Tuning the optoelectronic properties of cyanine and ketocyanine dyes by incorporation of 9,9-di-n-propylfluorenylindolenine

Article abstract of DOI:10.1016/j.dyepig.2013.09.003

The synthesis and properties of novel cyanine and ketocyanine dyes incorporating 9,9-di-n-propylfluorenyl units is reported. While bathochromic shifts of order 100 nm are observed in cyanines for addition of each vinylene unit (lengthening the methine cha

Full text of DOI:10.1016/j.dyepig.2013.09.003

Accepted Manuscript
Tuning the optoelectronic properties of cyanine and ketocyanine dyes by
incorporation of 9,9-di-n-propylfluorenylindolenine
R.D. Jansen-van Vuuren, P.C. Deakin, S. Olsen, P.L. Burn
PII:
S0143-7208(13)00328-8
10.1016/j.dyepig.2013.09.003
DYPI 4077
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 16 August 2013
Accepted Date: 4 September 2013
Please cite this article as: Jansen-van Vuuren RD, Deakin PC, Olsen S, Burn PL, Tuning
the optoelectronic properties of cyanine and ketocyanine dyes by incorporation of 9,9-di-n-
propylfluorenylindolenine, Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.09.003.
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ACCEPTED MANUSCRIPT
Graphical abstract
18

ACCEPTED MANUSCRIPT
Tuning the optoelectronic properties of cyanine and ketocyanine dyes by incorporation of 9,9-
di-n-propylfluorenylindolenine
Author names and affiliations: R. D. Jansen-van Vuuren,a P. C. Deakin,b S. Olsen,a,c P. L. Burna,b
aCentre for Organic Photonics & Electronics (COPE), The University of Queensland, 4072
Queensland, Australia
bDepartment of Chemistry, Chemistry Research Laboratory, University of Oxford, Mansfield Rd,
OX1 3TA Oxford, United Kingdom
cSchool of Mathematics & Physics, The University of Queensland, 4072 Queensland Australia
bCorresponding author. Tel: +61 7 3346 7614
Email addresses: p.burn2@uq.edu.au (P. L. Burn)
Abstract:
The synthesis and properties of novel cyanine and ketocyanine dyes incorporating 9,9-di-n-
propylfluorenyl units is reported. While bathochromic shifts of order 100 nm are observed in
(keto)cyanines for addition of each vinylene unit (lengthening the methine chain) it was observed
that addition of the fluorenyl unit enabled fine tuning of the UV-visible absorption spectrum. The
hypsochromic shift in the UV-visible absorption spectrum of the ketocyanines relative to the
corresponding cyanine dyes was found to be consistent with the Dewar-Knott colour rule. The
oxidation ('Highest Occupied Molecular Orbital energy') and reduction ('Lowest Unoccupied
Molecular Orbital energy) potentials were also qualitatively consistent with the Dewar-Knott colour
rule, with the 'HOMO energies' of the comparative dyes being the same and the LUMO energies of
the ketocyanines being destabilised relative to the cyanine dyes.
Keywords: Cyanine, Ketocyanine, Organic Semiconductor, Fluorenyl, Photodiodes, Solar Cells
Highlights
•
Cyanine and ketocyanine dyes have been synthesized incorporating a fluorenyl unit within
the indolenine end-group.
•
Inclusion of the 9,9-di-n-propylfluorenyl units enables fine-tuning of the optoelectronic
properties of cyanine and ketocyanine dyes.
1

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•
The absorption properties of the dyes follow the Dewar-Knott colour rule with the
electrochemical properties of the materials experimentally confirming the theory.
1. Introduction
The development of organic semiconductors has experienced remarkable progress over the past
decade [1] as their use in a range of applications including photovoltaic (PV) cells [2,3], light-
emitting diodes [4], thin-film transistors [5,6], and organic photodetector (OPD) arrays for image
sensing [7-11] has been realized. Organic semiconductors offer many potential benefits for these
applications including the prospect of simple processing methods (e.g., solution deposition), the
possibility of lightweight, thin, and flexible devices, and the existence of a wide selection of organic
materials, which allows for tuning of the physical and optoelectronic properties [12,13].
Recent reports have highlighted the incorporation of organic dyes in small molecule organic
PV (OPV) [14-16] and OPD devices [10,17,18] due to their strong light absorption and good
solution processability. One class of dye that has received attention with respect to OPVs are the
cyanines, a group of charged polymethine dyes [19-22]. A strong emphasis of current OPV research
has been on the development narrow optical gap materials, often called 'low band-gap' materials,
that are designed to absorb as much of the solar spectrum as possible [23,24]. The creation of
cyanine dyes with a narrow optical gap is therefore a core objective for organic PV applications,
with a particular focus on materials that absorb in the Near-Infrared (NIR) [25,26]. The long-
wavelength absorption peak of cyanines is typically controlled through the number of methine
linkages, with a red shift in the peak wavelength of ~100 nm per additional methine group generally
observed.[21] However, the stability of cyanine dyes decreases with an increase in the polymethine
length [27], with the methine bridges being susceptible to oxidation.
Ketocyanines are the neutral cyanine equivalents, which have been studied as the light
absorbing materials in OPDs for visible image-sensing applications [11]. The advantage of having a
non-charged dye is that counterion migration, which can degrade device performance [21,28], is
avoided. Counterion migration is a significant issue for OPDs, which are typically operated under a
reverse (negative) bias [29]. An interesting feature of the ketocyanine dyes is that they generally
have a narrow full-width-half-maximum, which is important to achieve spectrally selective colour
sensors [30, 31]. However, a second requirement for spectrally selective colour sensors is that the
absorption maximum of each sensor needs to be at different and specific wavelengths [32]. As in
the case of the cyanine dyes, the wavelength selectivity can also be achieved at a coarse level by
increasing the number of methine units although the same issues of stability arise as the number of
double bonds increases.
2

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An alternative strategy for tuning the absorption of cyanine and ketocyanine dyes is to
extend the conjugation of the indolenyl moiety of the dyes. Thus, in this manuscript we describe the
synthesis and optoelectronic properties of fluorenylindolenine cyanine 2 and fluorenylindolenyl
ketocyanine 4 and compare them with their parent cyanine and ketocyanine dyes, 1 and 3,
respectively (Figure 1). A further advantage of the ‘fluorenyl moiety’ is that the n-propyl chains
attached to the bridgehead benzyl-position improve the solubility of the dyes. Finally, while the
structure-property relationships of cyanine dyes have been studied [33], this work will impart
valuable information for predicting the properties of the less well-known ketocyanines.
2. Experimental
2.1. General information
Unless otherwise noted, all chemicals were obtained from commercial suppliers (Sigma Aldrich,
Alfa Aesar, Unilab) and used as received. Molecular sieves (3Å and 4Å) were activated for use by
heating at 250 °C under reduced pressure (10-1-10-2 mbar) for 24 h. All solvents were distilled prior
to use. Acetonitrile was dried over anhydrous potassium carbonate for 24 h, dried over 3Å
molecular sieves for 24 h before distilling onto and storing over 4Å molecular sieves. Pyridine was
dried by refluxing over calcium hydride for 30 min followed by fractional distillation onto activated
4Å molecular sieves. Tetrahydrofuran was freshly distilled over sodium benzophenone ketal.
Column chromatography was performed with Kieselgel 60 230-400 mesh silica purchased from
Merck (cat. number 1.09385.9025). Materials that were purified with >5% methanol in chloroform
mixtures were further dissolved in the minimum dichloromethane and passed through a plug of
Celite to remove any dissolved silica gel. Thin layer chromatography (TLC) was performed on
aluminium plates coated with silica gel 60 F254. 1H and 13C NMR spectroscopy was performed on
Bruker Avance AV-300, AV-400 or AV-500 MHz spectrometers: AR H = aryl H; IND H =
indolenyl H; VIN H = vinyl H; br = broad; CP H = cyclopentanone H; FL H = fluorenyl H; PR H =
n-propyl H. Coupling constants are given to the nearest 0.5 Hz. UV-visible spectroscopy was
performed on a Cary 5000 UV-Vis spectrophotometer as a solution in spectroscopic grade solvent:
sh = shoulder. FT-IR spectroscopy was performed on solid samples using a Perkin-Elmer Spectrum
100 FT-IR Spectrometer with an ATR attachment. Melting points (Tms) were measured in a glass
capillary on a Buchi B-545 melting point apparatus and are uncorrected. Microanalyses were
carried out on a Carlo Erba NA 1500 Elemental Analyser. Matrix-assisted laser-desorption
ionisation time of flight (MALDI-ToF) mass spectrometry were carried out on a Voyager DE STR
MALDI-ToF spectrometer from dithranol (as matrix) in positive reflectron mode. Electrospray
ionisation mass spectrometry was performed on a BRUKER HCT 3D Ion Trap mass spectrometer.
Glass transition temperatures (Tg) were determined using a Perkin-Elmer Diamond Differential
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Scanning Calorimeter. Thermal stability was determined by Thermal Gravimetric Analysis (TGA)
under a nitrogen atmosphere using a Perkin-Elmer STA 6000 Simultaneous Thermal Analyser.
Thermal decomposition values were reported as the temperature corresponding to a 5% sample
mass reduction [Td(5%)].
2.2. Electrochemical Measurements
Electrochemistry was performed on an Epsilon Basi C3 Cell Stand with a glassy carbon working
electrode, platinum counter electrode and silver reference electrode in 0.01 M silver nitrate in
acetonitrile. Cyanines: oxidations and reductions were measured in fresh dry-distilled acetonitrile.
Samples (1 mM) were dissolved with 0.1 M tetra-n-butylammonium tetrafluoroborate as the
supporting electrolyte and degassed for 15 min with argon immediately prior to the electrochemical
measurements. Ketocyanines: Oxidations were measured in freshly distilled dry dichloromethane
[distilled over calcium hydride (5% w/v) under nitrogen] and reductions were measured in freshly
double distilled dry tetrahydrofuran (first distilled over sodium/benzophenone, then from lithium
aluminium hydride). Samples (1 mM) were dissolved with electrochemical grade tetra-n-
butylammonium perchlorate (0.1 M) (Alfa Aesar cat. number 043999) as the supporting electrolyte,
and degassed for 15 min with argon immediately prior to the electrochemical measurements. E
½
values are reported relative to the ferrocenium/ferrocene standard [34] which was measured on the
same day using the same solvent and measurement conditions as the respective samples. Where the
cyclic voltammetry showed that the redox processes were chemically reversible but lacked a clearly
defined Epa or Epc, Differential Pulse Voltammetry (DPV) was used to determine the E1/2.
2.3. Organic Synthesis
2.3.1. 1,1',3,3,3',3'-Hexamethylindotricarbocyanine iodide (1)
Triethylamine (190 ꢀL, 1.36 mmol) was added to a mixture of 1,2,3,3-tetramethyl-3H-
indolium iodide (211 mg, 0.70 mmol), N-(5-anilino-2,4-pentadienylidene)aniline hydrochloride
(100 mg, 0.35 mmol) and acetic anhydride (130 ꢀL, 1.38 mmol) in methanol (10 mL), and the
resultant mixture deoxygenated by three vacuum-nitrogen purging cycles. The reaction was then
stirred at room temperature in the dark for 16.5 h followed by removal of the volatiles in vacuo. The
crude material was taken up in the minimum amount of dichloromethane (<10 mL) and precipitated
into ether (200 mL). The precipitate was isolated by centrifugation (3 min at 3000 rpm) followed by
decantation of the supernatant and drying. The precipitate was collected and further purified by
column chromatography over silica using methanol/chloroform (1:9) as eluent followed by removal
of the solvents and recrystallization from an ethanol/petroleum mixture to give 1 as a green
4

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1
crystalline solid (77 mg, 41%), which had an identical H NMR spectrum and ES-MS as the
literature [35].
2.3.2. Di-tert-butyl-1-(9,9-di-n-propyl-9H-fluoren-2-yl)hydrazine-1,2-dicarboxylate (6)
n-Butyllithium (1.6 M, 6.4 mL, 10 mmol) was added slowly to a stirred solution of 9,9-di-n-
propyl-2-bromofluorene, 8 (2.8 g, 8.5 mmol) in tetrahydrofuran (50 mL) at -78 °C, under nitrogen.
After stirring for 30 mins, di-tert-butylazodicarboxylate (4.31g, 18.6 mmol) dissolved in
tetrahydrofuran (50 mL) was added to the reaction mixture. After stirring at -78 °C for 2 h, the
mixture was allowed to warm to room temperature and stirred for a further 18 h. Glacial acetic acid
(1.81 mL) was added slowly and the mixture stirred for a further 4 h at room temperature. Ether (50
mL) and water (50 mL) were added. The aqueous layer was separated and extracted with ether (2 x
60 mL). The combined organic extracts were then washed with brine (60 mL), dried over
magnesium sulphate, filtered and the solvent completely removed. The crude material was purified
by column chromatography over silica using dichloromethane as eluent to give 6 as a white
amorphous powder (2.42 g, 64%); Td(5%) 220 °C; Tg 50 °C; (Found C, 72.35%; H, 8.35%; N, 5.9%.
C29H40N2O4 requires C, 72.5%; H, 8.4%; N, 5.8%); νmax (solid)/cm-1 1705 (C=O); λmax (THF) 247
nm sh (log ε/dm3 mol-1 cm-1 3.57), 253sh (3.75), 281(4.28), 289sh (4.24) , 306 (4.13), 313sh (4.05);
δH (400.1 MHz, CDCl3) 0.65 (10 H, brm, PR CH2 & CH3), 1.50 (18 H, brs, 2 x tert-Bu), 1.92-1.98
(4 H, brm, PR CH2), 6.63 and 6.84 (tautomers) (1 H, br s, N-H/O-H), 7.26-7.44 (5 H, m, FL H),
7.61-7.68 (2 H, m, FL H); δC (100.6 MHz, CDCl3) 14.4, 17.1, 28.0, 28.15, 28.17, 42.7, 55.3, 81.5
(br), 82.1, 119.4, 119.5, 122.8, 126.7, 126.8, 138.7 (br), 140.5, 141.4 (br), 150.9 (br), 153.8 (br),
155.4 (br) (the doubling up of peaks is due to the different tautomers); m/z [ESI: 5% DCM, MeOH]
found 479.4 (100%), 480.3 (32%), 481.3 (6%), 482.2 (1%); C29H40N2O4 requires 480.30 (100%),
481.30 (32%), 482.30 (1%), 482.31 (5%).
2.3.3. 2-(9,9-Di-n-propyl-9H-fluoren-2-yl)hydrazin-1-ium chloride (7)
A solution of concentrated hydrochloric acid (4.1 mL) in 1,4-dioxane (12.5 mL) was added
to a solution of 9 (2.0 g, 4.1 mmol) in 2-propanol (22.7 mL). The reaction mixture was heated at 85
°C for 1 h. After cooling to room temperature, the solvent was completely removed to give 7 as a
pink amorphous solid (1.2 g, 95%); mp 172-175 °C; (Found C, 72.0%; H, 8.1%; N, 8.6%.
C19H25ClN2 requires C, 72.0%; H, 7.95%; N, 8.8%); νmax (solid)/cm-1 3324 (NH), 3220-2500 (NH
and CH); λmax (EtOH) 306sh nm (log ε/dm3 mol-1 cm-1 4.30), 316 (4.35), 336sh (4.28), 349sh
(4.01); δH (400.1 MHz, DMSO-d6) 0.48-0.65 (10 H, m, PR CH2 & CH3), 1.81-1.97 (4 H, m, PR
CH2), 6.91 (1 H, dd, J = 8 Hz, J = 2 Hz, FL H), 7.04 (1 H, d, J = 2 Hz, FL H), 7.20-7.28 (2 H, m,
FL H), 7.37 (1 H, d, J = 8 Hz, FL H), 7.66-7.70 (2 H, m, FL H), 8.28 (1 H, br s, NH), 10.14 (3 H, br
5

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+
s, NH3 ); δC (100.6 MHz, DMSO-d6) 14.2, 16.9, 42.0, 54.8, 109.3, 113.4, 119.1, 120.3, 122.8,
126.3, 126.9, 134.5, 140.4, 145.1, 149.6, 151.5; m/z [ESI: 5% DCM, MeOH] found: 281.1 (100%),
+
282.1 (34%); C19H25N2 requires 281.2 (100%), 282.2 (21%), 283.2 (2%).
2.3.4. 2,3,3-Trimethyl-9,9-di-n-propyl-3,9-dihydroindeno[1,2-f]indole (8)
Concentrated sulphuric acid (1.4 mL) was added dropwise to a mixture of 7 (1.22 g, 3.86
mmol) and 3-methylbutan-2-one (1.25 mL, 11.6 mmol) in ethanol (28 mL). The reaction mixture
was heated at 85 °C for 48 h. After cooling, the solvent was removed and the residue was taken up
in diethyl ether (50 mL). The organic layer was washed with aqueous potassium hydroxide (5%
w/v, 50 mL), and the aqueous layer was extracted with diethyl ether (2 x 50 mL). The combined
organic extracts were dried over magnesium sulphate, filtered and the solvent removed. The residue
was purified by column chromatography over silica using dichloromethane as eluent to give 8 as a
viscous yellow oil (721 mg, 56%); (Found C, 86.7%; H, 8.8%; N, 4.3%. C24H29N requires C, 87.0;
H, 8.8; N, 4.2); λmax (THF) 224sh nm (log ε/dm3 mol-1 cm-1 4.36), 235 (4.18), 243 (4.15), 253sh
(4.04), 259sh (4.12), 291 (4.40), 299 (4.39), 311 (4.34), 318 (4.26), 325 (4.32); δH (400.1 MHz,
CDCl3) 0.63-0.70 (10 H, m, PR CH2 & CH3), 1.37 (6 H, s, C(CH3)2), 1.93-2.00 (4 H, m, PR CH2),
2.30 (3 H, s, CH3), 7.24-7.34 (3 H, m, FL H), 7.48 (1 H, brs, FL H), 7.57 (1 H, d, J = 0.5 Hz, FL H),
7.66 (1 H, m, FL H); δC (100.6 MHz, CDCl3) 14.4, 15.5, 17.1, 23.4, 42.8, 53.2, 55.0, 112.5, 114.5,
119.1, 122.8, 126.4, 126.6, 138.3, 141.1, 144.8, 150.9, 151.1, 153.5, 187.9; m/z [ESI: 5% DCM,
MeOH] found 332.2 (100%), 333.2 (26%), 334.2 (5%); C24H29N requires 331.2 (100%), 332.2
(26%), 333.2 (3%).
2.3.5. 1,2,3,3-Tetramethyl-9,9-di-n-propyl-3,9-dihydroindeno[1,2-f]indol-1-ium iodide (9)
A mixture of 8 (111 mg, 0.34 mmol) and iodomethane (0.25 mL, 4.02 mmol) in acetonitrile
(5 mL) was heated under nitrogen at 85 °C for 22 h. The reaction mixture was cooled to room
temperature before the solvent and excess iodomethane were completely removed in vacuo. The
residue was sonicated with diethyl ether (15 mL) for 5 min before the solvent was decanted and the
residual solid dried to give 9 as a yellow/green amorphous powder (150 mg, 95%); mp 235-237 oC;
(Found C, 63.2%; H, 6.8%; N, 2.95%. C25H32IN requires C, 63.4%; H, 6.8%; N, 3.0%); λmax (THF)
225sh nm (log ε/dm3 mol-1 cm-1 4.14), 237 (3.95), 253sh (3.74), 261 (3.72), 268sh (3.75), 298sh
(3.93), 320 (4.29), 337 (4.28), 382sh (3.59); δH (300.1 MHz, CDCl3) 0.60-0.73 (10 H, m, PR CH2
& CH3), 1.72 (6 H, s, C(CH3)2), 1.99-2.07 (4 H, m, PR CH2), 3.19 (3 H, s, CH3), 4.39 (3 H, s,
NCH3), 7.38-7.41 (3 H, m, FL H), 7.56 (1 H, brs, FL H), 7.73-7.77 (1 H, m, FL H), 7.79 (1 H, brs,
FL H); δC (100.6 MHz, CDCl3) 14.3, 17.1, 17.3, 23.3, 37.4, 42.5, 54.1, 56.0, 109.7, 114.0, 120.4,
6

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127.3, 128.7, 138.7, 140.9, 143.7, 151.4, 153.1, 194.8; m/z [ESI: MALDI ToF: DITH] found 346.2
(100%), 347.2 (61%), 348.2 (11%). C25H32N+ requires 346.2 (100%), 347.3 (27%), 348.3 (4%).
2.3.6. 1,3,3-Trimethyl-9,9-di-n-propyl-2-[(1E,3E,5E,7E)-7-(1,3,3-trimethyl-9,9-di-n-propyl
indeno[1,2-f]indol-2{1H,3H,9H}-ylidene)hepta-1,3,5-trien-1-yl]-3,9-dihydroindeno[1,2-f]indol-1-
ium iodide (2)
Triethylamine (90 ꢀL) was added to a stirred solution of 9 (150 mg, 0.32 mmol), N-[5-
(phenylamino)-2,4-pentadienylidene]aniline monohydrochloride (45 mg, 0.2 mmol) and acetic
anhydride (0.90 mL, 9.5 mmol) in methanol (4.5 mL). The reaction mixture was deoxygenated by
three vacuum-nitrogen purging cycles, and left to react at room temperature for 23 h under nitrogen,
before the solvent was removed. The crude product was precipitated by dissolving in
dichloromethane (8 mL) and adding this solution to diethyl ether (60 mL), before centrifuging the
suspension at 3000 rpm for 15 min. The supernatant was decanted and the residue dried to give 2
(48 mg, 34%) as a copper-orange coloured solid; Td(5%) 233 °C; UV-vis (THF): λmax (THF) 280 nm
(log ε/dm3 mol-1 cm-1 4.81), 366 (4.06), 402 (3.97), 416 sh (3.78), 475sh (3.54), 736sh (4.74), 803
(5.07); δH (400.1 MHz, CDCl3) 0.60-0.70 (20 H, m, PR CH2 & CH3), 1.74 (12 H, brs, C(CH3)2),
1.91-2.03 (8 H, m, PR CH2), 3.73 (6 H, brs, N-CH3), 6.28 (2 H, br d, VIN H), 6.68 (2 H, br m, VIN
H), 7.02 (2 H, s, FL H), 7.27-7.36 (6 H, m, FL H), 7.60 (2 H, s, FL H), 7.65-7.68 (2 H, m, FL H),
7.80 (3 H, br m, VIN H); m/z [ESI: 5% DCM, MeOH] found: 753.5 (100 %), 754.5 (58%), 755.5
+
(17%), 756.5 (4%). C55H66N2 requires 753.5 (100%), 754.5 (60%), 755.5 (18%), 756.5 (4%).
2.3.7. (2E,5E)-2,5-Bis[(E)-2-(1,3,3-trimethyl-9,9-di-n-propylindeno[1,2-f]indol-2-{1H,3H,9H}-
ylidene)ethylidene]cyclopentanone (4)
A solution of 9 (102 mg, 0.22 mmol) and enaminocyclopentanone 10 (21 mg, 0.1 mmol) in
pyridine (2.1 mL) was heated at reflux for 1.5 h. The reaction mixture was then allowed to cool to
room temperature before being diluted with water (40 mL), filtered and the residue dried in vacuo.
The crude product was purified using column chromatography over silica using hexane:ethyl
acetate mixtures (1:0 to 1:4 to 2:3) as eluent to give 4 as a purple/gold solid (107 mg, 25%); Td(5%)
330 °C; Tg 145 °C; νmax (solid)/cm-1 1567 (C=O); λmax (THF) 277 nm (log ε/dm3 mol-1 cm-1 5.15),
346br (4.50), 420sh (4.17), 454sh (4.47), 492 (4.85), 527 (5.08); δH (300.1 MHz, toluene-d8) 0.61-
0.85 (20 H, m, PR CH2 & CH3), 1.67 (12 H, s, C(CH3)2), 1.85-2.00 (8 H, m, PR CH2), 2.66 (6 H, s,
N-CH3), 2.79 (4 H, m, CP H), 5.34 (2 H, d, J = 13 Hz, VIN H), 6.50 (2 H, s, FL H), 7.18-7.53 (10
H, m, FL H), 8.23 (2 H, d, J = 13 Hz, VIN H); m/z [ESI: 5% DCM, MeOH] found: 795.6 (100.0%),
796.6 (60.5%), 797.6 (21.0%) (MH+). C57H66N2O requires 794.5 (100.0%), 795.5 (62.4%), 796.5
(19.3%), 797.5 (4.0%).
7

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3. Results and Discussion
3.1. Synthesis
Indotricarbocyanine 1 was synthesised in a 41% yield using a procedure that has been successfully
used for shorter cyanine dyes [36] (Scheme 1). The synthetic route to ketocyanine dye 3 has
previously been reported [37]. The strategy used to obtain fluorenylcyanine 2 and
fluorenylketocyanine dye 4 is shown in Scheme 2, with the salt 9 the common intermediate.
Transmetallation of 2-bromo-9,9-di-n-propylfluorene 5 with n-butyllithium, and subsequent
reaction of the anion formed with di-t-butylazodicarboxylate (DBAD) provided the N-t-
butoxycarbonyl (t-BOC)-protected hydrazine 6 in a 64% yield. Deprotection via acid hydrolysis
resulted in the formation of the hydrazine hydrochloride 7 in a yield of 95%. The Fischer-Indole
method [38] facilitated the ring-closure and formation of fluorenylindolenine 8 (56% yield), which
was subsequently methylated to form the iodide salt 9 in a 95% yield. To form the fluorenylcyanine
2
the iodide salt
9 was reacted with N-[5-(phenylamino)-2,4-pentadienylidene]aniline
monohydrochloride whereas to form 4, 9 was reacted with enaminocyclopentanone 10. 2 was
purified by precipitation and isolated in a 34% yield, whilst chromatography over silica resulted in
1
the isolation of 4 in a 25% yield. It is important to note that the H n.m.r. spectrum of 4 was more
1
complicated than expected for the all trans product. The complexity of the H n.m.r. spectrum was
similar to that observed for the parent chromophore 2, with the extra signals arising from different
geometric isomers [11]. In addition, small signals were sometimes observed in the aldehydic region,
indicating that care had to be taken to avoid hydrolysis of the ketocyanines.
3.2. Properties of the materials
The first stage in the analysis of the materials was determination of their thermal properties by
Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA). TGA was
undertaken at a scan rate of 50 °C/min under nitrogen and the decomposition temperature was taken
as the temperature corresponding to a 5% weight loss [Td(5%)]. The ketocyanines were found to be
more stable than the cyanines with decomposition temperatures approximately 100 °C higher [1
(203 °C) and 2 (233 °C) versus 3 (345 °C) and 4 (330 °C)]. The greater stability of the ketocyanine
is directly related to the presence of the cyclopentanone unit rather than the conjugated vinylene
units in the cyanines. It is interesting to note that the extension of the indolenyl unit did not change
the thermal stability significantly. DSC analysis (scan rate 100 °C/min) showed that only 4 had a Tg
(145 °C), which was more than 100 °C below the melting point of the parent dye 3.
8

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We next investigated the optical properties of the materials with the solution (tetrahydrofuran)
absorption spectra of 1-4 shown in Figure 2. The short wavelength features at around 350 nm for
the dyes are due to π-π* transitions of the arylindolenyl unit (a fluorenyl unit absorbs at 250-320 nm
[39]). The strong longer wavelength absorptions observed for both sets of compounds are reported
to be associated with a donor-acceptor Intramolecular Charge Transfer (ICT) transition that occurs
over the polymethine-bridge. For 1 and 2 (the cyanines), the donor and acceptor refer to the two
nitrogen centres: for each resonance contributor, one of these will be positively charged, therefore
acting as the acceptor, whilst the other nitrogen is the donor, and a ‘push-pull’ feature enables the
ICT to occur [33]. In the case of the ketocyanines 3 and 4, the ICT takes place from the amine
donor to carbonyl acceptor, and the shoulder observed at 480-490 nm is due to interaction of the
two chromophores within the bichromophoric system [40]. For the long wavelength ICT
absorptions in moving from indolenyl to the fluorenylindolenine, the Full Width at Half Maximum
(FWHM) increases for the cyanine dyes by ~30 nm, from 66 nm for 1 to around 95 nm for 2, while
for the ketocyanines dyes 3 and 4 the FWHM is essentially the same at 64-65 nm. It is important to
note that these FWHM are sufficient to enable imaging with colour constancy in photodiode
applications [32].
However, in both series in moving from the indolenyl to the fluorenylindolenine moiety,
there is a red shift in the absorption maxima – from 743 nm to 802 nm for 1 and 2, and 502 nm to
527 nm for 3 and 4. The red shift between cyanines 1 and 2 and the ketocyanines 3 and 4 caused by
the introduction of the fluorenyl 'end group' is the same at 0.12 eV. The addition of an extra vinyl
moiety within the cyanine chromophore changes the absorption maxima bathochromically by
approximately 100 nm [41, 42], that is, for a parent dye absorbing at 743 nm, an additional vinyl
unit would be expected to reduce the optical gap by 0.2 eV. The observed red shift by the addition
of the fluorenyl 'end group' is significantly smaller than expected for an equivalent physical
extension of the charge conjugation path of the parent cyanine chromophore, and hence the results
show that modification of the indolenyl unit enables fine-tuning of the absorption wavelength. In
contrast, there is a large difference between the cyanine and ketocyanine chromophores that have
nominally the same number of methine carbons between the two nitrogen atoms (most easily seen
when the ketocyanine is drawn in the enolate form with a positive charge on one of the nitrogens).
Comparing the change in the optical absorption wavelength in energy units (eV) between the
cyanine 1 (λmax = 1.67 eV) and the ketocyanine 3 (λmax = 2.47 eV) yields a shift of 0.80 eV that can
be attributed to the influence of the oxy substituent upon the excitation energy of the underlying
cyanine 1. The same shift was also observed between cyanine 2 (λmax = 1.55 eV) and ketocyanine 4
(λmax = 2.35 eV). These observations indicate that the effect of the oxy substituent in the middle of
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the methine chain (termed the meso-position in the chain) and the fluorenylindole end-groups are
each affecting local, independent and additive changes in the spectra of the dyes studied. The
hypsochromic shift observed between dyes 1 and 3 (equivalently, between 2 and 4) is close to the
range of energy shifts that are observed in chemically similar cases. For example, it is somewhat
smaller than the shift obtained upon comparison of Michler’s Hydrol Blue [λmax = 615 nm (2.02 eV)
in glacial acetic acid [43] with Michler’s Ketone [λmax = 372 nm (3.33 eV) in the same solvent [44],
and is similar in size to the shift between Michler’s Hydrol Blue and Auramine-O [λmax = 441 nm
(2.81 eV), also in glacial acetic acid [44]. The observation of a hypsochromic shift upon substitution
at the meso-position in the chain with a strongly electron donating oxy-substituent is consistent with
the Dewar-Knott colour rule [45, 46]. The Dewar-Knott colour rule is based on the structure of the
Hückel orbitals of an odd-alternate hydrocarbon reference species [45] (the cyanine and
ketocyanines of this work have five 'double bonds'). The rule has recently been shown to also be
consistent with more modern electronic structure calculations for a series of dyes similar to those
studied here [47]. In this analysis, the highest occupied orbital (HOMO) of the reference has a node
at the meso-position, while the lowest unoccupied orbital (LUMO) does not. Substitution at the
meso-position in the chain with electro-donating groups therefore destabilizes the LUMO while
leaving the HOMO energy unchanged. If the electronic excitation is modelled as a one-electron
excitation between these levels, this leads to the prediction of a blue shift, which is observed
experimentally in this work. Thus, for the UV-visible absorption spectra of the dyes it can be
concluded that the extension of the heterocyclic end-groups to include a fluorenyl moiety leads to a
bathochromic shift that is the same for both cyanine and ketocyanine cases studied. The magnitude
of this shift is significantly smaller than the bathochromic shift that is often observed in cyanines
upon the lengthening of the polymethine chain (around 100 nm per vinylene unit [41, 42]). This
argues against the interpretation of the bathochromic shift arising from the fluorenyl 'end groups'
being due to the extension of the effective conjugation length of the polymethine. Instead, the shift
can be explained by relative stabilization of the resonating charge at the chain termini (the iminium
cation) by interaction with the polarizable fluorenyl π-electron system. A similar mechanism has
been invoked by Spasokukotskii et al. to explain bathochromic shifts of similar size for cyanines
containing indolyl, benzoxazolyl and thiazolyl end-groups [48].
In a final aspect of the analysis, the redox properties of the materials were studied by cyclic
voltammetry (CV) and, where necessary, Differential Pulse Voltametry (DPV) in solution. Both
reduction and oxidation measurements for 1 and 2 were carried out in acetonitrile and, in both
cases, referenced against the ferrocenium/ferrocene couple. In all cases, four scans were carried out
and no reduction in the current was observed, indicating that the redox processes were chemically
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reversible. The cyclic voltammograms for the oxidative processes of 1 and 2 are shown in Figure 3.
The E1/2s for the first oxidation of 1 and 2 were resolved with DPV and found to be -0.2 V for 1 and
0.0 V for 2. The oxidation of the iodide counteranion has been previously reported to occur at
around 0.4 V [49] and is therefore masked by the oxidative processes of the dyes. The E1/2s for the
first reduction of 1 and 2 were also resolved with DPV and found to be at -1.1 V for 1 and -1.0 V
for 2. The E1/2s for the first oxidation and reductions of 3 and 4 were 0.0 V and -0.1 V (oxidations)
and -2.2 V and -1.3 V (reductions), respectively (see Figure 4). Thus the electrochemical results
qualitatively support the Dewar-Knott rule in that the oxidation potentials ('HOMO energy levels')
are essentially the same for the cyanine and ketocyanine dyes while the 'LUMO energy' is
destabilised (reductions at more negative potentials) for the ketocyanine dyes relative to the cyanine
chromophores. That the electrochemical data are consistent with the Dewar-Knott rule in this way is
a significant result. It suggests that the electrochemical and photophysical processes can be
described by a common compact set of frontier orbitals. There is no obvious a priori reason to
expect this because the photophysical process is electron number conserving while the
electrochemical process is not.
4. Conclusion
Cyanine and ketocyanine dyes incorporating 9,9-di-n-propylfluorenyl units into end groups of the
chromophores have been synthesized. The thermal, optical and electrochemical properties were
investigated experimentally and compared with the parent chromophores. It was found that
inclusion of the fluorenyl unit only had a small influence on the thermal properties of the dyes but
led to a red shift in the absorption spectra compared with the parent dyes, thus enabling fine-tuning
of the optical properties. The origin of the shift was understood to arise from stabilization of the
resonating charge at the chain termini by interaction with the polarizable fluorenyl π-electron
system. However, the largest difference in the absorption spectrum was observed between the
equivalent cyanine and ketocyanine chromophores with the latter showing a strong hypsochromic
shift. The hypsochromic shift could be explained by the Dewar-Knott colour rule and
electrochemical analysis supported the explanation, with the HOMO energies of the equivalent dyes
being the same with the LUMO energies of the ketocyanines being destabilised relative to the
cyanines.
5. Acknowledgements
We thank EPSRC for financial support (Project number EP/E01075X/1). Professor Paul Burn is UQ
Vice Chancellor's Senior Research Fellow. Seth Olsen acknowledges the support of the Australian
Research Council through Discovery Project DP110101580 and the award of an Australian
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Research Fellowship. We also acknowledge the University of Queensland for providing funding for
Ross Jansen-van Vuuren (UQRS and UQIRA) and Brasenose College, University of Oxford for
funding Peter Deakin (Senior Germaine Scholarship).
Figures and Schemes
Figure 1. Chemical structures of cyanine 1, fluorenylcyanine 2, ketocyanine 3 and
fluorenylketocyanine 4.
Figure 2. Normalised solution absorption spectra of 1, 2, 3 and 4 (in THF).
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Figure 3. (a) Oxidation and (b) reduction cyclic voltammograms of 1 and 2, obtained with scan
rates of 100 mV/s (oxidation of 1 and 2, and reduction of 2) and 50 mV/s (oxidation of 2).
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Figure 4. (a) Oxidation and (b) reduction cyclic voltammograms of 3 and 4, obtained with scan
rates of 100 mV/s (oxidation of 4 and reduction of 3 and 4) and 200 mV/s (oxidation of 3).
Scheme 1. Synthetic route to cyanine 1.
Scheme 2. Synthetic routes to fluorenylcyanine 2 and fluorenylketocyanine 4.
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Highlights
•
•
•
Cyanine and ketocyanine dyes containing 9,9-di-n-propylfluorenyl moieties are
synthesized.
The 9,9-di-n-propylfluorenyl units enable tuning of the optoelectronic properties of the
dyes.
The absorption properties of the dyes follow the Dewar-Knott colour rule.