Synthesis and photophysics of ambipolar fluoren-9-ylidene malononitrile derivatives

Article abstract of DOI:10.1021/jo901869g

(Figure Presented) The efficient synthesis of ambipolar 2,7- and 3,6-disubstituted fluoren-9-ylidene malononitrile derivatives (4 and 9) is described. Structure-activity relationships depend on the position of substitution. Population of the S₁ excited state in the 2,7-disubstituted FM derivatives is achieved via higher states, as evidenced from the UV-vis absorption and emission spectra. The results are supported by TDDFT calculations. Charge transfer states were the main deactivation path of the excited states of ambipolar 9b in polar solvents, as evidenced by fluorescence spectroscopy. 2009 American Chemical Society.

Full text of DOI:10.1021/jo901869g

pubs.acs.org/joc
malononitrile (FM) derivatives had been integrated as
Synthesis and Photophysics of Ambipolar
Fluoren-9-ylidene Malononitrile Derivatives†
photosensitizers for carbazole-containing conductive poly-
mers.3,4 While looking for improvements in the electron
affinity of electron-deficient fluorene derivatives, a series of
nitro-containing FOs and FMs were mainly developed by
Perepichka’s and Bryce’s research groups.5 These efforts
recently led to amazingly low band gap materials if cova-
lently attached to electron-rich tetrathiafulvalene (TTF).6
Leandro A. Estrada and Douglas C. Neckers*
Center for Photochemical Sciences at Bowling Green State
University, Bowling Green, Ohio 43403
neckers@photo.bgsu.edu
Received August 28, 2009
The efficient synthesis of ambipolar 2,7- and 3,6-disub-
stituted fluoren-9-ylidene malononitrile derivatives (4
and 9) is described. Structure-activity relationships de-
pend on the position of substitution. Population of the S1
excited state in the 2,7-disubstituted FM derivatives is
achieved via higher states, as evidenced from the UV-vis
absorption and emission spectra. The results are sup-
ported by TDDFT calculations. Charge transfer states
were the main deactivation path of the excited states of
ambipolar 9b in polar solvents, as evidenced by fluores-
cence spectroscopy.
FIGURE 1. Design and purpose of target compounds.
Substitution at positions 3, 6, and 9 offer, in theory, good
electronic communication through the fluorene skeleton,
making it suitable for antenna-type systems were proper
electron donor and acceptor groups incorporated
(Figure 1). The rigid fluorene carbon frame should aid in
electron and energy transfer processes in a similar fashion as
well-studied compounds for this purpose.7,8 Though transi-
tion-metal-catalyzed C-C coupling9 reaction processes have
enabled the incorporation of a plethora of substituents in
(3) Hoegl, H.; Barchietto, G.; Tar, D. Photochem. Photobiol. 1972, 16,
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(4) Perepichka, I. F.; Mysyk, D. D.; Sokolov, N. I. In Current Trends in
Polymer Photochemistry; Allen, N. S., Edge, M., Bellobono, I. R., Selli, E.,
Eds.; Ellis Harwood: New York, London, 1995.
Current efforts leading to new multiphoton-absorbing
materials are reflected by the number of organic ambipolar
compounds recently reported.1 Fluorene has become a fa-
vorite synthon in this.2 2,7-Derivatives mainly focus on the
delocalizing π-electron density via byphenylene, whereas
derivatives at C-9 usually improve solubility, processability,
and interchain charge and energy transfer in polymer com-
posites. Incorporating electron-withdrawing groups at C-9
stabilizes the LUMO of fluorene, making it remarkably
electrophilic. Thus, fluorenone (FO) and fluoren-9-ylidene
(5) Perepichka’s work in “Electron Acceptors of the Fluorene Series”
involves a total of 13 parts published in different scientific journals. Among
the ones involving FMs, see: (a) Part 5. Perepichka, I. F.; Popov, A. F.;
Orekhova, T. V.; Bryce, M. R.; Vdovichenko, A. N.; Batsanov, A. S.; Goldenberg,
L. M.; Howard, J. A. K.; Sokolov, N. I.; Megson, J. L. J. Chem. Soc., Perkin
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J. Chem. Soc., Perin Trans. 2 1997, 537. (c) Part 7. Perepichka, I. F.; Kuz'mina,
L. G.; Perepichka, D. F.; Bryce, M. R.; Goldenberg, L. M.; Popov, A. F.; Howard,
J. A. K. J. Org. Chem. 1998, 63, 6484. (d) Part 10. Perepichka, I. F.; Popov, A. F.;
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and references therein.
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Published on Web 10/08/2009
DOI: 10.1021/jo901869g
r
2009 American Chemical Society

Estrada and Neckers
JOCNote
SCHEME 1. Synthetic Route for the Synthesis of Compounds
4-9a
TABLE 1. Photophysical Properties Recorded in Toluene
λabs (nm)
[ε (M-1 cm-1)]
λem max
|(nm)
τF
(ns)
krad
(s-1
c
cmpd
)
ΦF
%
4a
320 [120000]
550 [1700]
350 [120000]
560 [2500]
290 [98000]
475[10000]
346 [96000]
470 [32000]
b
540
550
615
620
9.98a
6.89a
1.00b
3.18b
1.00 ꢀ 108
0.06
4b
9a
9b
1.45 ꢀ 108
9.96 ꢀ 108
3.15 ꢀ 108
0.7
0.9
2
aλexc = 340 nm. λexc = 370 nm. ΦF was measured against 0.1 M
c
solution of quinine sulfate in 1 M H2SO4 (aq) as fluorescence standard.
extraction in the decarbonylation step under basic condi-
tions, as previously suggested by Ipaktschi et al.12 The yields of
SC dramatically improved (3-4-fold) when sublimed 7 was
used as starting material. Apparently, Pd(0) is deactivated by
complexation with the o-quinone competing with the oxidative
C-Br addition step. Although this hypothesis is yet to be
proved, it is known that Pd coupling of dihalophenanthrene-
9,10-dione (DHPD) gives irreproducible results, and the yields
are often low.18 Tour et al. was the first group to successfully
derivatize DHPD after generation of 9,10-diethyleneglycolk-
aReagents and conditions: (i) I2, H5IO6, AcOH/H2SO4/H2O, Δ, 98%;
(ii) hν (λ = 350 nm, P = 100 W), Bz2O2, Br2, PhNO2, 77%; (iii) KOH/
KMnO4, H2O, Δ, 69%; (iv) phenylacetylene (a) or 3,6-di-tert-butyl-9(4-
ethynylphenyl)carbazole (b), PdCl2(PPh3)2, CuI, Et3N, DMF, 90 °C,
12 h, 80-99%; (v) malononitrile, pyridine (anhyd.), 90 °C, 2 h, 68-91%.
carbons 2 and 7 for further exploration of fluorene electronic
properties, there are few reported examples of such substitu-
tions at 3,6-positions.10-12
etal under acid conditions prior to Pd coupling.19 Mullen et al.
€
recently reported a similar approach: protection of DHPD with
Me2SO4 and regeneration of the o-quinone with CAN.20
The target compounds were obtained after Knoevenagel
condensation (KC) of FO derivatives with malononitrile in
anhydrous pyridine.21 SC after KC was avoided since func-
tionalization of 2,7-dibromo-FM under SC conditions leads
to replacement of one of the nitriles for an acetylide.22
Photophysical properties are reported in toluene in
Table 1. UV-vis absorption spectra show intense absorp-
tion bands in the UV-blue region for each compound
(Figure 2). The high energy absorption profile for 4a is
similar to the π-π* absorption of 2,7-bis(phenylethynyl)-
fluorene (BPF).23 This allows the assignation of these bands
to locally excited states of the chromophore since electronic
communication between BPF and malononitrile units must
be poor in the ground state. The lower energy weak bands are
assigned to forbidden intramolecular charge transfer (ICT)
transitions between high energy occupied orbitals of the
aromatic donor and a low energy unoccupied orbital of the
dicyanovinylene (DCV) acceptor. This is similar to the case
of FM reported by Rault-Berthelot.24 The 4b absorption
band red shifts with respect to 4a given the increased con-
jugation conferred by the carbazole units; however, 4b dis-
plays the same characteristics as compound 4a: allowed
Herein, the synthesis and characterization of an ambipolar
3,6-disubstituted FM derivative were pursued. Carbazole
was selected as the electron donor because of its electronic
properties and processability,13 while phenylacetylene (PA)
was chosen as the electronic bridge.14 FMs substituted at the
2,7-positions and compounds lacking of carbazole were also
synthesized for comparison purposes.
The synthetic route is outlined in Scheme 1. Carbazole-
containing PA was synthesized according to Hou’s proto-
col.15 2,7-Diiodofluorenone (2) was prepared by iodination
of fluorenone with a mixture of I2/H5IO6 under acid condi-
tions.16 3,6-Dibromofluorenone (7) was synthesized by a
modified Bhatt’s protocol starting from phenanthrene-
9,10-dione (6).17
Purification of 7 via Soxhlet extraction with toluene
initially reported by Bhatt, and subsequently adapted by
others,10,11 is likely to leave <5% of starting material as
impurity. This is hard to detect by 1H NMR. A little amount
of 6 proved to be detrimental toward efficient Sonogashira
couplings (SCs) since the Pd catalyst loading is usually
around the same low mole percentage. Analytically pure 7
was obtained after two sublimation steps and final Soxhlet
π
ArfπAr* and forbidden πArfπ*DCV transitions.
(10) (a) Brunner, K.; van Dijken, A.; Brner, H.; Bastiaansen, J. J. A. M.;
Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126, 6035.
(b) Wu, Z.; Xiong, Y.; Zou, J.; Wang, L.; Liu, J.; Chen, Q.; Yang, W.; Peng,
J.; Cao, Y. Adv. Mater. 2008, 20, 2359. (c) Yeh, H.-C.; Chien, C.-H.; Shih,
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R. Helv. Chim. Acta 1998, 81, 1821. (b) Ipaktschi, J.; Hosseinzadeh, R.; Schlaf, P.;
Eckert, T. Helv. Chim. Acta 2000, 83, 1224.
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Hung, C.-Y.; Alemany, L. B.; Sasaki, T.; Morin, J.-F.; Guerrero, J. M.;
Kelly, K. F.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 4854.
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Anson, C. E.; List, E. J. W.; Mullen, K. Angew. Chem., Int. Ed. 2008, 47,
8292.
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(14) Bunz, U. H. Chem. Rev. 2000, 100, 1605.
(15) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem. Soc. 2006,
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Hornback, W. J.; Jiang, D.; Ornstein, P. L.; Thompson, M. L.; Tromiczak,
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Soc. 2006, 128, 3789.
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2003, 365.
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lson, L.-O.; Batsanov, A. S.; Bryce, M. R. J. Am. Chem.
~
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E. G.; Wu, Z.; Zarrinmayeh, H.; Zimmerman, D. M.; Castano-Mansanet,
(23) Birckner, E.; Grummt, U.-W.; Goller, A. H.; Pautzch, T.; Egbe,
D. A. M.; Al-Higari, M.; Klemm, E. J. Phys. Chem. A 2001, 105, 10307.
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1997, 436, 85.
J. Org. Chem. Vol. 74, No. 21, 2009 8485

Estrada and Neckers
JOCNote
FIGURE 3. Normalized emission spectrum of solutions of the
ambipolar compounds in toluene at 25 °C.
FIGURE 2. Normalized UV-vis absorption spectrum of solutions
of the ambipolar compounds in toluene at 25 °C. Inset: 425-650 nm
region shows the weak absorption bands for the series compounds.
excimers become repulsive and separate immediately after
photon emission.29
FM derivatives 9a and 9b show strong absorption bands
around 400 nm and medium intensity bands centered ca.
475 nm. Interestingly, compounds with similar structures to
9b, used as sensitizers for photoelectrochemical cells, show
comparable molar absorptivities and absorption profiles.25
The highest energy band for compound 9a centered at ca.
290 nm is similar to that of 1,2-diphenylacetylene.26 Even so,
the rest of the bands could not be assigned by comparison
with reported absorptions for similar compounds; this in-
dicates effective electronic communication between the 3,6-
positions and C-9. Additionally, none of the spectra for any
of the compounds shows much solvent dependency (see
Figure S2 in the Supporting Information), in accordance
with previously reported data for other types of aryl-
DCVs.27 However, the absorption band maximum centered
at ca. 470 nm of 9b blue shifts upon increasing solvent
polarity. This may indicate higher polarity of the ground
state compared to that of the excited state for this compound,
yet its red-edge bathochromic shift points toward a different
direction.
Radiative decay was found to be sensitive to solvent
polarity (see Figure S3, Supporting Information). Com-
pounds 4a and 4b showed bathochromic shifts with increas-
ing polarity, though in DMF, the emission maxima shifted to
shorter wavelengths. In the case of 9a, such an unusual shift
was not apparent. Compound 9b showed total emission
quenching in EtOAc, DCM, and DMF, contrary to the
high intensity displayed when dissolved in cyclohexane or
toluene. This suggests that charge transfer states are involved
as one of the possible nonradiative deactivation pathways.
Furthermore, the lack of emission at 300 K in MeTHF
compared with high emission intensity at 77 K suggests that
rotational modes could also be involved in the nonradiative
decay (i.e., dicyanoethylene isomerization). This finding
further explains the low fluorescence quantum yields of all
the compounds at room temperature. Transient absorption
spectroscopic measurements are underway in order to re-
solve completely the photophysics of these materials.
To gain more insight into such unique excited state
behavior, DFT calculations were performed for all carba-
zole-containing compounds. The B3LYP functional30,31 and
the 6-31G*32 basis set were used for the present computa-
tions. Geometry optimization of 4b and 9b showed that PA
units were planar to the FM skeleton, whereas carbazole
units were quite twisted from such a plane (see abstract
graphic). Localization of HOMO was found in the CPA
units and LUMO in the FM unit (Figure 4), in line with the
experimental behavior exhibited by the lowest energy transi-
tions. Additionally, the calculated ground state dipole
moment was higher for 4b (μg = 6.33 D) than for 9b (μg =
1.34 D). From TDDFT calculations, the first vertical transi-
tion for 4b is of lower energy than that of 9b and involves a
smaller oscillator strength ( f = 0.12, λ ∼ 750 nm for 4b; f =
0.30, λ ∼ 664 nm for 9b). Although this trend is consistent
The normalized fluorescence spectra in toluene are shown
in Figure 3. Most compounds show broad and structureless
emission bands, with maxima centered at λ > 500 nm.
Excimer formation was ruled out after concentration depen-
dence studies revealed the same appearance at low concen-
trations (120-1200 nM for 4a; see Figure S4 in Supor-
ting Information).28 Moreover, vibrational features (νIR
∼
0.19 eV) were found in the emission spectra for some of the
compounds when measured in cyclohexane at room tem-
perature and in glasses of methyl-THF at 77 K (see Figures
S3 and S5 in Supporting Information). Since the intermole-
cular distance between chromophores is the same before and
after radiative relaxation for any excimer, there is no possi-
bility of vibrational modes in the emission spectrum because
(25) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; De
€
Angelis, F.; Di Censo, D.; Nazeeruddin, M. K.; Gratzel, M. J. Am. Chem.
Soc. 2006, 128, 16701.
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(30) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
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(27) Katritzky, A. R.; Zhu, D.-W.; Schanze, K. S. J. Phys. Chem. 1991,
95, 5737.
(28) See that pyrene excimer formation is achieved at concentrations
higher than 1 μM.
(31) Lee, C.; Yang, W.; Parr, G. Phys. Rev. B 1988, 37, 785.
(32) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
(b) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.;
DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (c) Rassolov, V.;
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8486 J. Org. Chem. Vol. 74, No. 21, 2009

Estrada and Neckers
JOCNote
Experimental Section
General Procedure for Knoevenagel Condensation. Starting
fluorenone (0.50 mmol) and malononitrile (5.0 mmol) were
combined under argon atmosphere and suspended in 5 mL of
argon-saturated pyridine. After 1 h of vigorous stirring, the
system was heated to 70 °C and allowed to further react for 2 h.
Upon cooling to room temperature, 10 mL of absolute ethanol
was added and then the mixture was cooled at -20 °C overnight.
The formed precipitate was collected by vacuum filtration and
washed with chilled ethanol. After air drying at 60 °C for 4-5 h,
the title compound was obtained.
2,7-Bis(1-(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)-2-ethynyl)-
fluoren-9-ylidene malononitrile (4b). Dark-gray powder. Analy-
tically pure product was obtained after recrystallization from
DMF (82 mg, 78% yield): mp >250 °C; IR (neat, cm-1) 2215,
1603; 1H NMR (CDCl3, 300 MHz) δ (ppm) 8.59 (s, 2H),
8.14-8.17 (d, J = 1.2 Hz, 4H), 7.72-7.82 (m, 6H), 7.59-7.64
(d, J = 8.4 Hz, 6H), 7.47-7.53 (dd, J1 = 1.2 Hz, J2 = 8.7 Hz,
4H), 7.40-7.45 (d, J = 8.7 Hz, 4H), 1.49 (s, 36H); 13C NMR
(CDCl3, 75.5 MHz) δ (ppm) 32.0, 34.8, 77.2, 88.9, 91.6, 109.2,
113.0, 116.3, 120.7, 121.0, 123.6, 123.8, 124.7, 126.4, 129.7,
133.3, 134.6, 137.9, 138.7, 138.8, 141.2, 143.3; MALDI-TOF-
MS 982.62 (M•þ); HRMS calcd for [C72H62N4] 982.4977, found
m/z 982.4974.
3,6-Bis(1-(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)-2-ethynyl)-
fluoren-9-ylidene malononitrile (9b). Maroon colored powder.
Analytically pure product was obtained as a deep red solid after
reprecipitation from pyridine/ethanol (89 mg, 73% yield): mp
240 °C (dec.); IR (neat, cm-1) 2202, 1598; 1H NMR (CDCl3, 300
MHz) δ (ppm) 8.40-8.45 (d, J = 8.1 Hz, 2H), 8.15 (s, 4H),
7.76-7.84 (m, 6H), 7.60-7.67 (d, J = 8.1 Hz, 4H), 7.52-7.58 (d,
J = 8.4 Hz, 2H), 7.44-7.51 (d, J = 8.7 Hz, 4H), 7.38-7.44 (s,
J = 8.7 Hz, 4H); 13C NMR (CDCl3, 75.5 MHz) δ (ppm) 32.0,
34.8, 76.0, 77.2, 89.6, 94.2, 109.2, 113.4, 116.4, 120.5, 123.6,
123.75, 123.78, 126.4, 126.9, 129.8, 132.5, 133.4, 133.8, 138.8,
139.1, 141.7, 143.5; MALDI-TOF-MS 982.66 (M•þ); HRMS
calcd for [C72H62N4 þ Hþ] 983.5053, found m/z 983.5056.
FIGURE 4. DFT calculated HOMO and LUMO orbitals for com-
pounds 4b and 9b in vacuum.
with the experimental observations, the calculated oscillator
strength for 4b seems to predict higher photon absorption
than what is seen from the UV-vis spectra. This should not
be surprising given the limitations of TDDFT when treating
long-range CT transitions.33 The transitions dealing with
LUMOþ1 and LUMOþ2 for 4b, which involve CPA units,
accounted for the biggest oscillator strengths. This also
suggests that S1 in this compound is mainly populated via
higher energy excited states (Sn where n > 1). Conversely, all
of the transitions involving LUMO for compound 9b showed
moderate oscillator strength. Finally, the difference between
the first excited state and ground state dipole moments was
found to be remarkably different for both compounds (Δμ =
þ2.35 D for 4b, and Δμ = þ35.1 D for 9b). Such numbers
anticipate a sensitive bathochromic shift from such transi-
tion in compound 9b, in line with the observed red-edge
behavior. Therefore, its unexpected absorption maximum
blue shift must be caused by coalescence of two or more
absorption bands.
In summary, four novel fluorene-based ambipolar com-
pounds were synthesized in order to evaluate the electronic
communication of positions 2,7 and 3,6 with C-9 in the
fluorene core. Compound 9b containing carbazole units
displayed efficient charge separation in moderate to high
polarity solvents. The structural design of these molecules
would aid in the search of novel materials as candidates for
electron transfer systems in which good charge separation is
required for varied applications in electronic devices.
Acknowledgment. Dr. X. Cai and Dr. I. Schapiro are
acknowledged for helpful discussions, and Prof. A. Tarnovsky
and Mr. P. Z. El-Khoury for support with the computations.
The calculations were performed with the support of the Ohio
Supercomputers Center. L.E. thanks the McMaster Endow-
ment for a fellowship. This work was financed by the Endow-
ment Fund of the Center of Photochemical Sciences.
Supporting Information Available: Experimental procedures
and characterization data. UV-vis absorption and fluorescence
spectra in various solvents. Fluorescence spectra in MeTHF at
1
298 and 77 K. H and 13C NMR spectra of final compounds.
DFT calculation data in the gas phase. This material is available
free of charge via the Internet at http://pubs.acs.org.
(33) Dreuw, A.; Head-Gordon, M. Chem. Rev. 2005, 105, 4009.
J. Org. Chem. Vol. 74, No. 21, 2009 8487