934
J. Phys. Chem. 1984, 88, 934-950
Desirable Properties of Photovoltaic Dyes A. P. Piechowski, G. R. Bird,* Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903
D. L. Morel,+and E. L. Stogryn Exxon Research and Engineering Company, Linden, New Jersey 07036 (Received: June 7, 1982; In Final Form: April 12, 1983)
We have investigated the chemical variables which have a major influence on the behavior of organic dyes as the active ingredients in experimental photovoltaic cells. We have selected some 60 dyes from a larger set of candidate photovoltaic materials (reported separately) and have measured electrochemical redox potentials, surface adsorption, spectroscopic properties, peak yields of fluorescence in solution and, where appropriate, acid-base equilibria. Only one of these properties, adsorption to Al,O,, is specific to the particular inorganic electrodes of the test cell. This is a cell having a semitransparent aluminum electrode, a layer of amorphous or microcrystallinedye of thickness 20-200 nm, and a silver rear electrode: A1/Alz03/solid dye/Ag. The electrochemical measurements identify the reduction potential as the more critical variable and set a threshold of -1.3 V or more reducing vs. SCE for best performance. The adsorption studies indicate that it is desirable that the dye be strongly adsorbed to the oxide layer (A1203-A10(OH)),and that adsorption should bring the electron-rich end of the chromophore close to the oxide surface. The fluorescence measurements and the correlation of solution fluorescence yield with cell performance show that it is advantageous to use materials which do not have a facile route for direct internal conversion from the first excited singlet state to the ground state. In general, good materials will exhibit a high quantum yield of fluorescence in fluid solutions at room temperature. The pH measurements only underscore the need to avoid materials which will be protonated or deprotonated at acidic or basic sites on the inorganic substrate.
In the past few years, several reports of photovoltaic (PV) systems involving organic dyes in the solid state have The obvious goal of these efforts is to produce a low-cost panel system for harvesting solar energy as electrical power. One of the most frustrating qualities of this broad area of research is that the most stable materials (especially metal-free phthalocyanine4 (H2PC)) are also the most difficult to manipulate and alter. The most efficient organic systems to date are those described by Morel et aL5S6 In the preceding paper, Morel and his colleagues at the Exxon Research Laboratories have begun a systematic exploration of the connections between the chemical properties of dyes and their performance in prototype solar cells. In this paper we report an exploration of the quantitative physical and chemical properties of the same group of dyes, molecular properties measured apart from any photovoltaic configuration. This is a familiar effort, one which resembles the 100-year-long effort to understand the logical connections between dye chemistry and the performance of those cyanine and merocyanine dyes which act as long wavelength sensitizers for photographic silver halide ~ystems.~Modern color photography would be impossible without the use of these sensitizers. The effort to characterize and predict the behavior of photographic sensitizing dyes has reached a fairly satisfactory plateau in recent years. The most significant physical and chemical criteria which have evolved are as follows: 1. The redox levels of the dye must meet certain threshold conditions.6 Most important is the voltammetric half-wave reduction potential of the dye, which must be more negative than -0.85 V (vs. Ag/AgCI/KCI saturated) which is the threshold potential for a successful electron injecting dye. 2. The chromophoric core of the dye must be essentially planar and free from steric crowding effectsg which tend to twist the dye, predisposing it to photoisomerizationIO or internal conversion, instead of sensitization by electron injection. 3. The dye must be capable of adsorbing to one or more of the common silver halide faces in a close-packed, long edge-on configuration." Small groups projecting out of the plane of the chromophore play a critical role in "indexing" the packing of adjacent molecules to obtain the desired aggregate structure with its characteristic spectrum. 'Present address: A R C 0 Solar Industries, P.O. Box 4400, Woodland Hill, CA 91365.
0022-3654/84/2088-0934$01.50/0
4. The dye must be a stable material and a strong absorber. It must not contain chemical groups such as -SH which react with AgBr and grossly alter photographic behavior. The most significant failing of the photographic art to date is the failure to accomplish efficient sensitization with dye coverage in excess of a monolayer.12 This imposes a severe limitation on the sensitivity and resolution of current photographic materials. We have recently reported on the combined photoconductive and photovoltaic behavior of a small sample of merocyanines and related material^.'^ Notably, the limitation to monolayer systems does not dominate photovoltaic performance. Some representative sensitizing dye structure may be seen in Figure 1. When one attempts to apply the above criteria for photographic sensitization to the description of photovoltaic materials, many insights are obtained. There does appear to be a critical dye redox potential for the given electrode assembly ((A1/Alz03/dye/Ag) in the present work), and we have fixed a preliminary value for this threshold from a necessarily limited sample of dyes. Planarity and freedom from steric twisting are at least as improtant as in the photographic systems. Bonding of the dye to the AZO3 surface. is very important, and we have devised a simple chromatographic adsorption test. Chemical stability is far more important than in the sheltered (dark, cool) environment enjoyed by photographic materials; it is a pivotal point of our continuing search for new ~~~
~
V. Y. Merritt and H. J. Hovel, Appl. Phys. Lett., 29, 414 (1976). A. K. Ghosh and T. Feng, J . Appl. Phys., 44, 2781 (1973). K. Iriyama et al., Jpn. J . Appl. Phys., 19, Suppl. 19-2, 173 (1979). J. H. Sharp and 2.Popovic, J . Chem. Phys., 66, 5076 (1977). D. L. Morel et al., Appl. Phys. Lett., 32, 495 (1978). D. L. Morel et al., J . Phys. Chem., preceding article in this issue. (a) W. West, Photogr. Sci. Eng., 18, 35 (1974). (b) For a collection of literature on photographic sensitization, see the "Proceedings of the Vogel Centennial Symposium", distributed throughout Photog. Sci. Eng., 18 (1974). ( 8 ) (a) L. Costa, F. Grum, and P. B. Gilman, Jr., Photogr. Sci. Eng., 18, 261 (1974); (b) D. M. Sturmer, W. S. Gaugh, and B. J. Bruschi, ibid., 18, (1) (2) (3) (4) (5) (6) (7)
49, 56 (1974). (9) L. G. S. Brooker et al., Chem. Rev., 41, 325 (1947). (10) (a) A. L. Greenberg, Doctoral Thesis, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, June 1975. (b) S. S. Sandhu,
Doctoral Thesis, Rutgers, The State University of New Jersey, New Brunswick, NJ, 08903, May 1979. (11) C. E. K. Mees and T. H. James, "The Theory of the Photographic Process", 3rd ed, Macmillan, New York, 1966, chapter 12, pp 239-45. (12) (a) J. Spence and B. H. Carroll, J . Phys. Colloid. Chem., 52, 1090 (1948); (b) G. R. Bird, Photogr. Sci. Eng., 18, 562 (1974). (13) P. Yamin, A. P. Piechowski, G. R. Bird, and D. L. Morel, J . Phys. Chem., 86, 3796 (1982).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 935
Desirable Properties of Photovoltaic Dyes
-
,la,!I,N I - i f I .rar e z r. yl cr. Ia E a r b s cyanine+ ; A t s i s t e s o v e r c r w a e c c y a n i n e a y e , ar.d B p r o b a b l e a n t i sensitizer.
@
.
?re':oELr.aat u?n:ing form i n t r . e E X C I t e c s t a t e SI.
, / J
A s j r n e t r L c a l cyknine-like
9 - p h e n y l f l ~ o r o n h s i o n ; A fl'dcresc e i n a n a l o g l a c k i c g t n e o-3CCL' s u a s t i t u e n t c n :he psnczn-, -r.enyl rir,g. T L E d y e 1% a n 1 n e f f i c i c r . t Kuor.
cye
f r o m t h e squsryliun f a m i l y .
> , 3 #- d i $ t h y l - 4 - n c : h y l
triacartacyanir.e ; A p r s t a t y p e p n c t r o f r . a t i c : r e d >no g r e e n ! s e n e i t i z r r ; A = a i l t r m s f s m , 5 = m O n D - 0 1 s forn.
, :, 3 1- c i e - . h y l - %
lC--c~r.ylenethiarr. h n s u c c e s s f u l cirSocyanise+ a r t e r r ~ tt o eLirinare t o r e i c n a l i n t e r i t 1 c c n r e i t i c n by c z , r t i a l cvclirntion.
1
Figure 1. Some significant structures of typical cyanine and merocyanine dyes. M-1 is shown in the neutral resonant form and as the protonated and decolorized material. M-8 is shown in its two major resonant forms. Of these forms the left-hand form is the majority form in the ground state, while the right-hand form is the majority form in the excited state. Both forms contribute to both states, and suitable substituents such as electrondonating groups (CH3, OCH,) on the benzothiazole residue can shift the system toward equal contributions by both form in both state. This is the isoenergetic or cyanine-like condition. M-93is an example of a squarylium dye. This neutral dye has optical properties almost indistinguishable from a dicarbocyanine. This is most easily explained by the assumption that the negative charge resides on the oxygens and the positive charge resides primarily on the nitrogens. 3,3'-Diethyl-9-methyIthiacarbocyanine(+) is an example of a photographic sensitizing dye. The upper structure is the all-trans isomer, and the lower structure is the 8,9-mono-cis isomer. Steric hinderances between the sulfurs and the 9-methyl protons shift the thermal isomer equilibrium to 1(trans): 1.8(cis). The isomers interconvert thermally and by photoisomerization. N,N',8,1O-Tetramethylthiacarbocyanine(+) is an overcrowded and twisted antisensitizer. 9-Phenylfluoron is an example of an inefficient fluor (Q, = 0.18). The high fluorescence efficiency of fluorescein(2-) is lost when the o-COOH function is removed from the central phenyl ring to make the molecule shown here. The loss of fluorescence is associated with increased torsion of the unsubstituted phenyl group and with direct internal conversion to the ground state (S, So). The last dye, a partially cyclized thiacarbocyanine(+), represents an unsuccessful attempt to increase fluorescence yield by removing some of the possible isomerizations. The actual yield is essential identical for this molecule and the uncyclized material with hydrogens on the three central carbons.
-
dyes. Out of these factors we have selected the following tests as having descriptive and predictive power: 1. Determine chromatographic Rfvalues on neutral AI20, as an indication of adsorption strength. 2. Measure electrochemical redox potentials, especially E I I 2 ( R ) .Explore electrochemical mechanisms where possible. 2a. Measure acid-base transitions of dyes; correlate these with redox potentials. 3. Measure relative quantum yields of fluorescence in solution. A high quantum yield indicates some freedom from internal conversion by torsional motions. 3a. Collect any structural data (X-ray structures, where available) or make space-filling molecular models (CoreyPauling-Kolthun) to gain some understanding of steric crowding and distortion out of planarity. The remainder of this paper will be devoted to the correlation of these molecular factors with the performance of dyes which have been incorporated into prototype solar cells, as fabricated and tested at the Exxon l a b ~ r a t o r i e s . ~ . ~ While the simpler photographic correlation of dye structure with the formation of two-dimensional monolayers on AgBr is lost just as soon as one begins to comtemplate the three-dimensional PV dye systems, the problem of determining and securing the optimal crystal packing structures for photovoltaic elements remains. We are convinced that packing structures of dyes can be
closely correlated with exciton mobility or the lack of it. Morel et aL6 and Ghosh and Feng14 have identified exciton mobility as a key step in the chain of microscopic events leading to the separation of a charge-carrier pair, and evidence can be found in their work to establish that different dye packing structures exhibit entirely different patterns of exciton mobility. Two caveats must be given in relation to structures. First, many organic dyes are capable of forming more than one crystal structure; these are typically metastable vs. stable polymorphs, subject to phase transitions with temperature and time. Three crystal structures (a,p, and X-H,PC) are well-known in the phthalocyanines: and it is significant for the argument given above that these structures behave quite differently in photovoltaic tests. We have established the existence of phase transitions for some of the preferred merocyanine and squarylium dyes, and are investigating these as special cases. Second, many organic dyes have a tendency to crystallize as stoichiometric solvates, Le., crystal structures with an ordered inclusion of an integer or integer fraction number of solvent molecules per dye molecule. Solvates of water, methanol, acetonitrile, pyridine, and acetic acid have been encountered among cyaning dyes;15we have recently encountered solvates of methylene (14) A. K. Ghosh and T. Feng, J . Appl. Phys., 49, 5982 (1978). (1 5) D.L. Smith, Photogr. Sci. Eng., 18, 309 ( 1 974).
936 The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 chloride and of 2-propanol with merocyanine dyes.I6 The difficulty with solvates, as with metastable-stable phase transitions, is that they ultimately lose solvent and produce a disordered PV cell structure which is likely to be inefficient in the transport of excitons or charge carriers. Much of the Exxon work with prototype photovoltaic cells has employed vacuum deposition of analytically pure dyes, a sure guarantee against the presence of solvates. However, when dyes are solvent cast, the possibility of solvate disorder interfering with the test must always be considered. Vacuum deposition gives no security against metastable stable phase transitions, but these can frequently be detected by changes of absorption spectra and even by changes in the visual appearance of stored films. -+
Specific Tests for Photovoltaic Dyes In seeking correlations of molecular properties with performance of PV assemblies, one must be aware that the assembled cells do respond to a number of molecular variables and also to a number of constructional variables. Morel et al. have discussed the requirements for success with each step in a chain of events in order to arrive at a high quantum yield of photovoltaic action. Some of the molecule-device correlations are well separated and easily visualized, while other variable are scrambled together. At this moment, chemical attachment of dyes to A1203is such a wellseparated variable; however, the variables of chemical stability, crystal domain size, polymorphism, internal conversion, and exciton mobility are still partly scrambled. Adsorption of Dyes to A1203 One requirement of a candidate PV dye is that it makes some intimate contact with the aluminum electrode surfaces. The more critical surface is actually A1203, formed by deliberate aerial oxidation of the surface of the freshly evaporated AI electrode. Since ambient water vapor is also present, the surface presumably includes some fraction of AlO(0H) with exposed hydroxyl groups. Since porous alumina is one of the standard substrates used for thin layer chromatography (TLC) of organic compounds, we settled on the determination of the standard chromatographic retention (R,) of the dyes on a standard alumina material,” Baker-flex lB, as a measure of the dye-A1203 interaction. R, is simply the ratio of distance moved by the dye spot to distance of advance of the solvent front. A material with no affinity for the A1203substrate would advance with the solvent front, giving R, = 1.O; a material either chemisorbed to the substrate or simply insoluble in the eluting solvent would not move at all (R,= 0.0). The use of a good solvent (methanol) excludes the possibility of simple insolubility, and one has a numerical ranking of the strengths of adsorption with materials of lowest Rfbeing those most strongly adsorbed. When a PV cell is prepared by vacuum evaporation of the dye, the molecular contact between successive layers is enforced, and the layers simply must either remain in contact or delaminate. However, with the present PV dyes there seems now to be a requirement for a reliable, ordered contact more nearly resembling covalent bonding than simple physical adsorption. Historically, Morel et al. inferred from the combination of dyes M1 and M8 (the first and eighth merocyanines tested)6 that the addition of a nonconjugated carboxyl group to a functional molecule would produce an improvement in performance. This leads to a correlation of structure with performance. In the simple class of merocarbocyanine dyes, for which M1 and M8 are the prototypes (see Figure l ) , some seven pairs of molecules with ethyl and carboxymethyl groups attached have given the same test result, viz. the short-circuit current quantum yield (ax)is always higher for the CH2COOH dye. In only one of the seven pairs is the ratio values even close to unity; in every other case the imof aSc provements obtained with CH2COOH are large.
Piechowski et al. TABLE I: Correlations of Strong Adsorption (Low Chromatographic R f Values) with High Charge Carrier Quantum Yielda osc
dye pair CH,COOH/CH,CH,
Rp, R f , (CH,COOH), (CH,COOH) (Et) 3%
3% ~~
M-8IM-1 (benzothiazolc) M-1 S7/M-4 (benzoxazole) M- 169/M-9 (2-qu in olin e) M-43/M-136 (pseudoindolc) M-5 7/M- 14 8 (benzimidazole) M-7S/M-74 (merodicarbocyanine) M-l27/M-126 (5-nitrobenzothiazole) M-28/M-1 (carboxyethyl) M-54/M-1 (carboxyethyl on benzothiazole; ethyl on rhodanine)
0.02
0.64
16
3.8
0.04
0.68
8
0.6
0.01
0.65
5.5
0.4
0.02
0.65
0.02
0.71
0.6
0.56
0.04
0.64
2.4
OS4
0.18
0.01
16
0.06
0.02
0.64
5.4
3.8
0.04
0.64
0.54
3.8
a Rf(CH,COOH) is the chromatographic transport ratio which compares the movement of the carboxymethyl dye with the movement of the solvent front with CH,OH eluting the dye from Bakerflex 1-B alumina. A low Rfvalue indicates a high affinity of the dye for alumina. Rf(E.t) is the same for the ethyl-substituted member of the pair. QSc is an experimental ratio of charge carriers transported to photons absorbed at the wavelength of maximum absorption.
Short-circuit current quantum yield aSc is simply the ratio of electron charges flowing to photons absorbed; it is measured with the PV device feeding a current meter of very low impedance. It is a very significant and easily interpreted variable for PV devices, and will be used as the prime figure of merit in this paper. To determine aSc, one must know the flux of incident (monochromatic) photons and must be able to correct for both the incomplete light absorption by the dye and the partial absorption of light in the semitransparent A1 electrode layer which is situated between the light source and the dye. Our chromatographic tests add the confirmation that this increase in quantum yield with carboxy substitution is directly associated with a considerable strengthening of the attachment of the CH2COOH dyes to the alumina surface. In fact, these acidic dyes barely move at all when eluted by the reasonably good solvent methanol. Several other eluting solvents have given similar results. The comparison of Rfand structure with asc is shown in Table I; a listing of properties and preparations of all of the dyes appearing in this paper may be found in Table 11. Further trials by Morel et a1.6 have shown that both the manner and point of attachment of the dye to the substrate are significant. Dye M28 has a CH2CH2COOHgroup attached in this same 3 position on the rhodanine residue. Though strongly bonded to Al2O3, it drops back almost to the poorer performance of the weakly bonded ethyl-substituted dye M1. In dye M54 the ethyl and carboxyethyl groups of M28 are interchanged, and the performance drops well below that of M1. Both M28 and M54 are strongly bonded by alumina by their carboxy groups, as shown by the chromatographic test. A consideration of the spectroscopic basicity values determined by Platt will allow us to speculate on the ideal mode of attachment for PV dyes of this class. Platt’* assigned spectroscopy derived basicity values of 3700 cm-l to 3-alkylbenzothiazole and 8400 cm-I to 3-alkylrhodanine as end groups of carbocyanines in methanol. Note that the basicity values vary with solvent/environment, just as the absorption spectra of these unsymmetrical merocyanines vary with environment.
(16) B. Toby and J. Potenza, Rutgers University, private communication. (17) J. T. Baker Chemical Co., Phillipsburg, NJ 08865; aluminum oxide
1B flexible TLC sheets.
@SC
(Et),
(18) J. R. Platt, J . Chem. Phys., 25, 80 (1956).
Desirable Properties of Photovoltaic Dyes Judging from Platt’s basicities, one expects the rhodanine residue to donate an electron to the quaternary benzothiazole(+) residue so that the ground state of the dye is predominatly in the nonpolar form of Figure 1. The excited state is predicted to be in the dipolar form.ls On this basis, the rhodanine group is the electron-rich portion of the dye, especially in the excited state. To promote electron transfer, one wants to bring the rhodanine residue as close to the alumina surface as possible without grossly perturbing the chromophore. Thus the extra CH2 of the CH2CH2COOHgroup displaces the rhodanine group away from the alumina surface unnecessarily, and the inversion of alkyl and CH2COOH groups displaces it even further, producing inferior performance. An alternate hypothesis is that bonding with the carboxyls down may create a monolayer which differs in structure from the bulk solid dye and acts as a Forster acceptor for energy transfer from the bulk dye. Absorption spectra of very thin films of M-8 on A1203 lend some support to this hypothesis.
Electrochemical Potentials and the Threshold for Electron Injection It is a much simpler task to analyze the electrochemical threshold for electron sensitization in the silver halide photographic process than it is in the photovoltaic process. In the so-called M sensitization of silver halides, the dyes are applied at much less than monolayer coverage, and are adsorbed flat-on as isolated molecules. This strips away all variation associated with crystallinity, intermolecular interactions, packing structures, and exciton mobility. Further, many molecules which function poorly as close-packed aggregate sensitizers do work well in the M (monomer) mode, since the forces of adsorption immobilize the molecule and prevent the torsional motions which are associated with rapid internal conversion of S I So. Thus, although the classical sensitizer pseudoisocyanine (lI1’-diethyl-2,2’-cyanine) gives no observable fluorescence in fluid solutions at room temperature and sensitizes photographic latent image formation poorly in the unassisted aggregated state, it preforms well as a monomeric dye.lg (Here “unassisted” refers to the absence of photographic “supersensitizers”. These are additives to dye aggregates which increase the quantum yield of sensitization either by facilitating charge separation or by partitioning the aggregate and inhibiting torsional internal conversion.) As a final assist to the unscrambling of the photographic data, reactive molecules signal decomposition or reaction with AgBr by impairing the intrinsic (blue) sensitivity of their AgBr host. The classical use of dyes as redox indicators in titrations is not pertinent to the present problem, since most of these indicator systems undergo oxidation or reduction by two electrons per molecule. The dominant reactions of dye sensitization involve one-electron reactions, and equivalent reactions can be observed by time-dependent coulometric techniques in solutions. We have performed electrochemical measurementsz0on some 30 of the photovoltaic dyes, using a variety of techniques such as classical polarography with a dropping mercury electrode (DME), cyclic voltammetry (CV), and differential pulse polarography. In addition to the DME as the working electrode, we have used hanging mercury drop electrodes (HMDE), rotating and stationary platinum electrodes, and pyrolytic graphite electrodes. Here we will discuss only the classical measurements with the DME in solutions of ca. lo4 M dye in absolute methanol containing 0.05 M LiCl as background electrolyte, in order to present a consistent data set which can be correlated with earlier investigations on photographic cyanine sensitizers.21 One of us (A.P.P.) has presented a discussion on the detailed electrochemical mechanisms20a*b and will present a discussion on the correlation of ox-
-
(19) (a) A. V. Buettner, J . Chem. Phys., 46, 1398 (1967). (b) T. H. James, Ed., ”The Theory of the Photographic Process”, MacMillan, New York, chapter IO. See especially pp 257-60. (20) (a) A. P. Piechowski, J. Electrounal. Chem., 145.67-85 (1983). (b) A. P. Piechowski, unpublished results. (c) A. P. Piechowski and G. R. Bird, to be, submitted. (21) R. F. Large, “Photographic Sensitivity”, R. J. Cox,Ed., Academic Press, New York, 1973, p 241.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 937 idation and reduction potentials with spectroscopic energies.2h It seems at first surprising that the electrochemical variable which best explains dye sensitization of AgBr should be the potential for electrochemical reduction of the dye: dye
+ e-
-+
dye-.
Obviously, the direct electron sensitization reaction involves loss of an electron from the dye: dye
-
e-
+ dye+.
However, it is the correlation with reduction potential which does emerge,22and this is the best understood by considering that excited dye molecules (SI)may have enough excess energy to inject either an electron into the conduction band or a positive hole into the valence band of the substrate. The key question is whether a separated electron will tend to remain in the dye layer or be injected into the substrate or electrode. In principle, this question is settled by the relative electron affinities of the dye and the substrate, as measured by Nelson.23 The chemical variable which most closely correlates with electron affinity is the reduction potential of the ground-state dye. By exploring dyes which lie close to the boundary between preferred electron injection and hole injection into a substrate of AgBr at a known pBr-, Gilman has established an empirical correlation between the solution electrochemical potential scale and the physical scale of electron affinities.8 The zero of the electrochemical scale is found to lie at about -4.40 V below the vacuum level, and of course the signs of the electrochemical and physical scales are reversed. Thus a dye such as M-8 with a large, negative reduction potential ( E , = -1.3 1 V) will tend to transfer electrons to aluminum, since the electron affinity of the dye is approximately 3.09 eV and the Fermi level of aluminum is approximately 3.75 eV. Ghosh and Feng14 have given a detailed discussion of eIectron tunneling across the AI,?, layer, but have only considered the single dye M-8 and have not introduced materials with other values of electron affinity. Passing quickly over the possibility of individual dye deviations from a single, simple correlation of solution reduction potential with solid-state electron affinities, one needs only to note that the photographic correlations of reduction potentials with sensitizing action do work remarkably well. There is a more enigmatic component to these correlations, and it is the difference between reversible behavior of sensitizing dyes on substrates and irreversible behavior of the same dyes in solution.20s21Neither the merocyanines of the Exxon6-20study nor the great majority of efficient photographic sensitizers2’ satisfy the criteria for reversible oneelectron reactions in solutions. A detailed study of irreversibility will be. reported separately,20but we should note that the potentials entered into the present correlation are, in most cases, derived from reproducible but demonstrably irreversible electrochemical reactions in the CH,OH + 0.05 M LiCl solutions. The overall reaction scheme appears to be of the EC class (simple electron exchange followed by some chemical reaction removing one of the products).24 With some related materials, the chemical reaction has been shown to be a dimerization of the dye radicals followed by the subsequent reduction of the dimer (ECE type).25 The inescapable cqnclusion is that any correlation of solution redox potentials with PV action in the solid state is only an empirical correlation. Figure 2 shows a phenomenological plot of the short circuit (current) quantum yield and DME E l 1 2 ( R potentials, ) with each point representing a single dye. All of the simple binuclear merocyanines for which both a,, and Eredhave been measured are shown on this plot. The dyes are listed with the reduction potentials and GSc’sin Table 111. We have excluded the trinuclear dyes and squarylium dyes for two reasons: first, the E1l2(R)measurements on these dyes are unreliable as a result of low solubility; second, the exciton mobilities in these larger molecules can be shown to be quite different. Figure 2 is a ~~
(22) S . S. Collier and P. B. Gilman, Jr., Phorogr. Sci. Eng., 16,413 (1972). (23) R. C. Nelson, J . Opt. SOC.Am., 46, 1016 (1956). (24) Albert J. Fry, “Synthetic Organic Electrochemistry“, Harper and Row, New York, 1972, pp 82-90. (25) Su Moon Park and A. J. Bard, J . Electrootad. Chem., 77, 137 (1977).
938
Piechowski et al.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
TABLE 11: Preparations, Properties, and Structure of the Merocyanine Dyes Used in This Work dye
structure
rll- 1
form i i hf-8 ~
syn recryst methoda solventb
mri. "C
hmax ,=
nni
Ad
1
265-6
521
A
2
229-30
490
compact, photographic sensitize1
Ad
3
M-9
slightly crowdcd, 3-H vs P-H
A
4
XI-10
twisted
A
4
51-13
good photographic scnsitizcr
A
1
3 09
4 94
M-26
as 1cI-8
A
1
303
5 20
A
1
284-5
520
I XI-4
R.1-8
517
i
M-28
266
560
614, 515
M-4 0
as 11-8
A
3
290-1
515
M-4 1
phenyl twisted out of plane
A
5
281
525
M-43
compact photographic sensitizer
2
2 24
498
M-4 7
slightly crowdcd as M-9
5
166
492
Desirable Properties of Photovoltaic Dyes
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 939
TABLE 11 (Continued) dye
structure
M-54
form
syn recryst mcthodU solventb
m u . "C
hmax:
nrn
as M-8
A
1
244-5
470
slight crowding, S-H-CH,
A
1
277-8
524
(0p = t L ?
1
COOH
M-55
M-57
4
M-5 8
as M-8,l
M-65
compact
M-6 9
509
1
246-8
625,420
3
251-3
533
M-72
A
252
5 06
M-73
A
21 1
513
M-74
flexible, prone t o cis-trqs photoisomerization
A
171-2
565
hl-75
as M-74
A
243
545
M-78
phenyl twisted out of plane
d
4 84
940 The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
Piechowski et al.
TACLE I1 (Coutirzfred) dye
structure
51-85
hf-93
as M-85,but h,N'-diethyl
form
nip, "C
~nl,wC
nin
compact; 0 bracketed by gem-dime thy1
d
>3 20
621
as hl-85
G
3 04
6 24
d
hf-102
hl- 106
spn recryst nicthoda solventb
as M-8
515
A
8
288-9
512
hl-107
A
4
180-1
502
M-108
A
4
164-6
505
M-109
G
3
hl-114
G
M-119
A
>300
670
28 2
6 28
2
231-9
523
hl-123
phenyl prcsumably twisted out of plane
A
1
290-2
561
hl-I24
compact, S-H,C
E
1
317
545
M- 126
as M-8,l
E
289
515
M-127
a3
M-8
E
307
505
subliiiied
5
Desirable Properties of Photovoltaic Dyes
The Journal of Physical Chemistry, Vol. 88, No. 5. 1984 941
TABLE I1 (Continued) form
structure
dye
syn recryst methoda solvent*
m p , "C
hnax,C
nni
M-133
phenyl presumably twisted out of plane
A
4
277-8
557
M-134
as M-1
E
3
303
508
A
2
M-136
I M-139
as M-8
E
5
293-4
537
M-141
as M-8
E
3
288-9
520
as M-8
E
3
288
518
E
2
245
497
E
3
284
511
A
4
181-3
512
8
288
525
>300
705
292
4 86
i M-143
M-144
A M-146
as M-8
M-148
M-150
as M-8
E
M-153
crowded
d
M-157
E
5
942
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
Piechowski et ai.
TABLE I1 (Continued) dyc
structure
form
syn recryst methodn solventb
mp, "C 260-2
536
&n,WC
nm
M-160
phenyl presumably rotated out of plane
A
sublimed
M-165
as M-8
E
5
302
533
M-166
compact, good S-H,C contact
E
8
3 03
53 1
slightly crowded, see M-9
E
3
284
563, 532
M-170
as M-169. see M-9
E
3
255
538
hi-171
slightly crowded, see M-9
E
1
291
563
crowded, see M-153
G
1
277
707
compact, see M-85
G
8
347
629
compact, see M-85
G
321
615
rigidized by O-H-.O
G
&A?""' M-167
e n
N -
hl-169
M-175
* ,
op.-+-rr 0
M-178 CI
Ei
CI
M-185
M-197
M-198
O H * * .I o
,
sublimed
Desirable Properties of Photovoltaic Dyes
The Journal of Physical Chemistry, Vol. 88, No. 5. 1984 943
TABLE I1 (Coririnued) dy c
structure
syn recryst method" solventb
form
51- 199
G
M-198
m p , "C
307
~Inax,C
nni
635 (CHCI,)
a Xlctliod A is detailed in L. S . G . Erookcr e t al.,J. An?. Chern. Soc., 7.3, 5332 (1951);iiicthod 3 , ibid., 6 3 , 3192 (1941); method C, E. E. Knott and L. A . \VilIiai:ls, U.S. Pdtent 2656 352 (1953);metliod D, J . M. Nys and T. 13. Chys, British Patent 785 901 (1954);inethod E, M,V. Dcichmcsitcr. Z. P. Sytnek, and I-. B. Lifshitq, Z/z. Obsiiclr. Khinz., 22, 166 (1952);method I-, L. S. G. Brooker, U.S. Patent 2454 629 Solvcilt: 1, pyridine; 2, toluene; 3, (1948): inethod G, H. I:. Sprcngcr and \V, Ziegcnbcin.nnjie~u.C/iem., i r r r . Edit. Engl., 1. 530 (1968). Solution spectra were occtic acid:4, ethanol; 5, cthylcnc glycol monoinethyl ether; 6. acetone; 7, benzene/heptanc; 8, forniic acid/H,O. taken in methanol, except where indicated otherwise. These dyes were obtained commercially, mainly from the Aldrich Chemical Co., Gallard-Schlessinger Cml. Mfg. C o . , and more recently from the Japanese Research Institute for Photosensitizing Dyes Co., Ltd., Okayama 700, Japan. This last organization does indeed sell dyes in spite of the name, and has on eno~rnoiisinventory of photosensitizing materials. LOG (Current Ouantw Yield) v* Holf-uav.
R d ~ o t l o nPotentlol
r I. 0.
A
0.
"I
2
-0.
0 J
-I.
1.10
1.28
-El/2Cr>
1.3
1.4
vs SCE Cvol t
1.m
1.88
d
Figure 2. The first reduction potentials (irreversible) of a group of merocyanine dyes plotted against the "short circuit current quantum yield" 0, of the same dyes operating in photovoltaic cells. The vertical lines connect identical chromophores which differ only in the 5-rhodanine substituent (C2HSlower vs. CHzCOOH upper). The slanting lines mark the envelope of best performing dyes and suggest a preferred reduction potential of -1.30 V or less. Dyes are listed in Table 111 and shown in Table 11.
restricted correlation within a related family of organic chromophores. As a reminder that several other variable besides E,,,(R) influence cell performance, the dyes of Table I which differ only in substitution of CHzCOOH for CH,CH, are connected by nearly vertical lines on this plot. (The lines are essentially vertical because the substitution of CHzCOOH for CHzCH3has negligible effect on the value of E l j 2 ( R )or on the absorption spectrum.) Chemical affinity for A1,03 is just one of the many other variable scrambled into this plot. In such a situation with multiple variables, we may hope that the upper envelope of the plot, as defined by the best performers at each Erd, may reveal the desired correlation. On this basis, the rather steeply slanting line on the left-hand side of the plot intersects a plateau at about -1.3 V. This resembles the plots for photographic sensitizer performance, and we take the reduction potential of -1.3 V vs. SCE as a critical potential for the sensitization of the Al/A1203 electrode. If more dyes had been available, we would have expected to see a flat plateau in the range from -1.3 to -1.6 V. However, as one moves into this region, a group of dyes having similar transition energies must become increasingly vulnerable to decomposition by simple aerial oxidation; this and the acid-base reactions discussed below may be partial explanations of the gentle decline shown by the few dyes in this region. The correlation of photovoltaic action with reduction potential is diammetrically opposed to the correlation between cell per-
TABLE 111: Reduction Potentials of the Dyes"
M -1 M-4 M- 8 M-9 M-10 M-13 M-26 21-40 111-41 M-43 111-47 kI-55 h.1-57 hr-s 8 M-65 M-69 hl-72 51-73 hI-74 51- 7 8 2.1-85 xi-102 M-107 wio8 hl- 1 09 hl-119 hl-127 M-134 M-136 M-139 M-141 111-143 &I-144 H-148 111-157 M-160 M-165 M-169 M-170 M-171 hl- 124
-1.32 -1.42 -1.31 -1.30 -1.18 -1.19 -1.33 -1.30 -1.28 -1.34 - 1.44 - 1.44 -1.60 -1.46 -1.26 -1.22 -1.39 -1.14 -1.22 -1.10 -1.41 -1.24 -1.20 -1.16 -1.14 -1.24 -1.28 -1.32 -1.37 -1.33 -1.32 -1.32 -1.57 - 1.44 -1.32 -1.26 -1.53 -1.40
4.77 0.6 16 0.47 0.17 0.38 2.3 0.3 0.52 16 0.38 7.1 0.6 1.79 22 2.89 1.7 0.44 0.54 0.32 9.8 0.39 0.67 0.72 nd 0.45 0.18 5.4 0.06 2.9 14.8 10.5 10.6 0.56 8 10.8 10.9 5.5 7.9 nd
0.67
3.60 t 0.2 07-CH,) 4.86 BzI-COO3.79 (CY-CH,)
3.60
i
0.5
4.58 BZI-Et
1.95 (4.2 COOH)
a Lists all of the potentials determined vs. saturated calomel electrode (SCE) for the first electrocheniical reduction (+1 e-) of the dyes in CH,OH 0.05 M LiCl at the surface of a dropping mercury clectrode (DME). The quantum yields of the same dyes for generating charge carriers from absorbed photons in solid photovoltaic cells are shown in the last column. The correlation of yields and reduction potentials is shown in Figure 2.
+
formance and oxidation potentials offered by Chamberlain, Cooney, and Dennison.26 These workers made cells which were structurally similar to the Exxon cells, and used four dyes (their B-E) common to the present investigation. However, they doped (26) G. A. chamberlain, P. J. Cooney, and S.Dennison, Nafure (London), 289, 45 (1981).
944
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
their cells with iodine by vapor diffusion. Given this striking chemical change, it is reasonable that they should invoke charge transfer behavior and seek correlations with the oxidation potentials of the dyes and also with the measured ionization potentials of the solid dyes.27 The contrast between the two approaches to cells with superficially similar contacting electrodes is heightened by the fact that Chamberlain et al. report high values of +sc for four days which are the inefficient members of the C2H5/ CH2COOH pairs reported in our Table I and shown in Figure 2. It seems best to view these contrasting results as belonging to superficially similar but chemically different systems. (One of the referees has raised a question as to possible action of iodine on the electrode materials of the cell. One would certainly expect formation of a surface layer of AgI on an Ag electrode. The Al/A1203is probably stable, through aluminum and I2 would react if the oxide layer were broken.) The microporosity of evaporated cell structures has been noted and used to advantage in our investigations. When a dye is suspected of undesirable acid-base reactions, it is possible to test this by exposing the cell to vapors, NH3 for example. When this is done with the dye M-8, performance falls off and then rises slowly. We assign this to the transient formation of a mixed phase of ordinary dye and dye -NH,+, a system which loses NH, and reverts to the original ordered microstructure. We have tried to avoid this kind of sensitivity to loss or uptake of volatile materials, as in the avoidance of solvated crystal systems.
Protonation of Merocyanine Dyes Protonation of cyanine dyes has been studied extensively. The acid-base reaction is of interest for its own sake and because protonation can be correlated with reduction potentials within a family of dyeszs It would be most undesirable to test a dye having an acid-base transition at pH 4-5 without some awareness of the possibility of protonating the dye during the course of the test. Protonation of the open chain cyanine dyes has been shown to occur on the carbons of the central polymethine chain and to cause a complete disruption of the conjugated system.2s Our limited results on the merocyanines suggest the same sort of reaction, as none of the open chain protonated dyes have shown any visible absorption whatsoever. This evidence argues against protonation at the rhodanine oxygen of the simple merocyanines (M-8), since one would expect some visible absorption from the resulting extended azapolyene. Buffers of citrate and phosphate were initially used for our study, but it became apparent that these large polyanions could form complexes with the protonated dyes, shifting the acid-base equilibrium in the direction of excessively high pKa values. For example, M-8H' has an apparent pKa = 3.66 in a citrate buffer, but pKa C 1.0 in a simple HCI/KCI system. This sort of result casts doubt on some of the earlier studies on the cyanines. To eliminate this error, we chose to work at a constant ionic strength of 0.20 with buffers of the small monovalent ions formate, acetate, and ammonium, using HCl/KCl for the lowest pH's. Our determinations were all done by measuring the spectra of these buffer solutions with added 5% aliquots of dye in ethanol, a procedure which minimized solubility difficulties with some of the dyes. The spectroscopic procedure is identical with that used by Tobey with methyl red,29and is simplified by the lack of absorption in the visible region by the protonated merocyanine dyes. The results of the protonation studies are shown in Table IV. We note that M-8 is protonated only in the neighborhood of pH 0, but that the dyes M-69 and M-57 with methyl substituents CY and /3 to the 3-ethylbenzothiazole group have pKa values of 3.8 and 3.6, respectively. This is quite comparable to the result of central ( p ) alkyl substitution in the corresponding thiacarbocyanines. The presence of an electron-donating group in the chain favors protonation of the chain. The two benzimidazole dyes (27) G.A. Chamberlain and R. E. Maluas, Faraday Discuss. Chem. Soc., 70,'299 (1980). (28) (a) A. H.Herz, Phofogr.Sci. Eng., 18,207 (1974);(b) P.Beretta and A. Jaboli. ibid., 18. 197 (1974). (29) S.W.Tobey, J . Cheh. Ed:, 35 (lo), 514 (1958)
Piechowski et al. TABLE IV: Acid Constants for Protonation-Deprotonation of Some Merocyanines and Related Dyes dye M-8 M-55 M-69 M-57 M-148 M-124
M-109
type benzothiazolerhodaninc mcrocarbocyanine p-CH, analogue of M-8 a-CH, analogue of M-8 benzimidazole analogue of M-8 M-57 with CH,COOH replaced by C,H, N-a-cyclized M-8 analogue
benzothiazole squarylium dye
pKa," at 24 1 "C +_
E,,(R), V
0.7
t
0.3
-1.31
3.6
t
0.2
-1.44
3.8
+_
0.2
4.9
i
0.15
-1.60
4.6
t
0.15
-1.57
2.0 t 0.4
3.6
t
0.5
definite evidence of overlap with CH,COOH
Ma
a pKa values listed are the acid constants for protonation-deprotonation of the chromopliores of the dyes listed. The protonated form is decolorized and inactive, as shown in Figure 1. The first reduction potentials are also shown (Table III), since these arc related to the pKa values within a family of dyes.
M-148 and M-57 have pKa values of 4.6 and 4.9 for the N-ethyl and N-carboxymethyl dyes, respectively, and these dyes show a trend in the direction of large pK, values as do the symmetrical benzimidazolocarbocyanines, which actually protonate on the alkaline side of neutrality. The benzimidazole merocyanines are encroaching on this zone of ambient protonation, and this may explain their mediocre values of aSc. In performing these studies of acid-base equilibria, we had hoped that it would be possible to observe at least subtle absorption shifts as the nonconjugated carboxymethyl group was protonated. No such shifts were observed for M-8 or for several other carboxymethyl dyes. We do note that the pK, values for the two benzimidazole dyes differ significantly in a direction which would indicate that the carboxymethyl group of M-57 is deprotonated in the pH range where the chromophoric protonation of M-57 occurs, pH 4-6. Further support for this inferrence is obtained from the partially cyclized M-8 analogue, M-124. This dye exhibits a chromophoric transition in the neighborhood of pH 2, but the slope of a plot of pH vs. the log of the concentration ratios is too shallow to fit a single, simple protonation. Here we believe that the acid-base conversions of the chromophore and the nonconjugated carboxyl are overlapped, and that the protonation of the carboxyl group does affect the intensity of the absorption band. It suffices for the moment that the pKa values of the merocyanine dyes do fall into a correlation with the solution reduction potentials. This correlation can be helpful when an individual dye is so nearly insoluble as to render polarographic measurements unreliable. Our one measurement on a squarylium dye, M-109, pKa = 3.6 k OS, cannot be simply correlated with the binuclear merocyanines measured.
Spectroscopic Properties, Especially Fluorescence Yields The merocyanines and squaric acid dyes have extinction coefficients ranging from 70000 to over 200000 (mol/L)-' cm-I. These strengths of extinction are similar to the cyanines, and both groups of dyes also show an increase of extinction coefficient with increasing chain length as well as an increase in absorption wavelength. The deviations of the merocyanines from the simple behavior of symmetrical cyanines (the Brooker deviation^)^^ can be described in detail by Platt's theory, which takes account of the unequal basicity values of the two end groups. It is implicit (30)L. G. S. Brooker et al., J . Am. Chem. SOC.,67, 1875 (1945). The interested reader should note the entire 'Colour and Constitution" series by Brooker and co-workers in this journal.
Desirable Properties of Photovoltaic Dyes in this treatment that when the end group basicities approach equality, the strength and shape of the absorption band will become indistinguishable from a cyanine. This is observed. With the squaric acid dyes such as M-93,we return to the case of symmetrical end groups, but seem to lose the ionic charge of the cyanines. In fact, the dye must exist largely in resonant structures like those shown in Figure 1, with a negative charge shared between the two squaric acid oxygens and a positive charge shared between the nitrogens of the two end groups. Such a structure reestablishes the charge mobility of a cyanine, and it is remarkable to see how precisely the old empirical rules for predicting the spectra of cyanine dyes work with the squaric acid dyes. As an example, consider M-93 and the corresponding dicarbocyanine (pentamethine cyanine) with the same two Nethylpseudoindole end groups. The additive rule1*for the dicarbocyanine dye gives 5 X 49 nm = 245 nm for the five CH groups and 2 X 197 nm for the two pseudoindole groups for the sum of 639 nm. The dicarbocyanine absorbs at 636 nm in methanol and the squaric acid dye absorbs at 625 nm. For M-109 the calculated wavelength is (2 X 202 5 X 49) = 649 nm, and the actual squaric acid dye absorbs exactly at 649 nm in methanol. We need to know more than the peak extinction of a dye to predict how it may absorb as an amorphous or microcrystalline layer, and part of the missing knowledge lies in the integrated absorption of the dye. It has been shown that this integrated absorption is conserved when a dye is adsorbed as a surface monolayer, even with large spectral shifting of the a b ~ o r p t i o n , ~ ~ and one rather expects that the absorption will be conserved in a thin polycrystalline film. The variable of significance is the dipole strength, as established by integrating the molar extinction coefficient c in In X or equivalently in In v over the spectrum of solution extinction coefficient values. This integral is approximated by a summation of values of c(X)/X over a set of equal wavelength intervals
+
We have found it convenient to perform this integral, but to discuss the merits of dyes in terms of the simple integral 1, rather than the dipole strength D. The difference is trivial, but the simple integral has the dimensions of molar extinction coefficient, and is easily visualized in relation to the peak extinction coefficient. The integral has values of 9600 (mol/L)-I cm-I for M-1in pyridine (peak absorption of c = 89 000 at 526 nm) and 18 000 (mol/L)-’ cm-I for M-93 in ethanol (peak absorption of 327 000 at 624 nm). By contrast, ordinary textile dyes have integrals on the order of 4000 (mol/L)-’ cm-I. This integral gives us the dipole strength of the dye, and also allows us to calculate the Einstein coefficient A and its reciprocal, the radiative lifetime. Thus, from the data on M-1above and the fluorescence peak location at 550 nm, we may calculate that the radiative lifetime is 6.0X s. The corresponding figure for M-93 in pyridine is 5.5 X s. There is an ancient photographic correlation that “overcrowded” sensitizing dyes make very poor sensitizer^.^ These dyes have steric hindrances which twist the chromophore out of planarity. They can be recognized by a broadening of the absorption band and a decrease in peak extinction coefficient. Another characteristic of these overcrowded materials is an absence of appreciable fluorescence in solution, a proof that some other process competes successfully with fluorescence to dispose of the energy of the excited state. Since typical radiative lifetimes for cyanines and merocyanines are on the order of 5 X lo4 s, this competing process must have a rate constant greater than 10” s-I to so diminish the fluorescence. We now understand this competing process to be a torsional motion of the two end groups of the molecule, a torsion which-if continued-would ultimately produce a cis isomer of (31) W. D. Pandolfe and G.R. Bird, Photogr. Sci. Eng., 18, 340 (1974).
The Journal of Physical Chemistry, Vol. 88. No. 5, 1984 945 the dye.1° This isomer need not exist (Le., it may have hopeless steric overcrowding), but rotation toward a 90° twist generally takes the molecule toward an excited state energy minimum lying below the threshold for sensitizing action. There are exceptions among short wavelength sensitizer^.^^ In studying the photoisomerization of cyanine dyes, one gains a respect for these torsional motions as a mechanism for dissipating quanta which might otherwise sensitize. As the introduction of very slight steric crowding occurs through a small chemical substituent (CH,, see Figure l), the quantum yield of photoisomerization of a thiacarbocyanine dye can go from 10% (3,3’-diethylthiacarbocyanine(+) in methanol, -90 “C) to 51% (3,3’-diethyl-9-methylthiacarbocyanine(+)), as the corresponding quantum yield of fluorescence drops from 40% to 2.5%. Thus a drop in fluorescence quantum yield is a sensitive indication of processes involving molecular torsion and torsional isomerization. Sauter3, has found a particular case of a merocarbocyanine which does undergo photoisomerization about the bonds in the dimethine chain, but most merocarbocyanines apparently do not have a possible second isomer which is free of excessive steric hindrance. O’Brien, Kelly, and Costa34surveyed the fluorescence quantum yields of a number of cyanine sensitizing dyes in methanol at room temperature, and we have conducted a similar study on the merocyanines and squaric acid dyes available from the PV investigation. O’Brien et al. found the following major factors in the fluorescence of cyanines: 1. Most of the useful photographic sensitizing dyes have surprisingly low yields of fluorescence in solution. For example, 9-methyl-S,5’-diphenyloxacarbocyanine(+), a prototype green sensitizer, has a fluorescence quantum yield of 1.0% and 9methylthiacarbocyanine(+), a prototype panchromatic sensitizer, has a fluorescence yield of 0.1% (see Figure 1). 2. Partially cyclizing the flexible central chain of a trimethine cyanine accomplished very little toward raising the fluorescence quantum yield. Apparently the cyclizing groups (8,lO ethylene, Figure 1) increased the steric strain on nearby uncyclized bonds, and simply shifted the locus of torsional internal conversion. 3. Fully cyclizing the central portion of typical sensitizers raised the quantum yields above 50% in all cases. (We would suspect that the yields approached loo%.) 4. Increasing the inertial mass of the end groups on a dye subject to torsional internal conversion did increase the fluorescence yield somewhat. 5. Yields of intersystem crossing to the triplet state are very low for this class of molecules, 3% for the fully cyclized dyes with the longest fluorescence lifetimes, and 1% for the flexible dyes with shorter lifetimes. The very low fluorescence yields of the prototype dyes come as something of a surprise. These low yields indicate that the process of direct internal conversion SI So has, in solution, a rate 100 to 1000 times faster than the rate for fluorescence. This seems a poor way to design a candidate sensitizer, since this rapid internal conversion process may also compete with the desired act of sensitization. Even if one argues that most of the sensitizer molecules in an aggregate or crystallite are intercalated or “clamped” between their van der Waals neighbors and so are unable to undergo torsional internal conversion, there are still those molecules which lie at terrace steps on the host crystal or at dislocations in the polycrystalline dye. Some of these may be pretwisted by their immediate environment, and so predisposed to act as centers for internal conversion. If this were not the case, the generalization that “overcrowded dyes make poor sensitizers“ ought not to hold. The fluorescence data reported in Table V for the new merocyanines and squaric acid dyes is presented in a slightly unusual format. We found the common practice of giving “relative
-
(32) D. M. Sturmer and W. S. Gaugh, Photogr. Sci. Eng., 19, 273 (1975). (33) F. J. Sauter, private communication, as quoted by D. M. Sturmer and D. W. Haseltine in “The Theory of The Photographic Process“, 4th ed, T. H. James, Ed., MacMillan, New York, 1977, chapter 8. (34) D. F. O’Brien, T. M. Kelly, and L. F. Costa, Photogr. Sci. Eng., 18, 76 (1974).
Piechowski et al.
The Journal of Physical Chemistry, Vol. 88, No. 5. 1984
946
TABLE V: Peak Fluorescence Intensities (d@/dh),,,
in Methanol and Pyridinea
methanol
pyridine
€a1
€a,
(ni ol/L)-’ dye
M- 1
il,,,,
3‘% A,, nni
M-8 M-9
3.8 16 0.47
M-10
0.17
M-28 M-4 1 M-4 3 M-55 M-57 M-65
5.4 0.52 16 7.1 0.6 22
M-6 9 M-74 M-75 M-85 M- 106 M-109 M-114 M- 123 M-124 M-127 M-133 M-134 M-139 M-141 M-144 M-146 M-148 M-150 M-153
2.9 0.54 2.4 9.8 11.1 no test 0 5.8 no test 0.18 1.9 5.4 2.9 14.8 10.6 15 0.56 14.6 11.5
M-157 M-160 M-165 M-166 M-167 M-169
8 10.8 10.9 10.1
M-170 M-171 M-175
7.9 no test 5.1
M-178 M- 185 M-I 97 M-198 M- 199
5.5
cni-’
hf,nrn
dd)/dh
ha, nin
(inol/L)-’ cni-‘
hi,
nrn
54 1 544 b
8.3 x 10-3 3.3 x 10-3 219000 561, 67 000, 529.5 72000 538 67000 564 563 124000 578 707, 5 646
500 533 518 414, 622 537 638 520 673 638 580
300000 87000
655 548 6 90 660 614
521 517 564, 532 614, 5 74 520 525 498 524 509 610
93 000 83 000 110000, 66 000 95 000 107 000 71 700 92 000 99 000
21.9 x 10-3 6.4 x 10-3 0.27 x 10-3 18.0 x 1 0 . ~ 1.2 x 1 0 - 3 9.4 x 10-3 3.4 x 10-3 2.0 x 10-3 1.5 x 10-3 3.6 x 10-3 5.9 x 10-3 < 3 x 10-3 3.3 x 10-3 1.4 x 10-3 5.8 X 9.7 x 10-3 93 x 10-3 3.4 x 10-3 0.96 x 10-3 < I .6 X
563
structure
ds/dh
4.0 x 1 0 . ~ 65 000 92000 118000 97000
NO, torsion phenyl out-of-plane
crowdcd
519 568 567 559
6 1 000, 71 000 58000
phcnyl out-of-planc
13.3 x 10-3
brackctcd planar cro\vdcd
c r owd cd
0.7 3.3 3.8
745, 675 644 631 647
132 000, 7 7 000 205000 285000 235000
660 646 664
1.7 4.3
639 640
263000 290000
655 656
cromdcd brackctcd planar brackctcd planar hydrogen bondcd plnn a r brackctcd pla~lar hrackctcd planar
Qsc is the quantum yield for charge carrier production from adsorbed photons in photovoltaic cclls, as listed in Tablc 111. the wavelength and extinction coefficient a t the absorption maxiinurn in solution. Not observed.
fluorescence” unsatisfactory, but could not perform complete quantum yield determinations for the large number of new compounds. The peak fluorescence intensities reported below are given relative to Rhodamine B in ethanol set arbitrarily equal to 1.OO peak intensity. (Taking the literature value for the quantum yield of Rhodamine B in ethanol as 70% and comparing the peak fluorescence with the wavelength integral @ = 1 (d@/dX) dX = 0.70, we find for Rhodamine B (d@/dX),,, = 0.0147 nm-l.) In the determination of these peak intensities, corrections have been made for the wavelength variation of the exciting photon flux, for the wavelength variation of detector quantum efficiency and detector monochromator efficiency, for partial absorption of exciting radiation, for partial reabsorption of the peak fluorescence wavelength, and for solvent refractive index. In reporting the peak fluorescence yields (d@/dX)max, we are taking advantage of the general similarity of the shapes of the fluorescence emission bands of this family of molecules. No striking change would occur if integrated quantum yields were
ha and ea arc
determined. Note, however, that these peak fluorescence yields can rise above 1.OO since the Rhodamine B standard has a reported fluorescence quantum yield of just 70%, and since some of the symmetrical squaric acid dyes have fluorescence peaks slightly sharper than Rhodamine B. Thus the determined peak fluorescence of M-197 in pyridine of 1.32 need cause no surprise. Our findings on the merocyanines are very similar to those of O’Brien et al. on the cyanines. Most of the peak fluorescence intensities are of order 1% or less relative to Rhodamine B. In the merocyanines we have the added dimension of dissimilarity (basicity difference) between the end groups, and this produces a very appreciable effect on the fluorescence. In the merocarbocyanines, we can construct a series of dyes based on 3-alkylrhodanine and a succession of substituted benzothiazole nuclei. In this series, the fluorescence rises steadily as the electron-donating power of the benzothiazole substituents rises. This is also the direction of increasing Platt basicity of the benzothiazole group, and represents a shift toward equal basicity for the two end groups.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 947
Desirable Properties of Photovoltaic Dyes
TABLE VI: Effect of Ring Substitution on Fluorescence, Current Efficiency, and Half-WavePotentialu
L
group dye fluor int 'P,, O/c E1,z(W3v
5-CF3 bl-134 1.2 x 5.4 -1.28
5-F M-146 1.5 x 10-3 15
5CH, M-141 3.4 x 10-3 14.8 -1.33
5-H M-8 3.3 x 10-3 16 -1.31
5,6-CH3 hl-150 5.9 x 10-3 14.6 b
5 ,6-OCHZO?4-139 9.4 x 10-3 2.9 -1.37
a A correlation of solution fluorescence yields froni Table V and PV quantum yields of charge carriers from Table 111 against systematic chemical substitution in a family of dyes. Three unresolved overlapping DCP waves.
We take this to mean that flexibility and torsion of the central chain are minimized when the end groups are balanced and the bonds in the central chain are all of order 1 in both ground state and excited state. Minimum flexibility of the ground state and excited state should be associated with maximum fluorescence yield. The series is shown in Table VI. We have no ready explanation for the peaking of this series for 9, in the midregion, and can only note that the two variables of reduction potential and freedom from internal conversion are running parallel in the series. This series of molecules was deliberately synthesized for the purpose of examining the effect of electron-donating and -withdrawing substituents, and it is a point of mild frustration that the electron-accepting ability of the overall molecule is varied only about 0.1 V over the series. The effects of torsional strain and of partial cyclization may be seen very directly in a series of molecules synthesized with substituents on the central chain relative to M-8, which has simply protons on the groups a and to the benzothiazole group. This series is shown in Table VII. It will be helpful to compare this merocyanine fluorescence intensity series with the known behavior of a series of red and panchromatic sensitizing dyes based on symmetrical carbocyanine structures. The 9-CH3 (p) dye is the prototype for a series of useful panchromatic sensitizers as shown in Figure 1. Substitution at this position does introduce some steric strain. This was shown by the crystallographic investigation of Mastropaolo and P0ter1z.a~~ as well as by the studies of quantum yields of fluorescence and photoisomerization by Greenberg and SandhuIO in the carbocyanine sensitizers. The 9-CH3 group seems not only to be. needed for a shift in reduction potential but also for some subtle steric indexing effect. Presumably a related dye would be better behaved if one could somehow have these effects without the steric strain. The 8, IO-dimethylthiacarbocyanine dye (Figure 1) has been studied by Brooker et al.36 Models indicate that there is a severe steric strain between the N-CH3 or N-CH2CH3group and the 8(m)-CH3 group, and the spectra obtained suggest a severe twisting of the molecule. It is assumed to be an antisensitizer, a member of a class of cyanines and merocyanines which can degrade the quantum yield of sensitization even when present only as a minority additive to a monolayer of a high yield sensitizer. Thus we were not surprised to observe that the corresponding a-CH, substituted merocyanine M-69 has the lowest fluorescence yield and the poorest photovoltaic performance of this series. The 9-phenyl (9-8) substituent produces markedly less strain than the 9-CH3 Dyes based on 9-phenylthiacarbocyanine(+) are used as relatively long wavelength red sensitizers for photographic color papers, even though the 9-phenyl group must interfere somewhat with the normal adsorption of a thiacarbocyanine dye to a silver halide surface. In considering the 9-phenyl group, we must note that it introduces two effects rather than one. It is a bulky group, and it also represents a new con(35) D. Mastropaolo and J. Potenza, Rutgers University, private cornrnunication. (36) L. G. S.Brooker et al., J . Photogr. Sci., 1, 173 (1953). (37) K. Nakao, K. Yakeno, H. Yoshioka, and K. Nakatsu, Acta Crystal!ogr., Secr B, 35, 415 (1979).
TABLE VII: Effect of Chain Substitution on Current Efficiency and Fluorescencea h
dye M-8 M-69 M-55 M-160 M-124
substituent a$= H CH, P=CH, P=phenyl N-CH,CH,-a
re1 fluor,b current quantum % yield, 70 0.33 0.035 0.091 0.14 0.64
16 2.9 7.1 10.8 no test, dye separates from electrode
compact twisted hindered hindered cyclized
a A correlation of solution fluorescence yields and PV quantum yields of charge carriers against the systematic introduction of steric hindrance (substituciits, toy line) for a family of related dyes. Relative fluorescence is measured at maximuin, taken relative to Rhodamine B set equal to 1.00 at peak; see text. Note: Both the fluorescence in solution and the device quantum yield are diminished by distortion of the planar dye.
jugated group capable of some torsional motion relative to the plane of the chromophore. This kind of torsional motion can be seen as a major cause of internal conversion in the case of the fluorescein dianion and 9-phenylfluoron (fluorescein without the pendant o-COOH group on the central phenyl ring, see Figure 1). The removal of this carboxy group from fluorescein drops the fluorescence yield from 91% to 18% without producing any significant effect on the shape or intensity of the absorption band or in the relative shape of the fluorescence band.38 This chemical change has also been shown not to increase intersystem crossing to the triplet state, so direct internal conversion stands as the one remaining mechanism. When one makes a molecular model of fluorescein it is obvious that the o-carboxy group greatly reduces the torsional freedom of the 9-phenyl group, and that removal of this group increases the torsional freedom of the 9-phenyl group. The group is not fully and freely rotating even then, as the ortho protons are hindered by the 1,8 protons of the xanthene structure. Drexhage has commented on similar effects in the rhodamine laser dyes.39 From these considerations, we infer that the 9-phenyl group of M-160 is moderately well "pinned" by hindrances with the benzothiazole sulfur and the rhodanine oxygen. The behavior of the partially cyclized dye M-124 rather closely resembles that of the 8,10-CH2CH2-cyclizedthiacarbocyanine (no. 11 in O'Brien's series,34see Figure 1). The fluorescence yields are 2% for the unsubstituted thiacarbocyanine, 0.1% for the 9-CH3 dye, and 2% for the 8,lO-cyclized dye. We find a peak fluorescence of 0.64% for M-124, to be compared with 0.33% for the parent ~~~
(38) L. Lindquist and G. W. Lundeen, J . Chem. Phys., 44, 171 1 (1966). (39) K. H. Drexhage, J . Res. Nutl. Bur. Stand., Ser. A, 80, 421 (1976).
948
Piechowski et al.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
dye M-8. Partial cyclization helps, but not a great deal. There appears to be ample torsional freedom in both of these partially cyclized dyes, and the very act of introducing the cyclizing group must increase the steric strain on the adjacent bonds along the chain. The inertial effect can be seen from a comparison of M-8 with M-165. The change from benzothiazole to a-naphthothiazole raises the fluorescence intensity from 0.33% to 0.58%. We would like to attribute the relatively high yield of the two butenolide dyes, M-123 and M-133, to the inertial effect of the pendant phenyl group on the ring system which replaces rhodanine, but this must remain as a conjecture for the moment. The intensities of these dyes are 2.2% and 1.8%, respectively. Here one can actually hope to gain meaningful fluorescence data from compounds which are polarographically impure, in the special case of colorless impurities and total concentration (dye plus impurities) too low for significant quenching of fluorescence. Some of the major variations of fluorescence output can be correlated with steric hindrances revealed by space-filling molecular models, particularly the Corey-Pauling-Kolthun models. None of the simple merocyanine dyes with 2-quinolyl or 4-quinolyl groups give any observable fluorescence whatever, and this failing can be correlated with steric strain between the ring protons and the a,j3 protons of the central chain. Nakatsu et al. have reported a series of crystal structuresa of cyanines and merocyanines, including a dye (M-11) which matches our nonfluorescent M-9. They comment only that “all of the molecules are almost planar except for the methyl groups of the N-ethyl groups”. However, we note two short intramolecular contacts in their structure of M-9, a quinolyl 3H to @-Hdistance of 2.09 A, and an a-H to rhodanine ring S distance of 2.89 A. Given these short contacts and the complete absence of fluorescence, we suspect that M-9, M-10, and related molecules are twisted in solution. The remark that the fluorescence of these dyes is
