Synthesis and spectral properties of near-IR polymethine dyes derived from tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-2,4,7-trisulfonate

Article abstract of DOI:10.1016/j.dyepig.2017.03.039

A vinylogous series of anionic polymethine (cyanine) dyes has been synthesized based on tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-2,4,7-trisulfonate. It has been found that their longest-wavelength and most intense electronic transition has the p

Full text of DOI:10.1016/j.dyepig.2017.03.039

Accepted Manuscript
Synthesis and spectral properties of near-IR polymethine dyes derived from
tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-2,4,7-trisulfonate
Irina V. Kurdyukova, Alexander A. Ishchenko, Dmitriy D. Mysyk
PII:
S0143-7208(17)30136-5
10.1016/j.dyepig.2017.03.039
DYPI 5866
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 20 January 2017
Revised Date: 16 March 2017
Accepted Date: 17 March 2017
Please cite this article as: Kurdyukova IV, Ishchenko AA, Mysyk DD, Synthesis and spectral properties
of near-IR polymethine dyes derived from tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-2,4,7-
trisulfonate, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.03.039.
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Synthesis and spectral properties of near-IR polymethine dyes derived from
tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-2,4,7-trisulfonate
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Irina V. Kurdyukova *, Alexander A. Ishchenko , and Dmitriy D. Mysyk
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Institute of Organic Chemistry, National Academy of Sciences of Ukraine,
Murmanskaya str. 5, Kiev, 02660 Ukraine
E-mail: irina.kurdiukova@gmail.com
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Donetsk National Technical University, Artema str., 58, Donetsk, 83001 Ukraine
E-mail: mysyk@yandex.ru
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A vinylogous series of anionic polymethine (cyanine) dyes has been synthesized based on
tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-2,4,7-trisulfonate. It has been found that their
longest-wavelength and most intense electronic transition has the polymethinic origin, unlike the
analogous polynitrofluorene dyes. As established, the corresponding absorption band, in contrast to
that of classical cyanines, remains quite intense and narrow in the near-IR spectral region even in
highly polar solvents. The nature of electronic transitions and relevant molecular orbitals as well as
excitation-induced electron density redistributions have been studied by the DFT and TDDFT
methods at the CAM-B3LYP/6-31G(d,p) level.
Keywords
Tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-2,4,7-trisulfonate,
anionic
polymethine dyes, Vis/NIR absorption and fluorescence spectra, DFT and TDDFT calculations,
molecular orbitals, electronic transitions.
Introduction
Most of the near-IR polymethine dyes contain heterocyclic moieties as end groups [1-5]. If their
electron-donor ability deviates upwards or downwards from the median value, the charge
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distribution between the polymethine chain and end groups becomes less uniform [6]. As a result,
dye solvation is enhanced due to static interactions between chromophore charges and solvent
dipoles [6, 7]. This causes, in turn, a weakening and broadening of the polymethine dye absorption
[6]. The afore-mentioned loss in charge-distribution uniformity drastically increases as the
polymethine chain is lengthened [6] and, therefore, it is problematic to achieve a highly intense and
selective absorption with long-chain polymethines, especially in highly polar solvents. This
limitation is quite typical of near-IR absorbing polymethine dyes, as their long-wavelength
absorption is mostly attributed to the long polymethine chromophore [3, 6].
Previously we reported the synthesis of deeply coloured anionic and zwitter-ionic polymethine
dyes, with electron-acceptor substituted fluorene nuclei as end groups, which showed nearly solvent
polarity-insensitive absorption bands [8-11]. The nitro group was initially chosen as an electron-
acceptor substituent but the corresponding dyes suffer from a number of shortcomings limiting their
application. When introduced into the dye chromophore, the nitro group gives rise to additional
close-lying energy levels and, accordingly, to electronic transitions with strongly overlapping
absorption bands [10] which cause a decrease in intensity and selectivity. The loss of these typically
polymethinic superior spectral properties finding diverse applications [1, 5, 12-14] takes away much
of the practical value of nitro-substituted dyes. Besides, the polymethine dyes derived from
polynitrofluorene are poorly soluble in standard solvents. All this makes it challenging to synthesize
fluorene dyes containing other strong electron acceptors instead of nitro groups.
The octafluoropentoxysulfonyl group, –SO OCH (CF ) CHF , has shown much promise in the
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chemistry of fluorene-derived cyanines. It displays a remarkable electron-withdrawing ability by
both inductive (σ = 0.67) and resonance (σ = 0.21) effects (cf. to the nitro group with σ = 0.58 and
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σR= 0.20), being the second strongest electron acceptor known, after polyfluoroalkyl sulfones such
as the trifluoromethansulfonyl group, CF SO , with σ = 0.73 and σ = 0.31) [15]. As a result, this
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substituent affords milder reaction conditions and higher stability of the corresponding polymethine
dyes [10]. Moreover, it is easily NMR and MS identifiable, and its amphiphilic properties provide a
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good dye solubility in solvents of wide-ranging polarity. The polymethines with fluorene end
groups, each bearing two octafluoropentoxysulfonyl substituents [9], were found to exhibit single-
band UV\Vis absorption spectra.
Though the 9-methylene-group reactivity in 2,7-di(octafluoropentoxysulfonyl)-9H-fluorene is
significantly increased by the two very strong electron acceptors, it proved, however, insufficient to
perform the cyanine condensation under standard reaction conditions. It is only in the presence of
strongly basic condensing agents such as DBU that the reaction proceeds [9]. To avoid the use of
harsh bases threatening the stability of the resulting dyes, it appears necessary to increase the CH
acidity of the fluorene substrate even more, e.g., by introduction of another electron-acceptor
substituent. To this end, we address here a synthetic route to tris(2,2,3,3,4,4,5,5-octafluoropentyl)-
9H-fluorene-2,4,7-trisulfonate as well as the preparation and study of the corresponding symmetric
anionic dyes.
Experimental
Electronic absorption spectra were recorded on a Shimadzu UV-3100 spectrophotometer
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(optical path length 1 cm) for solutions with a solute concentration of 1⋅ 10 mol L . Fluorescence
spectra were recorded on a Solar CM 2203 spectrofluorometer. Solvents were purified according to
known procedures [16]. Dichloromethane was stabilized by the addition of 1% anhydrous ethanol.
For column chromatography silica gel 60 (Fluka) and alumina 100 (Merck) were used. The purity of
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the dyes was monitored by TLC on Silufol UV-254 plates with acetonitrile as eluent. H NMR
spectra were recorded on a Varian VXR-300 spectrometer (299.945 MHz) with TMS as internal
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standard. C NMR spectra were recorded on a Bruker Avance III spectrometer (500 MHz).
Melting points (uncorrected) were determined in open capillaries.
The Gaussian 9.0 program package [17] was employed for quantum chemical DFT/CAM-
B3LYP/6-31G(d,p) calculations with the preliminary ground-state geometry optimization using the
same basis set. The wavelengths and oscillator strengths of electronic transitions were calculated by
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the TDDFT method. Theoretical spectra were simulated taking into account the electronic transitions
with the oscillator strength above 10% of its highest value in the spectrum.
The calculations were performed using the computational facilities of the joint computational
cluster of the SSI "Institute for Single Crystals" and the Institute for Scintillation Materials of the
National Academy of Sciences of Ukraine incorporated into the Ukrainian National Grid.
Synthetic procedures
9H-Fluorene-2,4,7-trisulfonyl trichloride (1)
Step А Potassium 9H-fluorene-2,7-disulfonate
A mixture of fluorene (5 g, 30 mmol) and concentrated sulfuric acid (d = 1.84 g/mL, 12 mL,
0.23 mol) was stirred under heating in an oil bath maintained at 120-125 °С. The mass started to
harden in about 10 minutes. The bath temperature was raised until the mass started melting and the
molten mass was stirred for 30 minutes to avoid overheating. After cooling, water (55 mL) was
added portionwise until the reaction mass was completely dissolved. Then the solution was carefully
neutralized with an aqueous solution of potassium carbonate, heated to boiling, and filtered. The
filtrate was left in ice water. The precipitate of potassium 9H-fluorene-2,7-disulfonate was filtered
off and washed first with a 1% aqueous solution of potassium chloride (50 mL) and then with cold
water (10 mL). Potassium 9H-fluorene-2,7-disulfonate was recrystallized from a 1% aqueous
solution of potassium chloride, using 6 mL of solution per gram of salt. After cooling down to room
temperature, the reaction mass was further cooled in ice water. The precipitate was filtered off and
washed with ice water (10 mL). The salt was recrystallized from water to give a maximum yield of
88% [18].
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Potassium 9H-fluorene-2,7-disulfonate (6.0 g, 14.91 mmol) was added to chlorosulfonic acid
(30 mL, 0.45 mol); potassium chloride impurities do not affect the chlorosulfonation [18]. The
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solution was heated for 3 h in an oil bath maintained at 115-125 С. After cooling, the excess of
chlorosulfonic acid was destroyed by adding dropwise, with stirring, cold concentrated sulfuric acid
(30 mL), and the resulting mass was poured onto ice (400 g). The sulfonyl chloride precipitate was
filtered off and thoroughly washed first with water and then with 2-propanol (10 mL). The product
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obtained (6.0 g), with mp. 199-202 С, was recrystallized from 1,4-dioxane (25 mL). Yield 4.8 g
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(70.7%). Mp. 205-206 С.
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H NMR (CD CN, 300 MHz), δH / ppm: 4.45 (2H, s, -CH -), 8.39 (1H, d, J=8.4 Hz, Ar-H),
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8.52 (1H, s, Ar-H), 8.75 (1H, s, Ar-H), 8.82 (1H, s, Ar-H), 9.00 (1H, d, J=8.4 Hz, Ar-H). Found: C,
33.70; H, 1.50; Cl, 22.90; S, 20.71%; molecular formula C H Cl O S requires C, 33.82; H, 1.53;
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Cl, 23.03; S, 20.83 %
Tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-2,4,7-trisulfonate (2)
To a solution of freshly distilled pyridine (0.35 mL, 4.35 mmol) in 2,2,3,3,4,4,5,5-
octafluoropentanol (8 mL, 0.06 mol) was added 9H-fluorene-2,4,7-trisulfonyl trichloride 1 (0.50 g,
1.08 mmol). The reaction mass was heated for 1 h in an oil bath maintained at 90-100 °С. After
cooling, the resulting solution was diluted with 2-propanol (20 mL) and treated with concentrated
hydrochloric acid (1 mL). A gel-like precipitate formed throughout the reaction volume. It was
allowed to settle down, then filtered off, washed twice with 2-propanol (2 x 5 mL), and finally with
water to give the crude product (0.86-0.87 g). It was dissolved in boiling 2-propanol (15 mL) and
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crystallized on standing. Yield 0.80-0.82 g (71-72%). Mp. 177.0-177.5 °C. IR (KBr), ν̃ / cm : 3096,
1382, 1182, 1036, 803, 592. The filtrate was distilled with steam. The organic phase was then
separated, washed with water, and fractionally distilled to regenerate the excess of
octafluoropentanol.
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H NMR (Acetone-d , 300 MHz), δH / ppm: 4.54 (2H, s, -CH -), 4.94 (2H, t, J=13.5 Hz, -
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OCH -), 5.02 (2H, t, J =13.8 Hz, -OCH -), 5.05 (2H, t, J=13.5 Hz, -OCH -), 6.70 (1H, tt, J1=51.0
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Hz, J2=5.4 Hz, -CHF ), 6.76 (2H, tt, J1=51.0 Hz, J2=5.4 Hz , -CHF ), 8.24 (1H, d, J=8.4 Hz, Ar-H),
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8.47 (1H, s, Ar-H), 8.69 (1H, s, Ar-H), 8.81 (1H, s, Ar-H), 8.92 (1H, d, J=8.4 Hz, Ar-H). Found: C,
31.91; H, 1.48; S, 9.02%; molecular formula C H F O S requires C, 32.07; H, 1.54; S, 9.17%.
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Tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9-(dimethylaminomethylene)-9H-fluorene-2,4,7-
trisulfonate (3)
Compound 2 (210 mg, 0.20 mmol) and DMF (40 drops or 0.5 mL, 0.21 mmol) were
dissolved in acetic anhydride (1 mL), heated with a burner and put in a bath maintained at 50-125 °C
for 1 h. Then the reaction mixture was cooled to 50 ° C and diluted with water. The resulting
precipitate was filtered off, dried, and chromatographed on silica gel with dichloromethane-toluene
(9:1). The solvent was removed on a rotary evaporator. Yield 160 mg (72%). Mp. 173-174 °С. IR
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(KBr), ν
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H NMR (Acetone-d , 300 MHz), δH / ppm: 3.71 (6H, s, -NMe ), 4.87 (2H, t, J=13.8 Hz, -
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OCH -), 4.94 (4H, t, J=13.8 Hz, -OCH -), 6.70 (1H, tt, J1=51.0 Hz, J2=5.4 Hz, -CHF ), 6.78 (2H, tt,
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J1=51.0 Hz, J2=5.4 Hz, -CHF ), 7.89 (1H, d, J=8.7 Hz, Ar-H), 8.42 (1H, s, Ar-H), 8.55 (1H, s, Ar-
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H), 8.91 (1H, s, Ar-H), 9.00 (1H, s, -CH=), 9.05 (1H, d, J=8.4 Hz, Ar-H). Found: C, 33.61; H, 1.88;
S, 8.59%; molecular formula C H F NO S requires C, 33.74; H, 1.92; S, 8.72%.
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(E)-tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9-[(E)-3-(N-phenylacetamido)allylidene]-9H-
fluorene-2,4,7-trisulfonate (4)
Compound 2 (347 mg, 0.33 mmol) and N-[3-(phenylamino)allylidene]benzenaminium
chloride (78 mg, 0.30 mmol) were dissolved in acetic anhydride (4 mL) and refluxed until the
starting materials were completely dissolved and then another 4 min. The reaction mixture was
diluted with water and the formed oil was rubbed to powder and dried. The obtained crude product
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(380 mg) was chromatographed on silica gel with dichloromethane. After elution of impurities, the
pure major fraction was eluted with dichloromethane containing 2% of ethanol. Evaporation of the
solvent afforded the product having a very strong adhesion to glass and metal. It was triturated under
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n-hexane and filtered out. Yield 250 mg (68 %). Mp. 222-223 °С. IR (KBr), ν̃ / cm : 3084, 1695,
1610, 1381, 1302, 1179, 1034, 801, 586.
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H NMR (Acetone-d , 300 MHz), δH / ppm: 2.09 (3H, s,MeCO), 4.82 (2H, t, J=13.5 Hz, -
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OCH -), 4.98 (т, J=13.5 Hz, 4H, -OCH -), 6.24 (1H, t, J1=13.2 Hz, -CH= β-H chain), 6.67 (1H, tt,
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J1=51.0 Hz, J2=5.4 Hz, -CHF ), 6.77 (2H, tt, J1=51.0 Hz, J2=5.4 Hz, -CHF ), 7.55-7.64 (2H, m, Ph-
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H), 7.66-7.82 (3H, m, Ph-H), 7.98 (1H, s, Ar-H), 8.11 (1H, d, J=7.2 Hz, Ar-H), 8.55 (1H, s, Ar-H),
8.56 (1H, s, Ar-H), 8.71 (1H, d, J=7.2 Hz, Ar-H), 8.78 (с, 1H, Ar-H). Found: C, 38.22; H, 2.00; s,
7.77%; molecular formula C H F NO S requires C, 38.40; H, 2.07; F, 37.38; N, 1.15; S, 7.89%.
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(E)-Tris(2,2,3,3,4,4,5,5-octafluoropentyl)-9-[(2E,4E)-5-(N-phenylacetamido)penta-2,4-dien-
1-ylidene]-9H-fluorene-2,4,7-trisulfonate (5)
Compound 2 (242 mg, 0.23 mmol) and (E)-N-[(2E,4E)-5-(phenylamino)penta-2,4-dien-1-
ylidene]-benzenaminium chloride (60 mg, 0.21 mmol) were dissolved in acetic anhydride (3 mL)
and refluxed for 2 min. The Vis absorption spectrum of the reaction mixture exhibited a band with a
maximum at 474 nm (in acetone). The reaction mixture was diluted 10 times with water and the
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precipitate formed was rubbed to powder. Yield 255 mg (97%). Mp. 145-146 °С. IR (KBr), ν̃ /cm :
3079, 1569, 1381, 1178, 1035, 801, 586.
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H NMR (Acetone-d , 300 MHz), δH / ppm: 1.95 (3H, s, MeCO), 4.88 (2H, t, J=13.8 Hz, -
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OCH -), 5.01 (4H, т, J=13.8 Hz, -OCH -), 5.64 (1H, t, J=14.1 Hz, -CH= β-H chain), 6.67 (1H, tt,
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J1=51.0 Hz, J2=5.4 Hz, -CHF ), 6.73 (1H, tt, J1=51.0 Hz, J2=5.4 Hz, -CHF ), 6.75 (2H, tt, J1=51.0
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Hz, J2=5.4 Hz, -CHF ), 7.41-7.52 (3H, m, Ph-H), 7.50 (1H, t, J=14.1 Hz, -CH= β-H chain), 7.63-
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7.74 (2H, m, Ph-H), 7.67 (1H, t, J=13.2 Hz, -CH= γ-H chain), 8.14 (1H, s, Ar-H), 8.16 (1H, t,
J=12.6 Hz, -CH= α-H chain), 8.29 (1H, d, J=10.8 Hz, -CH= α-H chain), 8.60 (1H, d, J=8.7 Hz, Ar-
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H), 8.69 (1H, d, J=8.7 Hz, Ar-H), 8.95 (1H, s, Ar-H), 9.03 (1H, s, Ar-H). Found: C, 39.39; H, 2.07;
N, 7.59%; molecular formula C H F NO S requires C, 39.53; H, 2.18; S, 7.72%.
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Hydrogen (E)-2,4,7-tris(2,2,3,3,4,4,5,5-octafluoropentyloxysulfonyl)-9-[(2,4,7-
tris(2,2,3,3,4,4,5,5-octafluoropentyloxysulfonyl)-9H-fluoren-9-ylidene)methyl]-9H-fluoren-9-ide (6)
Compounds 2 (105 mg, 0.10 mmol) and 3 (111 mg, 0.10 mmol) were mixed and rubbed until
uniform consistency was achieved. The mixture was dissolved under heating in dry ethanol (1.5
mL), followed by addition of 0.15 mL (1.10 mmol) of triethylamine. The reaction mixture was
refluxed for 10 min and then another portion of absolute ethanol (1.5 mL) and a portion of
triethylamine (0.15 mL) were added. Heating was continued until the solution became dark green in
colour. For completion of the slow reaction, the reaction mixture was allowed to stand for 2 h, then
boiled for 5 min and left overnight. The next day the solvent was evaporated and the residue was
chromatographed on alumina, initially with pure dichloromethane and then, after elution of
impurities, with dichloromethane containing first 10% and then 50% of ethanol (to increase the
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elution strength). Yield 70 mg (33%). Mp. 266-267 С. IR (KBr), ν̃ / cm : 3442, 2923, 1558, 1373,
1178, 1037, 801, 584.
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H NMR (Acetone-d , 300 MHz), δH / ppm: 4.72 (4H, t, J=13.5 Hz, -OCH -), 4.82 (4H, t,
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J=13.5 Hz, -OCH -), 4.91 (4H, t, J=13.5 Hz, -OCH -), 6.70 (6H, tt, J1=51.0 Hz, J2=5.4 Hz, -CHF ),
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7.86 (2H, t, J=8.7 Hz, Ar-H fl), 8.31 (2H, s, Ar-H), 8.42 (2H, s, Ar-H), 8.67 (2H, s, Ar-H), 9.11 (2H,
d, J=8.7 Hz, Ar-H fl), 9.63 (1H, s, -CH= ).
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C NMR (Acetone-d , 125 MHz), δH / ppm: 63.56 (t, J=27 Hz), 63.92 (t, J=27 Hz), 64.38 (t, J=27
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Hz), 107.29 (tt, J1=250 Hz, J2=30.7 Hz), 111.86, 113.43, 115.46, 118.23, 120.30, 120.47, 125.72,
125.92, 127.45, 128.15, 128.67, 129.53, 130.67, 131.89. Found: C, 32.32; H, 1.40; S, 8.99%;
molecular formula C H F O S requires C, 32.49; H, 1.44; S, 9.13 %.
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Tetramethylammonium 2,4,7-tris(2,2,3,3,4,4,5,5-octafluoropentyloxysulfonyl)-9-{(1E,3E)-3-
[2,4,7-tris(2,2,3,3,4,4,5,5-octafluoropentyloxysulfonyl)-9H-fluoren-9-ylidene]prop-1-en-1-yl}-9H-
fluoren-9-ide (7)
Compounds 2 (105 mg, 0.08 mmol) and 4 (122 mg, 0.08 mmol) were dissolved in dry
ethanol (2 mL), heated, and treated with triethylamine (0.3 mL, 2.20 mmol). The reaction mixture
was refluxed until it became bright blue in colour and the starting materials were completely
dissolved. After evaporated to dryness, the reaction mixture was washed with water and the resulting
residue was filtered off, washed three times with water, and dried to give a solid (220 mg). It was
chromatographed on alumina with dichloromethane containing triethylamine (10 drops per 50 mL).
After elution of impurities, the major fraction was released slowly with dichloromethane containing
up to 10% of acetonitrile or ethanol. The crude product (160 mg) was dissolved in acetone (3 mL)
and the resulting solution was filtered and treated dropwise with a solution of tetramethylammonium
bromide (130 mg) in water (5 mL). Then the reaction mixture was diluted with another 5 mL of
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water and left overnight in a refrigerator at 4 С. The product appearing as a fine powdery precipitate
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was filtered off and dried. Yield 50 mg (30%). Mp. 214-215 С. IR (KBr), ν̃ / cm : 2924, 2854,
1552, 1418, 1178, 1035, 796, 582.
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H NMR (Acetone-d , 300 MHz), δH / ppm: 3.46 (12H, s, N-CH ), 4.92 (8H, t, J=13.5 Hz, -
6
3
OCH -), 5.02 (4H, t, J=13.5 Hz, -OCH -), 6.69 (2H, tt, J1=51.0 Hz, J2=5.4 Hz, -CHF ), 6.72 (4H, tt,
2
2
2
J1=51.0 Hz, J2=5.4 Hz, -CHF ), 7.88 (2H, d, J=9.0 Hz, Ar-H fl), 7.89 (1H, t, J=14.0 Hz, -CH= β-H
2
chain), 8.38 (2H, s, Ar-H), 8.71 (2H, d, J=12.9 Hz, -CH= α-H chain), 8.93 (2H, s, Ar-H), 9.07 (2H,
d, J=8.7 Hz, Ar-H fl), 9.12 (2H, s, Ar-H fl).
1
3
C NMR (Acetone-d , 125 MHz) , δH / ppm: 56.06 (t, J= 4.5 Hz), 65.46 (t, J=27 Hz), 65.80 (t, J=
6
26.3Hz), 66.09 (t, J=26.3 Hz), 109.01 (tt, J1=250 Hz, J2 =30.7 Hz), 111.26, 111.54, 111.67, 113.34,
114.17, 115.27, 120.18, 122.44, 127.88, 129.48, 130.97, 133.42, 143.84. Found: C, 34.14; H, 1.90;
S, 8.65%; molecular formula C H F NO S requires C, 34.30; H, 1.96; S, 8.72%.
6
3
43 48
18 6
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Tetramethylammonium 2,4,7-tris(2,2,3,3,4,4,5,5-octafluoropentyloxysulfonyl)-9-
{1E,3E,5E)-5-[2,4,7-tris(2,2,3,3,4,4,5,5-octafluoropentyloxysulfonyl)-9H-fluoren-9-ylidene]penta-
1,3-dien-1-yl}-9H-fluoren-9-ide (8)
Compounds 2 (105 mg, 0.10 mmol) and 5 (125 mg, 0.10 mmol) were dissolved in dry
ethanol (2.5 mL) and triethylamine (0.30 mL, 2.20 mmol) was added to the solution. The reaction
mixture was heated until a stable bright blue colour developed. After cooling, it was evaporated and
the residue was chromatographed on alumina with dichloromethane containing triethylamine (10
drops per 50 mL). The major fraction was released under stronger elution conditions using
dichloromethane containing up to 10% of ethanol or acetonitrile. The resulting solid was dissolved
in acetone and treated with an aqueous solution of tetramethylammonium bromide, followed by
о
standing overnight in a refrigerator at 4 С. The product was filtered off and dried. Yield 83 mg
о
-1
(37%). Mp. 132-133 С. IR (KBr), ν̃ / cm : 2924, 1597, 1382, 1178, 1129, 1035, 801, 585.
1
H NMR (Acetone-d , 300 MHz), δH / ppm: 3.46 (12H, t, J=7.5 Hz, N-CH ), 4.90 (8H, t,
6
3
J=13.5 Hz, -OCH -), 5.00 (4H, t, J=13.5 Hz, -OCH -), 6.69 (2H, tt, J1=51.0 Hz, J2=5.1 Hz, -CHF ),
2
2
2
6.74 (4H, tt, J1=51.0 Hz, J2=5.1 Hz, -CHF ), 7.49 (2H, t, J=13.2 Hz, -CH= β-H chain), 7.85 (1H, t,
2
J=13.0 Hz, -CH= γ-H chain), 7.85 (2H, t, J=9.0 Hz, Ar-H fl), 8.27 (2H, t, J=13.5, -CH= α-H chain),
8.36 (2H, s, Ar-H), 8.82 (2H, s, Ar-H), 8.03 (2H, d, J=9.0 Hz, Ar-H fl), 9.05 (s, 2H, Ar-H fl).
1
3
C NMR (Acetone-d , 125 MHz) , δH / ppm: 55.50 (t, J= 3.9 Hz), 64.89 (t, J= 27 Hz),
6
65.23 (t, J= 26.5 Hz), 65.53 (t, J= 27 Hz), 108.47 (tt, J1=250 Hz, J2 = 30 Hz), 110.71, 111.34,
112.64, 118.37, 119.63, 121.86, 123.95, 127.17, 128.80, 130.35, 132.83, 139.53, 143.81, 144.92,
146.16, 155.11, 157.72, 162.57. Found: C, 34.79; H, 1.99; S, 8.51%; molecular formula
C H F NO S requires C, 34.97; H, 2.03; S, 8.62%.
65
45 48
18 6
Tetrabutylammonium 2,4,7-trinitro-9-[(1E,3E,5E)-5-(2,4,7-trinitro-9H-fluoren-9-
ylidene)penta-1,3-dien-1-yl]-9H-fluoren-9-ide (10)
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2,4,7-Trinitrofluorene (45 mg, 0.15 mmol) [19] and (E)-N-[(2E,4E)-5-(phenylamino)penta-
2,4-dien-1-ylidene]benzenaminium chloride (22 mg, 0.08 mmol) were dissolved in pyridine (1 mL)
with heating. The reaction mixture was allowed to boil, followed by addition of piperidine (0.1 mL).
The solution which became ink-blue was heated for another minute, treated with an aqueous solution
of tetrabutylammonium iodide (50 mL), and left overnight. The precipitate formed was filtered off,
washed twice with diethyl ether (2 x 5 mL), and dried to give the crude product (60 mg, 88%). It was
dissolved in dichloromethane with heating and the resulting solution was filtered. The insoluble
о
residue was washed twice with dichloromethane (2 x 10 mL). Yield 27 mg (38%). Mp.>270 С.
-1
IR (KBr), ν
̃
/ cm : 3098, 2958, 1558, 1501, 1449, 1404, 1315, 1103, 1046, 923, 705.
1
H NMR (Pyridine-d , 400 MHz), δH / ppm: 1.08 (12H, t, J = 7.2 Hz, -CH ), 1.45 (8H, sex, J
5
3
= 7.6 Hz, -CH -), 1.88 (8H, qu, J= 7.9 Hz, -CH -), 3.48 (8H, t, J=8.0 Hz, N-CH ), 5.97 (2H, t, J =
2
2
3
12.0 Hz, -CH= β-H chain), 7.42 (2H, t, J = 12.0 Hz, -CH= γ-H chain), 7.63 (2H, d, J = 12.4 Hz, -
CH= α-H chain), 7.78 (2H, d, J = 8.8 Hz, Ar-H), 7.98 (2H, s, Ar-H), 8.09 (2H, s, Ar-H), 8.43 (2H, d,
J=8.8 Hz, Ar-H fl), 8.56 (s, 2H, Ar-H fl). Found: C, 62.11; H, 5.59; N, 10.69%; molecular formula
C H N O requires C, 62.31; H, 5.67; N, 10.82 %.
47
51
7
12
Result and Discussion
Hitherto, only Cremlyn et al. [20] have succeeded to obtain trisulfonated fluorene, in 45%
yield, as a result of a chlorosulfonation with an excess (20 eq) of chlorosulfonic acid. We have found
an alternative access to 9H-fluorene-2,4,7-trisulfonyl-trichloride. In the three-step procedure
proposed here (Scheme 1), fluorene is first sulfonated in an excellent yield to potassium 9H-
fluorene-2,7-disulfonate [18] which in turn is treated with phosphorus pentachloride to give 9H-
fluorene-2,7-disulfonyl-dichloride. The latter is then almost quantitatively converted to 9H-fluorene-
2,4,7-trisulfonyl-trichloride 1 in a chlorosulfonic acid solution at 120 °С.
A two-step version of the synthesis of 1 implies that potassium 9H-fluorene-2,7-disulfonate is
at once chlorosulfonated by heating it in aqueous chlorosulfonic acid so that the 2- and 7- sulfonato
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groups are converted to chlorosulfonyl groups and the third such group is introduced at position 4. In
this reaction, the yield of compound 1 is likewise close to 100% and depends on the amount of the
KCl impurity in the sulfonate substrate. Recrystallization gives more than 70% isolated yield of the
desired trisulfonyl trichloride 1, with mp 205-206 °С (and not 185 °С, contrary to Ref. 20).
The chlorosulfonyl group is quite sensitive to nucleophilic reagents and has not the above-
mentioned advantages of polyfluoroalkoxysulfonyl groups. Therefore, it should be esterificated with
a polyfluoroalkanol prior to cyanine synthesis [15]. To obtain tris(2,2,3,3,4,4,5,5-octafluoropentyl)-
9H-fluorene-2,4,7-trisulfonate 2, compound 1 is reacted with octafluoropentanol (see Scheme 1).
Scheme 1.
2
2
92
93
294
295
296
297
298
299
300
The additional electron-acceptor substituent in the 2,4,7-tri(polyfluoroalkoxysulfonyl)fluorene
derivative makes the 9-methylene group acidic enough to enter into the cyanine condensation under
standard conditions, in contrast to the insufficient reactivity of the 2,7-disubstituted analogues [9].
The corresponding hemicyanines 3-5 were first obtained according to Scheme 2; they were
then reacted with a second molecule of trisubstituted fluorene 2 to give dyes 6-8 according to
Scheme 3.
Scheme 2.
1
2

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3
01
02
303
Scheme 3.
3
3
04
05
306
307
308
309
310
311
312
313
As we showed before [9], anionic polymethine dyes readily form contact ion pairs with a
strong electrostatic interaction between the cation and the anionic moiety. If the cation has a labile
H-atom, the anion is protonated and the dye bleaches as a result of the broken conjugation in the
chromophore [9]. For the sake of stability of the dyes synthesized, they all (except compound 6)
were isolated with the aprotic tetramethylammonium cation as counterion. Interestingly, dye 6
obtained and isolated as an H-acid proved unexpectedly stable. This is probably due to the fact that
the small proton can easily be located between two π-electron-rich fluorene nuclei and has no
tendency to bind covalently to positions 9, 9’.
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3

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For comparison, we have also synthesized the pentamethine dye 10 containing 2,4,7-
trinitrofluorene [19] end groups. It is notable that the corresponding cyanine condensation is very
demanding in terms of reaction conditions: those used successfully for the preparation of analogous
dyes with dinitro- and tetranitro- fluorene end groups [10] (more severe and milder, respectively,
than expected for the trinitro derivative) both failed to afford appreciable yields of the desired dye
10. It appeared logical to use a base of intermediate strength between triethylamine and DBU but an
attempted reaction with ethyldiisopropylamine and dicyclohexylethylamine proved unsuccessful.
Likewise, the use of higher-boiling solvents, N,N-dimethylacetamide or DMSO instead of ethanol or
acetic anhydride had no effect. Eventually, dye 10 was obtained by heating the reaction mixture in
pyridine with piperidine as base followed by precipitation with an excess of a tetrabutylammonium
iodide solution (see Scheme 4).
Scheme 4.
3
26
27
3
As can be seen, dye 10, like the previously described tetranitrofluorene dyes [10] and in
contrast to polymethines 6 – 8, was prepared by the one-step procedure. This may be attributable to
the higher CH acidity of compound 9 as compared to 2 as a result of the better conjugation of nitro
than of polyfluoroalkoxysulfonyl groups with the fluorene core (which overcompensates the
markedly higher electron-withdrawing ability of the latter).
328
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332
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334
335
All the synthesized dyes are stable both in the solid state and in solution. The wavelengths of
a
f
their Vis/NIR absorption and fluorescence maxima, λ
and λ max, as well as their molar extinction
max
coefficients ε, Stokes shifts ꢀλ or ꢀν
̃s, and fluorescence quantum yields φ in different solvents are
s
listed in Table 1.
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Importantly, dyes 6 – 8 are rather deeply coloured though they have only one to five methine
units in the chromophore (see Table 1). The anionic pentamethine dye 8 has an absorption maximum
at λ = 820 nm in DMF, whereas the analogous dyes derived from bare and 2,7-
di(polyfluoroalkoxysulfonyl) substituted fluorene absorb light at shorter wavelengths, viz. λ = 740
[21] and 790 nm [9], respectively, under the same conditions. A shorter-wavelength absorption as
compared to dye 8, at λ = 642 nm, is also observed for the anionic pentamethines terminated with the
electron-acceptor heterocyclic residues of N,N-diethylthiobarbituric acid (known to significantly
deepen the colour) [22]. Moreover, the absorption maxima of classical cationic pentamethine
cyanines derived from indolenine, benzothiazole, and benzoxazole [12] are also ꢀλ = 150-180 nm
blue-shifted as compared to compound 8.
Table 1. Vis/NIR spectral characteristics of dyes 6 – 8 and 10.
a
-5
f
Dye Solvent
λ
, ε × 10 , λ , ꢀλ (ꢀν
̃s)
φ,
max
max
s
-
1
-1
-1
nm
654
93
648
89
649
89
Toluene 730
M cm
nm
730
nm (cm )
76 (1592)
%
6
CH
2
Cl
2
0.403
0.118
0.398
0.108
0.388
0.118
0.499
0.515
0.228
0.09
4
EtOH
DMF
4
4
7
764
34 (610)
51 (899)
6
5
75
50
CH Cl2
728
57
723
56
1.132 779
0.179
0.54
2
5
EtOH
1.127
5
0.188
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5

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DMF
723
56
CH CN 718
1.073
5
0.193
1.020
1.105
0.952
1.008
0.466
0.340
0.572
0.475
0.368
0.544
3
8
1
CH
2
Cl
2
832
828
820
950
880 48 (655)
0.24
EtOH
DMF
0
CH Cl
2 2
6
4
81
15
DMF
923
6
4
75
16
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Like the polymethine dyes derived from bis(2,2,3,3,4,4,5,5-octafluoropentyl)-9H-fluorene-
2,4,7-disulfonate [9] or classical cyanines [12], and in contrast to polynitrofluorene dyes (dye 10 and
those previously studied [10]), polymethines 6 – 8 exhibit absorption spectra with a clearly
dominating “cyanine” band in the visible or near-IR region (see Fig. 1). In addition, there is a blue-
shifted weak band in their spectra and its presumed impurity origin is ruled out by the fact that it
remains completely unchanged (including the intensity ratio to the main band) after recrystallization
and chromatography. As the separation of the short-wavelength band from the main one (ꢀν
̃
= 4990,
-1
4220, and 3930 cm for dyes 6, 7, and 8, respectively) far exceeds the valence vibration frequency
-1
of the chromophoric carbon atoms (1300 сm ) [13], we may assume that the two bands arise from
different electronic transitions of the dye. This conjecture is verified by the analysis of the
fluorescence spectra of compounds 6 – 8 (recorded with the accumulation in view of the
fluorescence quantum yields less than 0.1%). The fluorescence spectra (see Fig. 2) are excitation-
wavelength independent, each displaying solely a mirror image of the long-wavelength absorption
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band. As exemplified by dye 6, the fluorescence excitation spectra almost coincide with the
absorption spectrum (see Fig. 3). Taken together, all this unambiguously proves that the short-
wavelength low-intense absorption band is caused by another electronic excitation of the same
compound.
3
65
66
3
Fig. 1. UV\Vis absorption spectra of dyes 6 (1), 7 (2), 8 (3), and 10 (4), measured in CH Cl .
2
2
367
3
3
68
69
a
b
c
3
70
71
Fig. 2. Normalized Vis absorption and fluorescence spectra of dyes 6 (a), 7 (b), and 8 (с), measured
3
in CH Cl .
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3
Fig. 3. Fluorescence excitation spectra registered at 710 nm (1) and 760 nm (2) for dye 6,
374
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378
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380
381
382
383
384
385
386
387
388
389
390
measured in CH Cl , along with the corresponding absorption spectrum (3).
2
2
The noticeable broadening of the fluorescence spectrum of dye 6 in comparison to its
absorption spectrum may well be of instrumental origin, since the spectrofluorometer sensitivity
appears insufficient to adequately measure so weak emission (with a quantum yield of 0.09). As
expected, no fluorescence at all can be detected for dye 10 (under the same measuring conditions as
for dye 8) because the nitro group is known to be a strong fluorescence quencher [23]. With the
lengthening of the conjugated chromophore from the mono- via tri- to penta- methine chain, the
main absorption band shifts bathochromically, its vibrational shoulder becomes more pronounced,
and the band intensity increases markedly. The latter effect is more notable in going from the mono-
to tri- methine (6 → 7) than from the tri- to penta- methine dye (7 → 8), since the first lengthening
step is likely to provide the main contribution to steric release and molecular planarization. Indeed,
o
the calculated interplanar angle between the two fluorene end groups is much larger for dye 6 (37.9 )
o
than for 7 and 8 (18.3 and 13.0 , respectively). For comparison, analogous nitro substituted dye 10
demonstrates much less steric hindrance and hence a considerably smaller value of the
o
corresponding interplanar angle (9.8 ) – cf. Fig. 3a to 3b.
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8

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3
91
92
a
b
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
Fig. 4. Optimized geometries of dyes 8 (with truncated fluoroalkyl substituents) and 10.
As the polymethine chain is lengthened, the second weak absorption band of dyes 6 – 8 also
shifts to the red, but contrary to what is observed for the main band, it grows more diffuse and even
less intense. The two bands thus become more resolved affording better absorption selectivity.
The first and second vinylene shifts (the corresponding shifts to longer wavelengths in going
from dye 6 to 7 and from 7 to 8) in the absorption spectrum are respectively equal to 74 and 104 nm
in dichloromethane, and ꢀλ = 74 and 97 nm in DMF. The first vinylene shift is much lower than the
typical value for classical cyanines (about 100 nm) [4, 5, 12] and this discrepancy is probably due to
some colour-deepening structural peculiarities of dye 6 (in view of the normal value of the second
vinylene shift). As already mentioned, this monomethine demonstrates the most significant deviation
from molecular planarity as compared to its higher vinylogues. Intuitively, it is clear that twisting
around a bond with the sufficiently high ground-state and lowered excited-state π-order (as
calculated for compound 6) should result in a smaller molecular energy gap and hence a red-shifted
absorption, since the energy rise from the broken π-conjugation is larger in the ground than in the
excited state. This is really the case for dye 6 as evidenced by the simulated effect of its interplanar
angle between the fluorene nuclei on the calculated absorption spectrum. An increase in the angle
from 38° to 45° and 60° causes the respective ꢀλ = 7 and 21 nm bathochromic shifts of the long-
wavelength band.
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The first and second vinylene shifts in the fluorescence spectrum are respectively equal to ꢀλ=
49 and 101 nm in dichloromethane. As with absorption bands, the low first vinylene shift arises from
the twisted structure of monomethine 6. The respective Stokes shifts for dyes 6, 7, and 8 decrease
-1
from ꢀνs̃ = 1592 via 899 to 655 cm (CH Cl ). The general decreasing trend is attributed to the
2 2
decay of vibronic interactions with the lengthening of the polymethine chain in symmetric cyanines
[12] and, in addition, a remarkably large Stokes shift for dye 6 is probably due to the fact that it
undergoes much stronger structural changes on excitation as compared to dyes 7 and 8. Indeed, the
lengthened chromophore bonds in the excited-state structure enable a considerable release of steric
hindrance and hence pronounced planarization of the monomethine, unlike its higher vinylogues
which are nearly planar already in the ground state.
Polymethines 6 - 8 are characterized by a slight negative solvatochromism: if dichloromethane
is replaced by a more polar solvent such as ethanol, DMF, or acetonitrile, both long- and short-
wavelength bands are very slightly blue-shifted. It is noteworthy that, irrespective of the solvent
used, the dyes concerned exhibit the narrow and intense long-wavelength absorption band shaped
typically of cyanines [12]; the band shape is much the same in dichloromethane and in aprotic
nucleophilic (DMF) and electrophilic solvents (acetonitrile) – see Fig. 5. It also remains unchanged
in highly electrophilic protic ethanol thus suggesting a negligible, if any, H-bonding effect on the
dye spectra. Of particular significance is the fact that the long-wavelength band of dyes 6 - 8, unlike
that of most near-IR polymethines [3, 6, 7], retains high selectivity and intensity as the polymethine
chain is lengthened (see Table 1 and Fig. 2).
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443
444
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447
448
449
450
451
452
453
Fig. 5. Vis absorption spectra of dye 7, measured in CH Cl (1), DMF (2), acetonitrile (3), and
2 2
ethanol (4).
Due to the presence of polyfluoroalkoxysulfonyl groups, dyes 6 – 8 are soluble in weakly polar
solvents such as toluene (in contrast to compound 10). Their electronic spectra are substantially
influenced by the low-polarity environment. For instance, the absorption band of dye 7 becomes
broader and weaker, and inflected several times in toluene as shown in Fig. 6. Such spectral effects
are comprehensible in terms of formation of contact ion pairs and their associates [13] as is usually
the case for ionic dyes in solvents with a relative permittivity lower than 5. Since these associates are
readily decomposed by increasing solvent polarity, it proves sufficient to add as little as a few drops
of a polar ionizing solvent (e.g., acetonitrile) to a toluene solution of dye 7 to restore its high-intense,
narrow, and typically shaped “cyanine” absorption band.
The spectra of solutions containing contact ion pairs are essentially concentration-dependent
since ion-pair formation is enhanced with increasing concentration. As an example, the long- and
-5
short-wavelength absorption bands are almost equally intense at the dye concentration 1 × 10 mol/L
(see Fig. 6), whereas the former band gains and the latter loses intensity on tenfold dilution
favouring ion pair dissociation (see Fig. 7). It appears that the short-wavelength absorption band is
associated just with contact ion pairs. This is consistent with the fact that the fluorescence spectra of
dyes 6 – 8 recorded in weakly polar solvents at low dye concentrations (to avoid reabsorption)
contain a single long-wavelength band irrespective of the excitation wavelength, as exemplified by
dye 7 in toluene (see Fig. 7).
2
1

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4
54
55
4
Fig. 6. UV\Vis absorption spectra of dye 7, measured in toluene (1), acetonitrile (2), and
-
5
456
457
458
459
460
toluene with 3 vol % acetonitrile (3). Dye concentration is 1 × 10 mol/L.
We have thoroughly analyzed the electronic structure of the dyes under study and rationalized
their spectral properties based on the DFT/CAM-B3LYP-calculated wavelengths, λmax theor, and
oscillator strengths, ftheor, of the relevant electronic transitions, S → S (see Table 2).
0
n
4
61
62
4
Fig. 7. Normalized Vis absorption and fluorescence spectra of dye 7, measured in toluene. Dye
-
6
463
464
465
466
467
concentration is 1 × 10 mol/L.
Table. 2. Quantum chemical data for the electronic transitions in dyes 6 – 8, 10, and 11 (obtained by
the DFT/CAM-B3LYP method). S → S transitions are omitted as practically forbidden.
0
2
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2

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λ
f
S → S
0
max theor
theor
n
nm
6
7
8
503.5
0.629
0.274
0.485
0.327
0.631
1.278
0.320
0.251
0.522
0.896
1.901
0.203
0.322
0.304
0.795
1.086
0.922
0.128
0.561
2.217
0.001
0.533
S → S
0
1
4
2
2
2
00.0
72.0
67.7
55.8
S0 → S3
S0 → S9
S0 → S10
S0 → S12
534.6
S → S
0 1
4
2
2
2
24.8
75.8
73.7
56.0
S0 → S3
S0 → S7
S0 → S10
S0 → S13
604.8
S → S
0 1
4
2
2
2
53.0
71.1
62.3
60.9
S0 → S3
S0 → S12
S0 → S14
S0 → S15
1
0
1
646.9
S → S
0 1
4
3
3
98.8
99.1
26.2
S0 → S3
S0 → S5
S0 → S10
1
524.9
S → S
0 1
3
2
64.3
53.3
S0 → S3
S0 → S10
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483
We consider dye 8 first, since its spectrum is the most interpretable (the least complicated by
steric effects). Its longest-wavelength electronic transition, S → S , is highly allowed (f = 1.901)
0
1
theor
and contributed mainly by the HOMO-LUMO excitation (69.2%) – see Fig. 8. It involves electron-
density transfer from even to odd carbon atoms of the polymethine chain (one of the α-carbon atoms
of the chain is numbered 1) and, to a much lesser degree, to the chain-closest conjugated atoms of
the fluorene nuclei. This transition is, therefore, clearly polymethinic and corresponds to the intense,
narrow, and specifically shaped long-wavelength “cyanine” band of dye 8 (cf. to Fig. 2c). The
second calculated transition of the dye, S → S , is significantly less allowed (f = 0.203), arises
theor
0
3
predominantly from the HOMO-LUMO+2 excitation (67.9%), and is mainly characterized by
electron-density transfer from the polymethine chain to positions 1 and 4 of the fluorene nuclei (see
Fig. 9). Thus it should be assigned to the weak short-wavelength band of dye 8. It should be noted
that the second electronic transition of classical cationic cyanines, S → S , is not observed in the
0
2
spectra of anionic fluorene dyes due to its negligible intensity. Since the corresponding oscillator
strengths are less than 0.01 (e.g., ftheor = 0.0059 has been found for dye 8), the S → S transitions are
0
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excluded from further consideration as practically forbidden.
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Fig. 8. HOMO (a), LUMO (b), and LUMO+2 (c) of dye 8.
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b
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Fig. 9. S → S (а) and S → S (b) transition-associated electron-density shifts in the dye 8
0 1 0 3
molecule (yellow and blue spheres denote, respectively, an increase and decrease in atomic electron
3
densities; the electron density isosurface is plotted at a difference value of 0.003 e/Å ).
For comparative analysis, the same quantum-chemical calculation has been performed for
analogous pentamethines containing the bare fluorene (dye 11) and 2,4,7-trinitrofluorene end groups
(dye 10); some computational results are presented in Fig. 10.
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Fig. 10. S → S (а, c) and S → S (b, d) transition-associated electron-density shifts in the
0
1
0
3
molecules of dyes 11 (а, b) and 10 (c, d). For plotting details see the legend to Fig. 9.
The nature of the S → S and S → S electronic transitions in dye 11 as well as the
0
1
0
3
percentages of contributing electronic configurations are much the same as in dye 8 (cf. Fig. 10a, b
to Fig. 9). At the same time, the calculated first transition wavelengths suggest that the absorption of
dye 8 is considerably red-shifted from that of dye 11, which is consistent with the experiment (cf.
λ = 820 nm for 8 to 740 nm for 11 in DMF) The bathochromic effect is qualitatively explicable in
terms of the Dewar-Knott rule [24, 25]: though the fluorene backbone atoms bearing
polyfluoroalkoxysulfonyl groups in dye 8 appear inactive in the S → S transition, they are
0
1
conjugated to the atoms with large LUMO coefficients (i.e. gaining electron density in the S state –
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see Fig. 10a, c), and hence electron-acceptor substituents at positions 2, 4, and 7 should also
decrease the molecular energy gap.
Another spectral distinction between the two dyes is that the excitation-induced electron-
density transfer from the polymethine chain to fluorene nuclei is somewhat larger in dye 8 (due to
the electron-withdrawing action of polyfluoroalkoxysulfonyl substituents); as a result, absorption
intensity decreases for the long-wavelength band and increases for the short-wavelength band in
comparison with dye 11.
Like pentamethines 8 and 11, their analogue 10 with 2,4,7-trinitrofluorene end groups displays
the S → S and S → S electronic transitions arising from the respective major excitations,
0
1
0
3
HOMO-LUMO (68.6%) and HOMO-LUMO+2 (68.3%). It should be noted, however, that the two
transitions interchange their character. Being substantially contributed by the nitro groups, the
LUMO and, accordingly, the S → S transition to a considerable degree lose their polymethinic
0
1
nature – see Fig. 10c. It is also seen from Fig. 1 that the first absorption band, weak and broadened,
is likely to correspond to the charge transfer from the polymethine chain to the nitro groups. The loss
in intensity and selectivity of this band detracts much from the practical value of its extremely long
absorption wavelength of λ = 950 nm. Vice versa, the S → S transition in dye 10 becomes more
0
3
polymethinic than in 8 or 11 (cf. Fig. 10d to Figs. 9b or 10b) since the electron-density transfer
beyond the polymethine chain is reduced in the former.
In contrast to nitro groups, polyfluoroalkoxysulfonyl substituents do not interfere
significantly in the nature of the electronic transitions in fluorene-based polymethine dyes, so that
their long-wavelength band remains clearly polymethinic. The spectra of trimethine 7 and
monomethine 6 are much the same as that of pentamethine 8, including the origin and electronic
configuration percentages of both transitions. The only difference quite expected for the lower
vinylogues of dye 8 is that their oscillator strengths are decreased for the long-wavelength
polymethinic S → S transition and increased for the short-wavelength S → S transition as
0
1
0
3
compared to 8 (see Table 2).
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The S → S transition in monomethine dye 6 is accompanied by the electron-density shift
0 1
from positions 9, 9' to 2, 2'- and 7, 7' of the fluorene backbone and also to the central methine unit.
Upon the S → S transition, electron density likewise shifts within the fluorene backbone, from
0
3
positions 9, 9' to 1, 1'- and 4, 4', as well as to the methine unit. As already mentioned, the long-
wavelength S → S transition is additionally red-shifted due to the twisted structure of the dye
0
1
molecule and gives rise to the lowered values of the first vinylene shift both in absorption and
fluorescence spectra.
The calculated vinylene shifts in the dye series 6 - 8 are larger for the long-wavelength (ꢀλ =
30 and 70 nm) than for the short-wavelength band (ꢀλ = 25 and 28 nm), in agreement with the
experimentally observed trend (cf. Table 2 to Table 1). This effect is easy to understand taking into
account the localization of MOs relevant to the two transitions concerned (see Fig. 8). The S → S1
0
transition mainly involves essentially polymethinic HOMO and LUMO, both approaching the Fermi
level and each other with the lengthening of the polymethine chain. Contrary to this, of two MOs,
HOMO and LUMO+2, participating in the S → S transition, only the former approaches the Fermi
0
3
level as the polymethine chromophore is lengthened, whereas the latter is practically insensitive to
its length. Thus, the wavelength of the S → S transition increases faster in the vinylogous series.
0
1
As a result, the two absorption bands move apart with the lengthening of the polymethine chain: the
difference between the band maximum positions has been calculated to increase, respectively, from
ꢀλ = 103.5 via 110 to 152 nm for dyes 6, 7, and 8.
The growing separation between the bands along with the increasing ratio of the first to
second transition oscillator strengths in the vinylogous series are consistent with the well-known
experimental fact that the polymethinic transition increasingly dominates the cyanine dye spectrum
as the polymethine chain is lengthened.
Absorption of the fluorene nuclei in dyes 6 - 8 arises from the transitions to higher excited
states; it is detected at λ = 260-275 nm irrespective of the polymethine chain length (see Table 2).
2
8