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A new dual fluorescence with rhodamine B lactone
Volume 43. number 1
CHEMICAL PHYSICS LEFRS
A NEW DUAL FLUORESCENCE
i October 1976
WITH RHODAMINE B LACTONE
Uwe K.A. KLEIN and Friedrich W. HAFNER lnstitut f& Physikdische Chemie der Universitit Stuttgart, Stuttgart, Germany Received 16 June 1976 The wavclcngth of the rhodamine in dipole moment of about 25 debye form is also observed, although only these results are interpreted in turns
B lactonc ftuoresccna: is strongly dependent on solvent polarity, indicating an incxwsc on excitation. In mcthylcne chloride and acctonitrile the fluorcscenrr of the quinoid the lactonc form is cxdtcd. With the help of quantum yiclds,and lifctimc measurcmcntc of a simple photochcmical scheme.
I. Introduction The equilibrium between the two ground state forms of rhodamine B at interfaces has been known for a long time [l]. Ramette and SandeU (21 made a quantitative study showing the absorption spectra of both forms, the quinoid form (I) in water (&ax = 553 nm, em, = 1.1 x 105 M“ cm-‘) and the lactone form (II) in ben=316nm,emw = 1.8 xld’M_’ cm-‘). zene (A,,
r----
__---’
--
1 f I
2. Experimental
i I
L____-_____
_I 11
l---------
I 4 L-1 I
They observed the well known fluorescence of the quinoid form in water and a weak blue fluorescence of the lactone form in benzene. If the equilibrium constants in ground and excited state are different, dual fluorescence could be observed analogously to the well known prototropic fluorescence change [3,4 J _Since the equilibrium in the ground state depends on soIvent polarity one could expect the same to apply in the excited state. it is interesting that dual fluorescence is known in several compounds which can form quinoid structures, for example p-N-N’dimethyiaminobcnzo nitrile. Various theories have been put forward to account for this phenomenon [S-14).
-l
7
To a 1 0m2 M solution of the hydrochloride of rhodamine B p-a., Merck, in 0.01 n NaOli, an equaI volume of cyclohexane was added. After stirring for 2 hours about 70% of rhodamine B was in the cyclohexane layer. The separated cyclohexane solution was evaporated and the precipitated rhodamine B lactone needIes were dried over CaCI, in MCUO.On dissolving the slightly red crystalline product in a less polar solvent, we found no absorption of the quinoid form; in methanol or ethanol, however, we obtained the absorption of the quinoid rhodamine B, given in the literature [2,1 SJ. indicating a pure product. All solvents used were cuefuI!y dried by passing them through an Al,O,, basic (Merck), column, as we found a large effect on quantum yield and IXetime
Volnmc 43, number 1
CHEMICALPHYSICS LETTERS
of the rhodamine b lactone due to smsll amounts of dissolved watdr. Absorption spectra were measurod with a spectrophotometer DMR 10, Zeiss, and flcorescence spectra wzre obtained using a Hitachi-Perkin-Elmer spectrophotometer MPF3L with a red sensitive multiplier, R 446 F Hamamatsu. All fluorescence spectra were corrected using the fluorescence standards given by Iippert et al. [16]. Fluorescence lifetimes were delermined with a single photon nanosecond spectrometer, Ortec system. Excitation wavelength was 3 13 nm and the emission wavelength was selected by Schott intcrference fdters. All measurements were made on freshly prepared solutions at room temperature without degassing.
1 October 1976 300’ .
r_mto-‘r
250 -
nr.
I&n9 t
4.
3.
30000
35coO
LOO00 -
45000 V(cm+)
3. Results
Fig. 1. Absorption spectrum of rhodamine B lactonc (RhB-L) in cyclohexane.
The absorption spectrum of rhodamine B lactone (RhB-L) in cyclohexane is shown in fig. 1_The spectrum is similar in the &her solvents studied, except that the shoulder at 31500 cm-’ is not visible in the more polar solvents. There is also a small red-shift in sm._ and increase in extinction coefficient of the first absorption band. Unlike the absorption spectra the fluorescence spectra show a very large solvent effect as may be seen in fig. 2. On increasing solvent polarity the lactone fluorescence can be shifted by up to 5000 wavenumbers,
whilst the quantum yield at fust increases and then decreases. In methylene chloride both the lactone and quinoid fluorescences can be clearly seen. In acetonitrile almost all fluorescence comes from the quinoid form (RhB) but at high amplification a small lactonc fluorescence can be observed. The spectra in fig. 2 are uncorrected so that the height of the quinoid band relative to that of the lactone is reduced by a factor of about 7 and the positions of the maxima arc also affected. True quantum yields and fluorescence maxima arc shown in table 1 together with other absorption
1
Fig.
142
2. Fluorescent
spectra of rho&mine
B lactcne (RhB-L) in
various solvents.
Volume 43, number 1
CHEMICAL PHYSICS LEl-fERS
I October 1976
Table 1 Solvent
Absorption
Fluorescence quinonc
lactonc %bs (cm’)
%bs (M-r cm’)
ro(calc) (ns)
A--.--
pL (cm-‘) _-
nL
$$,,
;k)
n’Qu
:Zg
cyclohexane
32300
13700
8.3
25150
0.007
1.2
isooctane
32300
13800
8.9
25100
0.007
1.3
decaline ndibutyl
32300 32300
13800 14200
8.3 10.6
24800 23300
0.011 0.014
1.7 2.8
ether
32200
15700
11.2
22650
0.022
5.8
benzene
31950
15300
9.9
22350
0.025
6.2
dioxane
32050
16000
12.5
21050
0.052
14.4
chlorobcnzcne mcthylene chloride
31630 31700
14800 15400
12.1 16.3
20950 19900
0.047 0.026
15.6 16.3
17500
0.35
Pcctonitrilc
31700
15800
18.9
19100
3.000,
2.3
17500
0.22
1.9
methanol
18400
110000
4.9 _-
17480
0.68
3.1
ether
fluorescence parameters. The increase in quantum yield of the lactone form is accompanied by an increase in lifetime. In methylene chloride, however, the lifetime is not reduced even though the quantum yield is less, but in acetonitrile the lifetime is only one seventh that in methylcne chloride. Whilst the lifetimes of both forms are similar in acetonitrile they are very different in methylene chloride. The lifetime of the quinoid form in methylene chloride agrees with the value given by Berlman [ 17). All decay curves were found to be simple exponential.
and
4.
3.2
--_
differences. Either the emitting state is not formed directly on excitation or there must be a competition between a radiationless transition from the excited FrankCondon state, rate constant kl, and a solvent relaxation prccess, rate constant k, (see fig. 3). It is not possible that the observed fluorescence arises from the primarily excited state if the Strickler-Berg equation [ i8] is valid. As a first limiting case one may assume that all the excited molecules are very quickly converted to the emitting state. Then the natural Lifetimes ro(exp.) = 7~ / vL determined from the experimental data are those of the emitting state. In the second limiting case the calculated natural lifetimes
Discussion
order to elucidate the nature of the states involved we first calculated natural lifetimes of the’longwave absorbing band of rhodamine B lactonc in the solvents used by aid of the Strickler-Berg equation [ 181 (see table 1) In
1 +X)
= 2 88 x It-j-g,,2 -
If(‘)” 5 d; Jj@)di+j3 s ’ -
RhB-L*_e
k3
-
-
Rhfl*
L-22,
(1)
The values of the natural lifetimes obtained from the measured quantum yields and fluorescence decays are more than twenty times greater than the calculated values from the absorption and fluorescence spectra. There are two limiting cases which may explain these
,
RhB - Lv
RhB
Fig. 3. Reaction scheme for the dual fluorescence of rhodzminc B (RhB-L: ladonc form, RhB: quinoid form). Asterisk denotes the excited state.
143
Volume 43. number 1
CHEMICAL PHYSICS LETTERS
so(caIc.) may describe the emitting stare. Then the relation between rO(calc.) and ro(cxp.) is given by 1 -f&xp.)
VL =-=-7L
k2 k,
+
1 k, +alc.)’
ti
(2)
WC think the second case applies. In less polar solvents the measured lifetimes 7L arc smaller than the calculated natural lifetimes whereas in dioxane, chlorobenzene and methylene chloride 7L and 70(calc_) arc the same, within experimental error and the approximations of the Strickler-Berg theory [ 181 -‘In these three solvents the real quantum yield of the lactone fluorescence should therefore be unity, which is in accordance with the fact that on cooling these solutions no change in lifetime occurred. In the less polar solvents, however, the lifetimes 7L = l/(kl,L + k,,) increase on cooling but they do not approach the calculated natural lifetimes (k,,: rate constant of fluorescence of the lactone form; kDL: rate constant of radiationIc;s deactivation). The lifetime of the lactone in acetonitrile is nearly equal to that of the quinoid because when the two states ere of similar energy there may be a fast equilibrium between them. It is well known that the solvent relaxaticn process due to changes of dipole moment on excitation causes a red shift of the fluorescence spectrum [7,19-221. The strong dependence of the Stokes shift U & - 5 L of the rhodamine B lactone on the solvent parameter Af, A~=(E - 1)/(2e + 1) - (n* - l)/(ti*
+ 1).
(3)
is shown in fig. 4. According to Lipper t’s theory [ 19) the relation is given by
abs
--3
L
=
1 October 1976
[2&
- pc)*/hca3] Af.
(4)
Except for benzene and dioxane a good straight line was found. Taking a radiusa of 7 A there is a difference of about 25 debye between ground and excited states of the rhodamine B lactone. The excited state of the quinoid form can be reached on further increase of solvent polarity. In very polar solvents only the quinoid form is present in the ground state, so that the dual fluorescence on exciting the Iactone form can only be observed in a narrow range of solvent polarity, for exampie in methylene chloride and acctonitrile. Both fluorcscenccs have the same excitation spectrum, which agrees well with the absorption spectrum of the lactone, indicating that unlike the case of Michlcr’s ketone we are not exciting two species [23-251. The ratio of quinoid and lactone fluorescence does not depend on excitation wavelength. A dual fluorescence from two singlet states S, and S, is involved. This is a further exception to Kasha’s rule (261 and it may be characteristic of molecules which can form intramolecular charge resonance states. As mentioned in the previous section
the lifetimes
of the two forms in mcthylene
chloride differ considerably and the fluorescences have simple exponential decay. We therefore postulate a third channel of deactivation of the primarily excited Franck-Condon state leading to the charge resonance state (see fig. 3). Eq. (2) is then modified to qL = [k-&
+ k, + k3)l 7L/7&alc.).
(5)
From eq. (5) one can calculate the fraction of excited molecules F2 = kc,/Ziki which reaches the relaxed Iactone state. Using the fluorescence quantum yield vQu of the quinoid form when the lactone form is excited, we can also determine the fraction of molecules Fj = k3/Ziki which occupies the quinoid state. ~~ = k3/(k,
+ k, + k3) = vQu/J)Qu(m=-)s
(6)
where qqu(max-) = kFqulSkFQu + kDQu)
I
0
0.1
0.2
0.3 -Af
F& 4. Stokes shift (G,bs-~L) of RhB-L as a function of the solvent parameter
144
Af.
(7)
(kFm >kDQu - rate constants of emission and deactivation of the quinoid form respectively) is the quantum yield of the qulnoid form on direct excitation. We determined qqu(max.) in methylene chloride and acetonitrile using the hydrochloride of rhodamine B. (The quantum yields of rhodamine B and its hydrochloride were found to be practically equal in methanol and ethanol.) In methylene chloride the quantum yield
Volume 43, number 1
CHEMICAL PHYSICS LE-ITERS
1 October 1976
References
(11 D. Dcutsch, 2. Physik. Chcm. 136 (1928) 353. (21 R.W. Ramctte and EB. SandeU. J. Am. Chem. Sot 78
-- ._._._ F?
0
“-.-I--x-.
01
.-._
0.2
03 -
At
Fig. 5. Fractions of molecules Fi = ki/Ziki which arc deactivated via channels kt , k2, k3 as a function of the solvent parameter 4/. T)Qu(max.) is about 0.75; in acetonitrile qqu(max.) is about 0.23 which is almost the same value as obtained on exciting the lactone form (see table 1). The fraction of excited molecules F, = k, /Xi&,. which are immediately deactivated via other states (probably triplets) then is Fl
=
1 -F2-F3.
(8)
Fig. 5 shows the dependence of the fractions Fi = ki/Etki on the solvent parameter. Up to Af = 0.2 F, and F2 are constant at about 95% and 5% respectively. On increasing Af a sudden decrease in F, is a’ccompanied by a corresponding increase in F3. ‘he probability that the excited molecules are deactivated via the diabatic channel k, will bc diminished when the quinoid state. which can be reached by an adiabatic pathway k3, lies energetically below the relaxed lactone state. Absolute values of k , k and k3 which may be in the region of 101o-lO1 $ s- f may be determined by laser picosecond spectroscopy. After the transition from the excited state to the quinoid ground state there must be a ring closure, rate constant kc, giving the lactone as no absorption of the quinoid form is detectable after irradiation. Flash experiments could be used to investigate this process and determine values of kc.
Acknowledgement We thank Dr. M. Hauscr and Mr. D.J. Miller for many helpful discussions and Professor critically reading the manuscript.
Dr. H.E.A. Kramer
for
(19%) 4872. 36 (1949) 186; Z ElckI31 Th. FGrster, Natunvissenschaften trochem. Angew. Physik. Chcm. 54 (1950) 42,531. (41 A. Weller. Progr. Reaction Kinetics 1 (1961) 18Y. 151 E. Lippert, W. Liidcr, F. MoU, W. Nigelc, H. Boss. H. Prigge and 1. Seibold-Blankenstein, Angcw. Chem. 73 (1961) 695. [6 j E. Lippert, W. Liidcr and ii. Boos, in: Advances in mokcular spectroscopy, ed. hlmgini (Pcrgamon Press. Oxford, 1962). p. 443. (7) J.B. Birks. Photophysia of aromatic molecules (Wiley. New York, 1970) p. 164-168. 188. (81 K. Rotkicwia. K.H. Grcllman and Z.R. Grabowski. Chem. Phyr Letters 19 (1973) 315. (91 O.S.-Khalil, R.H. Hofeldt and S.P. hld;lynn, Chem. Phyr Letters 17 (1972) 479; Spcctry. Letters 6 (1973) 147. (lo] N. Nakashima and N. Matap, Bull. Chcm. Sot Japan 46 (1973) 3016. [ 111 N. Nakashima, ii. Inouc, N. hfataga and C Yamanaka, Bull. Chem. Sot Japan 46 (1973) 2288. (121 W.S. Struve, P.M. Rcntzcpis and J. Jortncr. 1. Chem. Phys. 59 (1973) 5014. (131 W.S. Struve and P.M. Rartrcpis, J. Chem Phyr 60 (1974) 1533; Chem. Phyr Letters 29 (1974) 23. (141 E.M. Kosower and H. Dodiuk. 1. Am. Chem. Sot 98 (1976) 924. (IS] M.I. Sncgov, 1.1. Reznikova and A.S. Chcrkasov. Opt Spcctry. 36 (1974) 55. (161 E Lippert. W. Nigele, I. S&old-Blankcnstein. U. Steiger and W. Voss, Z. AnaL Chem. 170 (1959) 1; in: LandctltBiirnstein Group II. Volume 3, Lumincsancc of organic substances, edr A. Schmillen and R. Logier (Springer, Berlin, 1967) p. 228. (171 LB. Berlman, Handbook of fluorcsana spectra of arco: matic molcculcs (Academic Press, New York, 1971) p. 411. (181 S-J. Stricklcr and R-A. Bug, J. Chcm. Phyr 37 (1962) 814. (191 E Lippert, Z. Elcktrochcm. 61 (1957) 962. (20 J B.S. Ncporcnt and N.G. Bakhshicv, Opt Spcctry. S (1958) (21 j L%lot and A. Kawski. 2. Naturforsch. 18a (1963) IO, 256. (22 1 N. Ma(Jga and T. Kubota. Molecular interxtions and elec tronic spectra (Dekku, New York, 1970) p. 3fl. (231 W. KIGpfer. Chcm. Phyr Letters 11 (1971) 482. (241 P.R. Callisand R.W. Wilson. Chem. Phvz Letters t 3 (1972) 417. I251 W. Liptay. H.-J. Schumann and F. Pet&e, I. Luminesozna 12113 (1976) 793. !261 M..Kasha, Di&ssions Faraday Sot 9 (1950) 14.
I45
CHEMICAL PHYSICS LEFRS
A NEW DUAL FLUORESCENCE
i October 1976
WITH RHODAMINE B LACTONE
Uwe K.A. KLEIN and Friedrich W. HAFNER lnstitut f& Physikdische Chemie der Universitit Stuttgart, Stuttgart, Germany Received 16 June 1976 The wavclcngth of the rhodamine in dipole moment of about 25 debye form is also observed, although only these results are interpreted in turns
B lactonc ftuoresccna: is strongly dependent on solvent polarity, indicating an incxwsc on excitation. In mcthylcne chloride and acctonitrile the fluorcscenrr of the quinoid the lactonc form is cxdtcd. With the help of quantum yiclds,and lifctimc measurcmcntc of a simple photochcmical scheme.
I. Introduction The equilibrium between the two ground state forms of rhodamine B at interfaces has been known for a long time [l]. Ramette and SandeU (21 made a quantitative study showing the absorption spectra of both forms, the quinoid form (I) in water (&ax = 553 nm, em, = 1.1 x 105 M“ cm-‘) and the lactone form (II) in ben=316nm,emw = 1.8 xld’M_’ cm-‘). zene (A,,
r----
__---’
--
1 f I
2. Experimental
i I
L____-_____
_I 11
l---------
I 4 L-1 I
They observed the well known fluorescence of the quinoid form in water and a weak blue fluorescence of the lactone form in benzene. If the equilibrium constants in ground and excited state are different, dual fluorescence could be observed analogously to the well known prototropic fluorescence change [3,4 J _Since the equilibrium in the ground state depends on soIvent polarity one could expect the same to apply in the excited state. it is interesting that dual fluorescence is known in several compounds which can form quinoid structures, for example p-N-N’dimethyiaminobcnzo nitrile. Various theories have been put forward to account for this phenomenon [S-14).
-l
7
To a 1 0m2 M solution of the hydrochloride of rhodamine B p-a., Merck, in 0.01 n NaOli, an equaI volume of cyclohexane was added. After stirring for 2 hours about 70% of rhodamine B was in the cyclohexane layer. The separated cyclohexane solution was evaporated and the precipitated rhodamine B lactone needIes were dried over CaCI, in MCUO.On dissolving the slightly red crystalline product in a less polar solvent, we found no absorption of the quinoid form; in methanol or ethanol, however, we obtained the absorption of the quinoid rhodamine B, given in the literature [2,1 SJ. indicating a pure product. All solvents used were cuefuI!y dried by passing them through an Al,O,, basic (Merck), column, as we found a large effect on quantum yield and IXetime
Volnmc 43, number 1
CHEMICALPHYSICS LETTERS
of the rhodamine b lactone due to smsll amounts of dissolved watdr. Absorption spectra were measurod with a spectrophotometer DMR 10, Zeiss, and flcorescence spectra wzre obtained using a Hitachi-Perkin-Elmer spectrophotometer MPF3L with a red sensitive multiplier, R 446 F Hamamatsu. All fluorescence spectra were corrected using the fluorescence standards given by Iippert et al. [16]. Fluorescence lifetimes were delermined with a single photon nanosecond spectrometer, Ortec system. Excitation wavelength was 3 13 nm and the emission wavelength was selected by Schott intcrference fdters. All measurements were made on freshly prepared solutions at room temperature without degassing.
1 October 1976 300’ .
r_mto-‘r
250 -
nr.
I&n9 t
4.
3.
30000
35coO
LOO00 -
45000 V(cm+)
3. Results
Fig. 1. Absorption spectrum of rhodamine B lactonc (RhB-L) in cyclohexane.
The absorption spectrum of rhodamine B lactone (RhB-L) in cyclohexane is shown in fig. 1_The spectrum is similar in the &her solvents studied, except that the shoulder at 31500 cm-’ is not visible in the more polar solvents. There is also a small red-shift in sm._ and increase in extinction coefficient of the first absorption band. Unlike the absorption spectra the fluorescence spectra show a very large solvent effect as may be seen in fig. 2. On increasing solvent polarity the lactone fluorescence can be shifted by up to 5000 wavenumbers,
whilst the quantum yield at fust increases and then decreases. In methylene chloride both the lactone and quinoid fluorescences can be clearly seen. In acetonitrile almost all fluorescence comes from the quinoid form (RhB) but at high amplification a small lactonc fluorescence can be observed. The spectra in fig. 2 are uncorrected so that the height of the quinoid band relative to that of the lactone is reduced by a factor of about 7 and the positions of the maxima arc also affected. True quantum yields and fluorescence maxima arc shown in table 1 together with other absorption
1
Fig.
142
2. Fluorescent
spectra of rho&mine
B lactcne (RhB-L) in
various solvents.
Volume 43, number 1
CHEMICAL PHYSICS LEl-fERS
I October 1976
Table 1 Solvent
Absorption
Fluorescence quinonc
lactonc %bs (cm’)
%bs (M-r cm’)
ro(calc) (ns)
A--.--
pL (cm-‘) _-
nL
$$,,
;k)
n’Qu
:Zg
cyclohexane
32300
13700
8.3
25150
0.007
1.2
isooctane
32300
13800
8.9
25100
0.007
1.3
decaline ndibutyl
32300 32300
13800 14200
8.3 10.6
24800 23300
0.011 0.014
1.7 2.8
ether
32200
15700
11.2
22650
0.022
5.8
benzene
31950
15300
9.9
22350
0.025
6.2
dioxane
32050
16000
12.5
21050
0.052
14.4
chlorobcnzcne mcthylene chloride
31630 31700
14800 15400
12.1 16.3
20950 19900
0.047 0.026
15.6 16.3
17500
0.35
Pcctonitrilc
31700
15800
18.9
19100
3.000,
2.3
17500
0.22
1.9
methanol
18400
110000
4.9 _-
17480
0.68
3.1
ether
fluorescence parameters. The increase in quantum yield of the lactone form is accompanied by an increase in lifetime. In methylene chloride, however, the lifetime is not reduced even though the quantum yield is less, but in acetonitrile the lifetime is only one seventh that in methylcne chloride. Whilst the lifetimes of both forms are similar in acetonitrile they are very different in methylene chloride. The lifetime of the quinoid form in methylene chloride agrees with the value given by Berlman [ 17). All decay curves were found to be simple exponential.
and
4.
3.2
--_
differences. Either the emitting state is not formed directly on excitation or there must be a competition between a radiationless transition from the excited FrankCondon state, rate constant kl, and a solvent relaxation prccess, rate constant k, (see fig. 3). It is not possible that the observed fluorescence arises from the primarily excited state if the Strickler-Berg equation [ i8] is valid. As a first limiting case one may assume that all the excited molecules are very quickly converted to the emitting state. Then the natural Lifetimes ro(exp.) = 7~ / vL determined from the experimental data are those of the emitting state. In the second limiting case the calculated natural lifetimes
Discussion
order to elucidate the nature of the states involved we first calculated natural lifetimes of the’longwave absorbing band of rhodamine B lactonc in the solvents used by aid of the Strickler-Berg equation [ 181 (see table 1) In
1 +X)
= 2 88 x It-j-g,,2 -
If(‘)” 5 d; Jj@)di+j3 s ’ -
RhB-L*_e
k3
-
-
Rhfl*
L-22,
(1)
The values of the natural lifetimes obtained from the measured quantum yields and fluorescence decays are more than twenty times greater than the calculated values from the absorption and fluorescence spectra. There are two limiting cases which may explain these
,
RhB - Lv
RhB
Fig. 3. Reaction scheme for the dual fluorescence of rhodzminc B (RhB-L: ladonc form, RhB: quinoid form). Asterisk denotes the excited state.
143
Volume 43. number 1
CHEMICAL PHYSICS LETTERS
so(caIc.) may describe the emitting stare. Then the relation between rO(calc.) and ro(cxp.) is given by 1 -f&xp.)
VL =-=-7L
k2 k,
+
1 k, +alc.)’
ti
(2)
WC think the second case applies. In less polar solvents the measured lifetimes 7L arc smaller than the calculated natural lifetimes whereas in dioxane, chlorobenzene and methylene chloride 7L and 70(calc_) arc the same, within experimental error and the approximations of the Strickler-Berg theory [ 181 -‘In these three solvents the real quantum yield of the lactone fluorescence should therefore be unity, which is in accordance with the fact that on cooling these solutions no change in lifetime occurred. In the less polar solvents, however, the lifetimes 7L = l/(kl,L + k,,) increase on cooling but they do not approach the calculated natural lifetimes (k,,: rate constant of fluorescence of the lactone form; kDL: rate constant of radiationIc;s deactivation). The lifetime of the lactone in acetonitrile is nearly equal to that of the quinoid because when the two states ere of similar energy there may be a fast equilibrium between them. It is well known that the solvent relaxaticn process due to changes of dipole moment on excitation causes a red shift of the fluorescence spectrum [7,19-221. The strong dependence of the Stokes shift U & - 5 L of the rhodamine B lactone on the solvent parameter Af, A~=(E - 1)/(2e + 1) - (n* - l)/(ti*
+ 1).
(3)
is shown in fig. 4. According to Lipper t’s theory [ 19) the relation is given by
abs
--3
L
=
1 October 1976
[2&
- pc)*/hca3] Af.
(4)
Except for benzene and dioxane a good straight line was found. Taking a radiusa of 7 A there is a difference of about 25 debye between ground and excited states of the rhodamine B lactone. The excited state of the quinoid form can be reached on further increase of solvent polarity. In very polar solvents only the quinoid form is present in the ground state, so that the dual fluorescence on exciting the Iactone form can only be observed in a narrow range of solvent polarity, for exampie in methylene chloride and acctonitrile. Both fluorcscenccs have the same excitation spectrum, which agrees well with the absorption spectrum of the lactone, indicating that unlike the case of Michlcr’s ketone we are not exciting two species [23-251. The ratio of quinoid and lactone fluorescence does not depend on excitation wavelength. A dual fluorescence from two singlet states S, and S, is involved. This is a further exception to Kasha’s rule (261 and it may be characteristic of molecules which can form intramolecular charge resonance states. As mentioned in the previous section
the lifetimes
of the two forms in mcthylene
chloride differ considerably and the fluorescences have simple exponential decay. We therefore postulate a third channel of deactivation of the primarily excited Franck-Condon state leading to the charge resonance state (see fig. 3). Eq. (2) is then modified to qL = [k-&
+ k, + k3)l 7L/7&alc.).
(5)
From eq. (5) one can calculate the fraction of excited molecules F2 = kc,/Ziki which reaches the relaxed Iactone state. Using the fluorescence quantum yield vQu of the quinoid form when the lactone form is excited, we can also determine the fraction of molecules Fj = k3/Ziki which occupies the quinoid state. ~~ = k3/(k,
+ k, + k3) = vQu/J)Qu(m=-)s
(6)
where qqu(max-) = kFqulSkFQu + kDQu)
I
0
0.1
0.2
0.3 -Af
F& 4. Stokes shift (G,bs-~L) of RhB-L as a function of the solvent parameter
144
Af.
(7)
(kFm >kDQu - rate constants of emission and deactivation of the quinoid form respectively) is the quantum yield of the qulnoid form on direct excitation. We determined qqu(max.) in methylene chloride and acetonitrile using the hydrochloride of rhodamine B. (The quantum yields of rhodamine B and its hydrochloride were found to be practically equal in methanol and ethanol.) In methylene chloride the quantum yield
Volume 43, number 1
CHEMICAL PHYSICS LE-ITERS
1 October 1976
References
(11 D. Dcutsch, 2. Physik. Chcm. 136 (1928) 353. (21 R.W. Ramctte and EB. SandeU. J. Am. Chem. Sot 78
-- ._._._ F?
0
“-.-I--x-.
01
.-._
0.2
03 -
At
Fig. 5. Fractions of molecules Fi = ki/Ziki which arc deactivated via channels kt , k2, k3 as a function of the solvent parameter 4/. T)Qu(max.) is about 0.75; in acetonitrile qqu(max.) is about 0.23 which is almost the same value as obtained on exciting the lactone form (see table 1). The fraction of excited molecules F, = k, /Xi&,. which are immediately deactivated via other states (probably triplets) then is Fl
=
1 -F2-F3.
(8)
Fig. 5 shows the dependence of the fractions Fi = ki/Etki on the solvent parameter. Up to Af = 0.2 F, and F2 are constant at about 95% and 5% respectively. On increasing Af a sudden decrease in F, is a’ccompanied by a corresponding increase in F3. ‘he probability that the excited molecules are deactivated via the diabatic channel k, will bc diminished when the quinoid state. which can be reached by an adiabatic pathway k3, lies energetically below the relaxed lactone state. Absolute values of k , k and k3 which may be in the region of 101o-lO1 $ s- f may be determined by laser picosecond spectroscopy. After the transition from the excited state to the quinoid ground state there must be a ring closure, rate constant kc, giving the lactone as no absorption of the quinoid form is detectable after irradiation. Flash experiments could be used to investigate this process and determine values of kc.
Acknowledgement We thank Dr. M. Hauscr and Mr. D.J. Miller for many helpful discussions and Professor critically reading the manuscript.
Dr. H.E.A. Kramer
for
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