EMERSON, CONLIN,ROSENOFF, NORLAND, RODRIGUEZ, CHIN,AND BIRD
2396
The Geometrical Structure and Absorption Spectrum of a Cyanine Dye Aggregate
by E. S. Emerson, M. A. Conlin, A. E. Rosenoff, K. S. Norland, H. Rodriguez, D. Chin, and G. R. Bird’ Research Laboratories of the Polaroid Corporation, Cambridge, Massachusetts
08139
(Received July 6 , 1966)
We have investigated the aggregation of a cyanine dye which is a particularly clear experimental example for the formation of aggregates with blue-shifted absorption bands. The large aggregates have been observed by electron microscopy, optical microscopy, X-ray diffraction, and absorption spectroscopy. The separate absorption bands of smaller aggregates up to the pentamer have been clearly resolved and have been identified by semiquantitative use of the molecular exciton model. These resolved bands form a regular wavelength progression leading up to a band head for very large aggregates located 100 nm from the monomer peak. The dye in question is 3,3’-bis(P-~arboxyethy1)-5,5’dichloro-9-methylthiacarbocyanine, a photographic sensitizer. All of the experimental results are consistent with a deck-of-cards structure for the aggregates, with the long axes of molecules perpendicular to the axis of the aggregate. Our results suggest that counterions are not incorporated in the space between molecular planes, since the wavelengths of the band head and the resolved low-polymer bands are all insensitive to the identity of the counterion.
Introduction The cyanine dyes are a remarkable class of strong light absorbers, having both sharp absorption bands and large dipole strengths.2 These dyes are used in sensitizing the silver halide microcrystals in photographic films, so that the transparent crystals respond to light absorbed only by the dye. There is a great deal of confusion in the literature about the behavior of cyanine dyes, since they form very tightly bound aggregates and exhibit spectral shifts of the order of A100 nm as the aggregates formU3 These cyanine aggregates may be observed on the surfaces of silver halide “free-standing” in solution,4b or adsorbed to polymeric substrates.5 Photographic sensitizing [action is observed from both aggregates and isolated molecules adsorbed to silver halide surfaces. The dye which we wish to describe is a particularly tractable material for the study of aggregation. It forms only one of several possible classes of solution aggregates and it has aggregate absorption spectra which are unusually sharp-even for a cyanine dye-so that we have been able to resolve the separate absorpThe Journal of Physical Chemistry
tion peaks of the monomer, dimer, trimer, and tetramer in solution and to observe the band head of the large polymers a t the limit of the wavelength progression of the smaller species. In addition, we have been able to observe a kind of fine structure in the absorption band of the high-polymer aggregate which gives further information concerning the structure of the large aggregate. Most important, the high-polymer aggregate of this dye is so stable that it survives the rigorous conditions of examination under vacuum with the electron microscope. As a result, we are able to present the first electron micrographs of a dye aggregate of this class and to correlate these with the optical studies. The dye in question is 3,3’-bis(@-carboxyethyl)-5,5’(1) Correspondence concerning this paper should be addressed t o G. R. Bird. (2) J. R. Platt, J . Chem. Phys., 25, 80 (1956). (3) C. E. K. Mees, “The Theory of the Photographic Process,” The Macmillan Co., New York, N. Y., 1954. See Chapters 10-12
and especially p 450. (4) (a) J. F. Padday, Trans. Faraday SOC., 6 0 , 1325 (1964); (b) S. F. Mason, Proc. Chem. Soc., 119 (1964). (5) L. Stryer and E. R. Blout, J . A m . Chem. Soc., 83, 1411 (1961).
2397
GEOMETRICAL STRUCTURE OF A CYANINE DYEAQGREGATE
Coupled Dipole Theory and Ita Limitations The large spectral shifts in these aggregates may be described semiquantitatively by use of t.he molecular exciton model.’0 As long as one is dealing with planar aggregates (decks of cards), the dipole theory predicts a red shift relative to the monomer for a slightly tilted deck and a blue shift for a highly tilted deck.” The N-fold degenerate excited states of N dye molecules are split into N separated sublevels when the aggregate is formed, but the transition intensity accumulates in the longest wavelength transition for a “deck” tilted less than 54”44’ or in the shortest wavelength transition for tilt angles greater than this value, as is shown in Figure 2 for dimers. Two weaknesses in the conventional treatment of these linear aggregates of cyanine dyes by the molecular exciton model combine to render the results semiFiatire 1. A moleciilnr model of the monomeric dye :I,:~’-his(6-carh~~x~~tllyl>5,5’-dirhb~nl-~methylthincarbucyanine quantitative a t best. A classical point-multipole here depicted with hoth cnrhoxyl groups ionized. The scale of expansion” is used to simplify the coulombic potential the model is shown and t.he ideirtit.y of hnrkhone atoms is interaction between molecules. The truncation of iiidicnted in the lower formula (enrlxm stoma a t dl this expansion with the dipole-dipole interaction is of mrners iinless ot.hemise shown). doubtful validity in the case a t hand, since the transition density of a cyanine dye extends over most of its dichloro-9-methylthiacarbocyanine. Its chemical strucmolecular length, which in turn greatly exceeds the ture is shown in I’igurc 1 along with an indication of the molecular separation. In this situation, sums of size of the molccule as dcrivcd from molecular models higher order interactions such as V dipole-octupole and correlated with Whcatley’s crystallographic study may make contributions comparable to V dipoleon a closely related cyanine dye, 3,.l’-diethylthiacarbtr dipole. Then, too, the actual magnitude of interaccyanine bromide.” tions in cyanine dye aggregates is large enough to require extension of the perturbation calculation to Cyanine Aggregates second or third order. The need for including higher The photographic literature on cyanine dyes refers order perturbations may be seen from an extension of rather loosely to all aggregates which exhibit spectral a sample order-of-magnitude cal~ulation’~to the red shifts as J aggregates (probably “J” for Jelley, plausible packing parameter t = 3.4 A. The result one of the first workers to investigate these shifts).‘ is a perturbation energy greater than the starting The blueshifted aggregates arc designated as H agtransition energy-a proof that a firstorder perturbagregates (“H” for hypsochromic) and have often been tion calculation may be inaccurate. The use of the considered as low-polymer aggregates. We shall see that this is not aIivays true. Thc J aggregates can be much more complicated than H aggregates, since either (6) P. J. Whentley. J . Chon. Soc.. 3245, 4096 (1959). (7) E. E. Jelley. Nolure. 138. 1009 (1936): 139,631 (1937). planar or helical .J aggrcgates can form in solution. (8) W.West nnd 9. I’enree. J . Phya. Chcm.. 69, 1894 (1985). West and I’earces have recently published a definitive (9)&e ref 2, pp 399406. study of thc monomer-dimer equilibria of several cya(IO) (n) E. D. McRne and M.Knshn. J . Chem. P h w . . 28.721 (1958): nine dyes. This dimerization is the first step in the (b) “l’hyaicnl Procegses in Radiation Biology.” Academic Press h e . , formation of a high-polymer H aggregate. New York. N. Y..1964. pp 23-42. (11) Note that the tilt angle is measured between the line eonneeG One of the unique structural features of all successful ing the centem of molecules and the axes of the trnnsition dilmles. photographic sensitizing dyes is their totally planar which here coincide with the long axes of the individual dye moled e s . On this basis. R deck of cards lying in it0 box is highly tilted backbone s t r u c t ~ r e . ~Wheatley has obtained a value (a WD). The mme derk. stwd on its shorted- on n table and of 3 3 A for the separation of successive molecular 0). By eontrnst, nilawed to fnll over, is relatively untitted (o n deck nlbwed to fnll from stnnding on its long edge is still tilted n t planes in his crystal study. This is essentially the W0.since we are eoneerned with the angle between the l o w nxe8 graphite interlayer space (3.37 A) and we see no reason of the individunl cards (molecules) rind their line of canters. to assume any other packing parameter for the dis(12) &e ref 10b. P 29. tance between molecular planes in a cyanine aggregate. (13) See ref IOb. Table I.
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Volume 71, Xumkr 8 July 1967
2398
EMERSON, CONLIN,
SPECTRAL SHIFTS OF DIMERS
ROSESOFF,S O B L A N D ,
Av(dimer)
AS A FUNCTION OF TILT ANGLE
=
AT CONSTANT CENTER SEPARATION
I
IfU H
I J I
0
1
q.90.
I w3
4.54.44'
cosa
q
I
I
MONOMER ABSORPTION TRANSITION
h-l
I ~ O D l U G U E Z ,C H I N , A N D B I R D
(1112)
r3
(1 - 3 cos* e)
(1)
where Av is the spectral shift from monomer absorption, h is I'lanck's constant, 1' is the separation of molecular centers, a is the tilt angle between the line of ccuters and molecular long uxcs (see Figure Za), (m*), the transition dipole moment of monomer, equals 9.1% X 10-39J>t(dX/A), t is the mnlar extinction coefficient i n (moles/l.)-' em-', A is the wavelength,
I
I
GROUND STATE
cl.0'
ALIGNMENT OF TRANSITION DIPOLES (OR MOLECULAR LONG AXES) RELATIVE TO LINE-OF-CENTERS
Figure 2. The location of excited energy states in B simple dimer as a function of tilt angle. The dashed arrows indicate the strongly allowed optical transitions, and the wavy arrow indicates a rapid internal conversion which probably occurs after absorption into the I1 dimer, This figure is constructed from point-dipole theory as applied to a dimer with constant separation of moleciilar centers hut a variable angle of inclination. LINEAR POLYMERIC ACOREGmTES: ARRANGEMENT OF MOLECULAR LONGAXES
Figure 3. The orientation of transition dipole moment8 (of molecular long axes) for linear H and J aggregates.
above approximat.ions has already been questioned in less drastic cases." Nevertheless, we do obtain a descriptive ordering of energy levels and transition frequencies from the molecular exciton model. It should be noted that the dipoles which are being coupled are the transition dipoles, rather than anv static dipoles or group moments. The results for molecular exciton shifts in the dimer are given in eq 1 and the predicted spect.ral separation of the N-mer allowed transition from the monomer band is given in eq 2.'6
Figure 4. Au elertmn micrograph of Ihc hiflh-poyiner B K K X K R ~ ~of S 3,~'-lris(Bcnrh~r?ryethyl)-.~,h'-~li~hl~~rr~!l-methylthiiienrhocya,iiiie. S o t e the parallel-handed rod structure of the aggregates. This micrograph was recorded with B Jiitachi IIU-11 instrument, npernting a t 25,OOOx magnification. The final magnification on this page is IR,OOOX. The aggregates were stained with phosphotungstic acid and no shadowing material was applied for preparation of the sample. This micrograph is R transmission positive i n the sense thnt decreased electron transmission appears as darkening. The hnr at t.he hottom of the micrngrnph indicates a 1.lh.n interval in the sample. (14) (:I) J. Ferauson. J . Chcrn. Phya.. 44, 2077 (1900); (h) D. P. Cnhip: imd T. Thirunnmnclumdmn. I'roc. Phya. Snc.. 84. 781 (1984). (15) Strictly. the term (S - l ) / S is inneeurnte nnd should be reid:teed h s cos z/(.V 1). The depurture from (S - l)/.\' dei~ndeneeis smdl : ~ s dneed eonmin UR no more than the over-all fnilure of point-dip& theory. It miiy even be t h n t an (.\' - UIS e x ~ m s i o ntiikes l ~ t t e nerount r of the real situation with nonnelrest neialihor internetions included.
+
GEOMETRICAL STIWCTIXE OF A CYASISEDYEAGGREGATE
2399
shown in 1:igures 4-7. The following features may be noted from the electron micrographs. There is a characteristic banded structure seen in the electron micrographs of the aggregates. At the ends of one nf the rodlike aggregates, smaller fibrils may be seen. From examination of these fibrils and also of the bands, we ohtain a characteristic lateral repeat distance of 28 A. The breaks in the linear structure occur with a characteristic angle of 57'. The length of the aggregates is limited only by the mechanical details of preparation. In no case has any ordered electron diffraction pattern been obtained from the aggregates under vacuum. We have obtained a rather diffuse X-ray scattering peak from similar preparatinns in water and this peak is indicative of a distance of about 28 A. Studies on moist preparations with the optical microscope indicate that the aggregates are amber (absorption blue shifted) and that the absorption electric vector is aligned perpendicular to the long axis of the aggregate. Spectroscopic Studies
and XI, band.
At
are the limits of well-defined absorption
Av(N-mer)
N
= 2h-I----
N
1 (m') --(1 r3
- 3 cos'
a)
(2)
where N is the degree of piilymerization nf the aggregate in question and all nther symbols are as i n eq 1. The structnres assoeinted with blue and red shifts are shown i n Vigure 3, where the lines indicnte the long axes of the rodlike molecules. Electron Microscopy, Optical Microscopy, and Diffraction Studies The samples for electron microscopy were prepared by salting out or cooling nut the H aggregates from concentrated dye solutinns in alkaline water. Triethylamine and triethnnolamine were used interchangeably as bases. Characteristic electron micrographs are
Spectroscopic studies on the growing polymer system have enabled us to observe in a single solution the separate bands of the monomer and several of the low polymer species, along with the band head of the very large aggregates, When this dye is dissolved in ethanol or in methylene chloride, the spectrum of the monomer (independent of concentration and of temperature except for a slight sharpening of the vibrational fine structure on cooling) is obtained. By performing the integral Ge(dA/X) on monomer spectra, we have obtained a value of 12,100 (moles/l.)-' cm-l for the integrated absorption. This absorption integral is equivalent to a quantum mechanical dipole dtrength for the monomer transition of 1.11 X lo-" erg em3. When this same dye is examined in weakly alkaline solutions with modest amounts of salt added to maintain constant ionic strength, a spectrum with numerous bands is observed. When such a dilute HtO solution of dye is carried to a high temperature, the spectrum simplifies and moves in the direction of the monomer spectrum, as seen in Figure 8. As the solution is cooled, a series of new absorption peaks comes in one by one in a regular progression to shorter wavelengths. This change is shown in Figure 9. The level of polymerization is influenced by adding salt (NaCl, KBr, etc.), hut the absorption maxima seen in water are not shifted in wavelength by presence or absence of salt or by changes in the identity of the added amine or the salt. The locations of these successive absorption bands Volume 71, N u m k 8 Julv lw17
2400
EMERSON, CONLIN, ROSENOFF, SOI~LAND, ROOIWXJEZ, CHIN,AND BIRD
GEOMETRICAL STIIUCTUI~E OF A CYANINE DYEAGGREGATE
8
c:'.
..
:., ;: 1
0
'i
... r:
I!
._ -:.
r._
Figure i . A detail ul R Imlken swtion of nn aggregntc, e i t l i ~ r g ~f n~ml , the plntc ahr,wn i t ) F i g ~ 6. e The banded suhrtntrtiire is here shown more rlenrly. The shndow reveals R hrenk angle of S i " , which seems to be characteristic of this agp;regate. We have no simple explnnatioo lor this prirtiriilnr angle. Final magnification here is 317,OOOX.
2401
EMERSON, CONLIN, ROSENOFF, NORLAND, RODRIGUEZ, CHIN,AND BIRD
2402
2
Ee 8 2.1U'MOLAR S = 0.1 ern 2
1
I
0
WAVELENGTH (NANOMETERS)
Figure 8. An absorption spectrum of the essentially unassociated dye in Hz0 (+ M NaCl) a t very low concentration and relatively high temperature. The uncorrected extinction coefficient a t the main peak is e 92,000 (moles/l.)-l cm-'. The shoulder a t ca. 510 nm is a t least partly a contribution from a trace of dimer, but is mainly a vibrational satellite of the monomer band.
are listed in Table I along with the identification as to degree of polymerization. The spectral shift from the monomer peak is used to calculate an intermolecular separation, based on an assumed 90" angle. Note that this distance is obtained from the simple molecular exciton model with approximations as noted, but that the figures are at least in the right order of magnitude.
Table I: Spectral Shifts and Point-Dipole Packing Parameters rcalcd Amax,
ymx.
(a = 90°),
N
nm
cm -1
A
1
547 507 487 477 450
18,282 19 ,724 20 ,534 20 ,964 22 ,222
2 3 4 m
... 7.30 6.92 6.78 6.57
We have spent considerable effort trying to study spectroscopically the equilibrium coefficients*J6 for successive degrees of polymerization, but have been prevented from obtaining really satisfactory results by the overlapping of the successive transition regions from monomer to dimer, etc., and by the reactivity The Journal of Phgsical Chemistry
Figure 9. An absorption spectrum of the same dye in the same medium as in Figure 8. Here the total dye concentration has been increased lOOX and the optical path length decreased 1/1OOX to give the same total amount of dye in the path. The dashed curve shows the spectrum of the hot (85') solution with monomer (1) and dimer (2) present. When the solution is cooled to 5 O , aggregation increases and the trimer (3) and tetramer (4) bands are resolved and observed. One unusual feature of this dye is the sharpness of the polymeric bands, which allows complete resolution of the trimer and tetramer. This solution and the solution of Figure 8 have been made basic with a trace of "8, so both carboxy groups are ionized.
of this dye at high temperatures. This is about the only instance in which this particular dye is a poor case for experimental study. The low polymeric species form and dissociate readily during normal times of heating and cooling, so the low polymers behave as an equilibrium system. When a relatively concentrated solution of this dye is cooled, a band head is observed at about 450 nm, along with the monomer and resolved low-polymer bands. We assign this band head to a single-stranded linear high-polymer H aggregate with structure essentially identical with the structure of the trimer and tetramer. On long standing, a new band appears at 425 nm and we associate this with aggregates of two or more strands having their larger flat faces fused together. From these observations, we draw the following conclusions.
Structure of the H Aggregate The single rod-shaped aggregates of this dye are essentially perpendicular decks of cards. The individual molecules are oriented in these with long axis perpen(16) V. Zanker, 2. Physik. Chem. (Leipsig), 199, 225 (1952); 200, 250 (1952).
GEOMETRICAL STRUCTURE OF A CYANINE DYEAGGREGATE
dicular to the axis of the aggregate or very nearly so (a = 90”). Presumably, the carboxyethyl side chains in successive molecules are oriented alternately up and down, very much as in Wheatley’s structure, though we have no definite indication of this feature beyond inference from molecular models. The short axes of the individual dye molecules are defined by the N-S line on a single benzothiazole end group and may be perpendicular to the axis of the aggregate or may be somewhat tilted. Since the molecular exciton model is only semiquantitative in this situation, we are left with a working hypothesis that Wheatley’s 3.4-3.3-A interplanar spacing occurs and that the molecular repeat along the aggregate axis may be anything from 3.4 A (X-S axis lying in the perpendicular to the agggregate) to about 6.5 A (the N-S axis tilted out of the perpendicular to the aggregate). Here we are in disagreement with the explanation of “metachromism” given recently by McKay and Hi1ls0n.l~ These workers assign the H bands to interactions between the dye ion and the counterions. By contrast, we do not observe any marked influence on the location of dimer, trimer, tetramer, or a-mer bands in wavelength as the counterions are altered. This wavelength invariance suggests that the counterions are not packed between molecular planes in the aggregates. We attribute the failure to obtain electron diffraction patterns from these aggregates to a disordering which occurs under vacuum conditions. It is very probable that solvent molecules (water or amine) are incorporated into the aggregate and are removed under highvacuum conditions. This, too, would be in accord with Wheatley’s results. In some cases, the positive counterion may be lost as amine hydrochloride, leaving behind the betaine (over-all uncharged) form of the cyanine dye. In any event, the aggregate retains the close-packed over-all geometrical structure which it had in solution. These studies have been restricted to the free-
2403
standing a,ggregates in solution. The very fact that this dye is such a clear experimental case of H banding is associated with a seemingly minor structural feature, the presence of the 9-methyl group in the middle of the chain. If this methyl is changed to ethyl, a dye is obtained which readily J bands in solutions and in photographic emulsions, whereas the present dye J bands only on solid surfaces such as silver bromide or sodium chloride, and then with some difficulty. It is precisely this reluctance to J band that makes this dye such a clear experimental case for H aggregation. The striking distinction between the 9-methyl and the 9ethyl dyes will point out the fact that no structural modification can be regarded as trivial when a dye is to operate as a photographic sensitizer in a highly regular aggregate. The one most useful piece of information which can be extracted from our observations and the molecular exciton model is a crude ( N - 1)/N frequency shift for the N-mer aggregate. This form of spectral shift allows us to classify resolved spectra of a series of related aggregates. We shall utilize this type of classification on J aggregates in a forthcoming publication on photographic supersensitization and hypsochromic shifting.
Summary The spectrum and structure of a series of aggregates of a particular cyanine dye can be reconciled with a simple deck-of-cards structure. The low-polymeric aggregates show spectral blue shifts with Av ( N - 1)/N for the dimer, trimer, tetramer, and infinite aggregate. The aggregate seems to have a simple layered structure, with no counterions present between molecular planes. Electron micrographs of the large aggregates are particularly helpful in determining the structure.
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(17) R. B. McKay and P. J. Hillson, Trans. Faraday Soc., 61, 1800 (1965).
Volume 71, Number 8 J u l y 1967
