W. WEST AND SANDRA PEARCE
1894
The Dimeric State of Cyanine Dyes
by W. West and Sandra Pearce Research Laboratories, Eastman Kodak Company, Rochester, New York
14660
(Received December 11, 1964)
Dyes of the 3,3-diethylthiacyanine series dimerize over a certain concentration range, as shown by their adherence to the law of mass action for the monomer-dimer equilibrium. At higher concentrations, a more complex H-aggregate appears. The stability of the dimers, as measured by the free energy of dimerization, increases steadily with chain length. The dimeric spectrum consists of an intense P-branch, hypsochromic to the monomeric maximum, and a feeble, bathochromic N-branch. The monomers are linked with their molecular planes and chromophores approximately parallel. Mixed dimers, with relatively strong N-branches, are formed in aqueous solutions of certain dye pairs. The significance of dimerization with respect to fluorescence quenching, photochemical activity, and the "excimer" is discussed.
Introduction It is well known that solutions of dyes in polar organic solvents a t room temperature follow Beer's law over an extended concentration range, but that in water large deviations from the law are observed.' For many classes of dye in aqueous solution, the band of highest intensity in dilute solution beconies weaker as the concentration is increased, and new bands appear a t other wave lengths. These spectral changes have long been attributed to aggregation of the dye molecules in water to form dimers and higher polymers under the influence of the strong dispersion forces associated with the high polarizability of the chroniophoric hai in.^-^ The dominant role of water as the solvent most favorable to aggregation a t room temperature is no doubt associated with the effect of its high dielectric constant in reducing the repulsive force between the similarly charged dye cations or anions in the aggregate; the absence of aggregation in organic solvents of high dielectric constant at room temperature suggests that solvation interferes with the aggregation, and in such solvents aggregates are stable only a t low temperatures under conditions of high viscosity.' The peculiar solvent effects of water on the absorption spectra of dyes are illustrated in Figures 1 and 2, which show the visible absorption spectrum of 3,3'diethylthiacarbocyanine p-toluenesulfonate (dye 11, Table I) a t 22", dissolved in methanol and in water. The Journal of Physical Chemistry
Alcoholic solutions follow Beer's law over a t least the hundredfold concentration range from 10+ to M . The shape of the spectrum in alcoholic solution is t'ypical of the molecular spectrum of cyanine and other polymethine dyes; with rare exceptions, the band of longest wave length is the most intense, presumably representing approximately the 0 0 transition in a vibronic progression, whereas the pattern of subsidiary maxima or shoulders a t shorter wave lengths, differing from dye to dye in resolution and intensity, is set by the 1 0, 2 + 0, etc., transitions of a dominant vibrational mode of frequency approximately 1200 cm.-I. (See Table 111, under +
+
Avv.>
In water, Figure 2, the aggregation of the dye with increasing concentration is shown by the reduction in intensity of the molecular maximum a t wave length (1) Deviations from Beer's law have been observed for certain dyes of the triphenylmethane class in organic solvents of low polarity at
room temperature, attributed to the appearance of contact ion pairs at high concentration: V. F. Feichtmeyer and J. Schlag, Ber. Bunsenges., 68, 95 (1963). Deviations attributed to dimerization have been observed for certain cyanine dyes dissolved in a rigid isopropyl alcohol-pentane solvent at 77'K. by G. Levinson, W. T. Simpson, and W. Curtis, J. Am. Chem. Soc., 79, 4314 (1957), and polymerization of 1,l '-diethyl-2,2'-cyanine chloride in ethanol at 90'K. has been recognized from spectral changes by H. Zimmermann and G. Scheibe, 2. Elektrochem., 60, 566 (1956). (2) G. Kortum, 2.physik. Chem., B33, 1 (1936); B34, 255 (1936). (3) G. Scheibe, 2. angeu. Chem., 49, 563 (1936); 50, 212 (1937); 52, 631 (1939). (4) E. Rabinowitch and L. Epstein, J. Am. Chem. Soc., 63, 69 (1941).
THEDIMERICSTATEOF CYANINE DYES
20
1895
20
I
I
I
I
500
550
,600
1
18
16
15
14
12 d
b IO
d
blO
-
X
X
w
” 8
6
5 4
\
2
0 500
400
600
700
n 460
Wovelength in Millimicrons
Wavelength in Millimicrons Figure 1. Absorption spectrum of 3,3’-diethylthiacarbocyanine ptoluenesulfonate (dye 11)in methanol.
Figure 2. Absorption spectrum of 3,3’-diethylthiacarbocyanine p-toluenesulfonate (dye 11)in water.
Table I : Dye Formulas
centrations, two isosbestic points appear in the family of absorption curves, one a t shorter wave length and the other a t longer wave length than the molecular maximum. Over this range of concentrations, therefore, according to the well-known properties of the isosbestic point, two colored species are in equilibrium. The persistence of the monomeric band shows that one of these species is the dye monomer, and it will be shown that the other is a dimeric form of the dye. As the concentration of dye is further increased, aggregation of cyanine dyes usually proceeds beyond the dimeric stage. At this stage, the absorption curve of the solution does not pass through the isosbestic points characteristic of equilibrium between monomer and dimer alone, the dimeric maximum broadens and drifts to shorter wave lengths, and a more or less definite new band appears hypsochromic to the dimeric maximum. It seems reasonable to attribute such bands to a polymer of greater complexity than the dimerB5 This polymer has been called an “H-polymer,” characterized by an H-absorption band.6 The Hband of dye 11, illustrated for the highest concentration in Figure 2, is not well resolved from the dimeric
CzHs
CzHs
I 3,3’-Diethylthiacyanine ethylsulfate, n
=
0
I1 3,3’-Diethylthiacarbooyaninep-toluenesulfonate or chloride, n = l I11 3,3’-Diethylthiadicarbocyaninep-toluenesulfonate, n IV
3,3’-Diethylthiatricarbocyaninep-toluenesulfonate, n
~ ~ cI + H = c ~ n - c H
= 2 = 3
c1-
cza
CzHs
Pinacyanol chloride, n = 1
(C&)SN ‘
a
L
r
H
3
)
2
~ 1 “
Methylene blue
553 mp and the appearance of a new maximum a t about 510 nip, somewhat shorter than the vibrational shoulder of the molecular band. Over a range of con-
(5) G . Scheibe, Kolloid-Z., 82, 1 (1938).
(6) W. West and
B. H. Carroll, J. Chem. Phys.,
19,417 (1951).
Voluma 69, N u d e r 6 June. 196.5
w.WEST AND SANDRA PEARCE
1896
maximum, but sometimes a definite H-peak can be however, Sheppard and GeddeP were unable to observed, as in Figure 3 for dye V. find strict quantitative adherence of their deduced The exist'ence of dimers in aqueous solutions of a concentrations of monomeric and aggregated dye to number of dyes of the t h i a ~ i n e ~and p ~ xanthine ,~ c l a s ~ e s , ~ the requirements of the mass law for the monomer S of rhodamine B, acridine orange,IO proflavine," and dimer equilibrium, although they concluded that their other dyes has been proved by observations that as deviations arose, not because the dimerization hypothethe total concentration of the dye is changed, the consis was in itself incorrect, but because certain activity centrations of monomer and dimer, Cm and c d , respecfactors must be taken into account. In the present tively, adjust themselves in accordance with the law communication, we report data derived from the specof mass action tra of aqueous solutions of the vinylogous series of thiacyanine dyes listed in Table I which are in quantiCm 2 -=K tative agreement, within a reasonable limit of experi(1) cd mental error, over a restricted concentration range, with the requirements of the simple mass law applied where K is the dissociation constant of the dimer. to the nionomer e dimer equilibrium. Among cyanine dyes, similar evidence has been found Dimers of Thiacyanine Dyes. To find the confor the existence of the dimer of 1,l'-diethyl-2,2'centrations of monomer and dimer in an aqueous solupyridocyanine salts in a rigid glass of isopropyl alcohol tion, one must know in essence the molar extinction and isopentane at 77"K.12 For aqueous solutions of coefficient of the pure monomer at a suitable wave l,lt-diethyl-2,2'-cyaninechloride and its next higher length, usually that of the absorption maximum and, vinylog, pinacyanol chloride (dye V, Table I), Scheibe since the absorption bands of the monomer and dimer found satisfactory conformance to the law of mass overlap, one must also know the relative contributions action insofar as the plot of log Cd against log cm a t of the monomer and dimer to the observed density a t various total concentrations was linear with a slope of the measuring wave length. In principle, the molar 2. For a number of cyanine dyes in aqueous solution, extinction coefficient of the pure monomer in water can be found from measurements of the absorption spectra of solutions so dilute that dimerization is 20 I negligible. If the fraction of the total molar concentration c of the dye in solution present as monomer is a,the dimeric dissociation constant is
K=-
15
For cyanine dyes we find K to vary from some mole I.-'. If a solution in which 95% of the to dye is present as monomer is regarded as a tolerable approximation to a solution of the pure monomer, the concentration a t which this condition prevails is found from eq. 2 to vary from 2.8 X lov5 M for K = to 2.8 X for K = lom6. Aqueous solutions of cyanine dyes, especially those of longer chain length, are not stable in light or on heating, and, moreover, the dyes are adsorbed to the glass walls of volumetric
9
-b
2azc l - a
10
X
w
5
(7) R. Havemann, E. Nutsoh, and H. Pietsoh, 2. physik. Chem. (Leiprig), 219, 171 (1962). (8) K. Bergmann and C. T. O'Konski, J. Phys. Chem., 67, 2169
(1963). 0 400
500
600
Wavelength in Millimicrons Figure 3. Absorption spectrum of pinacyanol chloride (dye V) in methanol and in water, showing dimeric and H-aggregate bands in water.
The Journal of Physical Chemistry
700
(9) T.Forster and E. Konig, 2. Elektrochem., 61, 344 (1956). (10) V. Zanker, 2.physik. C h m . , 199, 256 (1952); 200, 250 (1952). (11) G. R. Haugen and W. H. Melhuish, Trans. Faraduy SOC.,60, 386 (1964). (12) See Levinson, Simpson, and Curtis, in ref. 1. (13) S. E. Sheppard and A. L. Geddes, J . Am. Chem. Soc., 66,2003 (1944).
1897
THEDIMERICSTATEOF CYANINE DYES
apparatus and of the measuring cell. It is, therefore, difficult to make accurate measurements of aqueous dye solutions at concentrations of M and less, requiring cell lengths of 10 cm. or more, even when precautions are taken to exclude actinic light in the preparation of the solutions and to correct for adsorption. Careful measurements of the absorption spectra of several cyanines in dilute aqueous solutions prepared with the exclusion of actinic light, rapidly made on an automatic spectrophotometer, gave values of the molar extinction coefficient at the monomeric maximum only slightly below that in methanol. Methanol solutions are considerably more stable than aqueous solutions, adsorption of dye to the glass walls is much reduced in these solutions, and, since Beer's law is followed, the molar extinction coefficient can be measured at a concentration allowing accurate measurement at a path length of 1 cm. Accordingly, we have assumed that, in general, the molar extinction coefficient of the pure monomer in water is practically equal to its value in methanol and, in cases where the dimerization is so great as to require absorption measurements of solutions ill or less, we regard the value of the alcoholic extinction coefficient as probably a closer approximation to that of the monomer in water than one obtained by extrapolation from values observed a t higher concentrations of aqueous solutions. We have attempted to allow for the overlap of the dimeric band on the monomeric band by a series of successive approximations, assuming in the first approximation that the monomeric band does not o;erlap the dimeric maximum and that the dimeric band is symmetrical about its maximum, overlapping the monomeric band. The contribution of the dimeric absorption to the observed optical density a t the wave length of the monomeric maximum a t any total concentration can therefore be read off, to a first approximation, and, on the assumption of additivity of the densities of the monomer and dimer at any wave length, a first approximation to the density of the monomer at its maximum can be determined. Overlap of the monomeric band with the dimeric band is now considered. From the absorption spectrum of a dilute solution of the dye in which dimerization is negligible, the ratio of the monomeric densities at the wave lengths of the monomeric and dimeric maxima can be found, and, from the first approximation of the density of the monomer at the monomeric maximum, the approximate contribution of the monomer to the density at the dimeric maximum can be calculated. A second approximation to the density of the dimer a t its niaximum can therefore be found; hence a second approximation to the density of the dimer at the monomeric
maximum can be made, and a second approximation to the monomeric density a t its maximum is obtained. From this, as before, a third approximation to the density of the dimer at its maximum is made, and the process is repeated. The approximations rapidly converge after the fourth or fifth cycle, and values are obtained for the densities of the monomer and the dimer at their respective maxima. The molar concentration of the monomer, cm, at any total concentration, c, can therefore be calculated from a knowledge of the molar extinction coefficient of the pure monomer and the cell thickness. The concentration of the dimer, cd, is obtained from the condition C cd
=
- Cm
2
(3)
Data for the calculation of the dissociation constant of the dimer of dye I1 are listed in Table 11; for the reasons already discussed, the molar extinction coefficient of the pure monomer in water was taken as 16.2 X lo4, the measured value in methanol. The total molar concentration, c, of the dye is listed in the first column; the thickness, t, of the cell used in the spectrophotometric determination of the monomeric and dimeric concentrations is given in the second column; the observed density, Dobsd, of the solution a t the monomeric maximum in the third column; the corrected density, Dm, of the monomer a t that wave
Table 11: Dissociation Constant of the Dimer of 3,3'-Diethylthiacarbocyanine p-Toluenesulfonate (Dye 11) a t 22'
1.00 1.93 2.00 2.88 3.00 4.00 5.00 10.00 20.00
1.0 0.5 0.5 0.1 0.2 0.2 0.2 0.05 0.05
1.19 1.01 1.04 0.28 0.58 0.71 0.86 0.32 0.51
1.13 0.92 0.94 0.24 0.52 0.63 0.74 0.27 0.38
3.28 0.15 0.40 3.25 0.42 3.21 0.70 3.12 0.70 3.66 1.03 3.66 3.82 1.36 3.20 3.36 c2.831 7.67 Av. 3.40 X 31ean dev. f0.17 X 0.70 1.14 1.16 1.48 1.60 1.94 2.28 3.28 4.66
length derived from successive approximations in the fourth column; the computed concentration of the monomer in the fifth column; the computed concentration of the dimer in the sixth column; and Volume 69,Number 6 June 1966
W. WESTAND SANDRA PEARCE
1898
the value of I< = Cm2/Cd in the seventh column. The values of K are constant within a mean deviation of ~ 5 % . At concentrations higher than 2 X M, the computed values of c, and cd no longer conform to the mass law for a monomer e dimer equilibrium, and at this concentration the H-band begins to appear. It follows from eq. 1 that, as one varies the total concentration of the dye participating in an equilibrium between the monomeric and dimeric states, a plot of log cd vs. log c, is h e a r with a Slope of 2. In Figure 4 such a plot made from the data of Table I1 is illustrated for dye 11. The line is the least-squares line from the data, and its slope, 1.99, is in accord with the existence of a monomer $ dimer equilibrium over the concentration range involved.
-6.0
I
I
I
I
1
1
-5.8
-5.6
-5A
h b a d = Emu
f (1 -
=
- Ed’)
CY(€,
a)Ed’
+
(4)
Ed’
where Ed‘ is the molar extinction coefficient of the monomers that are bound together in the dimer, Le., half the molar extinction coefficient of the dimer. Since e, and ed’ are constants a t a given wave length, the observed extinction coefficient is linear in a!, the intercept corresponding to CY = 0 being the value of Ed’. This relation for dye I1 at various wave lengths is illustrated in Figure 5. From the values of Ed’, taken from the intercepts a t a! = 0 of plots such as that of Figure 5, or found by direct calculation from eq. 4, one obtains the absorption spectrum of the pure dimer, illustrated in comparison with the spectra of the pure monomers, for dyes I1 and IV in Figures 6 and 7. Sin& lar curves have been found for the other thiacyanine dyes discussed in this paper. The dimeric spectra consist of two branches, one of high intensity, designated as the positive or P-branch, at the high-frequency side of the monomeric maximum and the other, the negative or N-branch, of relatively low intensity, near the monomeric maximum, usually on the low-frequency side. The significance of this structure is discussed later in this paper.
-5.2
. I )
-5.0
0
0 -4.8 .J
-4.6
/ //
14
-4.4
I
-4.2
/
t
c
-4.0
I/
I
I
I -
1
I
/
/
550 1
56c
530
Loglo c, Figure 4. Plot of log Cd us. log cm for aqueous solutions of dye 11. The slope of the least-squares line is 1.99.
510
A variation of the test for the existence of an equilibrium between monomer and dimer alone is to find that, as the concentration of the dye is changed, the observed extinction coefficient a t any wave length, Eobsd, is proportional to a,the mole fraction of the total dye present in the monomeric state, calculated from the mass law according to eq. 2.* If the monomeric and dimeric absorptions are additive The Journal of Phy&d Chemistry
570 5 0.6
500 4 80 . 6 1 0 0
0.2
0.4
0.0
0.6
1.0
a
Figure 5. Linearity in CY of observed molar extinction coefficient of aqueous solutions of dye 11.
THEDIMERIC STATEOF CYANINEDYES
2c
1
1899
Data for the P-branch of the dimeric spectra of thiacyanine dyes in aqueous solution are listed in Table 111. The first column designates the chain length (see Table I), the next two columns contain the wave length and wave number of the monomeric maximum,
I -
Monomer
.-5.
15
Table 111: Spectral Data for Dimers of Thiacyanine Dyes in Aqueous Solution a t 22"
5
E
hr~,
b a.
10
d I
E! X W
Ymi
om.-l
n
mp
0 1 2 3
422 23,697 553 18,083 648 15,432 753 13,280
Xv,
YY,
Avv,
ids
Vd,
AUd,
mp
om.-'
om.-'
mp
om.-'
om.-'
24,691 19,380 16,722 14,493
1106 1297 1290 1248
402 510 579 647
24,876 19,608 17,271 15,456
1179 1525 1839 2176
405 516 598 690
5
0 460
500
550
600
Wavelength in Millimicrons Figure 6. Absorption spectra of pure monomer and calculated spectrum of pure dimer of dye I1 (p-toluenesulfonate) in water.
I
I
I
c 25
'E 3
!
zs 2c b a
7
15
0 X W
IC
5
5 20
600
700
800
Wavelength in Millimicrons Figure 7. Absorption spectra of pure monomer and calculated spectrum of pure dimer of dye I V in water.
the fourth, fifth, and sixth columns refer, respectively, to the wave length and wave number of the vibrational shoulder in the monomeric spectrum and the frequency displacement of the shoulder from the maximum, and the last three columns refer to the wave length and wave number of the P-branch of the dimeric maximum and to its displacement from the monomeric maximum. Consistent with its interpretation as being in essence a vibrational interval determined mostly by a dominant vibrational mode in the conjugated chromophoric chain, the displacement of the molecular shoulder from the maximum does not vary much from dye to dye, but the separation of the dimeric maximum from the molecular maximum increases steadily with increasing chain length; in fact, as is shown in Figure 8, the displacement is linear in n. The dimeric maximum is always hypsochromic to the molecular shoulder, although for dyes of the shorter chain lengths, not by many millimicrons, but the separation increases with chain length. There can be little doubt that the shoulder and what we have called in this paper the dimeric maximum are distinct spectroscopic entities that happen to fall a t about the same wave length, probably because the energy of interaction associated with dinierization is similar in magnitude to the energy of the vibrational transition. The values of the dissociation constants of the dimers of the series of thiacyanine dyes are listed in Table IV, along with the free energy of dimerization, computed from the relation - A F = RT In K', where K' is the association constant of the monomer, equal to 1/K. The concentration at which the H-band appears determines the range over which the siiiiple mononierdimer equilibrium exists. Owing to the relative instability of dyes I11 and IV, solutions of these dyes contained 1 and 2y0 methanol, respectively. Since Volume 69, Number 6
June 1065
W. WESTAND SANDRA PEARCE
1900
Table IV : Dimerization and H-Aggregation of Thiacyanine Dyes
Dye
n
I
0
I1
1 2 3
I11 IV
XEthyl sulfate p-Toluenesulfonate p-Tolueoesulfonate p-Toluenesulfonate
K,
-AF,
mole 1.-1
295OX.
13 X 3.4 X 1.5 X 4.1 X
10-5 10-5 10-5 10-8
5250 6040 6510 7260
X-band perceptible st
1.0 X 10-2 cu. 2 X 10-5 ea. 1 X 10-5
upper excited state may, as pointed out by Lavorel,16 contribute substantially to concentration quenching of the fluorescence of dyes. Also, the metastable level of the dimer is nearer the triplet level of the dye than the upper level (and probably than the excited monomer level) ; hence, intersystem crossing may be facilitated through this path. At low teniperatures, the dinier might show a higher phosphorescence yield than the niono~iier,~’ and in photochemical reactions induced by the triplet state of dyes the quantum efficiency might increase with increasing concentration as dimer is produced, if the triplet energy is sufficient for the reaction. The diniers of the cyanine and other dyes are stable in the ground state and in the two excited states which give rise to the P- and N-branches, i e . , if the potential energy of inttiraction of the monomers linked together is plotted as a function of their distance apart, the curves of all these states possess minima. The condition could arise, however, in which the minimum of the ground state is very shallow or nonexistent, when no dimer woiild be formed in the ground state, or it would be formed only at low temperatures. The upper excited state could then be wholly repulsive, while the lower, stabilized by reasonance, could possess a minimum in its potential energy curve. Under these conditions, no dimeric absorption band would be observed at room temperature, but the lower excited state of the dimer could be populated by the product of reaction between an excited monomer and one in its ground state A fluorescence emission corresponding to the N-branch of the dimer could then occur, with relatively low quantum yield and decay period of the order of 10 t:mes that of normal dye monomeric fluorescence. These properties resemble those observed in the excited dimers or “exciiiiers” of perylene and other hydrocarbons. * 9
1901
Mixed Dimers. Besides dimers formed from two identical nionoineric molecules, the possibility of mixed dimers from two different monomeric dyes arises. I n such a case, the density of a mixture of two dyes in aqueous solution will not be additively composed of those of the separate dyes. Mixed diniers have been found in this way for binary mixtures of azo dyesz0and a mixed polymer (probably dimer) between methylene blue and acridine orange has been observed in aqueous solution.21 Mixed polymers between pairs such as acridine orange and thionine, and others have been found in aqueous solution in the presence of polyanions such as chondroitin sulfate.21 Mixed exciiiiers have also been observed in binary solutions of some aromatic hydrocarbons.22 Among cyanine dyes that form in aqueous solution, the reversible polyniers characterized by tLe sharp J-band bathochroniic to the molecular band, mixed J-aggregates have been f o ~ n d . ~ ~ * ~ * Evidence is presented here for the formation of mixed dimers between different cyanine dyes in aqueous solution. As an example, in Figure 10 are plotted the observed densities as a function of wave length of a mixture of 3,3’-diethylthiacarbocyanine p-toluenesulfonate (dye 11, n = 1) and of 3,3’-diethylthiadicarbocyaninebromide (dye 111, n = 2), each a t 1 X lov4M in water, and also, as the dotted line, the sum of the densities of aqueous solutions of the separate dyes at a concentration of 1 X M . The deviations from additivity are plotted as a function of wave length in Figure 11. There are large negative deviations at the positions of the monomeric and dimeric maxima, 11111and DIII of the dicarbocyanine, and a t the position of the nionomeric maximum of the carbocyanine, 5111,whereas a new band, D11.111, appears between the dimeric peaks DIII and DII of the symmetrical dimers. The positions of the monomeric and dimeric maxima of the individual dyes, and of the mixed dimer, are indicated along the wave length axis of Figures 10 and 11. The most obvious interpretation of DIIJII is that it is the absorption spectrum of a complex between the two dyes in solution, a nixed dimer. The absorption of a very dilute solution containing the two dyes is additive. In the more concentrated solutions, inspection of (17) E. C. McRae and M.Kasha, J. Chem. Phys., 28, 721 (1958). (18) T. Forster and K. Kasper, Z. Elektrochem., 59, 977 (1955). (19) B. Stevens and E. Hutton, Nature, 186, 1045 (1960). (20) D. R. Lemin and T. Vickerstaff, Trans. Faraday SOC.,43, 491 (1947). (21) M. K. Pal and M. Schubert, J . Phys. Chem., 67, 1821 (1963). (22) J. B. Birks and L. G. Christophorou, Nature, 196, 33 (1962). (23) C. Scheibe, Z . angew. Chem., 52, 631 (1939). (24) H. Ecker, KoZZoid-Z., 92, 35 (1940).
Volume 69, Number 6 June 1966
W. WESTAND SANDRA PEARCE
1902
1
*gOl
Loo
30
-
30
-
400
I /"
500
600
Wavelength in Millimicrons Figure 10. Absorption spectrum of a mixture of dye I1 and dye I11 in aqueous solution, each a t 1 X 10-4 M : , experimental; -------- additive.
,
Figures 10 and 11 shows that the mixed dimer is formed at the expense of the monomers and of the symmetrical dimers. These figures also show a considerable increase in the absorption of the solution of the mixture a t the long wave length side of the nionomeric maximum of the dye of the longer chain length. The condition of identical oscillators interacting in opposite phases that causes the negative branch of the absorption spectrum of the symmetrical dimer to be very feeble no longer holds in the mixed dimer. The interacting transition moments are no longer equal and even if directed in completely opposite directions would produce a nonzero resultant. The observed increase in the intensity of absorption of the mixed dimer on the long wave length side of the monomeric band of the dye of longer chain length is therefore in accord with the theory of dimeric absorption discussed earlier and supplies significant experimental verification of the theory. Mixed dimers showing these spectral characteristics have been observed in aqueous solutions of the dye pairs listed in Table V. The table shows the wave lengths of maximum absorption, XMD, of the mixed The Journal of Physical Chemistry
Wavelength in Millimicrons Figure 11. Deviations from additivity in the absorption spectrum of a mixture of dye I1 and dye I11 in aqueous M. solution, each a t a concentration of 1 X
dimers, the peak maxima of the two symmetrical dimers derived from the monomeric parents, the mean wave length of the parent dimeric maxima, and the difference, A, between the parental mean wave length and that of the mixed dimer. All of these data refer to the intense P-branches of the dimeric bands. This
Table V : Spectral Data on Mixed Dimers Mean of x of parent dimers, parents mr A, mr Amax,
Mixture
Thiacyanine dyes n = O,n = 1 n = O,n = 2 n = l,n = 2 n = l,n = 3 n = 2,n = 3
Methylene blue Thiacyanine ( n = 1) Pinacyanol, 10-4 M Thiacyanine (n = l), Pinacyanol, 10-5 U Thiacyanine (n = l),
AI dl
}
~ M of D
mixed dimer, mr
402,510 402,579 510,579 510,646 579,646
456 491 544 579 613
420 418 532 542 602
36 73 12 37
610,510
560
538
22
510,544
527
500
27
510,548
529
502
27
11
1903
THEDIMERICSTATEOF CYANINE DYES
branch of the mixed dinier spectrum, in all of the exaniples studied, shows a iiiaxiinuni a t a wave length less than the mean wave length of the corresponding symmetrical dimers. With respect to its syniinetrical parents, therefore, a mixed dimer exhibits in absorption a “deviation,” in the sense of the deviations found by B r ~ o k e for r ~ the ~ absorption of a monomeric cyanine or merocyanine dye derived from two different heterocyclic nuclei. In the latter case, the unsymmetrical dye exhibits a maximum a t a wave length shorter than the mean of those of the two maxima of the parents, the deviation being the greater, the greater the difference in basicity of the two heterocyclic nuclei. Both for mixed dimers and mixed monomers, the deviation arises froni the energetic inequality of the resonating structures froni which the actual states of the system are derived. The splitting between the upper and lower states derived from the resonance now includes a term for this energetic inequality and, although the additional separation effected by the resonance is less than for interacting identical structures, the net effect is a displacement of the higher-energy structure of the unsymmetrical system to a value higher than the mean of the energies of the corresponding two syinmetrical systems, and a hypsochromic displacement of the P-branch of the absorption band of the unsymmetrical system from the mean of the absorption wave lengths of the two symmetrical systems. Table V also shows that the deviation, A, for the mixed dimers derived from the thiacyanine series increases with increasing difference in the chain length of the mononiers linked together in the dimer. The energetic inequality between the resonating structures of the mixed dimer increases with increasing difference in the chain length, and the consequent increase in deviation is analogous to Brooker’s observation of
increasing deviation with increasing difference in the basicity of the two heterocyclic nuclei in unsymmetrical nionoineric d ~ e s . 2 ~ Besides the mixed dimers forined from cyanines containing the same heterocyclic nuclei, mixed dimers have been observed containing thiacyanine nionomers linked to 2,2’-cyanine inonoiiiers, and between a 2,2‘cyanine and methylene blue. Data on these systems are listed in the lower part of Table V. Among the thiacyaniiie dyes, the degree of niixed dimerization is greatest when the difference in chain length of the interacting inononiers is equal to one vinylene group. Uoreover, as is to be expected, the degree of niixed dimerization depends on the tendency toward self-dimerization of the nioiioiiiers. For example, the siniple thiacyanirie (n = 0) foriiis a mixed dimer more readily with the carbocyanine (n = l), than with the dicarbocyanine (n = 2), but the dinierization in this case is less than that between the dicarbocyanine and the tricarbocyanine (n = 3), both of which very readily form the corresponding symmetrical dimer. Experimental measurements were made on dye solutions a t 22” prepared in noiiactinic light and, in the case of dyes I11 and IV, without assisting solution of the dye by heating. The Beckinan DU and the General Electric automatic spectrophotometers were used, the high scanning speed of the latter being especially advantageous in the nieasureiiients of solutions of low stability.
Acknotuledgnzent. We are indebted to Dr. L. G. S. Brooker, of these laboratories, for the supply of dyes, and to RiIr. F. Gruni, also of these laboratories, for many of the measurements of the absorption spectra. (25) L. G . S. Brooker, Rev. Mod. P h y s . , 14, 289 (1942).
Volume 69, Number 6 June 1965
