glycol under acid conditions prior to Pd coupling.10 Sub-
sequent deprotection permits recovery of the dihalophe-
DBPDs (Table 1). All EA values were found to be remark-
ably similar to that of parent DBP.12
€
nanthrene-9,10-dione (DHPD) functionality. Mullen
reported a similar approach when PD was functionalized
with Si-terminated acetylenes: protection of DHPD with
Me2SO4, Sonogashira coupling, and regeneration of the
o-quinone with CAN.11
Table 1. Redox Potentials of DBPDs in DMF vs Fc/Fcþ
compd
E1red0 (V)
Epa (V)
IP (eV)
EA (eV)
PADBP27
CPADBP27
PADBP36
CPADBP36
DBP
ꢀ1.61
ꢀ1.64
ꢀ1.65
ꢀ1.62
>5.7
5.6
3.3
3.3
3.3
3.3
3.14
þ0.74
þ0.79
>5.7
5.7
Scheme 2. Final Synthesis of DBP Derivatives
All DBPD/THF solutions absorb light strongly at λ <
450 nm (Figure 1A, log(εabs) ∼ 4.48ꢀ4.84 at λmax).
PADBP27 exhibits two absorption bands with maxima
centered at 335 and 395 nm, respectively. The features of
the CPADBP27 spectrum resemble the coalescence of the
two absorption bands of PADBP27. In contrast, the
spectral features of 3,6-DBPs are similar to each other.
The only difference among these is the slightly red-shifted
absorption of CPADBP36’s red-edge. These observations
underline the small electronic contribution of Cz on the
band gap of the low energy transitions, although the
oscillator strengths are evidently enhanced by the expan-
sion of the π-system. Solvatochromic effects in the absorp-
tion spectrum were negligible, in parallel to parent DBP
(Figure S10, Supporting Information). This reflects small
GS dipole moments for the compounds.
Steady-state photoluminescence (PL) measurements on
a PADBP36/THF solution showed a narrow and struc-
tured signal in contrast to all other DBPDs (Figure 1B).
The significant red shift between CPADBPs and PADBPs
in THF reflects either important internal reorganization or
S1 energy stabilization due to solvation after incorporation
of Cz donors. Poor solubility of DBPDs in cyclohexane
and other aliphatic solvents restricted the calculation of
their internal reorganization energies. Most DBPDs ex-
hibit dual emission from the local excited (LE) and charge
transfer (CT) states (Figure S11, Supporting Information).
This is evident for the cases of PADBP27, CPADBP27,
and CPADBP36, where the CT band develops in parallel
with the increasing of solvent polarity. The lack of PL
signal in the case of both variants of CPADBP in DMF
solution highlights an efficient nonradiative decay (NRD)
mechanism. The nature of this NRD is uncertain, although
charge separation (CS) could occur at such a high polarity.
Similar systems with different acceptors behave this way.13
LippertꢀMataga plots (LMPs) reflect nonlinear dependence
of the Stokes shift with the Onsager solvent parameter
f(ε,n) for DBPs (Figure S12, Supporting Information).
While PA is compatible with Tour’s protocol, the intro-
duction of soluble CPA units leads to problematic regen-
eration of PD even with the use of strong acids such as
CF3CO2H or TfOH. This drawback prompted us to try an
€
alternative based on Mullen’sprotocol. We alsofound that
the solubility of DBP was improved after incorporation of
tert-octyl groups in the phenyl and Cz units of the coupling
partners. Regeneration of the PD unit with CAN is
difficult for Cz-containing compounds as well. These
results suggest that incorporating electron-rich heterocycle
derivatives in conjugation to PD might be inefficient in a
general sense.
The early derivatization of DHPD producing dihalodi-
benzo[a,c]phenazine (DHDBP) was effective for further
access of dipolar DBPDs. Tour and Jenekhe were able to
synthesize DBPDs via Pd-catalyzed cross-coupling reac-
tions in high yields.4,10 This counterintuitive finding sug-
gests that the poor solubility of DHDBP is irrelevant in
order for the reaction to proceed. Consequently, Cz units
were incorporated through this method after the synthesis
of convenient building blocks (Scheme 2).
Voltammetry experiments (CV and DPV) vs Fc/Fcþ
reveal that all DBPDs possess one single-electron reduc-
tion wave and, only for the case of Cz-substituted variants,
one reversible oxidation wave within the region scanned
(Figure S9, Supporting Information). This suggests loca-
lization of the LUMO in the DBP unit. The ionization
potential and electron affinity values could be taken as
references for the HOMO and LUMO energies for all
(12) Assuming Koopmans’ theorem is valid, the values of interest
were calculated using EA (eV) = Eonset(red) þ 4.9; IP (eV) = Eonset(ox)
þ 4.9. These formulas are based on the assumptions that the energy level
of SCE relative to vacuum is 4.4 eV and SCE reference with respect to
Fc/Fcþ is þ0.5 V.
(10) Shirai, Y.; Osgood, A. J.; Zhao, Y.; Yao, Y.; Saudan, L.; Yang,
H.; Yu-Hung, C.; Alemany, L. B.; Sasaki, T.; Morin, J.-F.; Guerrero,
J. M.; Kelly, K. F.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 4854.
(11) Qin, T.; Zhou, G.; Scheiber, H.; Bauer, R. E.; Baumgarten, M.;
(13) (a) Estrada, L. A.; Cai, X.; Neckers, D. C. J. Phys. Chem. A 2011,
115, 2184. (b) Estrada, L. A.; Yarnell, J. E.; Neckers, D. C. J. Phys.
Chem. 2011, 115, DOI 10.1021/jp-2011-00507j.
€
Anson, C. E.; List, E. J. W.; Mullen, K. Angew. Chem., Int. Ed. 2008, 47,
8292.
Org. Lett., Vol. 13, No. 13, 2011
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