Structure-property relationships in conjugated donor-acceptor molecules based on cyanoanthracene: Computational and experimental studies

Article abstract of DOI:10.1021/jo051183g

Two series of π-conjugated bipolar compounds, namely, 9-phenyl-10-anthronitriles (PAN series) and 9-phenylethynyl-10-anthronitriles (PEAN series), having inherent redox centers have been synthesized and their electronic absorption, fluorescence emission,

Full text of DOI:10.1021/jo051183g

Structure-Property Relationships in Conjugated Donor-Acceptor
Molecules Based on Cyanoanthracene: Computational and
Experimental Studies
Jui-Hsien Lin, Arumugasamy Elangovan,† and Tong-Ing Ho*
Department of Chemistry, National Taiwan University, Taipei - 106, Taiwan
hall@ntu.edu.tw
Received June 11, 2005
Two series of π-conjugated bipolar compounds, namely, 9-phenyl-10-anthronitriles (PAN series)
and 9-phenylethynyl-10-anthronitriles (PEAN series), having inherent redox centers have been
synthesized and their electronic absorption, fluorescence emission, and electrochemical behavior
have been studied. Electrochemiluminescence of these molecules bearing weak, strong, and spin-
polarized donors is also studied. The observed electronic properties are explained with the help of
results obtained from density functional theory (DFT- B3LYP/6-31G*) calculations. The structure-
property relationships of all the molecules are discussed.
Introduction
Charge recombination of electrolytically generated
radical ions in solutions leads to the formation of
electronically excited states of molecules by energetic
electron-transfer reactions at the electrified interface.
Such excited species emit energy in the form of fluores-
cence of a certain wavelength. This process is called
electrogenerated chemiluminescence or electrochemilu-
minescence (ECL).5 ECL emission is believed to occur via
one of the three different routes: directly from singlet
excited state (S-route), via triplet-triplet annihilation (T-
route), or via excimer formation (E-route).6 Excimers are
homodimers that exist in the photoexcited state of
molecules in solution.7 Many excimers are formed be-
tween molecules whose π-systems interact effectively,
leading to the formation of appropriate states, and they
possess structures prominently governed by their mono-
mer chemical structures. The formation of excited states
in the solid state by the application of an electric field
Organic π-conjugated donor-acceptor (D-A) molecular
materials have attracted significant attention due to their
potential application in electronics such as electrooptic
devices,1 light-emitting diodes,2 and field effect transis-
tors.3 The synthetic ability of organic chemists coupled
with the wealth of fabrication techniques available to
materials scientists and engineers contribute to the
realization of materials for our future needs. Suzuki
coupling and Sonogashira coupling approaches have
proved to be some of the best established techniques for
the synthesis of a wide variety of conjugated organic
materials.4
† Present address: Department of Chemistry, University of Wash-
ington, Seattle, WA 98195.
* Fax: +886-2-23636359.
(1) (a) Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.;
Robinson, B. H.; Steier, W. H. Science 2000, 288, 119. (b) Dalton, L.
R.; Steier, W. H.; Robinson, B. H.; Zhang, C.; Ren, A.; Garner, S.; Chen,
A.; Londergan, T.; Irwin, L.; Carlson, B.; Fifield, L.; Phelan, G.; Kincaid,
C.; Amend, J.; Jen, A. J. Mater. Chem. 1999, 9, 1905.
(2) (a) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J.
H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.;
Bre´das, J.-L.; Lo¨gdlund M.; Salaneck, W. R. Nature 1999, 397, 121.
(b) Kraft, A.; Grimsdale A. C.; Holmes, A. B. Angew. Chem., Int. Ed.
1998, 37, 402.
(4) (a) Wonag, K.-T.; Hung, T. S.; Lin, Y.; Wu, C.-C.; Lee, G.-H.;
Peng, S.-M.; Chou, C.-H.; Su, O. Y. Org. Lett. 2002, 4, 513. (b)
Yamaguchi, Y.; Kobayashi, S.; Miyamura, S.; Okamoto, Y.; Wakamiya,
T.; Matsubura, Y.; Yoshida, Z.-i. Angew. Chem., Int. Ed. 2004, 43, 366.
(c) Boydston, A. J.; Yin, Y.; Pagenkopf, B. L. J. Am. Chem. Soc. 2004,
126, 3724. (d) Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett.
2003, 5, 4245, (e) Pohl, R.; Anzenbacher, P., Jr. Org. Lett. 2003, 5,
2769. (f) Metivier, R.; Amengual, R.; Leray, I.; Michelet, V.; Genet,
J.-P. Org. Lett. 2004, 6, 739.
(3) Brown, A. R.; Pomp, A.; Hart C. M.; de Leeuw, D. M. Science
1995, 270, 972.
10.1021/jo051183g CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/05/2005
J. Org. Chem. 2005, 70, 7397-7407
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Lin et al.
(as in OLEDs) has been shown to follow a mechanism
similar to that in solution (ECL).8 Hence, the study of
electronic properties of more new donor-acceptor mo-
lecular systems is necessary to gain a better understand-
ing of the basis underlying the electron-transfer processes
occurring in the materials upon interaction with external
stimuli such as electromagnetic radiation and addition
and removal of electrons to and from the molecules.
In the present work, two series of compounds, namely,
biaryls (9-phenyl-10-anthronitriles, PANs) and biaryl-
ethynes (9-phenylethynyl-10-anthronitriles, PEANs), hav-
ing donor-acceptor character were synthesized and their
photophysical properties, electrochemical characteristics,
and ECL were studied. The donors are methoxy- (ie.,
anisole, An-), p-N,N-(dimethylamino)- (DMA-), p-(2,5-
dithienyl)pyrrolyl- (DTP-), and N,N-di-p-anisylamino-
(An2N-) substituted phenyls, and the acceptor is the
anthronitrile (AN) moiety. The approach is to provide the
fluorophore AN with incremental electron-donating
strength at the phenyl moiety from no donor to weak
(OMe) to strong (NMe2) donors and high-spin donors
[N-(2,5-dithienylpyrrolyl)phenyl- (DTP-P) and N,N-di-
p-anisylaminophenyl- (An2N-P)] in both series (the ethy-
nyl linkage is denoted by E).
electron acceptor. Moreover, 9-cyano-10-halo anthracenes
have been found to afford, upon electroreduction, radical
anion intermediates stable enough for the electrochemical
studies.13 Substitution of the halogen with electron-
donating groups via conjugation would help achieve new
materials having tunable properties. The anthronitrile
moiety can additionally impart intermolecular π-π in-
teraction, and by appropriate choice of conditions, the
chromophore activity can be controlled and the emission
can be tuned.
Results and Discussion
The chemical structures of the compounds studied in
the present work are listed in Chart 1. Compounds of
the PAN series were synthesized by palladium-catalyzed
Suzuki coupling14 of 9-bromo-10-cyanoanthracene (Br-
AN) with the corresponding donor-substituted phenyl-
boronic acids. The PEAN series of internal ethynes were
prepared by the coupling reaction of the corresponding
terminal aryl acetylenes with Br-AN under modified
Sonogashira conditions.15 A summary of the photophysi-
cal data of these compounds is presented in Table 1, and
their electrochemical characteristics are furnished in
Table 2.
The triarylamino and the dithienylpyrrolyl (high-spin)
donors have been chosen in view of the substantial
amount of research done on them by Sugawara,9a Jen9b
Yano,10 Blackstock,11 and Lambert12 on the synthesis and
study of adiabatic electron-transfer properties in certain
mixed-valence systems.
Photophysical Properties
The photophysical characteristics of the two series of
compounds are given in Table 1. The UV-vis absorption
spectra of all the compounds recorded in CH2Cl2 showed
an absorption at about 270 nm characteristic (â-band) of
an anthracene unit. The π-π* transition region of
electronic absorption spectra is shown in Figure 1. Figure
1a depicts the electronic absorption spectra of the PAN
series in which one can see the pronounced effect of
structural modification. While all molecules show struc-
tured π-π* that can be attributed to the local excitation
(LE) all centered around 410 nm, DMA-PAN and An2N-
PAN show a moderate charge transfer component as seen
from the appearance of a shoulderlike hump around 420
nm accompanied by some degree of loss of fine structure.
Surprisingly, the DTP-PAN (though bearing an electron-
donating DTP-) did not exhibit a similar trend; instead,
while retaining the major part of its vibrational fine
structure along the lines of PAN and AnAN in its
absorption, it shows an unusual band around 350 nm.
One explanation could be that this band might arise from
the probability of an intramoiety charge transfer from
the pyrrole (donor) moiety to the thiophenyl (acceptor)
moiety. The effect of extension of π-conjugation is evident
from the absorption spectra of the PEAN series. It can
be seen from Figure 1b that, for those PEANs having
poor donors (PEAN and AnEAN), the longest wave-
length of absorption maximum falls at about 440 nm as
structured bands at a wavelength that is red-shifted by
about 40 nm as compared with the PAN series. The
vertically excited lowest singlet state (the Franck-
Condon state) experiences no charge transfer as in PAN
The anthronitrile (AN) moiety has been chosen in view
of the fact that it is an excellent fluorophore and a strong
(5) (a) Faulkner, L. R.; Bard, A. J. Electrogenerated Chemilumi-
nescence. In Electrochemical Methods; John Wiley & Sons: New York,
1980; pp 621-627. (b) Faulkner L. R.; Bard, A. J. In Electroanalytical
Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10,
pp 1-95. (c) Knight, A. W.; Greenway, G. M. Analyst 1994, 119, 879.
(d) Knight, A. W. Trends Anal. Chem. 1999, 18, 47. (e) Richter, M. M.
Chem. Rev. 2004, 104, 3003. (f) Lai, R. Y.; Fabrizio, E. F.; Jenekhe, S.
A.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 9112. (g) Knorr, A.; Daub,
J. Angew. Chem., Int. Ed. 1995, 34, 2664. (h) Prieto, I.; Teetsov, J.;
Fox, M. A.; Vanden Bout, D. A.; Bard, A. J. J. Phys. Chem. 2001, A105,
520. (i) Oyama, M.; Okazaki, S. Anal. Chem. 1998, 70, 5079. (j)
Kapturkievicz, A. J. Electroanal. Chem. 1990, 290, 135. (k) Kapturk-
ievicz, A. J. Electroanal. Chem. 1991, 302, 13. (l) Faulkner, L. R.;
Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 691.
(6) Faulkner, L. R.; Bard, A. J. In Electroanalytical Chemistry; Bard,
A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 10, pp 1-95.
(7) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/
Cummings Publishing Co., Inc.: Menlo Park, CA, 1978.
(8) (a) Anderson, J. D.; et al. J. Am. Chem. Soc. 1998, 120, 9646. (b)
Armstrong, N. R.; Anderson, J. D.; Lee, P. A.; McDonald, E. M.;
Wightman, R. M.; Hall, H. K.; Hopkins, T.; Padias, A.; Thayumanavan,
S.; Barlow, S.; Marder, S. R. SPIE 1999, 3476, 178. (c) Armstrong, N.
R.; Whightman, R. M.; Gross, E. M. Annu. Rev. Phys. Chem. 2001, 52,
391.
(9) (a) Nakazaki, J.; Chung, I.; Matsushita, M. M.; Sugawara, T.;
Watanabe, R.; Izuoka, A.; Kawada, Y. J. Mater. Chem. 2003, 13, 1011.
(b) Kim, K.-S.; Kang, M.-S.; Ma. H.; Jen, A. K.-Y. Chem. Mater. 2004,
16, 1558.
(10) (a) Sato, K.; Yano, M.; Furuichi, M.; Shiomi, D.; Abe, K.; Takui,
T.; Itoh, K.; Higuchi, A.; Katuma, K.; Shirota, Y. J. Am. Chem. Soc.
1997, 119, 6607. (b) Yano, M.; Ishida, Y.; Aoyama, K.; Tatsumi, M.;
Sato, K.; Shiomi, D.; Ichimura, A.; Takui, T. Synth. Metals 2003, 137,
1275.
(11) (a) Stickley, K. R.; Blackstock, S. C. J. Am. Chem. Soc. 1994,
116, 11576. (b) Stickley, K. R.; Selby, T. D.; Blackstock, A. C. J. Org.
Chem. 1997, 62, 448.
(12) (a) Lambert, C.; No¨ll, G.; Schelter, J. Nat. Mater. 2002, 1, 69.
(b) Lambert, C.; No¨ll, G.; Schma¨lzlin, E.; Meerholz, K.; Bra¨uchele, C.
Chem. Eur. J. 1998, 4, 2129. (c) Heckmann, A.; Lambert, C.; Goebel,
M.; Wortmann, R. Angew. Chem., Int. Ed. 2004, 43, 5851. (d) Lambert,
C.; No¨ll, G. J. Am. Chem. Soc. 1999, 121, 8434. (e) Lambert, C.; No¨ll,
G. J. Chem. Soc., Perkin Trans. 2 2002, 2039.
(13) Heinze, J.; Schwart, J. J. Electroanal. Chem. 1981, 126, 283.
(14) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(15) (a) Elangovan, A.; Wang, Y.-H.; Ho, T.-I. Org. Lett. 2003, 5,
1841. (b) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 16, 4467.
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Electronic Properties of Donor-Acceptor Molecules
CHART 1. Chemical Structures of Compounds Studied in the Present Work: PAN Series (Top) and PEAN
Series (Bottom)
TABLE 1. Photophysical Data of PANs and PEANs
TICT state) is absent when the donor and acceptor are
separated by an ethynyl π-bridge. In fact, the DTP-
PEAN shows emission similar to that of PEAN. The
AnAN shows moderate charge transfer, but this does not
seem to effect a major shift in the emission wavelength
as compared with DMA-PAN. In Figure 2b, compounds
PEAN and DTP-PEAN emit closely at the same maxima
with vibrational fine structure, while AnEAN attains
moderate charge-transfer character given its structure-
less emission band again due to the subtle increase in
donor strength replacing H with OMe (Figure 2b).
Compounds DMA-PEAN and An2N-PEAN show vast
red-shifted fluorescence emission at 590 and 575 nm,
respectively, which can be assigned to an excited-state
charge-transfer transition.
abs
flu
λmax
∆EHL
λmax
b
compounds
(nm)
(eV)a
ꢀmax
(nm)
Φc
PAN
389
390
408
388
410
422
430
483
424
490
2.85
2.85
2.48
2.85
2.48
2.64
2.64
2.25
2.64
2.25
0.68
0.84
0.44
0.42
0.80
1.46
1.74
3.07
2.50
3.38
455
470
562
443
454
465
485
588
469
574
0.46
0.10
0.09
0.004
0.03
0.86
0.71
0.04
0.001
0.002
AnAN
DMA-PAN
DTP-PAN
An2N-PAN
PEAN
AnPEAN
DMA-PEAN
DTP-PEAN
An2N-PEAN
a ∆EHL: HOMO-LUMO gap calculated from the onset of the
visible absorption maxima. b (×104 M-1 cm-1). c Using coumarin
334 standard (Φ 0.69 in MeOH).16
On comparison of the data in Table 1, the PEAN series
shows absorption red-shifted by 30-90 nm as compared
with the PAN series. Similarly, the fluorescence spectra
show a 10-125 nm red shift as compared with PAN
series. Thus, extension of π-conjugation shifted the
electronic properties in the desirable region useful for
light-emitting applications. In general, the PEAN series
has been found to show absorption and emission sub-
stantially red-shifted (into the visible region) as compared
with PANs and free AN (λmaxabs ) 400 nm; λmaxflu ) 440).
and AnAN. For the PEAN having a stronger donor
(DMA-PEAN and An2N-PEAN), the absorption shifts
to >483 nm with a structureless feature. This vast red
shift can be attributed to absorption due to charge-
transfer transition at the lowest Franck-Condon excited
state. In this series also the DTP-substituted molecule
DTP-PEAN behaves similar to DTP-PAN in that the
LE band is retained with the appearance of an additional
band around 300-350 nm.
The fluorescence spectra of the PAN series (Figure 2a)
and PEAN series (Figure 2b) are interesting to study.
PAN and An2N-PAN show surprisingly similar emission
patterns, while DMA-PAN alone shows a charge-transfer
spectrum. While all compounds in the PAN series show
emission maxima close to 440 nm (the so-called B band),
compounds DMA-PAN and DTP-PAN show additional,
more intense A bands at 570 and 520 nm respectively.
This observation of dual fluorescence in these two
systems may be attributable to the existence of twisted
intramolecular charge transfer states (TICTs).17 It may
be noted that this kind of dual fluorescence (vis-a`-vis
(16) Reynolds G. A.; Drexhage, K. H. Optics Commun. 1975, 13, 222.
(17) (a) Rettig, W. Electron-Transfer I. In Topics in Current Chem-
istry; Mattay, J., Ed..; Springer-Verlag: Berlin, 1994; Chapter 5, p 253.
(b) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, Z. R. Chem. Phys.
Lett. 1973, 19, 315. (c) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk,
A.; Cowley, D. J.; Baumann, W. Nouv. J. Chim. 1979, 3, 443. (d) Rettig,
W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971. (e) Rettig, W.; Maus,
M. In Conformational Analysis of Molecules in Excited States; Waluk,
J., Ed.; Wiley-VCH: New York, 2000; Chapter 1, pp 1-55. (f)
Grabowski, Z. R.; Rotkiewicz, K. Chem. Rev. 2003, 103, 3899. (g) Yang,
J.-S.; Liau, K.-L.; Wang, C.-M.; Hwang, C.-Y. J. Am. Chem. Soc. 2004,
126, 12325. (h) Klock, A. M.; Rettig, W. Polish J. Chem. 1993, 67, 1375.
(i) Baumann, W.; Schwager, B.; Detzer, N.; Okada, T.; Mataga, N. Bull.
Chem. Soc. Jpn. 1987, 60, 4245.
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Lin et al.
TABLE 2. ECL and Electrochemical Data for PANs and PEANs Recorded in Acetonitrile and Dichloromethane with 50
mM TBAP at a Scan Rate of 50 mV/s
Ep,ox (V)
Ep,red (V)
∆EHL (eV)a
-∆H° (eV)b
compound
λmaxECL (nm, eV)
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
MeCN
CH2Cl2
PAN
456
480
538
535
512
552
547
588
590
2.72
2.58
2.30
2.32
2.42
2.25
2.27
2.11
2.10
2.72
1.79
1.71
1.68
0.89
0.86
1.62
1.49
0.90
0.96
1.00
1.80
1.48
1.08
0.86
0.86
1.14
0.82
0.97
0.87
0.90
-1.13
-1.32
-1.09
-1.28
-1.36
-0.92
-0.89
-0.90
-1.00
-1.05
-1.55
-1.53
-1.58
-1.56
-1.37
-1.24
-1.32
-1.35
-1.30
-1.29
2.92
3.23
2.77
2.17
2.22
2.54
2.38
1.80
1.96
2.05
3.35
3.01
2.66
2.42
2.43
2.38
2.14
2.32
2.17
2.19
2.76
2.87
2.61
2.01
2.06
2.38
2.22
1.64
1.80
1.89
3.19
2.85
2.50
2.26
2.07
2.22
1.98
2.16
2.01
2.03
AnAN
DMA-PAN
DTP-PAN
An2N-PAN
PEAN
AnPEAN
DMA-PEAN
DTP-PEAN
An2N-PEAN
a HOMO-LUMO gap calculated as the difference between the two peak potentials. b Calculated using the equation ∆H° ) Ep,ox
Ep,red - 0.16.5k
-
FIGURE 1. UV-visible absorption spectra of PAN series (a) and PEAN series (b) in CH2Cl2 (10-5 M).
FIGURE 2. Fluorescence emission spectra of PANs (a) and PEANs (b) in CH2Cl2 (10-5 M).
The photoluminescence quantum yields (Φ measured
in CH2Cl2) are moderate to quite high for the PEAN
series as compared with the PAN series, especially the
weak donor-substituted ones (Table 1). In both series,
weak donor-substituted compounds show higher Φ than
strong donor-substituted compounds. The optical band
gap (∆EHL) calculated from the onset of the absorption
maxima are found to be in the exploitable region for
emitting devices. The ∆EHL has been decreased to near
2.25 eV for ethynes as compared with directly linked
biaryls for which it remains between 2.48 and 2.9 eV.
These properties arise due to the donor-acceptor char-
acter of these molecules and extension of π-conjugation
particularly in PEAN series.
The existence of ICT character can be understood very
well by testing the ability of the solute molecules in
undergoing shifts in their emission maxima with increas-
ing solvent polarity. Hence, the solvatochromic spectra
of these fluorophore materials were checked by choosing
different solvents of varying polarity ranging from non-
polar hexane through highly polar DMSO. The solvato-
chromic fluorescence spectra of all compounds are pro-
vided in Figure 3. It is interesting to note that the general
solvatochromic trend in both the series is almost identi-
cal; however, a case by case analysis is worthwhile.
Neither PAN nor PEAN experienced any solute-solvent
dipole-dielectric interaction as seen from the absence of
ICT fluorescence bands, though there is a very mild red
7400 J. Org. Chem., Vol. 70, No. 18, 2005

Electronic Properties of Donor-Acceptor Molecules
FIGURE 3. Fluorescence solvatochromic spectra of PANs (left column) and PEANs (right column) recorded in various solvents
of differing polarity. DMA-PEAN and An2N-PEAN are not fluorescent in solvents higher in polarity than CH2Cl2.
shift in the emission maxima. Upon substitution of a
moderately strong donor, OMe group, the maxima gradu-
ally loose their vibronic structure and attain a structure-
less feature when going from less polar hexane through
J. Org. Chem, Vol. 70, No. 18, 2005 7401

Lin et al.
more polar DMSO. The effect of ethyne substitution is
well pronounced in the case of AnEAN as compared with
AnAN in that the red shift is more noticeable and well
resolved in the spectra of the former in comparison to
that of the latter.
A plot of the polarity (or the orientation polarizability,
∆f) of the solvents against the Stokes shift for selected
compounds in various solvents is shown in Figure 4. The
observed linear correlation between the Stokes shift and
the ∆f illustrates the adherence of experimental data to
the Lippert equation in the case of AnEAN. Other
molecules show moderate deviation reflecting the obser-
vation of regular (and irregular) red shift in the emission
maxima (see Figure 3).
Electrochemistry. To determine the electrochemical
activity and to arrive at the reduction and oxidation
potential values of these compounds, cyclic voltammo-
grams (CV) were recorded. CV of PANs and PEANs (1.0
× 10-3 M) were recorded in acetonitrile as well as in CH2-
Cl2 with 50 mM tetrabutylammonium perchlorate (TBAP)
as a supporting electrolyte at a scan rate of 0.05 V/s, and
the reduction and oxidation peak potentials are shown
in Table 2.
The scan rates were varied from 50 to 125 mV in
increments of 25 mV in an attempt to see any variation
in the peak potential with change in scan rates. Figure
5 shows CV curves recorded for An2N-PAN and An2N-
PEAN, and, as expected, these compounds showed
reversible oxidation and reversible reduction one-electron
transfer reactions. No significant shift in the peak
potentials was observed with varying scan rates, and only
the current was found to be reduced. Among the two
series of compounds, PANs showed moderate to very
good electrochemical reversibility (see Supporting Infor-
mation for values) in both oxidation and reduction cycles;
in contrast, PEANs (except DTP-PEAN and An2N-
PEAN) showed irreversible oxidation in both MeCN and
CH2Cl2, although unsubstituted PEAN showed moderate
reversibility of oxidation reaction in CH2Cl2 but complete
irreversibility in MeCN.
Amino-substituted DMA-PAN and An2N-PAN show
two different emission maxima. The first emission band
is centered around 450 nm with fine structures for both
molecules. This band can be described as LE emission
and does not appear to be affected by the solvent
dielectric constant. The second, structured A band, which
is more intense than the structureless B band appearing
at a longer wavelength (>520 nm) and is likely due to
the TICT state, alone undergoes a shift to the much lower
energy region with increasing solvent polarity and with
concomitant reduction in the intensity. This kind of TICT
is not observed in the case of ethynyl counterparts of
these two chromophores, namely, the DMA-PEAN and
An2N-PEAN. This could be due to the fact that the
ethynyl bridge allows for conformational twisting with
respect to its long axis, and as a result, ICT in the twisted
state is not possible and only normal ICT is observed.
The absence of dual-fluorescence in PEANs confirms that
the observation of A bands in DMA-PAN and An2N-PAN
is due to the TICT transition. The ICT in the ethyne-
linked donor acceptor molecules is more noticeable, as
stated above, than in directly linked PANs with a larger
red shift (>100 nm) going from hexane to CH2Cl2. An
additional interesting point is the failure to observe any
influence of solvent polarity on the emission behavior of
dithienylpyrrole-substituted PAN and PEAN. Both DTP-
PAN and DTP-PEAN show similar patterns of spectra
in the solvatochromic studies in that there is no pro-
nounced shift in the emission maximum upon increasing
the dielectric constant of the media, though only CH2Cl2
causes a shift in the emission of DTP-PEAN (500 nm
band), and this could be from an excimer state as also
noticed in the case of DTP-PAN.
A fluorophore, by the virtue of its electronic structure,
is considered to be a dipole. This dipole (with a dipole
moment µ) in solution can interact with its surroundings,
the medium characterized by its dielectric constant (∈)
and refractive index (n). This interaction produces changes
in energy of both the ground state and excited state of
the molecule. As a result, the fluorophore emits light of
different energies, and the difference between the ground
state and excited-state energy levels is given as the
Stokes shift. This Stokes shift is a property of the solvent
refractive index and dielectric constant. The influence of
the local molecular environment on the optical property
of the molecules studied here can be understood by using
the Lippert equation; a model that describes the interac-
tions between the solvent and the dipole moment of
chromophore.18
In less polar CH2Cl2, the reversible reduction peak
potentials, which correspond to the reduction at the
9-cyanoanthracene moiety, occur in the vicinity of around
-1.3 V for PEANs, whereas they occur at about -1.55
V for PANs. The extension of conjugation via a C-C
triple bond has altered the reduction potential favorably.
Computation
The observation of the electronic properties can be
better understood by analyzing the results obtained from
density functional calculations carried out using Becke’s
three parameter set with Lee-Yang-Parr modification
(B3LYP) with the 6-31G* basis set of theory.19 The
calculated highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO)
surfaces of the molecules PAN and AnAN both have
twisted structures in the energy-minimized gas-phase
geometry. Their HOMO and LUMO are both localized
on the acceptor AN moiety. All electronic properties are
hence governed by these orbitals, and, as a result, we
observe structured absorption and LE emission for these
two molecules. For DMA-PAN and An2N-PAN, the
HOMO is localized in the donor site (NMe2 and An2N-)
νabs - νflu ) 2/hc[∆f]{(µ* - µ)2/a3} + Constant
where
(18) (a) Lakovickz, J. R. Principles of Fluorescence Spectroscopy
Plenum Press: New York, 1983; p 187 (b) Lippert, Von E. Z.
Electrochem. 1957, 61, 962.
(19) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(b) Beck, A. D. J. Chem. Phys. 1993, 98, 5648.
∆f ) [(∈ - 1)/(2∈ +1)] - [(n2 - 1)/(2n2 + 1)]
and a is the radius of the chromophore
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Electronic Properties of Donor-Acceptor Molecules
FIGURE 4. Linear correlation of orientation polarization (∆f) of solvent media with Stokes shift (∆ν) for PAN (a), AnAN (b),
PEAN (c), and AnEAN (d). See Supporting Information for data tables of all compounds.
FIGURE 5. CV traces of An2N-PAN (left) and An2N-PEAN (right) recorded with reference to Ag/Ag+ at different scan rates
in CH2Cl2 (10-3 M with 50 mM TBAP). Color code: red, 50 mV; blue, 75 mV; green, 100 mV; crimson, 125 mV; navy blue,
150 mV.
groups with some degree of orbital coefficients on the
phenylene bridge, whereas the LUMO is located mainly
on the acceptor AN moiety. Therefore, probable intramo-
lecular charge transfer (ICT) transition can be expected
in these two systems. Considering DTP-PAN, there
seems to be no involvement of phenylene bridge, and
there is no adequate contribution to either the HOMO
or the LUMO. Thus, the donor π-electron remains cut
off from the LUMO coefficients, and hence there can be
no communication between the two parts. This is evident
from the observation of absorption and emission spectra
that do not reveal any ICT character for this particular
compound.
The same trend can be observed for the PEAN series
(Figure 7). The poor-donor-substituted PEAN, AnEAN,
DMA-PEAN, and An2N-PEAN have orbital contribu-
J. Org. Chem, Vol. 70, No. 18, 2005 7403

Lin et al.
FIGURE 6. HOMO (bottom) and LUMO (top) orbital surfaces of (from left to right) PAN, AnAN, DMA-PAN, DTP-PAN, and
An2N-PAN.
FIGURE 7. HOMO (bottom) and LUMO (top) orbital surfaces of (from left to right) PEAN, AnEAN, DMA-PEAN, DTP-PEAN,
and An2N-PEAN.
FIGURE 8. HOMO-LUMO energy levels of PANs and
PEANs. Molecule code: 1, PAN; 2, AnAN; 3, DMA-PAN; 4,
DTP-PAN; 5, An2N-PAN; 6, PEAN; 7, AnEAN; 8, DMA-
PEAN; 9, DTP-PEAN; 10, An2N-PEAN. This energy level
FIGURE 9. Comparison of experimental [optical (UV-vis
diagram shows the influence of the donor substituents on the
λmax/eV) and electrochemical] energy gaps with theoretical
HOMO and LUMO energies.
values. Compound code: 1, PAN; 2, AnAN; 3, DMA-PAN; 4,
DTP-PAN; 5, An2N-PAN; 6, PEAN; 7, AnEAN; 8, DMA-
PEAN; 9, DTP-PEAN; 10, An2N-PEAN.
tions similar to those of PAN, AnAN, DMA-PAN, and
An2N-PEAN, respectively. They additionally have con-
tribution from the ethynyl linkage, and thus the ethyne
participates in the electronic properties of these systems.
The ethyne bridge does not seem to affect in any manner
the properties of DTP-PEAN. The orbital coefficients of
the latter indeed remain similar to that of DTP-PAN,
despite the extension of π-conjugation by the C-C triple
bond and hence the observed electronic properties.
The energy levels of HOMO and LUMO are of interest
for study. As seen from Figure 8, both the HOMO energy
and LUMO energy increase on increasing the strength
of electron donors for the two series. However, the trend
breaks at DTP-substituted PAN as well as PEAN, where
the LUMO level is reduced when compared with the rest.
The energy gaps, values in electronvolts (eV), obtained
from the theoretical calculations and UV-visible absorp-
tion maxima and electrochemical data, are depicted in
the graph in Figure 9. Apart from a slight variation for
each compound, there is a satisfactory correlation of the
7404 J. Org. Chem., Vol. 70, No. 18, 2005

Electronic Properties of Donor-Acceptor Molecules
the DFT calculations, the HOMO-LUMO gap as ob-
tained from the DFT results and electrochemical results
appears to differ vastly as with the DTP-substituted
molecules (DTP-PAN, 4, and DTP-PEAN, 9), which also
have wide difference in their energy gap values.
The intriguing electronic nature of the DTP-substituted
systems is well augmented in the theoretical dipole
moment, as shown in Figure 10. While the dipole
increases upon increasing the electron donor strength,
it reduces drastically in the case of DTP-substituted
molecules in both PAN and PEAN series. The highest
dipole among all compounds rests with the NMe2-
substituted molecules. The dipole vector always remains
directed toward the cyanoanthryl subunit. Calculated
dipole moments are in good agreement with the observed
red shift in the emission maxima of the compounds. As
the dipole moment increases from PAN to DMA-PAN,
there is a gradual shift in the emission wavelength upon
increasing the solvent polarity when going from PAN to
AnAN. This applies to ethynyl counterparts as well
(Figure 3). Again, as the dipole, as well as the emission
maxima, of DTP-substituted molecules diminishes greatly
in both DTP-PAN and DTP-PEAN, they do not vary
with increasing solvent dielectric constant, indicating
that these molecular dipoles do not interact with the
medium and hence the observed spectra.
FIGURE 10. Calculated dipole moments of compounds.
Compound code: 1, PAN; 2, AnAN; 3, DMA-PAN; 4, DTP-
PAN; 5, An2N-PAN; 6, PEAN; 7, AnEAN; 8, DMA-PEAN;
9, DTP-PEAN; 10, An2N-PEAN.
data from theory and UV-vis absorption maxima in that
all compounds show a similar trend except DTP-substi-
tuted molecules in PAN and PEAN series. While the
observed electronic properties of all compounds can be
well explained by the orbital coefficients obtained from
FIGURE 11. DFT-calculated changes in electrostatic potential surface, spin polarization, and spin density maps obtained after
adding an electron to (one-electron reduction, left column) and removing an electron from (one-electron oxidation, right column)
DTP-PAN.
J. Org. Chem, Vol. 70, No. 18, 2005 7405

Lin et al.
FIGURE 12. ECL spectra of AnAN (left) and DMA-PAN (right) recorded in CH2Cl2 (10-3 M) containing TBAP supporting
electrolyte (50 mM).
It is of interest to us to investigate the nature of the
charged molecules by DFT calculations in an attempt to
gain maximum insight into their electronic structure. An
expensive unrestricted HF model was to be incorporated
for the optimization of the equilibrium geometry of these
charged molecules. Figure 11 shows typical surfaces of
one molecule, namely, DTP-PAN.
with 50 mM tetrabutylammonium perchlorate as a
supporting electrolyte in a cell setup similar to the
previously published one.21 The electrodes were pulsed
between first oxidation and first reduction peak poten-
tials at various pulse intervals in order to generate
radical ions and induce annihilation reaction. Satisfac-
tory ECL spectra were obtained at a 10-3 M concentration
of sample solution, while no ECL was observed at lower
or higher concentrations.
The ECL maxima (Table 2) for PAN, AnAN, DMA-
PAN, and DMA-PEAN are very close to the photolumi-
nescence maxima recorded in Table 1. They can be
considered as emitting from their monomeric S1 state
(-∆H values higher than ECL emission maximum in eV).
All other molecules in both the PEAN and PAN series
show ECL emission red-shifted by >80 nm, and they have
energy insufficient to populate the singlet state; thus,
they followed the T-route (triplet-triplet annihilation)
to produce the emitting state.
The appearance of the ECL spectra of AnAN and
DMA-PAN is interesting (Figure 12). Even though
AnAN displays predominantly monomer ECL, there is
a latent tendency for this molecule to produce the excimer
state as seen from an inherent poorly generated shoulder
around 525 nm that exists within the monomer emission
band. DMA-PAN shows a shoulder at a higher energy
area of the spectrum, indicating the tendency for forma-
tion of a blue-shifted ECL. Full formation of blue-shifted
ECL with respect to photoluminescence spectra of struc-
turally related compounds have been well studied and
documented from our labs.22
The potential density surface of the neutral molecules
is shown at the center, and the spin density and spin
surfaces are shown, respectively, on top and bottom. The
structures on the left column represent the result of one-
electron reduction, and those on the right represent the
outcome of one-electron oxidation. As evident in Figure
11, the reduction process adds an electron to the mol-
ecule, and as a result of the strong electron-withdrawing
nature of the acceptor AN moiety, the electron reaches
this part of the molecules and resides on it. The electro-
static potential shows more (red) negative density at this
part. The spin density, as well as the spin surface on the
left-hand side, depicts clearly the existence of the added
electron at the acceptor moiety AN. On the other hand,
removal of an electron from the neutral molecules pro-
duces an oxidized species, and the positive charge (the
radical cation) resides at the donor part showing the oxi-
dation occurring on this moiety. The donor moiety attains
lass electron density (more blue), and the corresponding
spin density map and the spin surface further confirm
this observation. As a result of removal of an electron,
the donor and acceptor moieties attain coplanarity, but
the bridge phenylene remains uninvolved and twisted.
The ethyne counterpart of the above molecule, DTP-
PEAN, showed a slightly different trend as the charging
produced species in which the phenylene unit, which
retained a 63.39° dihedral angle with the acceptor in its
neutral form, attains coplanarity with the acceptor AN
moiety upon receiving an electron.20 Thus, the spin
polarization can be achieved by using this kind of donor
and acceptor to produce molecules suitable for electronic
applications.
Conclusion
Conjugated donor-acceptor molecules based on 9-cy-
anoanthracene have been prepared by Suzuki coupling
(21) (a) Chen, F.-C.; Ho, J.-H.; Chen, C.-Y.; Su, Y. O.; Ho, T.-I. J.
Electroanal. Chem. 2001, 499, 17. (b) Elangovan, A.; Chen, T.-Y.; Chen,
C.-Y.; Ho, T.-I. Chem. Commun. 2003, 2146.
(22) (a) Elangovan, A.; Yang, S.-W.; Lin, J.-H.; Kao, K.-M.; Ho, T.-
I. Org. Biomol. Chem. 2004, 2, 1597. (b) Elangovan, A.; Chiu, H.-H.;
Yang, S.-W.; Ho, T.-I. Org. Biomol. Chem. 2004, 2, 3113. (c) Ho, T.-I.;
Elangovan, A.; Hsu, H.-Y.; Yang, S.-W. J. Phys. Chem. B 2005, 109,
8626. (d) Elangovan, A.; Lin, J.-H.; Yang, S.-W.; Hsu, H.-Y.; Ho, T.-I.
J. Org. Chem. 2004, 69, 8086. (e) Elangovan, A.; Kao, K.-M.; Yang,
S.-W.; Chen, Y.-L.; Ho, T.-I.; Su, O. Y. J. Org. Chem. 2005, 70, 4460.
Electrochemiluminescence. ECL spectra were re-
corded in acetonitrile at 1.0 and 0.10 mM concentrations
(20) Optimization of radical cation failed after 3 days, 25 iterations.
7406 J. Org. Chem., Vol. 70, No. 18, 2005

Electronic Properties of Donor-Acceptor Molecules
and Sonogashira coupling approaches in order to get
directly linked and enthyne-bridged bipolar centers. All
10 compounds show interesting electronic characteristics.
While directly linked Suzuki coupling products show
absorption properties comparable with free AN, their
ethyne-bridged counterparts possess more favorable prop-
erties in that the electronic absorption and fluorescence
emission maxima are very much shifted to the longer
wavelength regions. N,N-Dimethylaminophenylanthroni-
trile (DMA-PAN)17 provides a guideline for the inter-
pretation of the observed photophysical properties of the
whole set of molecules. As stated,17a,h this molecule
exhibits TICT in medium-polarity solvents such as
benzene, methylene chloride, and THF, while it shows
normal LE emission in other solvents. Similar behavior
is observed in the case of triarylamino donor-bearing
anthronitrile, An2N-PAN, but not in dithienylpyrrole-
bearing phenylanthronitrile, DTP-PAN. There appears
to be no electronic communication between the DTP
donor and AN acceptor. The ethynes on the other hand
exhibit no TICT character in either the triarylamino
donor system or the DMA donor system. The adherence
of selected molecular systems to the Lippert equation
explains the molecular environment dependence of the
Stokes shift. Very good correlation has been made for the
true ICT molecule, but poor correlation was noticed for
poor- or non-ICT systems.
monomer ECL do so from the singlet state, while those
exhibiting excimer ECL do so via the triplet-triplet
annihilation route as explained by the difference in the
ECL emission energy and annihilation enthalpy change.
Density functional theory has been found to provide
support for the observed electronic properties of the
systems studied in this work. Extensive open-shell
calculations for all molecules require expensive compu-
tational power, though we have been able to provide the
results of open-shell calculations for the DTP-based
system, while closed-shell calculations for all molecules
offer excellent correlation with the observed electronic
properties. The power and use of quantum chemical
calculations have been well established. The present
study offers opportunity for further development of novel
functional organic materials for electronic applications
in addition to providing a basic understanding of the
mechanisms governing the electron-transfer processes in
π-conjugated donor-acceptor arrays.
Acknowledgment. Financial support for this work
was provided by the National Science Council, Taiwan.
Supporting Information Available: Complete ref 8a,
general experimental methods, characterization data, 1H
NMR, 13C NMR, multiscan CV and raw ECL, spectra all
compounds, Cartesian coordinates for all neutral molecules
and radical ions of DTP-PAN and DTP-PEAN, and solvato-
chromic data tables (XLS). This material is available free of
charge via the Internet at http://pubs.acs.org.
The electrochemical behavior has been well docu-
mented and was found to be affected by the introduction
of both variable donor strengths and an ethyne bridge.
All but one molecule show ECL emission; those showing
JO051183G
J. Org. Chem, Vol. 70, No. 18, 2005 7407