Viscosity dependent radiationless relaxation rate of cyanine dyes. A picosecond laser spectroscopy study

Chemical Physics 61 (1981) 257-269 North-Holland Publishing Company

VISCOSITY DEPENDENT RADIATJONLESS RELAXATION RATE OF CYANLNE DYES. A PKOSECOND LASER SPE~ROSCOPY STUDY Villy SLJNDSTReM and Tomas GILLBRO Dicision of Physical Chemistry, Uniaersity of Urn&, S-901

87 Urn&,

Sweden

Received 19 May 1981

The viscosity dependent radiationless relaxation of several cyanine dyes has been studied by picosecond laser spectroscopy. It was found that the relaxation rate is proportional to n-=_ The value of CI. however, is net constant for a certain dye molecule, but is strongly dependent on the kind of solvent used. In n-alcohols for instance ~1 is typically about 1. In glycerol/methanol or glycerol/water mixtures on the other hand cz =OS_ A comparison is made with literature data on orientational relaxation lifetimes of some dyes in similar solvents. It is shown that the radiationless relaxation of cyanine dyes and the orientational relaxation of for instance xanthene dyes changes in roughly the same way as the solvent is changed. This is taken as proof of the proposal that a torsional motion of the heterocyclic quinolyl rings is the main course of the viscosity dependent relaxation of the cyanine dyes studied.

1. Introduction The viscosity and temperature dependent electronic relaxations of polymethine dyes have lately been the subject of several picosecond spectroscopy investigations [l-18]. Both absorption and emission techniques employing single pulse or repetitive laser sources have been used in these studies. Quantum yield measurements with more conventional techniques have also been used [19,20]. The great interest shown by picosecond spectroscopists in the photophysics of these dye molecules is due to their potential use as mode-locking dyes [6, 21-241. The excited singlet state lifetimes of the polymethine dyes have been shown to be very sensitive to solvent viscosity and molecular structure [l-20]. Increasing the solvent viscosity lengthens the excited state lifetime and increases the fluorescence quantum yield. Structural features of importance are polymethine chain length, the degree of steric hindrance between the quinoline rings and the rigidity of the molecule [lS, and references therein]. The efficient deactivation of the excited singlet state has been shown to proceed mainly 0301-0104/S1/0000-0000/S02.50

conversion to the ground state [13, 25, 261. The picture used to explain the ex?erimental observations is as follows: Decreased double bond character and loss of resonance ener,T in the polymethine chain in the excited state result in rotation of the quinoline rings to a new equilibrium position where decreased steric hindrance is balanced by the loss in resonance energy. In the new equilibrium position the rate of internal conversion will probably be high and the excited molecule rapidly deactivated_ This is a view similar to that used to explain the photophysics of trans-stilbene [27-291. The n dependerce of the relaxation lifetime (7) for various cyanine dyes in difIerent solvents is reported to be close to either v”~ or 17’. In the case of T = Cna’s [li] the results were seen as a confirmation of the validity of the Fiirster and Hoffman (FH) theory [30] developed for the relaxation of triphenylmethane dyes. Mialocq et al. [13] obtained a 7’ dependence of the relaxation rate of pinacyanol in water/giycerol mixtures. The same viscosity dependence for the relaxation of DODCI in unbranched alcohols has been reported by Jar-and& [lS]. These results are in agreement with the theory via internal

@ North-Holland

258

V. Smdstrc%n, T. GiXbro / Radiationless relaxation of cyanine dyes

of Oster and Nishijima (ON) [31]. Recently, McCaskill and Gilbert [32] successfully applied Kramers’ equation [33], for the rate of passage over a potential barrier of a particle subject to the irregular forces of a surrounding medium, to the problem of conformational relaxation in l,l’-binaphthyl. In this case one obtains a different viscosity dependence of the relaxation rate depending on the shape and size of the potential barrier and the magnitude of the drag coefficient. In this paper we present picosecond absorption studies on several cyanine dyes in various solvents and solvent mixtures at different temperatutres. It is shown that the viscosity dependence of 7 for a particular dye molecule critically depends on the particular solvent system used. The models discussed above [30-321 ‘are thus not able to provide a complete explanation for the experimentai results. We therefore suggest that a new model should be used in which the radiationless relaxation rate is considered as a function of the rotational diffusion lifetime of some part of the molecule rather than solvent viscosity. Such an approach to the problem also seems to be justified in the light of recent rotational diffusion studies of some laser dyes in various solvents [34-361, where it is found that the same dye might have different rotational diffusion lifetimes (T& in solvents of the same viscosity. The reason for this behaviour might be different solute-solvent interactions, e.g. hydrogen bond-strength, and/or invalidity of the hydrodynamic stick boundary condition. In this paper we want to stress the relationship between the viscosity dependence of radiationless relaxation (induced by torsional vibrations) and rotational diffusion.

2. Experimental The recovery kinetics of ground state bleaching after picosecond pulse excitation as a function of soIvent viscosity of several polymethine dyes was studied. The picosecond pulses were

delivered by a synchronously mode-locked, cavity-dumped dye laser. A detailed description of this system has been presented elsewhere [37, 421, so only a short account will be given here. The picosecond pulse train from the dye laser was split in two parts. Ca. 95% was used to excite the sample and ca. 2% was used for the analyzing light. Time resolution was achieved by a motor driven variable delay in the excitation beam. The modulation imposed on the analyzing beam through the chopping of the excitation beam was detected by a photomultiplier and a phase sensitive amplifier. The output of the amplifier was recorded on a chart recorder as a function of optical delay. The pulses used in this work were ca. 2-5 ps, carrying 2.5-5 nJ of energy. In order to obtain the shortest pulses a mode-locking dye (DQQCI or DODCI) was added to the laser dye [37]. In most experiments 2 repetition rate of 82 kI-Iz resulting in an average power of ca. 0.5 mW was used. The sample was contained in a 0.21 mm cell The diameter of the focused laser beams in the sample was adjusted to 2200 pm yielding an excitation degree of -1%. In some experiments a flow cell system was used to check the possible. influence of photoproducts on the measured kinetics. The flow speed commonly used resulted in a change of sample within the excitation volume once every 30 pulses. An Oxford Instruments DN 704 cryostat maintained constant temperature to within the excitation volume once every 30 pulses. An Oxford Instruments DN 704 sample was kept at about 0.5 resulting in a concentration of ca. 104 l@. Varying the concentration by a factor 50 around this value had no detectable effect on the recorded kinetics. The elects of rotational relaxation on the Ievel kinetics were also checked by performing some of the measurements with the polarization of the analyzing light rotated 54.7’ relative to ?hat of the excitation light to yield a rotation-free signal [38,39]. These experiments demonstrated that the rotational relaxation did not aEect the measured !ifetimes within experimental error under the conditions just mentioned and also summarized in table 1.

V. Sundsmim,

7: GiNbro / Radiaxionless relaxation of cyanine dyes

259

Table 1 Summary of studied dyes and experimental conditionsa’

l,l’-diethyl-4,4%yanine

I

Vismsity range (cP)

A,, (nm)

295-20s 296*1

0.55 1.16-11.4 2.0-25

588 588 596

E?z

296-196 296-196 296zt 1 296-L 1 296*1 29611

0.5.5-20 1.2-27.5 0.55-11.7 2.0-25 1.7-11.6 18.5

595,602,619 602 597,607,619 596 597 595

MeOH EtOH

296-182 296-220

0.55-9.6

1.2-7.2

603 603

MeOH EtOH

292-254

0.6-1.12 1.16

633 63:

MeOH EtOH n-alcohols GIyjMeOH Etg

296rl 300-193 29611 29651 296rl

OS5 1.05-18.3 0.55-8.0

2.0-25 18.5

586, 603,619 586.603 586.595,607,619 596 595

EtOH

296-197

12-13.7

600

EtOH

307-226

0.92-5.8

632

Solvent

Dye

Temperature range K)

iodide MeOH EtOH Gly/MeOH

QQ

E:-Nd-CH=(_jN-Et

29611

-

l,l’-diethyl-2,2’-carbocyanine

iodide MeOH EtOH n-alcohols Gly/MeOH G~Y/HzO

l.l’-diethyl-2,4’-carbocyanine

iodide

I,l’-diethyl-4,4’-carbocyanine

iodide

1,3’-diethyl-4,2’-oxacarbocyanine

1,3’-die~yI-2,2’-thiacarbocyanine

iodide

iodide

= I’ / -cHic”-c”=<~yJ or) “: Et

I

296-Ll

:t

l.l’-diethyl-2,2’-dicarbocyanine

iodide

*’ Abbreviations used in the table: LMeOH= methanol, EtOH = ethanol. Gly = Glycerol, normal akOhOls, methanol, ethanol, butanol, hexanoi. octanol, decanof.

Etg= ethyh&yml,

n_abhols

=

The lifetimes, r, of the recorded kinetics were evaluated by performing a semilogarithmic plot of intensity versus time. r was obtained as the slope of a straight line by using the part of the recorded curve yielding a straight line, i.e. starting the plot sufficiently far away from t = 0 so that the excitation plllse had completely decayed. In this way we have measured lifetimes down to =5 ps employing the shortest pulses of =l ps. Thus, the current time resoIution of the apparatus was considered to be =5 ps. The time resolution could be improved by using some method of deconvolution. But this technique requires the knowledge of pulse shape which is not easily obtained. The effect of local heating on the recorded kinetics was also considered. The lifetime of DQOCI in a 70/30 (w/w) glycerol/methanol mixture was measured as a function of average laser power focused into the excitation volume of the sample. The pulse energy was kept constant. Increasing the power t.y a factor of ten, from 0.2 to 2 mW, caused a ca. 30% decrease in the measured lifetime. ConsequentIy it was conciuded that the laser power focused into the sample.should be kept below ca. 1 mW with our degree of focusing to avoid local heating effects. All the dyes (Koch-Light Ltd.) were used without further purification and the solvents were of analytical or spectroscopic grade. For the viscosity of pure methanoI and ethanol below room temperature values obtained from the literature [40] were used. In the experiments using higher alcohols or solvent mixtures at room temperature, the viscosities were measured. In the experiments to be described below, the variation in solvent viscosity was realized by four different methods, namely: (a) by varying the temperature of the sample, (b)by varying the composition of a glycerol/methanol mixture, (c) by varying the composition of a glycerol/water mixture, (d) by using a series of unbranched alcohols.

3. Results The rest&s obtained in the present work are summarized in table 2 together with results previously obtained by other authors. The room temperature lifetimes at a specified wavelength and the q-dependence of 7 are given _ The qdependence is obtained through the relation:

l/r=k

= CT-=_

Thus, if log T) is plotted versus log k a straight line of a slope -CY should be obtained_ The parameter (Y is used to characterize the viscosity dependence of the radiationless relaxation rate. We will now discuss the results obtained for each dye in some detail. For numerical values and references to the literature we refer to table 2. 3. I.

I, 1 ‘-diethyi-4,4’-cyanine iodide

The room temperature lifetimes in ethanol and methanol were too short to be accurately obtained directly from a log I versus t pIot with the pulses available in this experiment. Therefore these values were deduced from the viscosity dependence of the lifetime_ Very short fluorescence lifetimes ((5 ps) were also reported by Tredwell et al. [15] for some l,I’-diethyi-2,2’-cyanine isomers with varying substitution. The viscosity dependence of the observed reiaxation rate of cyanine was investigated according to methods (a) and (b), described in section 2. In both cases the log k versus log 17 plot yielded a sIope, LY,close to 0.5, see fig. 5 and table 2. The glycerol/methanol line is seen to be displaced towards higher rate coeflicients compared with the ethanol line. This is due to two effects; (i) a general trend towards shorter relaxation times in glycerol/methanol and glycerol/water mixtures at a certain viscosity compared with nalcohols of the same viscosity (see also figs. 2 and 3) and (ii) the wavelength dependence of the rates (see also fig. 4 and ref. [42]).

V. Sundstr&n, T. Gillbro / Radialionless Table 2 Summary

of room temperature

lifetimes and viscosity dependence

Dye

relaxation of cynninedyes

of ta) 7

iodide

l,l’-diethyl-2,2’-carbocyanine = pinacyanol

l,l’-diethyl-2.4’~carbocyanine l,l’-diethyl4,4’-carbocyanine = cryptocyanine

iodide

iodide iodide

1,3’-diethyl-4,2’-oxacarbocyanine iodide = DQGCI

1,3’-diethyI-2,Z’thiacarbocyanine iodide = T?IIA l.l’-diethyL2J’dicarbocyanine =DDCI

iodide

MeOH, 5X8 nm EtOH, 588 nm Gly/MeOH, 596 nm MeOH, 595 run MeOH, 602 nm MeOH, 619 nm EtOH, 602 nm

;.8b’t0.5 5.Prt0.5 see fig. 5 9.X*0.5 7.4zto.s S5 13.oco.5

Gly/MeOH, 596 nm Gly/HZO, 597 nm n-alcohols, 597 nm n-alcohols, 607 run n-alcohols, 619 nm Etg. 595 nm MeOH, 603 nm EtOH, 603 m-n MeOH, 633 nm

see fig. 2 see fig. 2 see fig. 2 C531 1531 59k2 9.5+0.5 16.0r0.5 4812

EtOH,

75*3

633 run

cz

at r.t. (ps)

this work l.L’-diethyl-4,4’-cyanine

261

MeOH, 586 nm MeOH, 603 nm MeOH, 619 nm EtOH, 586 nm EtOH, 603 nm Gly/MeO-H 596 nm Etg. 595 nm DIMSO, 570 mn n-akohols, 586 mn n-alcohols, 595 nm n-alcohols, 607 nm n-alcohols, 619 nm EtOH, 600 nm

12.6-~0.5 10.5*0.5 8.5&0.5 24~1 15s ho.5 see fig. 3 11215 41+2 see fig. 4 see fig. 3 E531 see fig. 4 9io.5

MeOH EtOH, 632 nm

27&2

ref.

this work”

ref.

0.48 rz = 0.98

13 [15]. 9[20] 5oc141

0.40

2 = 0.98

0.79 0.81 0.89 0.89

r) = YZ= rz = rz =

0.99 1.00 1.00 1 .oo

0.45 rz = 1.00 0.57 r2 = 0.99 0.91 r2=0.99 0 98 rz= 1.00 1:03r’=1.00

WI

1[13]

7.5 El33 10 C5]. 20 El ~30 IS], 22 111 10 [21, 80 [41 20, 165 [lS] 37 1191, 80 [14] 15.5 [l?]

0.95 rz = 1.00 1.05 YZ= 0.99 1.13 r’=0.99

22 [15] i5 [16] Cl71 120 [17]

0.91 r= = I.00 0.49 tz = 1.00

0.96

0.61 [17]

r2= 1.00

1.09rz=1.00 1.06 r== 1.00 0.97 i = 1.00 0.84 YZ= G-99 14 c13, 10 121 25 [6], 11.5 [19]

1.06 r= = 0.98

a) Abbreviations used in the table are the same as in *Able 1. ‘) Denotes extrapolated lifetimes from fig. 5. ” r2 is the coefiicient of determination. The closer r2 is to 1, the better fit.

A much slower relaxation rate (r=S ns in ethanol at room temperature) attributed to thermal reconversion of a photoisomer [41] was also observed in low viscosity solvents_ The amount of photoisomer formed decreases rapidly with increasing solvent viscosity [41]. The isosbestic point of normal isomer/photoisomer is at = 605 nm.

3.2. 1,l ‘-diethyl-2,2’-curbocyanine

iodide

(pinacyanoi}

The literature

values and the value obtained

by us for the relaxation rate of pinacyancl in ethanol strongly indicate that the correct value is ca. 13 ps at room temperature and A,,,= 600 nm. The value of 50 ps obtained by

262

V. Sundsrrfim, T. Gillho / liodiadontess

Paerschke et al. [14] is in conflict with other values. The viscosity of the solvent was varied in al! four ways mentioned in section 2. The detailed experimental conditions in each case can be found in table 1. In the more viscous solvents used at room temperature, extensive permanent photochemical bleaching of the sample occurred. A 0.2 mm flow cell was therefore used in these experiments. As an example

relaxation of cyanine dyes

of the recorded kinetics the GSR as a function of time for pinacyanol in ethanol. hexanol and 61.4% (w/w) glycerol/water at 619 nm are shown in fig. 1. Single exponential decays with T = 12.5, 54.2 and 30.2 ps respectively, are obtained. Going from ethanol to hexanol the increase in P is seen to follow the increase in n. By comparing the lifetimes obtained in hexanol and the glycerol/water sofutioa it is immediately

(a)

(cl

Fig. 1. Recorded GSR kinetics of pinacyanol at 619 nm in, (a) ethanol at 296 K, r = 12.5 ps; (b) hexanol at 296 K, + = 54 pi; glycerol/water

61:4% +/w)

at 296 K, 2 = 3G ps.

,

(c)

V. Swrdstnim,

T. Gillbro / Radiationless

evident that 7 does not follow the macroscopic viscosity. Although 7 is higher in the glycerol/water solution than in hexacol a lower value of 7 is obtained. This trend becomes partitularly evident in fig. 2 where the complete viscosity dependence of 7 is shown. Varying the temperature of a methanolic solution and the nalcohol series yields similar viscosity dependences, LT=0.79 and CY= 0.91 respectively. The viscosity dependences obtained in glycerol/methanol and glycerol/water solutions are on the other hand considerably weaker, a = 0.45 and cy =0.57 respectively. The linear relationship between T-I and 77-l in glycerol/water solutions obtained by Mialocq [13] is equivalent to a value CY= 1 in our plots. As can be seen from fig. 2 this relation is not confirmed by our measurements in the range of viscosities studied.

115

reIaxarion of cyanine dyes

263

The time resolved GSR of pinacyanol, in solvents of low viscosity also showed a nanosecond recovery lifetbe, similar to that observed in the l,l’-4,4’-cyanine recovery. As for 1,1’-4,4’cyanine it is attributed to the thermal decay of a photoisomer [41] formed by the picosecond excitation. The uhotoisomer has a red-shifted absorption relative to that of the normal form and an isosbestic point at 616~‘~ 1 nm in methanol [41]. 3.3.

l,l’-diethyl-2,4’-carbocyanine

iodide

This dye is structuralIy closely related to pinacyanol, see table 1. The relaxation rates are also seen to be very similar, see table-2. Previously obtained room temperature lifetimes are in rough agreement with our values. The vslues of CI as obtained by v2rying the temperature of

1

11.0

1

2 10.5

10.0

Fig. 2. Raie constant of pinacyanol GSR ES a function of viscosity in various sclvents. Q) a-alcohols at 597 nm, a=0.91; 0 methanol at 595 nm,~a = 0.79; 0 glycerol/methane; at 596 nn, cz = 0.45; A glycerol/water at 597 nm, cx= 0.57; ESethykxe glycol at 595 nm. 296 K.

264

V. Sundstr~m, T. Gillbro / Radiationless relaxarion of cyanine dyes

methanolic and ethanolic solutions are close to tinity, see fig. 5 and table 2.

3.5. 1,3’-diethyl-4,2’-oxacarbocyanine (DQOCII

3.4.

The room temperature lifetimes in methanol, T = 12.6 ps, and ethanol, T = 24 ps, at 586 nm and ethylene glycol, r = 112 ps, at 595 nm measured by us are in good agreement with the results reported by Tredwell et al. [X5] and Taylor et al, [17]. The shorter lifetimes at longer wavelengths, see table 2, are a consequence of the wavelength dependent relaxation rate [42]. The viscosity dependence of the relaxation rate was studied by methods (a), (b) and (d) described in section 2. From the log k versus log g plots shown in fig. 3 it is clear that varying the temperature of an ethanolic solution and the series of n-alcohols yields very similar viscosity dependences with (Y= 1. The parallel displacement of the two lines is yet

I, 1 ‘-diethyl-4,4’-carbocyanine (cryptocyanine)

iodide

A single exponential recovery is seen in both methanol and ethanol. Our data cannot confirm the two component decay reported by Tredwell et al. [lS]. There is a large variation in the lifetimes previously reported as can be seen in table 2. A comparison with these values is thus of little value. A value cr = 1.13 was obtained for the viscosity dependence, measured by varying the temperature of a methanolic solution. The viscosity range covered in this measurement was however narrow, due to permanent photochemical bleaching at lower temperatures.

iodide

1

115

1:a

1

H

10.5

10.0

9.5

2

5

10

20

50

VIScoslY/cP Fig. 3. Rate constant of DQOCI GSR as a function of q in various solvents. 0 n-alcohols at 595 m, p = 1.09; 0 glyczrol/methanoI at 596 IYIII,LI= 0.49; A ethanol at 603 mn, (1 = 0.91; B ethylene glyc& at 593 mq 296 K

V. Sundstrh,

T. Giilbro / Radiationless relaxation of cyanine dyes

another manifestation of the wavelength dependence of the radiationless relaxation rate. In the glycerol/methanol solutions a considerably weaker viscosity dependence is obtained, a = 0.49. This is a somewhat lower value than that obtained by Taylor et al. [l’i] by time resolved fluorescence measurements. The lifetime of DQOCI in ethyleneglycol at room temperature, 7 = 18.5 cp, falls closer to the glycerol/methanol line than the n-alcohol line, see fig. 3. The wavelength dependence of T is very clearly demonstrated in fig. 4 where log k is plotted versus log q for the n-alcohol series at two different wavelengths h = 586 nm and A = 619 nm. Two parallel lines with a disp!acement in k by a factor of =2 are obtained. As for pinacyanol and l,l’-4,4’-cyanine a nanosecond decay time of a photoisomer was also observed in DQOCI in solvents of lower viscosity [41]. 3.6.

1,3’-diethyl-,?2’-thiacarbocyanine

iodide

This dye is similar to DQOCI in structure, the oxygen in one of the quinolyl rings of DQOCI has been substituted for a sulphur atom. The observed room temperature lifetime is shorter than that for DQOCI. This is in accordance with the general finding of Fischer and Hamer [43,44] that for two similar dyes from the “this”- and “oxa”- groups the latter has a higher fluorescence quantum yield. The viscosity variation was realized through method (a), using ethanol as solvent. The viscosity dependence, cz = 0.84, is similar to that obtained for other dyes in thii solvent, see fig. 5 and table 2. 3.7. l,l’-diethyl-2,2’-dicarbocyanine (DDCI)

iodide

DDCI is the only dye of the dicivbocyanine family that was studied in this work. It was found two behave similarly to the dyes with shorter methine chain. The measured relaxation lifetime in ethanol at room temperature is in good agreement with the measurements of Arthurs et al. [21]. The lifetimes obtained by

165

Duguay and Hansen [I] and Mourou et al. [2] in methanol are in rough agreement with our data in ethanol when the linear relationship between lifetime and viscosity is considered (see fig. 5). The viscosity dependence was measured through method (a), using ethanol as solvent. We obtain (r = LOS which agrees with our findings for the carbocyanine dyes in the same solvent.

4. Discussion

As mentioned in the introduction various theories have been developed to explain the viscosity dependent radiationless relaxation (or fluorescence quantum yield) of some triphenylmethane dyes [30, 311. According to FH [30] the fluorescence quantum yield should vary as rl2’3, while ON [31] proposed that it should vary as q/T. Both groups claimed a good fit of their theoretical expression to the experimental data. The two models used different starting conditions. ON considered a free rotational diffusion of the phenyl groups about a C-C bond, to be responsible for the fast radiationless relaxation, while FH preferred to consider a damped oscillation of the phenyl groups in a potential well created by intramolecular forces. In order to obtain the qzr3 dependence of the fluorescence quantum yield FH further had to assume that the radiationless relaxation rate varies as (B-8,$, where 80 is the equilibrium dihedral angle of the ground state This assumption led to a rate law for the decay of S1 of the form elrp (-a?) which has never been observed in spite of numerous kinetic studies of triphenylmethane dyes [4_5-48]. Recently, another approach was successfully used in the treatment of the torsional motion of the naphthyl group about the C-C bond in l,l’binaphthyl [32]. McCaskill and Gilbert used a Kramers equation [33], which yields the rate constant of conformational change occurring in a large molecule, with an intramolecular potential barrier to this change, under the influence of a hydrodynamic drag and a random fluctuating force of the solvent. This description seems

V. Sundsfriim, T. Gillbro / Radiationless relaxation of cyonine dyes

266

11.5

11.0

9.5

0.5

7

2

5

10

20

50

VISCOSrrY/cP Fig. 4. Rate tonstan; of DQOCI 619-nm. a =0.97.

GSR a~ a function of T in n-alc0hok

to be more complete than those of FH and ON. According to Kramers’ equation the viscosity dependence of the rate consiant varies from case to case. In principIe one might tlnd a viscosity dependence of the relaxation rate anywhere between 7’ and ql_ Factors determining the type of viscosity dependence to be expected are the shape and height of the potential barrier and the quantity /3= .&/I, where & is an angle dependent drag coefficient, which is assumed to be proportional to the viscosity, and I is the reduced moment of inertia. These factors are however, in most cases not known. The evaluation of the drag coefitient in particular has been a matter of much controversy and it is still not completely clear when the slip or stick boundary condition should be used [49, 501, or if these boundary conditions are both oversiinpliflcations of the actual situation [LX]. It is evident from the above discussion that one

at two different wavelengths:

8 586 nm, a=0.96;

A

should not expect the reIaxation rate to follow a universal law of the form k = Cqmn, where a is a fixed constant independent of the solvent and solute used. Our experimental results on a wide variety of cyanine dyes in various soIvent systems clearly illustrate this point. The values of a range from 0.40 for l,l’-diethyl-4,4’-cyanine in methanol/glycerol mixtures to 1.13 for cryptocyanine in methanol at diiferent temperatures (see table 2). One notices, however, that LYvatues close to 1 are obtained for pinacyanol and DQOCI in n-alcohols. When the viscosity is changed by using glycerol/methanol or glycerol/water mixtures LYis considerably lower, =0.5. Using methanolic or ethanolic solutions at varying temperature gives a larger spread in Q! but most of the values lye in the range 0.8-1.1. One exception is l,l’-diethyl-4,4’-cyanine in ethanol where a! = 0.49. We do not believe that

267

115

1l.o

5

10.5

100

3.5 05

1

2

5

70

20

5c

vIsccsITY/cP Fig. 5. Rate constant of GSR as a function of 4 for several eyanine dyes. 0 1.1’-4,4’-cyanine in ethanol at 588 nm, P = 0.48; @ l,l’-4,4’-cyanine in glycerol/methanol at 595 nm, (I = 0.40; 0 DDCI in ethanol at 632 nm. a = 1.06: A THTA in ethanol at 600 nm. a = 0.84; 0 l,l-diethyl-2.4-carbocyanine in ethanol at 603 nm, o(= 1.05.

this large scatter in u is due to changes in the potential energy smface of the molecule from one solvent to another but rather to differcne interactions between the solute and the snrrounding solvent molecules, which is bound to influence the orientational relaxation of the solute. In order to stress this point further we will not attempt to make theoretical estimates of these interactions, since too many important parameters are still tmknown as mentioned above. Instead we will use a more empirical approach to the problem and investigate if there is any correlation between out rx-values and literature data on orientational relaxation of dye molecules in various solvents. In the work by Chuang and Eiienthal[52] and later by other authors [34-363 0n the’ orientationa! relaxation of rhodamine 6G in a range of n-alcohols it was found that rnr 1

.

followed the Stokes-Einstein equation, i.e. T.,~= 7~V/ kT, where V is the hydrodynamic volume of the molecule. The same relationship seems to hold also for eosin-y and rose bengal in alcohols [34, 361. The litera?ure data on the rotational diffusion of cyanine dyes are very rare and as far as we know all are on DODCI (and its phstcisomer) in alcoholic solution [39, 54, 551. Typical vaiues of the rotational correlation times are SO-95 ps (MeOH), 16% 200 ps (EtOH) and 320 ps (i PrOH), which indicates that 7&Cg also in this case_ We therefore suggest that a - 1 for the radiationless relaxation in n-alc~hcls as found by us corresponds to a situation in which the orientational relaxaticn f0110ws the Stokes-Einstein equation. Unfortunately we are not aware of any rotational difiusion study of dye molecules in glycerol/methanol or glycerol/water mixtures.

268

t’. Sw~dsrrt+n, T. Gillbro ! Radiationless relaxation of cymine dyes

However, in a recent publication, Rice and Kenney-Wallace [34] showed that in glycerol at various temperatures the rotational diffusion of the xanthene dye rhodamine 6G was substantially faster than expected by comparison with n-alcohols. This is equivalent to the low a ~0.5 value found by us, i.e. the radiationless relaxation time in glycerol mixtures is much faster than in n-alcohols of the same visc?sitjr. Ethylene glycol is another example of a solvent in which the rotational relaxation of rhodamine 6G is 2-3 times faster than expected from the Stokes-Einstein relation, znd by comparison with n-alcohols [34, 521. Eosin-y in ethylene glycol has a rotational relaxation time which is about 5 times smaller than in the n-alcohol of the same viscosity [34]. We find that the radiationless relaxation of pinacyanol and DQOCI in ethylene glycol is ~3-4 times faster than expected as compared with n-alcohols (see figs. 2 and 3). Altogether there is a qualitative relationship ‘between the radiationless relaxation times in cyanine dyes as observed by us and orientational relaxation times of xanthene and cyanine dyes as long as they are measured in the same solvent. It should however be mentioned that this comparison of two different kinds of dyes is not completely without objection, since they might have different short range interactions with the solvent molecules

and thus “feel”

a ditferent

[S6], e.g. for one dye the stick condition might apply while for another the slip condition might be the proper choice. In conclusion we have found further support for the proposal that the viscosity dependent radiationIess relaxation in most cyanine dyes is due to a torsional motion of the quinolyl groups about a C-C bond in the polymethine chain. Finally,‘it should be pointed out that it might of course be possible that +&hetorsional motion around a C-C bond is influenced by intra-molecular forces. The local forces exerted by the solvent molecules on the twisting groups may also be different from those exerted on a freely rotating molecule. especially if they have _ different functiozzal groops and shapes. It should therefore be of interest to compare the microviscosity

radiationless relaxation in cyanine dyes with the rotational diffusion of quinoline in the same solvents.

Acknowledgement

:

Financial support from the Swedish Natural Research Council and University of UmeQ is gratefully acknowledged. We also wish to thank Professor P-O. Kineli for encouraging discussions and support of this work and Mrs Eva Vikstriim and Mrs Ingegerd Hiigstram for their skilful assistance in preparing this publication.

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