Chemical Physics 103 (1986) 399-405 North-Holland, Amsterdam
FLUORESCENCE QUENCHING OF RHODAMINE 6G IN METHANOL AT HIGH CONCENTRATION
A. PENZKOFER and Y. LU 1
Naturwissenschaftliche Fakultät II - Physik, Universität Regensburg, 8400 Regensburg, FRG
Received 23 October 1985
The fluorescence lifetime of rhodamine 6G dissolved in methanol is measured over a wide concentration region from 10 ~5 to 0.6 mol//. The rapid reduction of fluorescence lifetime above 10 ~ 2 mol/«f is found to be mainly due to energy transfer to quenching centers. The decrease of the fluorescence lifetime is limited by the finite fluorescence lifetime of the quenching centers of ~ 1 ps.
1. Introduction The fluorescence quantum yield of dye solu• tions may be reduced by addition of quenching Molecules promoted to excited electronic states substances (impurity quenching [2-7]). Electrolyte relax to the ground state by radiative and radia- admixtures may cause aggregation of dye mole• tionless transitions. The relaxation of highly ex• cules (salting-out). Addition of molecules absorb• cited singlet (triplet) states to the first excited ing in the fluorescence region of the substance singlet (triplet) state generally occurs by radiation- results in energy transfer mechanisms (donor less transitions [1-7]. In the lowest excited singlet -acceptor complexes [2-6,14], sensitized fluores• state spontaneous emission competes with internal cence [2-6]). conversion (equi-energetic Sj-Sn transition and At high concentrations self-quenching of the vibrational relaxation within the S0 potential well) fluorescence is experienced. This concentration and intersystem crossing (S^T transition) [2-7]. quenching is due to the formation of dimers or
Relaxation of the lowest triplet state to the S0 other quenching complexes which have very fast ground state is spin forbidden and occurs mainly radiationless deexcitation channels [2-6]. The by the radiationless process of T-S0 intersystem quenching complexes behave like flexible mole• crossing. cules in low viscous solvents and relax by fast For rigid dye molecules in liquid solution at low internal conversion. They are often treated as concentration the spontaneous emission generally non-fluorescent species [15]. In some cases excited dominates and a high fluorescence quantum yield dimers fluoresce in a shifted wavelength region is observed. For flexible molecules the radia• (excimer emission) [3-6]. tionless internal conversion reduces the fluores• In this paper we measure the fluorescence life• cence lifetime and makes it strongly viscosity de• time and the fluorescence quantum efficiency of pendent [7-12]. S!-state depopulation by S^T rhodamine 6G in methanol within a wide con• intersystem crossing is generally slow [13]. The centration region. The strong decay of fluores• process is caused by spin-orbit coupling. The cence lifetime and fluorescence quantum efficiency presence of heavy atoms or paramagnetic mole• at concentrations above 10~2 mol/tf [15,16] is cules (e.g. 02) enhances the transfer [5-8]. found to be mainly due to energy transfer to bimolecular rhodamine 6G quenching centers.
1 On leave from Institute of Optics and Fine Mechanics, From the data a fluorescence lifetime of the Academia Sinica, Shanghai, P.R. China. quenching centers of « 1 ± 0.5 ps is extracted.
0301-0104/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 2. Experimental The fluorescence quantum yield is measured with the same experimental arrangement. Only the Rhodamine 6G from Kodak was used without streak-camera is operated in focal mode (no tem• further purification. The solvent was analytic grade poral deflection). The fluorescence signal SF is methanol from Merck. For concentrations C> proportional to the absorbed second-harmonic
3 10 ~ mol/tf a thin cell of adjustable thickness was light energy [SF oc qF(l — T)E, qF is the fluores• applied (inset of fig. 1). The path length was cence quantum efficiency, T the dye transmission adjusted to a transmission of r« 0.1 at X = 526.5 at second-harmonic frequency and E the incident nm (approximately S0-S, absorption maximum). second-harmonic pulse energy]. The proportional• This thin-cell arrangement hinders fluorescence re- ity constant is eliminated by normalizing the fluo• absorption and secondary fluorescence (high rescence signal to the fluorescence at low con• transmission in fluorescent region) [2,4] and centration (reference S0 oc q0(l — T0)E0, con• 3 amplified spontaneous emission (weak gain since centration 10" mol//, q0~0.9 [15]). From the high transmission) [17,18]. ratio SF/S0 = [qF(l - T)E]/[q0(l - T0)E0] the The experimental arrangement for the fluores• fluorescence quantum efficiency at concentration cence lifetime and fluorescence quantum efficiency C is obtained: measurements is depicted in fig. 1. Single picosec• T _ S (l - T )E ond second-harmonic light pulses (A/^4 ps, F F 0 0 4F = (1) fwhm) from a passively mode-locked Nd-phos- S0(1-T)E phate glass laser are used as excitation source. The T is the fluorescence lifetime and r is the fluorescence lifetimes are measured with a fast f raa radiative lifetime. The input pulse energy E was single-sweep streak camera (Hamamatsu model 8 kept below 10" J to avoid amplified spontaneous C1587 with M1952 plug-in, time resolution < 2 emission in the plane parallel to the cell window ps). The backward fluorescence light is directed to [18]. An increase in energy by a factor of 50 had the streak camera. The fluorescence traces are no influence on the fluorescence quantum ef• analysed with the Hamamatsu C2280 temporal ficiency. The fluorescence quantum yield may be disperser which is interfaced to the read-out vidi- used to determine the fluorescence lifetime. Use of con. eq. (1) with q0 = r0/rmd0 gives
BS (2) M. L. LASER SWITCH] V~ AMPLIFIER S0(1-T)E rrad0
r is the fluorescence lifetime of the reference PD1 0 3 ü rhodamine 6G-methanol solution of C0 = 10~ TC L1 HM SHanGu Ii
mol//. TRAD0 is the radiative lifetime at the refer•
T *g3k—-J--B-* ence concentration C0 while RAD is the radiative lifetime at concentration C. Since the integrated PD2 S0-S1 absorption spectrum is nearly independent
T 1 of concentration [19], RAD is nearly independent of
concentration and we use r . = TR„HN. SC OSC r
VI 3. Results Fig. 1. Experimental arrangement. SHG: KDP crystal for second harmonic generation; BS: beam splitters; Fl, F2: filters; In fig. 2 three fluorescence traces for different HM: 50% cube beamsplitter; LI: lens (/ = 5 cm); L2: lens dye concentrations are presented (C = 0.04, 0.1 (/ = 15 cm); PD1, PD2; photodetectors; OSC; transient dig• itizer; SC: streak camera; VI: vidicon; TC: thin cell. Inset and 0.2 mol//). They were obtained by streak shows schematic cross section of thin cell. camera measurements. Within the experimental T i i i i i i i i i i i i i i r accuracy the trailing parts of the fluorescence curves decrease exponentially. In fig. 3 the measured fluorescence lifetimes and fluorescence quantum efficiencies are pre• sented. The open circles give the fluorescence life• times. The open triangles indicate the fluorescence quantum efficiencies. The solid triangle represents the measured fluorescence quantum efficiency for a solid rhodamine 6G film on a glass plate. Be• cause of eq. (2), the triangles obtained by fluores• cence yield measurements (right-hand ordinate) also give the fluorescence lifetimes (left-hand ordinate). Up to 10"2 mol// the fluorescence lifetime is
< Fig. 2. Streak camera. Fluorescence traces for rhodamine 6G in methanol. Points taken from data of temporal analyser. Straight lines indicate exponential decay. Solid curve (closed circles): concentration C = 0.2 mol/^; T = 10 ps. Dashed curve (open circles): C = 0.1 mol/Y, T = 39 ps. Dash-dotted curve (trian• TIME t [psl gles): C = 0.04 mol/A T = 357 ps.
10"5 10~A 10~3 10"2 10"1 CONCENTRATION C [mot/I]
Fig. 3. Fluorescence lifetime and fluorescence quantum efficiency versus concentration for rhodamine 6G dissolved in methanol. Open
circles: measured Tf values; open triangles: measured fluorescence quantum efficiencies; solid triangle: fluorescence quantum efficiency of a rhodamine 6G film. Curve 1: influence of quenching centers on fluorescence quantum efficiency [eq. (10) with V= 3.5
3 nm ]. Curve 2: influence of quenching centers and diffusion on fluorescence quantum efficiency [eq. (12) with T0 = 3.9 ns, TJ = 5.6x10" 3 J s/m3 and T=295 K]. Curve 3: fluorescence quantum efficiency due to initial quenching centers, diffusion and
3 4 energy transfer [eq. (14) with C0 = 4x 10~ mol// and qQ = 2.25 X 10~ ]. Dashed curve: monomer fluorescence quantum efficiency alone [first term of eq. (14)]. independent of concentration (TF = 3.9 ± 0.5 ns). excited quenching centers (open circle and cross Then the fluorescence lifetime decreases rapidly within dashed circle). The excited monomers are with increasing concentration. At 0.4 mol/Y the assumed to relax with a fluorescence lifetime T0 fluorescence lifetime is shorter than the pump and a fluorescence quantum efficiency q0 ( T0 = pulse duration and the time-resolved streak traces tfoTrad> process A in fig. 4). The excited quenching give only an upper limit of the decay time. The centers are assumed to relax with a fluorescence fluorescence quantum yield measurement is appli• lifetime r0 and a fluorescence quantum efficiency cable over the whole concentration region. At a qQ (process B in fig. 4). concentration of 0.6 mol/Y a lifetime of Tf = 1.5 Within the lifetime of the excited monomers + 1 ps is found. For the solid film the fluorescence diffusion may bring an excited monomer and a yield measurements give a fluorescence lifetime of ground-state monomer near together. Both mole• 15 ± 5 ps. cules form an excited quenching center with life• time TQ (process C in fig. 4). The diffusion rate constant is given by the Debye equation [2-6]: 4. Discussion kdlf = 4*(2a)(2D)N= (SNAkBT/3V)C, (3)
The observed dependence of fluorescence life• where a is the interaction radius of a monomer, time on concentration is analysed with the model D = kBT/6 1/2 rescence lifetime Tf is /dif = 2(2Z>Tf) [20]. Within the lifetime Tq of the excited quenching center the diffusion continues and disintegrates the fraction of /k = kdi{/(kdi{ + 1/Tq) quenching centers to monomers (Q* -> M 4- M*). The effec• tive rate constant for diffusion-controlled quench• ing center formation becomes k'di{ = A:dif(l — /k), i.e. *dif = ^dif/(l +£dif7b)- (4) Since the following analysis gives kdif <^ TQ, kdi{ may be approximated by kdif. Besides diffusion an excited molecule may transfer its excitation energy to a nearby unexcited molecule getting itself unexcited [2-6]. This hop• ping mechanism may bring - after some steps - Fig. 4. Illustration of deactivation processes of excited mole• the excitation energy to a reaction center where cules. Open circles: unexcited molecules; crosses: excited mole• fast radiationless decay occurs (process D in fig. cules; dashed circles: reaction centers. Process A: excited 4). The energy transfer is generally due to monomer fluorescence. Process B: excited quenching center fluorescence. Process C: formation of excited quenching centers dipole-dipole interaction. The rate of dipole-di- 6 by diffusion. Process D: formation of excited quenching centers pole energy transfer kET is proportional to R~ 9 by energy transfer. where R is the distance between the dipoles. The rate &EQ of energy transfer to quenching centers is formed quenching centers (unbound dimers), xs, proportional to the rate of energy transfer kET and is given by [2,21,22]: to the mole fraction xQ of quenching centers [2]: xs = l-exp( -VNAC), (9) 1 (Ro 1 C R (5) where V is the volume of a quenching center. In the excitation process monomers and mole• R0 is the critical transfer distance of energy trans• cules in quenching centers are excited. The strong _1 fer (energy transfer rate kET = T0 ) and C0 is the radiationless deactivation of excitation in quench• critical transfer concentration. ing centers reduces the fluorescence light. The Within the lifetime rQ of the excited quenching fluorescence quantum efficiency reduces due to the centers, an energy back transfer to monomers oc• presence of quenching centers to curs (Q* + M -* Q + M*). The fraction of excited molecules that escape the quenching centers is 1 X ^F = ( -- Q)^O + ^Q f = k (l -x )/[k (l -*Q)+-1/T ]. The ef• B ET Q ET q = {l-x )(q -q ) + q . (10) fective rate of excited quenching center formation Q Q Q Q by energy transfer reduces to k = k x (l —/ ), E ET Q B The fluorescence intensity decays accordingly to i.e. /F(0 = /F(0)[(l-*Q)exp(-;A) (6) + xQ exp(-//rQ)], (11) where TQ/T0 = qQ/q0 is used. with T = T0. The fluorescence is composed of two The quenching centers consist mainly of two single-exponential decay components (monomers molecules which are so near together that they and quenching centers). interact mutually. They may be formed by the The inclusion of the formation of excited chemical reaction of dimer formation (bound quenching centers by diffusion approximately leads ground-state dimers) or they are formed by mole• to [6] cules which are occasionally (by statistics) near together without chemical binding (unbound qF = (l-xQ) +qQ (12) ground-state dimers). The mole fraction xQ of 1 ^ Kdif^ molecules in quenching centers (equal twice the fraction of quenching centers is composed of the and T of ^q. (11) becomes: mole fraction JCd of molecules in dimers and of T = (13) the mole fraction xs in statistical quenching 1 + «d.fC centers: Kdif = (7) The abbreviation r0k'dif/C ~ r0kdif/C is used in eq. (13). The mole fraction of molecules in bound dimers, The additional inclusion of energy transfer to JC , is obtained from the law of mass action for the d fluorescence quenching centers leads to chemical reaction M + M *± D. The dimerization constant is K = [D]/[M]2 = (X /2)/CJC^ = u d q ™(l-x )(q -q ) 2 F Q 0 Q xD/[2C(l — xD) ] and the mole fraction comes out to be X(l + /cdifC + xQ^Co 1/2 1 r =1-1 -1 D 1 + 1 + (8) " 4CKT 4CKD X + qQ (14) The mole fraction of molecules in statistically and r of eq. (11) becomes Eq. (11) represents the fluorescence decay of two components with different decay times T and T^ (T()-TQ) rQ. The fast decay component could not be re• 2 solved from the streak camera traces (see fig. 2), x{i + Kdifc * (^) + Q since rQ « 1 ps is faster than the pulse duration A/« 4 ps of the excitation source and faster than the response time of the streak camera. The time X (15) constant for energy transfer is time dependent at low concentrations in viscous media [23-27]. If the The calculated curves in fig. 3 clarify the impor• diffusion length /dif is short and the mean molecu• l/3 tance of the various fluorescence quenching lar distance x = (NAC)~ is large compared to processes. Curve 1 depicts the fluorescence the critical transfer distance R0i a fluorescence quenching due to the excitation of quenching decay proportional to exp( — £tx/2) is expected (£ centers [eq. (10)]. An analysis of the absorption is a constant) [23-27]. In our case the diffusion spectra of rhodamine 6G dissolved in methanol length within the radiative lifetime is /dif = 1/2 3 suggests that bound dimer formation is very weak 2(2£Brrrad/6 (xD = 0) and the quenching centers are due to s/m , a « 1 nm), the critical transfer distance is neighbouring molecules (xQ ~ xs) [19]. A volume R0 = 4.63 nm, and the mean molecular distance at 3 3 of V= 3.5 nm [eq. (9)] is assumed for the quench• the critical concentration of C0 = 4 X 10 ~ mol/Y ing centers [19]. (Curve 1 does not strongly depend is x = 7.46 nm. These data would lead to a 1/2 on whether xQ is due to xu or xs.) Curve 2 is exp(-£/ ) decay at C0. But at the critical con• obtained by considering the processes of excitation centration the deactivation by energy transfer is of quenching centers and of diffusion [eq. (12)]. still negligibly small, since the mole fraction of Curve 3 includes deactivation by reaction centers, quenching centers is small [xs(C0) = 0.0084, see diffusion, and energy transfer [eq. (14)]. The curve eq. (9)] and many energy transfer steps are neces• is fitted to the experimental quantum efficiency at sary to reach a quenching center. For concentra• 3 C = 0.1 mol/ 4 = 2.25 X 10~ ). The dashed curve represents the comes x < 4.4 nm, i.e. x < R0. All molecules are first term of eq. (14) (monomer fluorescence). within the critical region of energy transfer and A comparison of the calculated curves with the the fluorescence decay time r behaves time inde• experimental points indicates that the excitation pendent. The fluorescence decreases exponential in energy hopping to quenching centers is the domi• time as observed experimentally [see eq. (11) and nant relaxation mechanism in rhodamine 6G- fig. 2]. methanol solution at high concentration. The fluo• The radiationless depopulation of the excited rescence lifetime of the quenching centers is found quenching centers is thought to occur via internal to be about a factor of 4000 shorter than that of conversion. Ground state absorption recovery time the monomers. The solid rhodamine 6G film has a measurements (bleaching experiments) support this factor of 10 higher fluorescence quantum effi• assumption (absorption recovery time is found to ciency than the high concentrated solution. In the be approximately equal to fluorescence decay time) solid film all molecules have near neighbours so [18]. that xQ = 1 and the fluorescence light results from emission of the quenching centers. The fluores• 5. Conclusions cence lifetime of the quenching centers in the solid film is about a factor of 15 longer than in the The fluorescence of rhodamine 6G dissolved in methanolic solution. The reduced flexibility of the methanol could be measured up to the saturation molecule clusters in the solid reduces the radia• concentration (« 0.66 mol/*?) without dis• tionless relaxation rate. turbances by reabsorption, secondary fluorescence and amplified spontaneous emission due to the [6] J.A. Barltrop and J.D. Coyle, Principles of photochemistry application of a backward fluorescence measure• (Wiley, New York, 1978). ment technique with a variable thin cell. The rapid [7] K.H. Drexhage, in: Dye lasers, Vol. 1. Topics in applied physics, ed. F.P. Schäfer (Springer, Berlin, 1977). 2 decrease of fluorescence lifetime above 10 ~ mol/Y [8] V. Sundström and T. Gillbro, Chem. Phys. 61 (1981) 257. was found to be due to excited state energy hop• [9] B. Kopainsky, P. Qiu, W. Kaiser, B. Sens and K.H. ping to quenching centers. A finite lifetime of the Drexhage, Appl. Phys. B29 (1982) 15. quenching center fluorescence (two rhodamine 6G [10] A.T. Eske and K. Razinagvi, Chem. Phys. Letters 63 molecules in near contact) of ~ 1 ps was observed (1979) 128. [11] D.A. Cremers and M.W. Windsor, Chem. Phys. Letters 71 which limits the decay of the fluorescence lifetime (1980) 27. at very high concentrations. [12] W. Rettig and G. Wermuth, J. Photochem. 28 (1985) 351. [13] D.N. Dempster, T. Morrow and M.F. Quinn, J. Photo• chem. 2 (1973) 343. Acknowledgement [14] A. Weller, in: Fast reactions and primary processes in chemical kinetics, ed. S. Claesson (Wiley, New York, 1967). The authors thank Mr. L. Schleinkofer of [15] K.A. Selanger, J. Falnes and T. Sikkeland, J. Phys. Chem. Hamamatsu Photonics Europe GmbH for lending 81 (1977) 1960. them the streak camera system. They acknowledge [16] R.R. Alfano, S.L. Shapiro and W. Yu, Opt. Commun. 7 financial support of the Deutsche Forschungs• (1973) 191. [17] A. Penzkofer and W. Falkenstein, Opt. Quantum Electron. gemeinschaft. 10 (1978) 399. [18] A. Penzkofer, Appl. Phys. B (1986), to be published. [19] Y. Lu and A. Penzkofer, to be published. References [20] P.W. Atkins, Physical chemistry (Oxford Univ. Press, Ox• ford, 1982). [21] M.F. Perrin, Compt. Rend. Acad. Sei. 178 (1924) 1978. [1] F. Graf and A. Penzkofer, Opt. Quantum Electron. 17 [22] A. Boutaric and M. Roy, Compt. Rend. Acad. Sei. 211 (1985) 53. (1940) 201. [2] Th. Förster, Fluoreszenz Organischer Verbindungen [23] Th. Förster, Z. Naturforsch. 4a (1949) 321. (Vandenhoeck and Ruprecht, Göttingen, 1951). [24] M.D. Galanin, Soviet Phys. JETP 1 (1955) 317. [3] C.A. Parker, Photoluminescence of solutions (Elsevier, [25] U. Gösele, M. Hauser, U.K.A. Klein and R. Frey, Chem. Amsterdam, 1968). Phys. Letters 34 (1975) 519. [4] J.B. Birks and LH. Munro, in: Progress in reaction kinet• [26] K. Allinger and A. Blumen, J. Chem. Phys. 72 (1980) 4608. ics, Vol. 4, ed. G. Porter (Pergamon Press, Oxford, 1967) [27] N. Tamai, T. Yamazaki, I. Yamazaki and N. Mataga, p. 239. Chem. Phys. Letters 120 (1984) 24. [5] J.B. Birks, Photophysics of aromatic molecules (Wiley-In- terscience, New York, 1970).