. Thesebroad spectrawith an absorption maximum around 505nm were ascribedto the excimer of benzene‘,*q9.As shown in this figure, this absorption signal decays in a few ns region, while the fluorescence lifetime of benzene excimer is 27ns’.Actually, no spectralevolution nor decreasein absorbancewas observedwhen excited with two-photon absorption of 355nm picosecondlaser pulse at this temperature 9. The decay profile is plotted in Fig.2, demonstratingthe linear relation between the reciprocal absorbanceand the time after excitation. Further, the slope was unaffected by the excitation intensity, indicating that this decay profile is due to the second-order kinetics. A simple calculation of the absorbanceof liquid benzeneat 266nm in the ground state gives the value of more than 100 in a lcm optical cell. Accordingly, the one-photon absorptionof an intense 266nm picosecondpulse producesdensepopulation of the excited moleculeson the surface or‘the samplecell, which induceseffective deactivation due to S,-S, annihilation. Figure 1. Transient absorption spectra of liquid benzene excited with a picosecond 266nm laser light.
400
500 600 700 Wavelength / nm
The S,-S, annihilation rate constantwas determinedin the following manner. First, the concentration of the cxcimer was obtained by dividing rhc observed absorbanceby its extinction coefficient and by the effective cell icngth where the S,*S, abso;bancc at 266nm was 1. In addition, in pure liquid benzene as well as in solution, there exists rapid .. . zqurhbtlum between monomer and excimer, of which time constants of association and dissociation are in the order of a few pslo. Hence, the sum of the monomer and excimer concentrations obtained by the equilibrium constant at each temperature”was used as the concentration of the excited singlet speciesfor the analysis.Although this assumption may affect to 5omeextent the accuracy of the obtained rate constant,the error of this estimation woild not dependupon the temperature. Figure 2. lwerse of transient absorbance vs. delay time relation of benzene excimer, excited at 266nm and observed at 505nm in neat liquid state. High excitation intensity for closed circles and low for open circles. i
Figure 3 shows the temperaturedependenceof the S/-S, annihilation rate constants for pure liquid benzene(a) and benzene(50 ~01%)in cyclohcxane(h). In both cases,the rtite constant decreasesconsistently with the temperature,but an increase at iow temperature region was clearly observedin the caseof pure liquid benzene.It is worth noting that the similar temperature dependenceas in Fig.3 (-) was also observed for the energy transfer process from excited benzeneto the solute biphenyl in benzenesolutionl*, where the rise of S,*S, absorptionof the guest molecule was monitored. Accordingly, it could be concluded that the rare constantobtained by the presentmethod correspondsto the energy migration processesin liquid benzene. Moreover, these results suggest that some intermolecular interactionsresponsiblefor the energy migration processespeculiar to the pure benzenearise as the temperaturedecreases.
,l~l, ! , L, ,-;I 3.2
3.4
3.6
3
3.2
3.4
3.6
Figure 3. Tempsrature !?ependenceof rate constant of S,-S, annihilation.(a) neat benzene and (b) benzene(56M) in cyclohexsne.
3%
The activationenergyestimatedfor the energymigration rateconstantin the normal Arrheniustype temperaturedependenceregion was 3.5-4.5 kcaJ/moI,which was in the order of the recttvationenergyfor the moleculardi-ffusion I3 and for the dissociationof the excimer into monomer excited state 14. From theseresults, it may be concluded that the energy migration processesassistedby some molecularmotions is dominant at higher temperature region: On the contrary, anothermechanismmay be responsible for the energy migration processin purebenzene at’thetemperatureregion below ca.20-25°C. As mentioned in the introductorysection, the liquid structureof benzeneat 25°C which was very similar to that in solid was demonstratedby means cf X-ray dift?action4. Moreover,~an exciton statein liquid ‘benzenewas predictedon the basisof the similarity of the S )-So absorptionspectrumin liquid phaseto that in solid crystals; K series on S,*Sa transifion inducedin the crystalline field was also observedin liquid benzene and this series becamemore apparentandsharperwith temperaturelowering. From theseresults,it can be deducedthat the structureof liquid benzenein QsmaNdomain is similar to that of the solid. integrating aboveargumentswith the presentresults on the temperatureand the concentration dependences of the St-St annihilationrate constants,it is suggestedthat the increase of the rateLonstantin pureliquid benzenebelow ca.20-25°C is attributableto an zxciton state arising from the periodicity of the local liquid structure.As the temperatureincreasesfrom the meltirg point to the rmrn temperature,the thermal motion of liquid lea&, to an decrease in the effective exciton size ( reactionradius ). The negativeslopes observed in Fig.3 (a) might be relatedwith the destructionof this periodicity. Hence the efficiency of this pathway decreaseswith increaseof temperature.In the high temperatureregion, other mechanisms, such as Birks mechanismand/orthe molecular diffusion, would play a dominant role in the energymigration processes.On the other hand,for the diluted solution, it is predicted that somemolecularmotions in the excitation migration process is dominantover the temperatureregion as examinedhere.More detailedstudies on theseproblems relating to the effective size of the domainwith crystal-like structureare now going on, results of which will be publishedshortly. References 1. R.Voltz, Rad.Res.Rev.,1(1968)301. 3 -. J.B.Birks and J.C.Conte,Proc.Roy.Soc.London,Scr.A,303 (1968385. T.Ohnoand S.Kato, Bull.Chem.Soc..Jpn.,54(1981)1517. 3. 4. A.H.Narten,J.Chem.Phys.,48(1968)1630. 5. T.Inagaki,J.Chem.Phys.,57(1972)2526. 6. H.Mcsuhara, N.Ikeda, H.Miyasaka, and N.Mataga,J. Spectrosc.Soc.Jpn.,31( 1982)19; H.Miyasaka,H.Masuhara,and N.Mataga,Laser Chem.l( 1983)357. 7. J.T.RichardsandJ.K.Thomas,Chem.Phys.Letters,5(1970)527. N.Nakashima,M.Sumitani, LOhmine, and K.Yoshihara,J.Chem.Phys.,72(1980)2226. 8. 9. H.Miyasaka,HMasuhara,and N.Mataga,J.Phys.Chem.,89(1985) 1631. 10. J.B.Birks, C.L.Braga,and M.D.Lumb, Proc.Roy.Soc.Ser.A.,283(1965)X3. 11. R.B.Cundalland D.A.Robinson,J.C.S.FaradayTrans. Ii, 68(1972)1133. 12. HMiyasaka, F.Ikejiri, and N.Mataga,to be published. 13. R.E.RathbunandA.L.Babb, J.Phys.Chcm.,65(1961)1072. 14. F.HirayamaandSLipsky, J.Chcm.Phys.,51(1969)1939.