J. Phys. Chem. 1993,97, 49164925
4916
Electrochemistry and Electronic Spectra of Cyanine Dye Radicals in Acetonitrile J. R. Lenbard' and A. D. Cameron Imaging Research Laboratories. and Research Technical and Support Services, Eastman Kodak Company, Rochester, New York 14652-3208 Received: November 6. 1992
The stability of the radicals formed during the one-electron oxidation and reduction of cationic cyanine dyes depends strongly on the type and extent of substitution in the polymethine chain of the dye. Radical persistence is greatly improved by alkyl substitution at the methine carbons to electronically stabilize the radical and/or sterically inhibit radical-radical coupling reactions. Within a family of related dyes, radical dication half-lives range from to lo4s in acetonitrile. Electronic absorption spectra that were recorded for 25 electrochemically generated cyanine radicals indicate the main absorption band of the radical to be hypsochromically shifted from that of the parent dye molecule. Most cyanine radical dications also exhibit weak absorption in the nearinfrared region. The spectral results are in agreement with INDO molecular orbital calculations.
Introduction Cyaninedyes have found diverse applicationdue to their unique chemical and photophysical properties. These dyes are most widely known for their practical use as photosensitizers for photographic silver halides and inorganic semiconductor materials.l.2 Cyanine dyes have also functioned as model systems for studies of photoactivated electron transport in solution and in interfacial monolayer assemblies.3 In the biological areas, cyanineshave served as probes for the physical state and membrane potential of liposomes and synthetic bilayers4and as photoactive antiviral and antitumor agent^.^ In addition, a number of dyes from the cyanine class have been utilized as saturable absorbers for the Q-switching and mode-locking of lasers and as absorbing media in optical recording system^.^^^ Cyanine radical ions are often formed as initial products or as intermediary speciesduring the course of photoinitiated electrontransfer reactions. In applications involving a cationic cyanine dye as a heterogeneous photosensitizer, the initial product of the photoredox reaction is dependent upon the energetic positions of the dye redox levels relative to the valence and conduction band energies of the substrate material.8 For dyes with redox levels that are higher in energy than the valence and conduction band edges of the substrate, the sensitization reaction proceeds with the optically excited dye functioning as an electron injector. The initial dye product of photosensitization under these conditions has been demonstrated to be the corresponding cyanine radical d i ~ a t i o n .Alternatively, ~ in cases where the redox levels of the dye are poised lower in relative energy than the substrate band edges, sensitization can occur by a hole-injection mechanism whereby the singly reduced (neutral radical) dye intermediate would be produced.1° In homogeneoussystems, cyanines can sensitize the formation of initiators for photopolymerizationprocesses via an alkyl b o r a t e dye ion pair. A neutral cyanine radical has been shown to be a transient product of the dyeborate electron-transfer reaction.' I Cyanine radical ions are also formed in nonaqueous media during the deactivation of electronically excited cyanine triplet states.12 In the absence of foreign electron donors or acceptors, reduced dye (neutral radical) was shown to be generated via triplet-triplet disproportionationand by interaction between triplet and groundstate molecules. In the presence of certain electron donor or acceptor molecules the quenching of the carbocyaninetriplet state is sometimes accompanied by the production of neutral-radical and radical-dication species, respectively.
* Address correspondence to this author at the Imaging Research Laboratories. 0022-3654/93/2097-4916$04.00/0
Although the reactivity of these radicals can play a crucial role in determining the overall efficiency of cyanine dye-sensitized electron-transfer processes, relatively few studies have been directed toward understanding the structure and chemical reactivity of these species.I3J4 In this paper, the dication and neutral radicals derived from a series of symmetrical, cationic cyanine dyes are characterized by absorption spectroscopy, electrochemistry,and INDO semiempirical molecular orbital calculations. It will be shown that the neutral and dication radical forms of a dye exhibit quite parallel structureproperty relationships with regard to radical-radical coupling reactions. The persistenceof the cyanine radical depends on the extent of delocalization and on steric hindrance to dimerization.
Experimental Section A. Materials. Acetonitrile (CH3CN, MCB spectrograde) was dried over 4-A molecular sieves (Kodak Laboratory Chemicals, baked at 400 "C). Tetrabutylammonium tetrafluoroborate (TBABF4,Kodak Laboratory Chemicals)was recrystallized three times from ethanol/water, vacuum dried at 50 OC, and stored in a desiccator. Ferric chloride hexahydrate (FeC13,Kodak Laboratory Chemicals), ascorbicacid (Kodak Laboratory Chemicals), and methanol (Kodak Laboratory Chemicals) were used as received. Cyanine dyes were synthesized in the Dye Research Laboratory of Eastman Kodak Co. Dye salts obtained as halides were converted to their corresponding perchlorate or p-toluenesulfonate (pts) salts via recrystallization from water/methanol containing NaC104 or by ion exchange on an Amberlyst-A26 (Alfa) anion-exchange resin. The A26 resin was washed thoroughly with methanol and converted to a pts-exchange material by passing saturated aqueous sodiump-toluenesulfonate (Kodak, technical grade) through a packed column until the eluent was chloride-free. Dye salts were put on and eluted from the pts-cycled column in methanol (Kodak, spectrograde). B. Electrochemistry. AC voltammetry and slow-scan cyclic voltammetry experiments were performed by using a Princeton Applied Research Corp. (PAR) 173 potentiostat in conjunction with PAR Model 175 universal programmer, PAR 179 digital coulometer, and 124A Lock-in amplifier. A Hewlett-Packard Model 239A low-distortion oscillator was used in AC measurements. Formal oxidation and reduction potentials were obtained via phase-selectivesecond-harmonicAC voltammetry as described previously.l5 Current-voltage curves were recorded on a HewlettPackard Model 7045A X-Y recorder. The potentiostat used for fast-scan voltammetry experiments was constructed according 0 1993 American Chemical Society
Spectra of Cyanine Dye Radicals to the design described by Wightman.16 Data were collected with a Nicolet 4094C digital oscilloscope and transferred to an IBM PC for analysis by using a locally written ASYST-based program. Solutions for voltammetric examination contained 0.1 M TBABF4 and ca. 5 X lo4 M dye and were deaerated with argon prior to examination. A two-compartment,three-electrode voltammetry cell was utilized. Contact between the reference electrode and working electrode compartments was made with a luggin probe. Working electrodes were Pt disk (Bioanalytical Systems, 1.6-mm and 10-pm diameter) and were polished with 1-pm diamond paste (Buhler Metadi) or 0.05-pm alumina, rinsed with water, and dried before each experiment. All potentials were measured vs the NaC1-saturated calomel electrodeat 22 OC and converted to the Ag/AgCl reference by adding 40 mV. C. UV-Vis Near-IR Spectroscopy. Solutions of dyeradical ion were prepared by controlled-potential coulqmetrywith a PAR 337A (three-compartment)coulometriccell system equipped with a 67-cm2area Pt gauze working electrode (electrolysis time for >98% oxidation is ca. 2 min). For radical dications with a halflife under 2 min, the cell used was a tubular electrochemical flow-cell of design similar to that reported by Miner and Kissinger17except that the working electrodewas compressed Pt gauze (Fisher). In all electrolysis experiments the working electrode was poised at a potential 200 mV beyond the reversible oxidation or reduction potential of the dye. The electrolyzed solutions were rapidly pumped through 0.8-mm inner-diameter Teflon tubing to a 1-mm or 10-mm path length quartz spectral flow-cell (Helma) for absorption measurement. For unstable radicals the spectra were recorded while solutions were flowing through the electrolysis and spectral cells. Spectra for stable radicals and all kinetic measurements were performed under conditions of no-flow. For a few dyes, radical solutions were also prepared by chemical oxidation by addition of a slight excess of a ferric chloride/acetonitrile solution to a dilute (ca. 10-6 M) solution of dye. Spectra were obtained with a Varian 2400 UVvis-near-IR Spectrophotometer or with a Hewlett-Packard 8450A diode array spectrophotometer. D. Molecular Orbital Calculations. Initial dye geomtries were obtained by using PCMode1.I Fully optimized (RHF) structures were computed with MOPACi9 by using the MNDO/pm3 parameter set. Unpaired electron densities in the highest singly occupied molecular orbital of radical ions were calculated by using the INDO/rohf approximation.20 Optical transition energies, oscillator strengths, and orbital symmetries were calculated for the thiadicarbocyaninedyes containinga hydrogen substituent on the nitrogen of the benzothiazole nucleus. All calculations were performed by using a Solbourne processor Model 830-32 and an IBM RS/6000 Model 550 computer.
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4917
0.40
-0.48
-0.34
0.52
\0.14
0.40
-0.52
0.06
0.34
0.07
-0.14/
N
N 0.28
\-0.15
N 0.23
-0.36
-0.44
0.54
0.11
0
-0.44
0.28
-0.11
w
w
0.49
-0.49
0.15/
N 0.36
-0.23
Figure 1. Diagram for electrondistributionin SOMO of radicaldications derived from simple cyanine, carbocyanine, dicarbocyanine, and tricarbocyanine dyes.
dimethylindole nucleus. These positions of high radical density areconsistent with the relativedistributionof r-electron densities in the HOMO of the parent dye in its ground state. Cyanine radical dications have been shown to be susceptible to radical-radical dimerization at the even-methine carbon atoms of the polymethine chain.I3 For the vast majority of cyanine dyes, particularly those containing no methine chain substituents, this dimerizationreaction is rapid and irreversible. Deprotonation and further oxidation of the dimer can also occur under certain conditions, which leads to an overall "ECE-type" electrodereaction and formation of a fully oxidized, tetracationic bis-dye species. This cyanine-radical dimerization chemistry is described by eqs 1-4. -le-
dye'
* dye'+2
+le-
-
2 d ~ e " ~ dimer+4
(2)
-2H+
dimer+4
bis-dye+2
(3)
-2e-
bis-dye+2 + b i ~ - d y e + ~
(4)
2e-
Results and Discussion A. Oxidation. Electrochemistry. The one-electron oxidation of a cationic cyanine dye initially yields the correspondingradical dication. The electron density distribution in the singly occupied molecular orbitals (SOMO) of radicals of various methine chain length were calculated by using MOPAC. Dyes and corresponding radical species containing one, three, five, and seven methine carbon atoms are referred to as simple cyanine, carbocyanine,dicarbocyanine,and tricarbocyanine,respectively. Figure 1 schematically depicts the SOMOs for radicals derived from the series of symmetrical dyes derived from the benzothiazole nucleus. The odd-electrondensity at a given atom is proportional to the square of the orbital coefficient and is represented in Figure 1 by the size of the circle. For each radical dication, the electron density in the SOMO is seen to be symmetrically distributed and found to reside primarily on the even-numbered carbon atoms of the polymethine chain (c8, C1o, (212, and C14) and to a lesser extent on the nitrogen atoms. Analogous results are obtained for symmetricaldyes containing the benzoxazole, quinoline, and 3,3-
Subtle changes in dye structure that increase the steric congestion at the sites of radical couplingcan dramaticallyreduce the rate of the initial dimerization reaction (eq 2). Effective substituentmodifications can involveeither the polymethine chain or the heterocyclic nuclei. These structure-reactivity effects are readily demonstrated by using cyclic voltammetry. Figure 2 shows, for example, cyclic voltammogramsobtained in CH3CN/ TBABF4 for the oxidation of a series of related dyes that vary in chain length and substitution pattern. An unsubstituted, simple thiacyanine dye (Figure 2a) undergoes an irreversible oxidation process at 1.2 V vs Ag/AgCl. The oxidation reaction, labeled I, was found to be chemically irreversible in experiments using potential scan rates ranging from 20 mV/s to 10 V/s. A small reduction wave I1 is also observed during the cathodic portion of the voltammetric scan at 0.84 V. Process I1 is associated-with the reversible reduction of the oxidized form of the bis-dye product (eq 4). By increasing the rate of potential scan to 200 V/s, the dimerization reaction can be outrun and the cyclicvoltammogram assumes the features characteristic of a reversible one-electron
4918 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
TABLE I:
b A
a
Lenhard and Cameron Spectral Data for Cyanioe Radical Dications in
Acetonitrilel
15
10
05
I
0
V vs SSCE
1.5
10
I110
402
10 1
388.744
39. I4
2
1.569
414
I59
462.m
8.1.1.3
1
IC&
436
14
399,.
21..
4
1.120
57.6
4.1
4%. 590
2.6,0.36
J
1.113
Jn
19
W,.
4.F.
6
0.731
614
52
462..
1.5%.
1
1125
5J8
13 I
194.94
7.0. 1.2
8
0716
522
18.9
Iu). lo2
6.6. 1.2
9
0 482
664
/ IJ
166.-
4.2.-
1211
Mo
21.0
Jcn. Bm
6.8. 0.61
I1
0 745
578
I J.0
412.7%
8.2.0.39
I2
0 511
572
19.9
458. 118
11.8,0.21
13
0.811
494
131
430,116
51,071
I4
06J2
6M
13 3
536.811
61.14
I5
1.096
582
26 0
484.780
44%
16
1.294
t a
8.8
555,.
39..
I7
0.491
614
161
512.W
106,0.81
I8
0 425
M8
24 0
498.872
I 4 I . 0.56
19
014
634
21 8
J12.954
91.091
20
0.183
8Op
20 1
541.827
9 S , 0 12
0.430
lo8
28.6
528. 885
10 1. 0.32
0.50s
162
29 6
w.w
111.047
I
0
V vs SSCE
T
IS
n
I
d
C
05
I
I
10
05
0
V vs SSCE
I
-05
15
10
05
0
v vs SSCE
Figure 2. Cyclic voltammograms recorded at Pt in CH3CN/0.3 M TBABFd: (a) [dye] = 3.4 mM, Y = 0.2 V/s, 1.6-mm-diameterelectrode, S = 20 PA; (b) IO-pm-diameterelectrode, Y = 200 V/s, S = 20 nA; (c) [dye] = 3.0 mM, 1.6-mm-diameterelectrode, Y = 0.2 V/s, S = 10 pA; (d) [dye] = 0.8 mM, 1.6-mm-diameterelectrode, Y = 0.2 V/s, S = 2.5 rA*
oxidation reaction, curve b (Figure 2). These data indicate that the radical dication half-life under the specified conditions is ca. 25 ms. Similar behavior is noted for an analogous carbocyanine dye, as given by the voltammetric response of Figure 2c. Due to the increased length of the methine chain, the one-electron oxidation, process I, for this dye, occurs at a potential 0.47 V lower than that observed for the simple thiacyanine dye. Irreversible coupling of the carbocyanine radical dications occurs at the equivalent 8and 10-methine carbon atoms. The dimer derived from the thiacarbocyanine dye is not as readily deprotonated under the conditions of the experiment and the voltammetric wave I1 associated with the bis-dye product (at 0.43 V) is correspondingly small. Instead, a new reduction wave I11 appears at -0.3 V due to the direct, irreversible reduction of the dimer tetracation. Voltammetric data obtained at a series of potential sweep rates indicate that the rate of dimerization of the thiacarbocyanine radical dication is much faster than that of its lower homolog. The thiacarbocyaninedye exhibits reversible cyclic voltammetry only in experimentswhere the potential sweep rate exceeds 1000 V/s.*' As shown by the cyclic voltammogram of Figure 2d (solid curve), the addition of N-C trimethylene bridging groups at the 8- and 10-methinepositions dramatically enhancescarbocyanine radical dication stability. The 60-mV separation between the voltammetricpeak potentials (wave I) and theanodic-to-cathodic current ratio of unity are diagnostic of a reversible one-electron oxidation. The current-potential rdponse of Figure 2d was found to be independent of potential scan rate from 10 mV/s to 100 V/s and the anodic peak currents are accurately proportional to the square root of the scan rate. Coulometric experiments indicate the radical dication to have a half-life of 53 min in room temperature acetonitrile. To the extent that the addition of the alkyl groups at thesespecific methine carbonsstabilizethe HOMO of the parent dye, this substitution also results in a decrease of the one-electron oxidation potential (by 0.16 V) and a shift in the spectral absorption maximum to longer wavelengths (by 24 nm) relative to that of the unsubstituted analog. Radical persistence is attributed to electronic stabilization by the alkyl substituents and to steric hindrance to dimerization.
IO
0
21
0% 22
0
u
c.,
Data obtained in CH,CN/O.I M TBABF4; E,, is reported vs Ag/ AgCI; extinctioncoefficientemaxisin M-' cm-I. i-bu represents isobutyl. Radical is unstable, this value is a lower limit.
Other combinations of N-C and C-C alkylene bridging and chain-alkyl substitution can be used to stabilize cyanine radical dications. Table I shows chemical structures of dyes that form persistent radical dications in acetonitrile solution and exhibit
Spectra of Cyanine Dye Radicals
reversiblecyclicvoltammetryat nominal (0.1 V/s) potentialsweep rates.22 The dyes included in this series range from the simple cyanine to the tricarbocyanine class and encompass oxidation potentials E,, that vary from 0.18 to 1.57 V vs Ag/AgCl. As indicated by the variety of molecular structures represented in Table I, the type and extent of substitution necessary to afford a persistent radical depends on the length of the polymethine chain and on the nature of the dyes' heterocyclic nuclei. Except for the simple cyanines, MOPAC/pm3 calculations indicate that most of the Table I dyes exist in a relatively flat, all-trans configuration with the atoms of the heterocyclic nuclei and the polymethine bridge approximately coplanar. Intramolecular alkylene bridges are most effective at stabilizing the radical dication but only when they incorporate the even-methinecarbons. Simple alkyl substitution within the methine chain alone is generally not effective at preventing radical-radical coupling reactions from occurring. Cyclic voltammetry indicate, for instance, that the radical dication from an 8-ethyl-10-ethyl thiacarbocyanine analogue of dye 11was found to be particularly reactive, despite the strategic placement of the chain alkyl groups within the molecule. Dimerization in these systems requires a certain degree of methine bond rotation as the reaction site undergoes the necessary sp2 to sp3 rehybridization. N-C and C-C alkyl bridges can restrict rotation within the methine chain and thereby enhance radical persistence. For dyes containing the 2-quinoline, substituted-imidazole, or 3,3'-dialkylindole nuclei, steric interactions involving the two heterocycles, or the heterocycleand the hydrogens of the adjacent methine carbon atom, can also affect the molecular geometry and thereby influence both the rate and predominant siteof radical coupling. The effect of this "residual crowding" can be quite pronounced for the shortest chain length dyes, e.g., dyes 2,4,7, and 8, and their radical i0ns.23.~~ X-ray crystallographic data confirm that the symmetrical simple cyanines derived from these heterocycles are distinctly nonplanar. Dye 2, for example, was shown to have a 5 5 O angle between the two heterocyclic ring systems.25 In comparison, analogous dyes derived from benzothiazole or benzoxazole are nearly planar, with angles of less than 1So between heterocycle planes. Spectroscopically, this twisting about the central methine carbon in the crowded dyes results in lower extinction coefficientsand shifts in the maximum absorbance to longer wavelengths. As expected, the more persistent radical dicationsare obtained from dyes with the bulkier N-alkyllaryl or C-alkyl/aryl groups. At 10-4 M concentrations in acetonitrile, the radical derived from the N,N'-diphenylsubstituted benzimidazolocarbocyanine,dye 8 ( t l / 2= 4 min), is 105 times more stable than the corresponding radical dication for an N,N,N,N-tetramethyl-substituted analogue ( t ' / 2= 1 ms). The steric interactions in these crowded dyes are not entirely relieved by increasing the methine chain length. Carbocyanine dyes 7, 8, and 10 form persistent radical dications because of interactions that impede coupling at methine carbons 8 or 10. In corresponding chain-unsubstituted dicarbocyanine and tricarbocyanine dyes, however, dimerization can occur at the unhindered, even-methinecarbon atoms 10 and 12, respectively. Table I1 summarizes these effects by comparing the relative stabilities for homologous series of cyanine radical dications derived from indole, imidazole, and quinoline nuclei. For the 3,3'-dialkylindole family of dyes the reactivity of the respective radicals varies over 6 orders of magnitude. The dyes of Table I are also capable of undergoing a second oxidation reaction at applied potentials 0.5-0.7 V more positive than that required for removal of the first electron. The cyclic voltammogram of Figure 2d (dashed curve) shows the second anodic wave IV, which corresponds to the monoelectronic oxidation of the radical dication to the trication. For dye 11, and all other dyes examined, the formation of this trication was chemically irreversible. No voltammetric peak associated with
The Journal of Physical Chemistry, Vol. 97,No. 19, I993 4919
TABLE Ik Relative Stabilities for a Homologous Series of Cvanine Radical Dications in Acetonitrile'
X
n 0 1 2
3 0 1
0 1
C(Me)2 C(W2 C(Me)2 C(Me)2 NEt NEt C H 4 H CH-CH
R Et Et Me Me Me Me Et Et
Y
1112, Sb
H H H H 5,6-C1 5,6-C1 H H
2.1 x 102 2.1 x 102 5x104 1 x 10-4 1.5 x 102 1 x lo-' 6 X 10-I 5 x 10-5
a All solutions contain 0.1 M TBABF4 and ca. 5 X 104 M dye or radical dication. b Radicals with half-lives ill2 > 10 s were determined spectroelectrochemically. Other values were estimated from cyclic voltammetric data.
-.
trication dication reversion was observed on potential scan reversal in experiments using potential scan rates of up to 1000 V/s. Attempts to use much higher potential scan rates to outrun the follow-up chemistry were unsuccessful as this second anodic wave became distorted due to the effects of slow charge transfer. Absorption Spectra. Solutions of cyanine radical dications weregeneratedat a Pt electrodeby controlled potentialcoulometry using either stirred-solution or tubular-flow methodology. The tricarbocyanine radical dications 21 and 22, which contain evenmethine carbon atoms that are not completely hindered toward dimerization, were stable only for a few seconds at the concentrations (10-4 M) most convenient for electrolysis. Solutions of dye 21 and 22 radical dications were best prepared at extreme dilution by chemical oxidation. Figures 3-5 compare absorption spectra for solutions of select dyes from Table I that were recorded before and after exhaustive one-electron oxidation. The parent cationic dyes exhibit the spectral features that are characteristic of cyanines as a class. The prominent absorption band for the planar, symmetrical dye is a relatively narrow, intense r+r* absorption of the ]Al IB1 type. Distinct shoulders displaced ca. 1370 cm-' hypsochromic from the absorption maximum are evident in the spectra for most dyes and belong to the 0-1' vibronic transition. The frequency of this displacementhas been associated with the C = C and C-N stretching vibrations in the conjugated chain of the dye chromophore.26 The values for the molar extinction coefficients for these dyes range from 2 X lo4to 26 X lo4 M-I cm-I. Increased chain length and chromophore rigidization result in higher molar extinction values. In addition to the main spectral band, these dyes also exhibit one or more relatively weak absorption bands in the 300400-nm range. The radical dication form of a cyanine dye is also an intensely colored speciesand exhibitsseveralabsorption bands in the visible, UV, and near-infrared regions of the spectrum. The principal absorption band for the radical is hypsochromicallyshifted relative to the main band of the parent chromophoreand typically appears in 400400-nm region. Absorption maxima and extinctionvalues are listed in Table I. The molar extinction coefficients e, of the principal band of the radical dications of Table I are typically about one-half that observed for the parent dye molecule. The shape of the absorption envelope of the radical dication closely resembles that of the parent dye, except for a small shift (100200 cm-1 in the position of the 0-1' vibronic shoulder. The similarity in the spectral bandshapes supports the conclusion that there are no gross differencesin the overall structure (bond lengths, molecular conformation,etc.) between dye and radical ion. These bandshape similaritiescan be expected,particularly for the longer chain length dyes, as the loss of one electron is not expected to have a significant effect on bond order. Moreover, the molecular
-
4920 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
Lenhard and Cameron
Wavelength (nm) Figure3. Absorption spectra recorded before (- - -) and after (-) one-electron oxidation of simple cyanine dyes in CH,CN/O. 1 M TBABFd. Absorbance scales are arbitrary; see Table I for relative extinction values.
geometries for many of the dyes of Table I are fairly rigid, which would minimize structural deformations. The shape and position of this main optical absorption band for the radical dication suggests that it is associated with transitions involving the r-SOMO and lowest unoccupied r* molecular orbitals (LUMO). Other less prominent absorption bands appear in UV (340400 nm) and far-red/near-IR (600-1000 nm) regions of the radical dication spectra and their relative intensities are seen to be a function of molecular structure. The relatively weak bands that occur at wavelengths longer than that of the principal band of the radical or parent dye molecule appear to have several components. The spacing of the bands suggests, however, that they are vibronically coupled. The progression of vibronic transitions is particularly evident in the spectrum of Figure 4d that was obtained for the radical dication of dye 13. The series of four absorption bands in the region 540-750 nm have an average spacing of 1450 cm-1, while the bands at 430 and 356 nm have shoulders that are 1400 cm-1 removed from their associated peaks. The spectra recorded for the radical dications of dyes 8, 12, 18, and 19 also exhibit strong vibronic character in the near-IR, vis, and UV regions. The relative magnitudes of the near-IR to visible spectral bands for various radicals appears to be a function of the length of the methinechain. Theradical dications from the shortchain-length dyes 1, 2, and 4 show a long-wavelength band of higher relative extinction. This may be related in part, however, to chromophore planarity, as the simple cyanine dyes involve more twisted conformations compared to their planar, rigidized carbocyanine and dicarbocyanine counterparts. No detectable long-wavelength absorption bands were observed for the radical dication of the tricarbocyanine dye 20. These absorption bands in the red/near-IR and long UV, which do not appear in the spectra of the parent dyes, likely involve optical transitions associated with the T-SOMOand lower-energy, doubly-occupied r-orbitals. Table I11 shows the wavelengths and oscillator strengths
calculated for a C2" symmetry-constrained, chain-unsubstituted thiadicarbocyanine dye and its corresponding radical dication (and neutral radical) form. These resalts are compared to the spectral absorption data for the planar, triply-bridged dye 17, for which the best experimental data are a~ailable.~'Comparison of the calculated integral intensities with experimental extinction coefficients is facilitated by the fact that the half-widths of the experimental absorption bands observed for a given dye and its radical are very similar. The two observed absorptions for the radical at 900 and 5 12 nm clearly correspond to the calculated bands at 928 and 495 nm, respectively.28 The relative extinction coefficients for the near-IR band and visible band of the radical dication also correlate quite well with the calculated oscillator strengths. The calculations also successfully predict the oscillator strength of the radical dication to be roughly one-half that of the parent dye molecule. On the basis of the data presented, we can assign the long-wavelength band of the radical to a 2B2 21A2 transition involving the SOMO and highest filled r-orbital. Accordingly, the main visible band is assigned to a 2Bz 21A2 transition involving the SOMO and LUMO orbitals.29 The absorption maxima obtained for a vinylogous series of dyes and their respective radical dications are seen to shift to longer wavelengths as the methinechain length increases. Figure 6 illustrates the dependence of absorption wavelength on chain length for the series of substituted dyes derived from benzothiazole, i.e., dyes 3, 11, 12, 17, 18, and 20. The absorption maxima for the parent cationic dyes form a nonconvergent series with an observed shift of 115nm per vinyl group. The absorption maxima associated with each of the corresponding radical dications are alsosystematically related to the chain length. The near-IR (long wavelength) and the prominent visible absorption bands of the radicals in this series shift 159 and 46 nm per vinyl group, respectively. A similar relationship between wavelength and methine chain length was found for dyes derived from the 3,3'dialkylindole nucleus.
-
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4921
Spectra of Cyanine Dye Radicals
Absorbance
RadicalDecay Kinetics. Electrogenerated radical dications of Table I dyes exhibit half-lives that range from a few seconds to severalhours in room temperature acetonitrile. The large majority of these radicals undergo a decomposition reaction that involves partial regeneration of the parent dye. Figure 7, curves a and b, displays a series of absorption spectra periodically recorded during the gradual decay of the radical dications of dyes 13 and 18, respectively. Figure 7a shows that thedecrease in absorbance due to dye 13 radical dication at 7 16,648,430, and 356 nm with time is accompanied by reformation of the parent dye at 494 nm and by appearance of a product at 310 nm. A kinetic plot of -In absorbance at 430 nm vs time was found to be linear, establishing the fact that the reaction is first-order with respect to the ion radical (t'/*= 64 min). Comparison of spectra obtained before and after complete reaction show that exactly one-half of the original dye species is regenerated in the process. It is noteworthythat four clearly defined isosbesticpoints are apparent in the spectra of Figure 7a. Similar spectral changes are obtained followingthe controlledpotential oxidation of dye 18. The data of Figure 7b indicate that dye 18 radical dication (498 nm) also reacts to regenerate dye (668 nm) and a product that absorbs in the visible region at 540 nm.30 Additional experiments indicate radical decay to be greatly accelerated by the deliberate addition of water, hydroxide ion, or amine bases to the acetonitrile. For dye 18, the rate of radical dication loss exhibits a first-order dependence on the water concentration. The eventual decay of dication radical in acetonitrile solution follows the experimental rate law:
-d[radical dication]/dr = k,,[radical
dication] [base] (5)
In addition to the regenerated parent dye, the product of radical decay was identified as a dehydrogenated derivative of dye 18.31 Product assignment was made by thin-layer chromatography, electrochemical,and spectralanalysis of spent electrolysissolutions with comparison to authentic dehydrogenated dye. The overall
stoichiometry involves the consumption of two radical dications to regenerate one dye molecule and one molecule of product. The half-regeneration mechanism detailed in Scheme I is proposed to account for the formation of the noted products and is consistent with the observed rate law and stoichiometry. This reaction scheme consists of an initial proton loss from the radical dication to form the corresponding cation radical. The latter species is more easily oxidized than its precursor and undergoes subsequent homogeneouselectron exchangewith a second radical dication to regenerate 1 equiv of the parent dye. Loss of a second proton serves to aromatize the N-C hydrocarbon ring, yielding the dehydrogenated dye. The rate-determining step is considered to be the deprotonation of the radical dication at the carbon site in the hydrocarbon ring system, which is a to the nitrogen atom. Kinetic data for the decay of other sterically hindered cyanine radical dications are listed in Table IV. Most radical decomposition reactions were found to follow first-order kinetics (over two half-lives), and the reaction rate constants were found to be sensitiveto the presenceof baseS32Collectively, these experimental observations suggest that a rate-determining deprotonation of the radical dication is a common first step in the decomposition of these monooxidizedcyanine-dye systems. The reformation of dye during radical dication decay is evidence that homogeneous electron exchange between dication radical and cation radical also occurs to an appreciable extent. As the kinetic data show, however, the relative values for the fraction of dye regenerated vary over a wide range, which clearly indicates that reaction routes other than electron exchange are available to the transient cation radical.33 In this respect, it is important to note that simple resonance theory predicts the odd-electron density in such a cyanine cation radical to preside at the odd-numbered methine carbon atoms, where dimerization reactions for the molecular structures of Tables I and IV could proceed relatively unhindered. Dimerization would yield a dicationic molecule containing two quaternary
4922 The Journal of Physical Chemistry. Vol. 97, No. 19, 1993
Lenhard and Cameron
-
'2
-
: d - Dye 20
Dye16
r
il I
I I 1 I I I
I I I I
I
I
1 1
I I I I I I I
1 1
-
1 1 I
,I
I
/ I
-
8
/
/
I
I
I I
8
'\-
200
400
600
600
800 400
1000
800
Wavelength (nm)
-
Figure 5. Absorption spectra recorded before (- -) and after (-) one-electron oxidation of dicarbo- and tricarbo-cyanine dyes in CH3CN/O.lM TBABF4. Absorbance scales are arbitrary; see Table I for relative extinction values.
TABLE iIk Calculated Optical ABsorpti0118 and Assignmeate for a Thirdicarbacyrrnkre Dye. hn*x
-
chromophore parent cation dication radical
assignment
calcd
obsd
calcdb
obsd'
' A I 'BI (HOMO + LUMO) 'B2 21Az(filled 4 SOMO) 'B2- 21A2(SOMO- LUMO) 2A2+ 21B2(SOMO -+ LUMO) *A2 4 *2B2(filled 4 SOMO)
562 928 495 923 395
674 900 512
1.91 0.1 1 1.28 0.03 0.45
1.61 0.08 1.06
+
neutral radical
intensity
494
0.65
INDO calculations were made on a Cb-constrained, chain-unsubstituted thiadicarbocyanine chromophore. Observed values are those for dye 17. Calculated oscillator strength. Observed molar extinction coefficient X los.
1
2
3
4
No. of Methine Bonds
Figure 6. Dependence of absorption wavelength on the length of the polymethine chain for benzothiazole-derivedcyanine dyes ( 0 )and their radical dications (A,0 ) .
heterocyclic units that, by analogy with similar cyanine dimers, would be expected to absorb in the 300-360-nm range.I3 The observationthat UV (300-350nm)-ahrbing products areindeed formed during the decay of dye 4, 10, 11, 13, and 19 radical dications is consistent with coupling of reactive cation radical
species. Further details concerning radical dication reaction kinetics for crowded or bridged cyanine dye systems will be presented in a separate report. B. Reduction. Cyclic Voltammetry. The cathodic voltammetric response observed for most cyanine dyes indicates a oneelectron reduction reaction that is chemically irreversible. A comparison of cyclic and AC voltammetric data for a series of dyes bearing no methine chain substituents indicates the singly reduced forms of the dyes to be generally more reactive than their radical dication counterparts. INDO molecular orbital calculations predict the odd-electron density in the reduced dye, Le., the neutral cyanineradical, to be delocalized in an alternating manner that is similar to that seen in the dication radical, except that the radical character will preside specifically at the oddnumbered carbon atoms (9,11,13, etc.) of the methine chain and at the bridging (2-position) carbon in the heterocycle ring as shown in Figure 8. The ESR results obtained by Baer and Oehling for simple cyanine and carbocyanine dyes also indicate the spin densityin theneutral radical topredominateat theodd-numbered carbons of the polymethine Structural modifications that electronically stabilize or contribute steric congestion at the odd-methineatomsofacyaninedyeareseentoresult ina reversible cyclicvoltammetry response. In fact, structurereactivity patterns
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4923
Spectra of Cyanine Dye Radicals
-0.26
0.21
0.26
0
-0.06
-0.06
0.37
0.21
0.56
-0.56
-0.37
N
N -0.17
-0.09
400
500
600
700
0.04
0
-0.03
-0.07
-0.03
0.17
0.03
-0.09
Figure 8. Diagram for electron distribution in SOMO of cyanine neutral
1
300
0.07
800
radical.
Wavelength (nm) Figure 7. Absorption spectra periodically recorded during the decay of dye 13 (curves a) and dye 18 (curves b) radical dications in CH3CN/O.l M TBABFd.
TABLE V Spectral Data for Cyanine Neutral Radicals in Acetonitnil@ SVUClURb
SCHEME I 7
Q
f
t
k
J
IS
I
El
Radical Dication
23
dye no.
[radical] X lo4 M
rxnb order
tl/*, min
1 2 4 7 8 10 11 12 13 18 19
1.4 1.1 0.5 1.5 0.6 1.1 0.5 0.9 1.1 0.8 0.7
1st 1st 1st 2nd 2nd 1st 1st 1st 1st 1st 1st
2.5 7.7 60 14 4 28 53 38 64 231 130
%dye regend
4.8 X 1.5 x 1.9 X 1.2X 1.4 X 4.2 X 1.3 X 3.1 X 1.8 X 5X 8.8 X
lW3 10-3
367,454 345,530 325 620 312 354 312 472 310 540 245,340
23 78 33 47 11 74 13 45 50 45 52
lo4 lW3 10-2 lo" lo4 lo"
Radical solutions.were prepared at the given initial concentration by exhaustive electrolysis at a potential 200 mV anodic of Eo, for the dye. Reaction order with respect to radical dication. Rate constant obtained from decay kinetic plot, in s-I or M-I s-I for first- and second-order reactions, respectively. Ratio of [regenerated dye] to preelectrolysis [dye] X 100.
for the neutral radical completely parallel those described above for the corresponding dye radical dications. Although the products of cyanine neutral radical decay have not yet been isolated or structurallycharacterized,the available electrochemicaldata are fully consistent with a decomposition pathway that initiates with radical-radical coupling. Of the several hundred cyanine dyes examined,however, only a few chromophorestructureswere seen
5.9
-0.780
582
26.0
538
-
-0.482
608
8.8
456
-
-1.040
674
18.2
494
6.5
-1.111
592
2.6
456
-
El
u
q-.yJ$p
uN+l El
product(s)
L a x
k,,h,Cs-I
430
I
q&Q v
TABLE I V Decay Kinetic Data for Selected Cyanine Radical Dications in CH3CN/TBABFd8
13.1
CH,
16
17
558
-1.OOO
q$a&Q tH,
H'
p
24
El+,+
25
A
El
-
El
-0.860
704
25.0
570
12.1
-0.762
742
21.6
508
10.0
Data obtained in CH3CN/O.1 M TBABF4; Eredis reported vs Ag/ AgCI; extinction coefficient Cmax is in M-I cm-I. i-bu representsisobutyl.
to afford persistent neutral radicals. The chemical structures, reduction potentials, and spectral data for representative dyes are listed in Table V. According to the molecular-orbital picture of Figure 8, substitutionof an electron-withdrawinggroup at the odd-methine carbonswould be expected to stabilizethe neutral radical. Cyclic voltammetry experiments indicate that the addition of a cyano or trifluoromethyl group to the 9-position of carbocyanine dyes does increase radical stability and lead to a reversiblevoltammetric response. Contrary to expectation, however, 9-chloro-substituted thiacarbocyanine dyes exhibit irreversible slow-scan cyclic voltammetry, indicating that a single methine halogen atom does not appreciably stabilize the carbocyanine radical. The 9-alkyl or 9-aryl substitutionsin carbocyanine dye systems also did not significantlymodify the reactivity of the corresponding neutral radicals, even though such substituentsare known to alter the relative populations of the mono-cis and all-trans configu-
4924 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
Lenhard and Cameron
Fipre9. Absorption spectra recorded before (- - -) and after (-) one-electron reductionofvariouscyanine dyes in CH3CN/O. 1 M TBABFd. Absorbance scales are arbitrary; see Table V for relative extinction values.
rations found at equilibrium in alcoholic solutionsfor dyes of this chain length.34 A rerr-butyl-substituted dye, 23, was the only thiacarbocyanine examined that contained the necessary steric restriction to slow radical coupling reactions to the degree that allowed measurement of radical absorption spectra. On the other hand, a number of dicarbocyanine dyes, especially those that contained hydrocarbon rings incorporating the 9- and 11-methine positions, were found to form persistent neutral radicals. The neutral radical of dye 17, which has a neopentyl group and N-C trimethylene rings, and an analogous structure containing only the neopentyl substituent exhibited half-lives of ca. 15 s in acetonitrile. Ring strain is also an important factor that affects radical stability in dyes containingN-C hydrocarbon ring systems.Cyclic voltammetry data indicate that ring strain is more important during dye reduction than during its oxidation. Dye 17, for example, which contains N-C six-membered rings, exhibits reversible cyclic voltammetry during one-electron oxidation and one-electron reduction. In contrast, dye 18,which contains N-C fivemembered rings, can be reversibly oxidized, but appears completely irreversible during one-electron reduction. Absorption Spectra. UV-vis spectra were obtained for solutions of neutral radical that were generated by using the tubular electrochemical cell at maximum flow rates (ca. 20 mL/min). Although the dye-stock solutions and flow-cell apparatus were thoroughly degassed with argon prior to and during the coulometric reduction process, no special precautions were taken to further exclude oxygen from the system. The limited lifetime experienced by the neutral radicals in this study may be due in part to the residual oxygen that was present in the acetonitrile at concentrations ( M) that in some cases were comparable to that of the radical species. Although each of the dyes of Table V gives a reversible 'slowscan-rate" cyclicvoltammetric response, the neutral radical forms were not long-lived. The most persistent neutral radicals were
obtained for those derived from the 4,4'-quinoline dye 24 and the "decalin"-bridged thiatricarbocyanine dye 25. Under the experimental conditions described here these radicals had a mean half-life under 30 s. The neutral radical forms for the remaining dyes of Table V were stable for only a few seconds. Figure 9 compares absorption spectra for solutions of select dyes from Table V that were recorded before and after one-electron reduction. Becauseof the high flow rates utilized, the electrolysis reaction was typically less than 90%efficient and the absorption spectracontained bandsduetounreduceddye. Thedataof Figure 9 represent these absorption spectra after electronic subtraction of the absorption bands of unreduced dye. The principal absorption bands for the cyanine neutral radicals of Table V lie in the visible region, between 430 and 570 nm. The extinction coefficients for dyes 24 and 25 were determined to be ca. one-half that of the parent dye. Accurateextinction coefficient data for radicals 15,16, and 23 could not be obtained due to their high reactivity. The neutral radical of 17 rapidly decayed to a product that absorbs at 404 nm in acetonitrile. As indicated in Table 111, the experimental observationsfor dye 17 are consistent with INDO calculations for a thiadicarbocyanine neutral radical. Although the substitution patterns that afford stable cyanine neutral radicals contribute little to the stability' of the corresponding radical dication species,dyes 7,16, and 17 were observed to be moderately persistent in both their one-electron-oxidized and one-electron-reducedforms. A comparison of the electronic spectra for the dication and neutral radicals of a given dye (for example, Figure 4a compared to Figure 9a and Figure 5a compared to Figure 9b) reveals a marked similarity between the main spectral features: band positions, relative intensities, and vibrational structure. In sharp contrast to the spectra for radical dications, however, no detectable spectral absorption bands were observed in the red or infrared regions for any of the neutral radicals examined. Overall, the similaritiesin the spectra between the oxidized and reduced forms for a given cyanine dye are
Spectra of Cyanine Dye Radicals consistent with a pairing of the *-electronic states. This feature is like that reported for alternant hydrocarbon systems, in which the electronic spectra of the anion radical and cation radical are nearly ~uperimposable.~~
Conclusions In this study, the interplay between electronic and steric effects on the stability and persistenceof symmetricalcyaninedye radicals is demonstrated by using electrochemical and spectroscopic techniques. Persistent radicals are obtained for molecular structures in which the dominant radical decomposition pathway, that is, radical-radical dimerization, is sterically hindered. Molecular orbital calculations indicate the primary sites for radical-radical coupling in the dication to be the even-numbered carbon atoms of the polymethine chain, whereas for the neutral radical they would be principally localized at the odd-methine carbons. This alternating feature of cyanine *-radical system results in parallel structure-reactivity patterns for the dication and neutral-radical species. In acetonitrile solution, the cyanine radical produced by an electrochemical one-electron oxidation or reduction of a cationic dye exhibits an intense absorption band in the visible region that is short-wavelength-shifted from the main band of the parent dye. In planar, rigidized cyanine radical chromophores, molar extinction coefficientsas high as 13 X 104 M-1 cm-1 are observed. In addition, most of the radical dications examined here showed a second, albeit weaker, absorption in the near-infrared. The transition wavelengths determined for a series of radical dications and neutral radicals derived from a common heterocycle were found to be systematicallyrelated to the length of the polymethine chain. Spectroelectrochemicaldata are presented for the radical forms of a wide variety of cyanine dyes with the aim of including, or closely representing, those dyes that are often employed in literature studies of cyanine dye sensitized photoredox processes. The data given here should prove useful for the identification and characterization of cyanine radical species as intermediates or products of photoinduced electron-transfer reactions.
References and Notes (1) (a) Hamer, F. M. The Cyanine Dyes and Related Compounds; Wiley: New York, 1964; pp 706-742. (b) West, W.; Gilman, P. B. In The Theory of the Photographic Process; James, T. H., Ed.; Macmillan: New York, 1977; p 277. (c) Sturmer, D. P. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Interscience: New York, 1979; p 393. (2) (a) Memming, R.; Tributsch, H. J . Phys. Chem. 1971,75,562. (b) Arden, W.; Fromherz, P. J . Elecrrochem. SOC.1980,127,370. (c) Hada, H.; Yonezawa, Y.; Inaba, H. Ber. Bunsenges. Phys. Chem. 1981,85, 425. (d) Gerischer, H. Faraday Discuss. Chem. SOC.1974, 58, 219. (e) Fujishima, A.; Watanabe, T.; Honda, K. Chem. Lett. 1975,13. (f) Sonntag, L. P.;Spitler, M. T. J . Phys. Chem. 1985, 89, 1453. (9) Spitler, M.; Parkinson, B. A. Langmuir 1986, 2, 549. (h) Sakata, T.; Hashimoto, K.; Hiramoto, M. J . Phys. Chem. 1990, 94, 3040. (3) (a)Kuhn,H.PureAppl.Chem.1979,51,341. (b)Kuhn,H.;Mobius, D.; Bucher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. I, Part IIB. (c) Biesmans,G.; Van der Auweraer, M.; Cathry, C.;Schryver, F. C.; Yonezawa, Y.;Sato, T. Chem. Phys. 1992, 160, 97. (4) Dragsten, P. R.; Webb, W. W. Biochemistry 1978, 17, 5228. (5) (a) Oseroff, A. R.; Ohuoha, D.; McAuliffe, D.; Ara, G.; Shea, C.; Foley, J.; Cincotta, L. Photochem. Photobiol. 1985,41, 752. (b) Kozai, Y.; Nomura, H.; Tashiro, T. Chem. Pharm. Bull. 1982, 30, 3106. (c) ValdesAguilera, 0.;Cincotta, L.; Foley, J.; Kochevar, I. E. Photochem. Photobiol. 1987,45, 337. (d) Yashui, S. Shikizai Kyokaishi 1987,60, 212. Sieber, F. Photochem. Photobiol. 1987,46, 1035. ( e ) Davila, J.; Harriman, A,; Gulliya, K. S. Photochem. Photobiol. 1991, 53, 1. (f) Bunting, J. R. Photochem. Photobiol. 1992, 55, 8 1. (6) (a) Sibbet, W.;Taylor, J. R.;Welford, D. IEEEJ. Quantum Electron. 1981, QE-17,500. (b) Sibbet, W.; Taylor, J. R. IEEE J . Quantum Electron. 1984, QE-20, 108. (c) Meyer, M.; Mialocq, J. C.; Perly, B. J . Phys. Chem. 1990, 94, 98. (7) Matsuoka, M. In Topics in Applied Chemistry. Infrared Absorbing Dyes; Matsuoka, M., Ed.; Plenum Press: New York, 1990; pp 19-34. (8) (a) Gerischer, H.; Willig, F.; Spitler, M. T. In Proceedings ojthe 3rd Symposium on Electrode Processes; 1979; Bruckenstein, S., Mclntyre, J. D. E., Miller, B., Yeager, E., Eds.; The Electrochemical Society: Princeton, NJ,
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4925 1980; Vol. 80-2, pp 115-135. (b) Gilman, P. B. Pure Appl. Chem. 1977,47, 357. (c) Muenter, A. A.; Gilman, P. B.; Lenhard, J. R.; Penner, T. L. Mechanistic Consequences of Anomalously Efficient Spectral Sensitization by DesensitizingDyes;The International East-West Symposiumon the Factors Influencing Photographic Sensitivity, 1984; Paper C-21. (d) Tani, T.; Suzumoto, T.; Ohzeki, K. J . Phys. Chem. 1990, 94, 1298. (9) (a) Tani, T. Photogr. Sci. Eng. 1975, 19, 356. (b) Lenhard, J. R.; Muenter, A. A. In Photoelectrochemistry and Electrosynthesis on Semiconducting Materials; Ginley, D. s., Ed.; The Electrochemical Society: Pennington, NJ, 1988; Proceedings Vol. 88-14, pp 97-104. (c) Tani, T. J . Appl. Phys. 1987, 62, 2456. (IO) (a) Sturmer, D. M.; Gaugh, W. S.; Bruschi, B. J . Photogr. Sci. Eng. 1974,18,56. (b) Saunders, V. I. Photogr.Sci. Eng. 1977,21,163. (c) Willig, F.; Eichberger, R.; Bitterling, K.; Durfee, W.S.;Storck, W.; Van der Auweraer, M. Ber. Bunsenges. Phys. Chem. 1987, 91, 869. (d) Siegel, J.; Fassler, D.; Friedrich, M.; Grossmann, J.; Kempka, U.;Pietsch, H. J. Photogr. Sci. 1987, 35, 73. (1 1) (a) Chatterjee, S.; Davis, P. D.; Gottschalk, P.; Schuster, G. B. J . Am. Chem.Soc. 1988,110,2326. (b) Chatterjee,S.; Davis,P. D.;Gottschalk, P.; Kurz, M. E.; Sauerwein, B.; Yang, X.;Schuster, G. B. J . Am. Chem. SOC. 1990, 112,6329. (12) (a) Chibisov, A. K. J . Photochem. 1976,6, 199. (b) Mialocq, J. C.; Doizi, D. Chem. Phys. Lett. 1983, 103, 225. (13) (a) Lenhard, J. R.;Parton, R. L.J. Am. Chem.Soc. 1988,109,5808. (b) Parton, R. L.; Lenhard, J. R. J . Org. Chem. 1990, 55, 49. (14) (a)Baer,F.;Oehling,H.Org.Magn.Reson. 1974,6,421. (b)Oehling, H.; Baer, F. Org. Magn. Reson. 1977, 9, 465. (c) Siegel, J.; Friedrich, M.; Kuhn, V.; Grossmann, J.; Fassler, D. J . In$ Rec. Mater. 1986, 14, 215. (15) Lenhard, J. J . Imag. Sci. 1986, 30, 27. (16) Wipf, D. 0.;Wightman, R. M. Anal. Chem. 1988, 60, 2460. (17) Miner, D. J.; Kissinger, P. T. Biochem. Pharmacol. 1979,28,3285. See also: Igo, D. H.; Elder, R. C.; Heineman, W. R.; Dewald, H. D. Anal. Chem. 1991,63,2535. (18) Geometry optimization program PCModel was developed and distributed by Serena Software, Bloomington, IN. (19) Stewart, J. J. P. J . Comp. Aided Molec. Des. 1990, 4 , 1. (20) (a) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1973. 32, 111. (b) Edwards, W. D.; Zerner, M. C. Theor. Chim. Acta 1987, 72, 347. (21) Similar findings are reported for a related dye, see: Nomura, S.; Okazaki, S. Chem. Lett. 1990, 223 1. (22) Dyes 5, 21, and 22 may undergo dimerization at the unsubstituted even-methine carbons and hence are not as stable as the other dyes in this group. At 0.1 V/s the voltammetry is quasi-reversible. (23) (a) Brooker, L. G. S.; White, F. L.; Heseltine, D. W.; Keyes, G.H.; Dent, S. G.; Van Lare, E. J. J . Photogr. Sci. 1953, 1, 173. (b) Brooker, L. G.S.; White, F. L.; Sprague, R. H.; Dent, S. G.; Van Zandt, G. Chem. Rev. 1947, 2, 325. (24) Residua) crowding as defined in ref 23 refers to steric interactions that strictly involve the heterocycle and the hydrogens of the methine chain. (25). (a) Sturmer, D. M.; Gaugh, W. S. Photogr. Sci. Eng. 1975,19,273. (b) Smith, D. L.; Barret, E. K. Acta Crystallogr. 1971, 278, 969. (26) West, W.; Lovell, S. R.; Cooper, W. Photogr. Sci. Eng. 1970,14,52. (27) By useof the MOPAC/pm3 geometriesand thestandardspectroscopic parameterization in the INDO program package, the calculated absorption maxima for the parent dye molecules are systematically shorter than the observedvalues. For a homologous seriesof dyes a plot of observedvs calculated values is linear. (28) Calculations on a geometry-optimized dye 17 structure did not correlate with experimental data as well as thosecalculated by using standard bond lengths and bond angles on the symmetry-constrained molecule. (29) The symmetry assignments in Table 111for the optical transitions of the dication and neutral radicals of a C,,dicarbocyanine structure also apply to those transitions in simple cyanines. For corresponding optical transitions in C,,carbocyanine and tricarbocyanine radicals, the symmetries are reversed; *A2 21B2for the radical dication transitions and 'B? '1A2 for the neutral radical transitions. (30) The extent of dye regeneration is a function of the water impurity level of the electrolyte solution. In acetonitrile dried over molecular sieves, which contains water at the millimolar level, the percent dye regeneration is near 40% and deviation from first-order kinetics is observed after one half-life. (31) Authentic material was prepared by G. Proehl of these laboratories by reaction of dye 18 with dicyanodichlorobenzoquinone and characterized by NMR and mass analysis. The dehydrogenated derivative (A,,,,, = 534 nm; t = 3.5 X lo4M-I cm-l in methanol) exhibits a reversibleone-electronoxidation at 0.83 V vs SSCE in acetonitrile. (32) A few of these highly strained radicals, e&, 17, have been shown to undergo reversible dimerization. At room temperature the dimerization equilibrium lies far to the side of the radical dication. (33) Simple radical dication disproportionation seems unlikely for most of these dyes since the equilibrium constant, IC,.. as calculated from the peak potential data of Figure 2d, is about 1 X lo-". (34) West, W.; Pearce, S.; Grum, F. J . Phys. Chem. 1967, 71, 1316. (35) (a) Aalbersberg, W. I.; Hoytink, G.J.; Mackor, E. L.; Weijland, W. P. J. Chem. SOC.1959, 3049. Sioda, R. E. J . Phys. Chem. 1968, 72, 2322. (b) Distler, D.; Hohlneicher, G. Ber. Bunsenges. Phys. Chem. 1970, 74,960. (c) Shida, T.; Iwata, S. J. Am. Chem. SOC.1973, 95, 3473.
-
-
