Emission from regioisomeric bis(phenylethynyl)benzenes during pulse radiolysis

Article abstract of DOI:10.1021/jo900494j

(Chemical Equation Presented) Emission from charge recombination between radical cations and anions of a series of regioisomeric 1,4-, 1,3-, and 1,2-bis(phenylethynyl)benzenes (bPEBs) substituted by various electron donor and/or acceptor groups was measured during pulse radiolysis in benzene (Bz). The formation of bPEB in the excited singlet state (¹bPEB*) can be attributed to the charge recombination between bPEB^?+ and bPEB^?-, which are initially generated from the radiolytic reaction. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (-ΔH° ) for the charge recombination of bPEB^?+ and bPEB^?+ and excitation energy of ¹bPEB*. Since the degree of the π-conjugation in the S ₁ state and HOMO-LUMO levels of bPEB change with the substitution pattern of phenylacetylene groups on the central benzene ring and the various kinds of donor and/or acceptor group, the fine-tuning of the emission color and intensity of bPEB can be easily carried out during pulse radiolysis in Bz. For donor-acceptor-substituted bPEB, it was found that the difference in the charge transfer conjugated pathways between donor and acceptor substituents (linear-, cross-, and bent-conjugated pathways) strongly influenced the HOMO-LUMO energy gap.

Full text of DOI:10.1021/jo900494j

Emission from Regioisomeric Bis(phenylethynyl)benzenes during
Pulse Radiolysis
Shingo Samori, Sachiko Tojo, Mamoru Fujitsuka, Torben Ryhding,† Aaron G. Fix,†
Brittany M. Armstrong,† Michael M. Haley,† and Tetsuro Majima*
The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki,
Osaka 567-0047, Japan, and Department of Chemistry and the Materials Science Institute, UniVersity of
Oregon, Eugene, Oregon 97403-1253
majima@sanken.osaka-u.ac.jp
ReceiVed March 5, 2009
Emission from charge recombination between radical cations and anions of a series of regioisomeric
1,4-, 1,3-, and 1,2-bis(phenylethynyl)benzenes (bPEBs) substituted by various electron donor and/or
acceptor groups was measured during pulse radiolysis in benzene (Bz). The formation of bPEB in the
excited singlet state (1bPEB*) can be attributed to the charge recombination between bPEB• + and bPEB• -,
which are initially generated from the radiolytic reaction. This mechanism is reasonably explained by
the relationship between the annihilation enthalpy change (-∆H°) for the charge recombination of bPEB• +
and bPEB• - and excitation energy of 1bPEB*. Since the degree of the π-conjugation in the S1 state and
HOMO-LUMO levels of bPEB change with the substitution pattern of phenylacetylene groups on the
central benzene ring and the various kinds of donor and/or acceptor group, the fine-tuning of the emission
color and intensity of bPEB can be easily carried out during pulse radiolysis in Bz. For donor-acceptor-
substituted bPEB, it was found that the difference in the charge transfer conjugated pathways between
donor and acceptor substituents (linear-, cross-, and “bent”-conjugated pathways) strongly influenced
the HOMO-LUMO energy gap.
Introduction
Recently, we have found that various π-conjugated com-
pounds showed emission during pulse radiolysis in benzene
(Bz).3 In pulse radiolysis of M in Bz, the ionization of Bz to
Electron detachment from and attachment to a solute molecule
(M) generates a radical cation (M• +) and anion (M• -), respec-
tively. M• + and M• - are known as important ionic intermediates
in photochemistry, electrochemistry, and radiation chemistry.1
It is well-known that M in the excited states (M* ) 1M* (excited
singlet state) and 3M* (excited triplet state)) can be formed by
(2) (a) Faulkner, L. R.; Bard, A. J. Electroanalytical Chemistry; Marcel
Dekker: New York, 1977; Vol. 10, pp 1-95. (b) Bard, A. J.; Faulkner, L. R.
Electrochemical Methods Fundamentals and Applications, 2nd ed.; John Wiley
and Sons: New York, 2001; pp 736-745. (c) Richter, M. M. Chem. ReV. 2004,
104, 3003.
(3) (a) Samori, S.; Hara, M.; Tojo, S.; Fujitsuka, M.; Yang, S.-W.; Elangovan,
A.; Ho, T.-I.; Majima, T. J. Phys. Chem. B 2005, 109, 11735. (b) Samori, S.;
Tojo, S.; Fujitsuka, M.; Yang, S.-W.; Elangovan, A.; Ho, T.-I.; Majima, T. J.
Org. Chem. 2005, 70, 6661. (c) Samori, S.; Tojo, S.; Fujitsuka, M.; Yang, S.-
W.; Ho, T.-I.; Yang, J.-S.; Majima, T. J. Phys. Chem. B 2006, 110, 13296. (d)
Samori, S.; Tojo, S.; Fujitsuka, M.; Liang, H.-J.; Ho, T.-I.; Yang, J.-S.; Yang,
S.-W.; Majima, T. J. Org. Chem. 2006, 71, 8732. (e) Samori, S.; Tojo, S.;
Fujitsuka, M.; Lin, J.-H.; Ho, T.-I.; Yang, J.-S.; Majima, T. J. Chin. Chem. Soc.
2006, 53, 1225. (f) Samori, S.; Tojo, S.; Fujitsuka, M.; Splitler, E. L.; Haley,
M. M.; Majima, T. J. Org. Chem. 2007, 72, 2785. (g) Samori, S.; Tojo, S.;
Fujitsuka, M.; Splitler, E. L.; Haley, M. M.; Majima, T. J. Org. Chem. 2008,
73, 3551.
charge recombination between M• + and M• - (M• + + M• -
f
1
M* + M), after which M* deactivates to M in the ground-
state by emitting light (1M* f M + hνfl).2 This process is
significant for optoelectronic materials such as OLEDs because
electrochemical energies can be converted to photochemical
energies.
† University of Oregon.
(1) (a) Fox, M. A.; Chanon, M. Photoinduced Electron Transfer; Elsevier:
Amsterdam, 1988. (b) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401.
3776 J. Org. Chem. 2009, 74, 3776–3782
10.1021/jo900494j CCC: $40.75 2009 American Chemical Society
Published on Web 04/14/2009

Radiolysis Emission from Bis(phenylethynyl)benzenes
FIGURE 1. Chemical structures of bis(phenylethynyl)benzene derivatives (bPEBs). Arrows (i-iii) show three types of charge transfer conjugated
pathways in bPEBs.
give Bz radical cation (Bz• +) and formation of an electron (e-)
occur at the same time, followed by reaction with M to give
M• + and M• -, respectively. In our previous work, we have
proposed that the charge recombination of M• + and M• - gives
1M* and/or 1M2* as the emissive species for the organic
electrochemiluminescent molecules such as phenylquinolinyl-
ethynes,3aphenyl(9-acridinyl)ethynes,3bphenyl(9-cyanoanthracenyl)-
ethynes,3b,d arylethynylpyrenes,3c 9-cyano-10-(p-substituted
phenyl)anthracenes,3e and 1,2,4,5-tetrakis(arylethynyl)benzenes
(TAEBs).3f,g
For optoelectronic applications, organic compounds possess-
ing a high degree of π-conjugation with donor and/or acceptor
groups have been recognized as ideal materials.4-6 Changes in
the substituents and substitution pattern, electronic structure,
and conjugation can provide highly variable photophysical
properties for such materials. As a class of π-conjugated
molecules with remarkable optoelectronic properties, function-
alized phenylacetylene structures have received considerable
attention because of their characteristic conjugated pathways.6-8
In particular, 1,4-, 1,3-, and 1,2-bis(phenylethynyl)benzenes
(bPEBs ) na, nb, and nc) containing both donor and acceptor
functionality are an ideal class of molecules for studying the
differences between the linear- (path i), cross- (path ii), and
“bent”-conjugated (path iii) pathways to gain a better under-
standing of the geometric aspects of the charge-transfer path-
ways (Figure 1). By varying the substitution pattern of the donor-
or acceptor-substituted phenylacetylene group at the central
benzene ring, each charge-transfer pathway can be modified.
Therefore, the donor-acceptor-type bPEB structures can avoid
the complexity of the charge transfer pathways observed in
TAEB structures having two-donor and two-acceptor groups.
In addition, the HOMO-LUMO energy gaps of neutral 1a and
1c have been reported to be 7.043 and 6.874 eV, respectively,
which are much lower than that of 1b (7.338 eV).9 This feature
indicates that the emission wavelength of the bPEB structure
dramatically changes with the different types of branching.
Consequently, we can easily fine-tune the emission wavelength
and intensity by changing the various types of donor and
acceptor substituents and the substitution pattern of the central
arene. Some of these compounds are expected to be useful for
OLED materials because of the sufficiently large fluorescent
quantum yield (Φfl) and excess energy value (-∆H° - ES1)
(details are discussed below). In addition, bPEB structures can
rotate freely about their C-C single bonds,3f,g decreasing
π-orbital overlap, and thus formation of the face-to-face excimer
structure with less luminescence intensity cannot be expected.
The time-resolved transient absorption and emission measure-
ments during the pulse radiolysis of various bPEBs are useful
(4) (a) Goes, M.; Verhoeven, J. W.; Hofstraat, H.; Brunner, K. ChemPhy-
sChem 2003, 4, 349. (b) Thomas, K. R. J.; Lin, J. T.; Tao, Y.-T.; Chuen, C.-H.
Chem. Mater. 2002, 14, 3852. (c) Zhu, W.; Hu, M.; Yao, R.; Tian, H. J.
Photochem. Photobiol., A 2003, 154, 169. (d) Thomas, K. R. J.; Lin, J. T.;
Velusamy, M.; Tao, Y.-T.; Chuen, C.-H. AdV. Funct. Mater. 2004, 14, 83. (e)
Chiang, C.-L.; Wu, M.-F.; Dai, D.-C.; Wen, Y.-S.; Wang, J.-K.; Chen, C.-T.
AdV. Funct. Mater. 2005, 15, 231.
(5) (a) Elangovan, A.; Chen, T.-Y.; Chen, C.-Y.; Ho, T.-I. Chem. Commun.
2003, 2146. (b) Elangovan, A.; Yang, S.-W.; Lin, J.-H.; Kao, K.-M.; Ho, T.-I.
Org. Biomol. Chem. 2004, 2, 1597. (c) Elangovan, A.; Chiu, H.-H.; Yang, S.-
W.; Ho, T.-I. Org. Biomol. Chem. 2004, 2, 3113. (d) Elangovan, A.; Kao, K.-
M.; Yang, S.-W.; Chen, Y.-L.; Ho, T.-I.; Su, Y. O. J. Org. Chem. 2005, 70,
4460. (e) Yang, S.-W.; Elangovan, A.; Hwang, K.-C.; Ho, T.-I. J. Phys. Chem.
B 2005, 109, 16628. (f) Lin, J.-H.; Elangovan, A.; Ho, T.-I. J. Org. Chem. 2005,
70, 7397.
(6) (a) Jayakannan, M.; Van Hal, P. A.; Janssen, R. A. J. J. Polym. Sci.,
Part A: Polym. Chem. 2001, 40, 251. (b) Tykwinski, R. R.; Schreiber, M.; Carlon,
R. P.; Diederich, F.; Gramlich, V. HelV. Chim. Acta 1996, 79, 2249. (c)
Tykwinski, R. R.; Schreiber, M.; Gramlich, V.; Seiler, P.; Diederich, F. AdV.
Mater. 1996, 8, 226. (d) Wilson, J. N.; Hardcastle, K. I.; Josowicz, M.; Bunz,
U. H. F. Tetrahedron 2004, 60, 7157. (e) Wilson, J. N.; Smith, M. D.; Enkelmann,
V.; Bunz, U. H. F. Chem. Commun. 2004, 1700. (f) Wilson, J. N.; Josowicz,
M.; Wang, Y.; Bunz, U. H. F. Chem. Commun. 2003, 2962. (g) Miteva, T.;
Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F. Macromolecules 2000,
33, 652. (h) J. Zucchero, A.; Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc.
2006, 128, 11872. (i) Ojima, J.; Kakumi, H.; Kitatani, K.; Wada, K.; Ejiri, E.;
Nakada, T. Can. J. Chem. 1984, 63, 2885. (j) Ojima, J.; Enkaku, M.; Uwai, C.
Bull. Chem. Soc. Jpn. 1977, 50, 933.
(7) (a) Miller, J. J.; Marsden, J. A.; Haley, M. M. Synlett 2004, 165. (b)
Marsden, J. A.; Miller, J. J.; Shirtcliff, L. D.; Haley, M. M. J. Am. Chem. Soc.
2005, 127, 2464. (c) Spitler, E. L.; Shirtcliff, L. D.; Haley, M. M. J. Org. Chem.
2007, 72, 86. (d) Spitler, E. L.; Monson, J. M.; Haley, M. M. J. Org. Chem.
2008, 73, 2211.
(8) (a) Marsden, J. A.; Palmer, G. J.; Haley, M. M. Eur. J. Org. Chem. 2003,
2355. (b) Jones, C. S.; O’Connor, M. J.; Haley, M. M. In Acetylene Chemistry:
Chemistry, Biology, and Materials Science; Diederich, F., Tykwinski, R. R.,
Stang, P. J., Eds.; Wiley-VCH: Weinheim, Germany, 2004; pp 303-385. (c)
Marsden, J. A.; Haley, M. M. J. Org. Chem. 2005, 70, 10213. (d) Anand, S.;
Varnavski, O.; Marsden, J. A.; Haley, M. M.; Schlegel, H. B.; Goodson, T., III.
J. Phys. Chem. A 2006, 110, 1305. (e) Bhaskar, A.; Guda, R.; Haley, M. M.;
Goodson, T., III. J. Am. Chem. Soc. 2006, 128, 13972. (f) Slepkov, A. D.;
Hegmann, F. A.; Tykwinski, R. R.; Kamada, K.; Ohta, K.; Marsden, J. A.; Spitler,
E. L.; Miller, J. J.; Haley, M. M. Opt. Lett. 2006, 31, 3315. (g) Spitler, E. L.;
McClintock, S. P.; Haley, M. M. J. Org. Chem. 2007, 72, 6692.
(9) Zhang, W.; Huang, C. Mater. Chem. Phys. 2006, 96, 283.
J. Org. Chem. Vol. 74, No. 10, 2009 3777

Samori et al.
SCHEME 1. Synthesis of bPEB 6c
FIGURE 2. Absorption (solid lines) and fluorescence (broken lines)
spectra observed by the steady-state measurement of bPEB in Ar-
saturated Bz. All solutions were prepared at a dilute concentration (10-5
M).
to gain a better understanding of the emission mechanism,
providing valuable information for molecular design with
efficient luminescence character.
In this paper, we report the emission from the charge
recombination bPEB• + and bPEB• - of 21 different bPEB
derivatives (na, nb, and nc; n ) 1-7): nonsubstituted (1a-c),
methyl-substituted (2a-c), methoxy-substituted (3a-c), cyano-
substituted (4a-c), methyl- and cyano-substituted (5a-c),
methoxy- and cyano-substituted (6a-c), and dimethylamino-
and cyano-substituted bPEBs (7a-c). When n is the same, 1,4-
(na), 1,3- (nb), and 1,2-bPEBs (nc) are regioisomers (Figure
1). The detailed study of 21 different kinds of bPEBs potentially
leads to customization of optical band gaps and luminescent
efficiency for specialized materials applications.
TABLE 1. Emission Properties of bPEB in Ar-Saturated Bz
a
b
c
bPEB
λFl (nm)
Φfl
λFl (nm)
Φfl
λFl (nm)
Φfl
1
2
3
4
5
6
7
351
354
362
367
372
386
462
0.90
0.93
0.82
0.93
0.99
0.85
0.39
331
332
337
340
345
361
452
0.25
0.22
0.065
0.54
0.63
0.63
0.29
346
350
363
369
377
422
490
0.34
0.44
0.63
0.61
0.60
0.50
0.34
Results and Discussion
π-conjugation; thus, the absorption spectra of 1b is predicted
to resemble the absorption spectrum of diphenylacetylene
(DPA).16 A comparison of the absorption spectra of DPA and
1b show that, indeed, this is the case: the spectrum of 1b
reproduces the spectral width and vibronic structure displayed
by DPA. In contrast, when substitution occurs at the para
position (1a), the absorption band edge is red-shifted by 30 nm
with respect the band edge of 1b and appears at 355 nm. The
relatively large red shift and loss of the sharp vibronic structure
reflect the π-electron delocalization over the linear three-ring
chain. The ortho branching type 1c exhibits an absorption band
edge at 350 nm, which confirms that π-electron delocalization
occurs over the bent three-ring chain, although the red shift is
somewhat smaller than that of 1a. Compared to neutral 1, donor
types 2 and 3, and acceptor type 4, the corresponding absorption
bands of donor-acceptor-substituted bPEBs 5-7 are broadened
and show further bathochromic shifts, indicating the intramo-
lecular charge transfer (ICT) from the donor to the acceptor for
the HOMO-LUMO transition.7
Fluorescence was observed for all bPEBs (Figure 2). Com-
pared to neutral 1, donor-acceptor types 5-7 showed a large
red shift in the fluorescence maxima in Bz. This again indicates
ICT character for donor-acceptor-substituted bPEB in the S1
state (Table 1). 5-7 showed strong fluorescence solvato-
chromism, experiencing bathochromic shifts in switching to a
more polar solvent such as CH3CN. This also indicates the ICT
character for the donor-acceptor-substituted bPEBs in the S1
state. It should be noted that the fluorescence maxima of neutral
1 increase in the order 1b (331 nm) < 1c (346 nm) < 1a (351
nm), whereas those of donor-acceptor-type 7 increase in the
Synthesis. Of the 21 bPEBs in this study, compounds 1a-c,10
2a,11 3a-c,12 4a,13 6a,14 and 7a14 have been previously
described in the literature. The other 11 bPEB structures were
synthesized by sequential Sonogashira reactions using 1,2-, 1,3-,
or 1,4-bromoiodobenzene as the core arene, illustrated for 6c
in Scheme 1. Cross-coupling 4-ethynylanisole to 1-bromo-2-
iodobenzene afforded bromoarene 8c. This was in turn cross-
coupled to 4-ethynylbenzonitrile using the more reactive
catalytic system PdCl2(PhCN)2/t-Bu3P first reported by Buch-
wald and Fu,15 furnishing bPEB 6c in 36% yield for the two
steps. The remaining 10 bPEBs were prepared analogously in
ca. 20-40% unoptimized yield (see Supporting Information).
Steady-State Spectral Properties of bPEB. Normalized
steady-state absorption and fluorescence spectra of bPEB in Bz
are shown in Figure 2. It seems that the electronic structures of
bPEB strongly depend on the substitution pattern of the
phenylacetylene group because the regioisomers (ortho-, meta-,
and para-substituted isomers) have similar spectral shapes. It
is well-known that the meta branching type 1b disrupts the
(10) (a) Nguyen, P.; Yuan, Z.; Agocs, L.; Lesley, G.; Marder, T. B. Inorg.
Chem. Acta 1994, 220, 289. (b) Songkram, C.; Takaishi, K.; Yamaguchi, K.;
Kagechika, H.; Endo, Y. Tetrahedron Lett. 2001, 42, 6365. (c) Kovalenko, S. V.;
Peabody, S.; Manoharan, M.; Clark, R. J.; Alabugin, I. V. Org. Lett. 2004, 6,
2457.
(11) Drefahl, G.; Plotner, G. Chem. Ber. 1958, 91, 1280. This compound
was originally prepared by stilbene bromination/dehydrobromination. An
improved preparation via Sonogashira reactions as well as NMR spectroscopic
data are given in Supporting Information.
(12) Ye, F.; Orita, A.; Doumoto, A.; Otera, J. Tetrahedron 2003, 59, 5635.
(13) Dirk, S. M.; Tour, J. M. Tetrahedron 2003, 59, 287.
(14) Nguyen, P.; Lesley, G.; Marder, T. B.; Ledoux, I.; Zyss, J. Chem. Mater.
1997, 9, 406.
(15) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org. Lett.
2000, 2, 1729.
(16) Melinger, J. S.; Pan, Y.; Kleiman, V. D.; Peng, Z.; Davis, B. L.;
McMorrow, D.; Lu, M J. Am. Chem. Soc. 2002, 124, 12002.
3778 J. Org. Chem. Vol. 74, No. 10, 2009

Radiolysis Emission from Bis(phenylethynyl)benzenes
order 7b (452 nm) < 7a (462 nm) < 7c (490 nm). This result
clearly indicates that the bent-conjugated charge transfer
pathway (path iii) leads the fluorescence band to longer
wavelength than the linear-conjugated charge transfer pathway
(path i). Compared to cross- (path ii) and bent-conjugated (path
iii) charge transfer pathways, it seems that the linear-conjugated
charge transfer pathway (path i) leads a slight red shift when
bPEB has both donor and acceptor substituents. Consequently,
it is clearly evident that the difference in the charge transfer
conjugated pathways between donor and acceptor substituents
strongly influences the HOMO-LUMO energy gap, although
we cannot clearly investigate this because of the complexity of
the charge transfer pathways observed in TAEB structures in
our previous reports.3f,g,7,8
The fluorescence quantum yield (Φfl) values of the bPEBs
ranged from 0.065 to 0.99 and showed the tendency for para
(0.39-0.99), ortho (0.34-0.63), and meta isomers (0.065-0.63),
in decreasing order. Recently, it was reported that there was a
correlation between the Φfl and the magnitude (Aπ) of the
π-conjugation length in the excited singlet state for various
π-conjugated hydrocarbons including 1a, 1b, and nonsubstituted
TAEB.17 The relationship between Φfl and Aπ is expressed by
the following simple equation:
FIGURE 3. Emission spectra observed in the time range of 0-100 ns
during pulse radiolysis of bPEB in Ar-saturated Bz (10-3 M).
SCHEME 2. Proposed Mechanism of the Emission during
the Pulse Radiolysis of bPEB in Bza
1
Φfl )
+ 1
(1)
exp(-Aπ)
As mentioned above, the meta branching disrupts the
π-electron conjugation; thus, the Aπ value of nb should be
smaller than those of na and nc. The lower Φfl value of nb can
be reasonably explained by eq 1. On the other hand, for the
para type na, the π-electrons are delocalized over the whole
molecule, indicating the Aπ value of na should be larger than
those of the others. Although the π-electron delocalization also
occurs in ortho type nc, the degree of the π-conjugation is
assumed to be smaller than na. As a result, the Φfl value of nc
is midway between those of na and nb. The lower Φfl value of
7 compared with the others can be explained by the energy-
gap law.18 From the intersection point of the absorption and
fluorescence spectra, the excitation energies of 7a, 7b, and 7c
(ES1 ) 2.94, 3.18, and 2.88 eV, respectively) are estimated to
be smaller than those of other bPEBs (3.33-3.97 eV). The
increase of internal conversion rate is expected and responsible
for the low Φfl values of 7. The Φfl value of 3b is exceptionally
low (0.065). Methoxy groups are not known to quench
fluorescence in fluorescent molecules, such as in stilbenes,19a
coumarins,19b,c quinolinones,19c and fluoranthenes.19d The reason
for the strong fluorescence quenching of 3b is still unclear.
Emission Generated from Charge Recombination be-
tween bPEB• + and bPEB• -. Emission spectra were observed
after an electron pulse during the pulse radiolysis of bPEB in
Ar-saturated Bz (1.0 mM) (Figure 3). According to previous
reports,3 the emission of M indicates the generation of 1M* by
charge recombination of M• + and M• - during pulse radiolysis
in Bz. When N2O gas was added to the solution as the electron
scavenger, the emission intensity observed during the pulse
a For donor-acceptor-substituted bPEBs (5-7), 1bPEB* ) 1(A• - - D• +)*
(ICT state).
radiolysis of bPEB in Bz was reduced. This result clearly
indicates that bPEB• + and bPEB• - are responsible to the
emission during the pulse radiolysis. The same emission
mechanism was assumed for bPEB as shown the following
(Scheme 2).
Little or no emission was observed during the pulse radiolysis
of bPEB in 1,2-dichloroethane (DCE) or N,N-dimethylforma-
mide (DMF), indicating that bPEB• + and bPEB• - do not emit
light. In other words, both bPEB• + and bPEB• - must be formed
at the same time in order to emit light. In Bz, no transient
absorption band of bPEB• + and bPEB• - was observed im-
mediately after an 8 ns electron pulse. Therefore, it is assumed
that bPEB• + and bPEB• - immediately recombine to give
3
1
1bPEB* and bPEB*, and bPEB* emits light within a pulse
duration. The transient absorption spectra observed during the
pulse radiolysis of bPEB in Ar-saturated Bz can be assigned to
3bPEB* (triplet-triplet absorption).
Because of the considerable repulsion between the substitu-
ents induced by the rotation around C-C single bonds, a
π-stacked structure cannot be formed for the interaction between
bPEB• + and bPEB• -. Therefore, bPEB showed only monomer
emission, although some electrochemiluminescent molecules
showed both monomer and excimer emissions with less
luminescence intensity during pulse radiolysis as mentioned in
previous reports.3
Compounds 1-7 showed emission peaks at 373-513 nm
during the pulse radiolysis (Figures 2). The shape of the emission
spectra of bPEB was similar to those observed in the steady-
state measurements, although the emission spectra of bPEB were
(17) Yamaguchi, Y.; Matsubara, Y.; Ochi, T.; Wakamiya, T.; Yoshida, Z.
J. Am. Chem. Soc. 2008, 130, 13867.
(18) Kasha, M. Chem. ReV. 1947, 41, 401.
(19) (a) Roberts, J. C.; Pincock, J. A. J. Org. Chem. 2006, 71, 1480. (b)
Mateera, N. N.; Kode, R. A.; Redda, K. K. J. Heterocycl. Chem. 2002, 39, 1251.
(c) Charitos, C.; Tzougraki, C.; Kokotos, G. J. Peptide Res. 2000, 56, 373. (d)
Tucker, S. A.; Griffin, J. M.; Acree, W. E., Jr.; Tanga, M. J.; Bupp, J. E.;
Tochimoto, T. K.; Lugtenburg, J. A.; Van Haeringen, K.; Cornelisse, J. Polycyclic
Aromat. Compd. 1994, 4, 161.
J. Org. Chem. Vol. 74, No. 10, 2009 3779

Samori et al.
Em
TABLE 2. Emission Maxima (λ ) and Relative Emission
TABLE 3. Electrochemical Properties of bPEBs in CH3CN and
Annihilation Enthalpy Changes (-∆H°), ES1, and (-∆H° - ES1) of
bPEBs in Bz
max
Intensities (I) during Pulse Radiolysis of bPEB in Ar-Saturated Bz
(1.0 mM)
a
b
c
in CH3CN
in Bz
Ia
λ
(nm)
I
λ
max
(nm)
I
bPEB Eox (V) Ered (V) -∆H° (eV) ES1 (eV) -∆H° - ES1 (eV)
Em
Em
Em
bPEB
λ
(nm)
max
max
1
2
3
4
5
6
7
373
100
73.5
348
35.2
34.0
9.92
42.5
57.7
31.1
23.9
365
35.0
64.3
71.0
46.5
59.0
45.6
56.4
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5a
5b
5c
6a
6b
6c
7a
7b
7c
1.38a
1.65
1.69
1.45
1.62
1.56
1.25
1.32
1.33
1.57
1.74
1.69
b
1.63
1.60
1.27
1.32
1.34
0.60
0.60
0.59
-2.35a
-2.52
-2.38
-2.30
-2.52
-2.37
-2.35
-2.48
-2.39
-2.17
-2.36
-2.22
b
-2.39
-2.28
-2.24
-2.39
-2.28
-2.26
-2.36
-2.28
3.92
4.36
4.26
3.94
4.33
4.10
3.79
3.99
3.91
3.93
4.29
4.10
3.54
3.97
3.68
3.54
3.97
3.67
3.46
3.88
3.56
3.44
3.82
3.54
3.37
3.82
3.46
3.33
3.76
3.36
2.94
3.14
2.88
+0.38
+0.39
+0.58
+0.40
+0.36
+0.43
+0.33
+0.11
+0.35
+0.49
+0.47
+0.56
378
386
392
398
409
467
341
348
361
366
386
453
368
382
381
391
421
513
64.6
74.1
53.1
85.0
30.4
a I values of bPEB were determined from the total amount of the
emission spectra observed during pulse radiolysis, relative to I value of
1a in Ar-saturated Bz.
observed at the slightly longer wavelength due to the self-
absorption as a result of the high concentrations.3 Therefore,
the formation of bPEB* with ICT character can be assumed
for 5-7 during pulse radiolysis. The intensities (I) of the
radiolysis-induced emission spectra of 1-7 were determined
from the total amount of the emission observed during pulse
radiolysis and are summarized in Tables 2.
4.21
4.07
3.70
3.90
3.81
3.05
3.15
3.06
+0.39
+0.61
+0.37
+0.14
+0.45
+0.11
+0.01
+0.18
1
To elucidate the emission mechanism of the bPEBs, we
estimated the annihilation enthalpy change (-∆H°) value for
the charge recombination between M• + and M• -. This -∆H°
value is a criterion for whether 1M* can be formed by the charge
recombination or not.2 -∆H° is calculated by eq 2:20
a Reference 21. b Not dissolved in the solvent.
εs
-∆H° ) (Eox - Ered)εs - ∆Gsol - wa,µ + T∆S° (2)
where Eox and Ered are the oxidation and reduction potentials of
M, respectively. εs, ∆Gsol, and wa,µ represent the static dielectric
constant of solvent, the free energy change of solvation, and
the work required to bring M• + and M• - within a likely
separation distance, respectively. For bPEB in Bz, -∆H° can
be expressed using Eox and Ered measured in CH3CN by
simplified eq 3:3
FIGURE 4. Plots of I value versus Φfl for na (red square), nb (green
circle), and nc (blue triangle) in Bz.
-∆H° ) Eox - Ered + 0.19eV
(3)
and nc (n ) 5-7) can contain the conjugated pathways: linear-
(path i), cross- (path ii), and bent-conjugated (path iii) pathways,
respectively, as shown in Figure 1. The emission spectra of
1bPEB* with ICT character depends on the substitution pattern
by the steady-state measurement and during the pulse radiolysis
as shown in Figures 2 and 3, respectively. The radiolysis-induced
emission spectra of all bPEBs showed emission bands in the
region of wavelength where the fluorescence bands were
observed by the steady-state measurements. For 5-7, therefore,
the charge recombination between bPEB• + and bPEB• - gener-
The calculated -∆H° values for bPEB are listed in Table 3,
together with their oxidation and reduction potentials and ES1
values. -∆H° values for all bPEBs (3.05-4.36 eV) are
consistently larger than their ES1 values (2.88-3.97 eV),
indicating that the energy available in the charge recombination
is sufficient to populate all bPEBs in the S1 states.
As shown in Figure 4, the I value seems proportional to Φfl.
However, the I value of some bPEBs deviates from the fitted
line. The decrease of the I value was explained by the self-
absorption. In our previous paper,3 the energy differences
between -∆H° and ES1 (-∆H° - ES1 () +0.01-0.61 eV))
were also assumed as one of the key factors for the I value of
Ms. Therefore, these deviated plots are assumed to depend on
not only Φfl and the amount of the self-absorption but also the
(-∆H° - ES1) value.
1
ates bPEB* with ICT character as shown in Scheme 3. The
results also indicate that the difference in the charge transfer
conjugated pathways strongly influences the HOMO-LUMO
energy gap: the bent-conjugated charge transfer pathway (path
iii) leads the fluorescence band to longer wavelength than the
linear- (path i) and cross-conjugated (path ii) charge transfer
pathways when bPEB has both donor and acceptor substituents.
Consequently, it is evident that judicious choice of the donor/
acceptor unit permits control of the HOMO-LUMO energy gap,
and thus the emission wavelengths can be fine-tuned at around
450-650 nm in the visible region.
Fine-Tuning of the Emission Color and Intensity by
Changing the Substitution Pattern of Donor-Acceptor-
Subsituted bPEB. Donor-acceptor-substituted bPEBs na, nb,
(20) Gross, E. M.; Anderson, J. D.; Slaterbeck, A. F.; Thayumanavan, S.;
Barlow, S.; Zhang, S. R; Marder, H. K; Hall, M. F.; Nabor, J.-F; Wang, E. A;
Mash, N. R; Armstrong, R. M.; Wightman, Y. J. Am. Chem. Soc. 2000, 122,
4972.
(21) Kilsa, K.; Kajanus, J.; Macpherson, A. N.; Martensson, J.; Albinsson,
B. J. Am. Chem. Soc. 2001, 123, 3069.
As shown in Table 2, the I values of nb are lower than those
of na and nc. This difference can be explained by the degree
of the π-conjugation in S1 states: the Aπ value of nb is assumed
to be smaller than those of na and nc. Therefore, it is suggested
3780 J. Org. Chem. Vol. 74, No. 10, 2009

Radiolysis Emission from Bis(phenylethynyl)benzenes
SCHEME 3. Proposed Structure for the Formation of Donor-Acceptor-Type bPEBs in the S1 and T1 States during Pulse
Radiolysis in Bz
that the molecular design that extends the π-conjugation length
is desirable in order to achieve the efficient charge recombination
emission.
between donor and acceptor substituents, the linear- (path i),
cross- (path ii), and bent-conjugated (path iii) pathways, strongly
influence the HOMO-LUMO energy gap. In addition, our
results show that the degree of the π-conjugation influences the
luminescence intensity during pulse radiolysis. These specific
emission properties of donor-acceptor-substituted bPEB may
lead to customization of optical band gaps and luminescent
efficiency for specialized materials applications.
Conclusions
Through control of the substitution pattern of the regioiso-
meric electron donor- and/or acceptor-substituted bPEBs 1-7,
the fine-tuning of radiolysis-induced emission color and intensity
can be achieved. Using pulse radiolysis technique, the charge
Experimental Section
1
recombination between bPEB• + and bPEB• - gives bPEB* as
the emissive species, which has the emission wavelengths
350-650 nm in the visible region. It is clearly indicated that
the difference in the charge transfer conjugated pathways
Solution Preparation. Benzene was purchased from Nacalai
Tesque (spectral grade) and used as a solvent without further
purification. All sample solutions were freshly prepared in 1 mM
J. Org. Chem. Vol. 74, No. 10, 2009 3781

Samori et al.
concentration in Bz in a rectangular quartz cell (1.0 × 1.0 ×
4.0 cm, path length of 1.0 cm). These solutions were saturated
with Ar gas by bubbling for 10 min at room temperature before
irradiation. All experiments were carried out at room tempera-
ture.
Measurements of Steady-State Spectral Properties. UV
spectra were recorded in Bz with a Shimadzu UV-3100PC UV/vis
spectrometer. Fluorescence spectra were measured by a Hitachi 850
spectrofluorometer. The fluorescence quantum yields (Φfl) were
determined by using 1a (Φfl ) 0.90 in cyclohexane, λex) 360 nm)22
standard.
Pulse Radiolysis. Pulse radiolysis experiments were performed
using an electron pulse (28 MeV, 8 ns, 0.87 kGy per pulse) from
a linear accelerator at Osaka University. The kinetic measurements
were performed using a nanosecond photoreaction analyzer system
(Unisoku, TSP-1000). The monitor light was obtained from a pulsed
450-W Xe arc lamp (Ushio, UXL-451-0), which was operated by
a large current pulsed power supply that was synchronized with
the electron pulse. The monitor light was passed through an iris
with a diameter of 0.2 cm and sent into the sample solution at an
intersection perpendicular to the electron pulse. The monitor light
passing through the sample was focused on the entrance slit of a
monochromator (Unisoku, MD200) and detected with a photomul-
tiplier tube (Hamamatsu Photonics, R2949). The transient absorption
and emission spectra were measured using a photodiode array
(Hamamatsu Photonics, S3904-1024F) with a gated image intensi-
fier (Hamamatsu Photonics, C2925-01) as a detector. All emission
spectra were corrected for the spectral sensitivity of the apparatus.
To avoid pyrolysis of the sample solution by the monitor light, a
suitable cutoff filter was used.
Measurements of Electrochemical Properties. Oxidation (Eox)
and reduction potentials (Ered) were measured by cyclic voltammetry
(BAS, CV-50W) with platinum working and auxiliary electrodes
and an Ag/Ag+ reference electrode at a scan rate of 100 mV s-1
.
Measurements were performed in dry AN containing approximately
1 mM of AHs and 0.1 M tetraethylammonium perchlorate.
Acknowledgment. We thank the members of the Radiation
Laboratory of ISIR, Osaka University for running the linear
accelerator. This work has been partly supported by a Grant-
in-Aid for Scientific Research (Project 17105005, 19350069,
Priority Research (477), and others) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT)
of the Japanese Government and the U.S. National Science
Foundation (CHE-0718242). One of the authors (S.S.) expresses
his thanks for a JSPS Research Fellowship for Young Scientists
and the Global COE Program “Global Education and Research
Center for Bio-Environmental Chemistry” of Osaka University.
T.R. thanks the Lundbeck Foundation for a fellowship during
his time at Oregon.
Note Added after ASAP Publication. An additional ac-
knowledgment was added in the version published May 8, 2009.
Supporting Information Available: Experimental details for
1
all new compounds and copies of H and 13C NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(22) Sciano, J. C. Handbook of Organic Photochemistry; CRC Press: Boca
Raton, FL, 1989; Vol. l, p 231.
JO900494J
3782 J. Org. Chem. Vol. 74, No. 10, 2009