Synthesis and photophysics of dibenz[a,c]phenazine derivatives

Article abstract of DOI:10.1021/ol200354t

The synthesis of dipolar dibenz[a,c]phenazine (DBP) derivatives is described. The compounds possess little electronic communication between donor and acceptor units in the ground state regardless of the pattern of substitution. The dipolar derivatives deactivate mostly via electron transfer (eT) under polar conditions. Intersystem crossing is likely to compete for S₁ relaxation.

Full text of DOI:10.1021/ol200354t

ORGANIC
LETTERS
2011
Vol. 13, No. 13
3304–3307
Synthesis and Photophysics of Dibenz-
[a,c]phenazine Derivatives
Leandro A. Estrada and Douglas C. Neckers*
Center for Photochemical Sciences, Bowling Green State University, Bowling Green,
Ohio 43403, United States
neckers@photo.bgsu.edu
Received April 12, 2011
ABSTRACT
The synthesis of dipolar dibenz[a,c]phenazine (DBP) derivatives is described. The compounds possess little electronic communication between
donor and acceptor units in the ground state regardless of the pattern of substitution. The dipolar derivatives deactivate mostly via electron
transfer (eT) under polar conditions. Intersystem crossing is likely to compete for S1 relaxation.
Dibenz[a,c]phenazine (DBP) is a structural analogue
of the expensive and rare hydrocarbon dibenz[a,c]-
anthracene.1 Its synthesis via condensation of o-diamino-
benzene with phenanthrene-9,10-dione (PD) has triggered
interest in the optical and conductive properties of some of
its derivatives.2 After the realization of columnar phase
control in thin films and solidꢀliquid interfaces of large
polyaromatic hydrocarbons,3 most of the latest reports on
DBP derivatives (DBPDs) focus on applications as liquid
crystals.2 Consequently, improvement of their optical
properties via chemical modification are scarce.4ꢀ6 Deri-
vatization of DBP is promising given its skeletal rigidity,
electron affinity, and thermal stability. Establishing design
patterns for its better use in organic electronics is desirable.
To answer these questions, the synthesis of dipolar DBPDs
via incorporation of pendant carbazole (Cz) to the DBP
acceptor linked through phenylacetylene (PA) was pur-
sued. Cz-lacking compounds were also synthesized for
comparison purposes.
Scheme 1. Initial Synthetic Approach to DBPDs
(1) Clar, E. Polycyclic Aromatic Hydrocarbons; Academic Press: New
York, 1964.
(2) For representative examples, see: (a) Foster, E. J.; Babuin, J.;
Nguyen, N.; Williams, V. E. Chem. Commun. 2004, 2052. (b) Foster,
E. J.; Lavigueur, C.; Ke, Y.-C.; Williams, V. E. J. Mater. Chem. 2005, 15,
4062. (c) Foster, E. J.; Jones, R. B.; Lavigueur, C.; Williams, V. E. J. Am.
Chem. Soc. 2006, 128, 8569. (d) Lavigueur, C.; Foster, E. J.; Williams,
V. E. Liq. Cryst. 2007, 34, 833. (e) Lavigueur, C.; Foster, E. J.; Williams,
V. E. J. Am. Chem. Soc. 2008, 130, 11791. (f) Voisin, E.; Foster, E. J.;
Rakotomalala, M.; Williams, V. E. Chem. Mater. 2009, 21, 3251. (g) Lee,
E.; Huang, Z.; Ryu, J.-H.; Lee, M. Chem.;Eur. J. 2008, 14, 6957. (h)
Hong, D.-J.; Lee, E.; Jeong, H.; Lee, J.-K.; Zin, W.-C.; Nguyen, T. D.;
Glotzer, S. C.; Lee, M. Angew. Chem., Int. Ed. 2009, 48, 1664.
Preparation of DBPDs was planned around the ex-
pected high-yield condensation of o-diaminobenzene with
a suitable PD derivative (Scheme 1).7
PD derivatization via Pd-catalyzed CꢀC coupling is
known to be problematic.8,9 However, Tour circumvented
this issue by protecting the carbonyl groups with ethylene
€
(3) Wu, J.; Pisula, W.; Mullen, K. Chem. Rev. 2007, 107, 718.
(4) Zhu, Y.; Gibbons, K. M.; Kulkarni, A. P.; Jenekhe, S. A.
Macromolecules 2007, 40, 804.
(7) Kato, S.; Hashimoto, H.; Hida, M.; Maezawa, M. Yuki Gosei
Kagaku Kyokaishi 1957, 15, 399.
(8) Ciszek, J. W.; Tour, J. M. Tetrahedron Lett. 2004, 45, 2801.
(9) Estrada, L. A.; Neckers, D. C. J. Org. Chem. 2009, 74, 8484.
(5) Gautrot, J. E.; Hodge, P. Polymer 2007, 48, 7065.
(6) Dubey, R. K.; Kumpulainen, T.; Efimov, A.; Tkachenko, N. V.;
Lemmetyinen, H. Eur. J. Org. Chem. 2010, 3428.
r
10.1021/ol200354t
Published on Web 06/02/2011
2011 American Chemical Society

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
3305

Table 2. Photophysical Properties of DBPDs Recorded in Toluene and THF at Room Temperature after Photoexcitation at 370 nm
toluene THF
a
a
compd
λ
em (nm) τem1 (ns) ΦF krad ( ꢁ 107 sꢀ1) knrad ( ꢁ 107 sꢀ1
)
τem1 (ns) τem2 (ns) ΦF krad ( ꢁ 107 sꢀ1) knrad ( ꢁ 107 sꢀ1
)
PADBP27
CPADBP27
PADBP36
CPADBP36
500
600
420
600
2.44 0.106
3.35 0.177
<1.00 0.090
1.49 0.229
4.3
5.3
36.6
24.6
4.29
3.64 (3%) 25.6 (97%) 0.237
<1.00 0.135
13.4 (70%) 19.2 (30%) 0.304
0.124
2.9
0.9b
20.4
3.0
>9.0
15.4
>91.0
51.7
>13.5
2.3b
>86.5
5.2
a ΦF values against coumarin 153 in EtOH (ΦF = 0.73). b Value calculated from the PL lifetime of biggest contribution.
While the linear model reveals higher dipole moment
differences between the ground and excited state of
CPADBP36 with respect of the 2,7-variant, the opposite
was found for PADBPs.
electrontransfer(eT) was performedvia theRehmꢀWeller
equation.15,16 The 0ꢀ0 singlet gap of the donor (ES1 = 84
kcal/mol)17 and that of the acceptor (ES1 = 71 kcal/mol)6
revealed exergonic driving forces for charge separation
(CS) in these compounds, resulting in ꢀ27 and ꢀ14 kcal/
mol in a solvent of medium polarity such as THF. The
center-to-center distance is assumed to be 1.3 nm, and the
˚
radii for Cz and DBP are taken as 3.5 and 4.0 A, respec-
tively, after evaluation of their optimized geometries via
DFT calculations (Section S3, Supporting Information).
Therefore, SSIP formation should occur in the proposed
DBPDs if their singlet energies are high enough, the kinetic
barriers are not sufficiently high, or the competing deacti-
vation mechanisms (i.e., PL or ISC) are not significantly
faster. While this should favor CT as excited-state deacti-
vation mechanism in all dipolar DBPDs, ISC is yet to be
discarded given that the spinꢀorbit coupling in these
systems should be similar as it is dominated by the nature
of the acceptor unit.
Figure 1. (A) Normalized UVꢀvis absorption spectrum of
solutions of PzDs in THF at 25 °C. (B) Normalized PL spectra
of THF solutions of FMDs at 25 °C after excitation at 400 nm.
After evaluation of the optimized geometries at the
B3LYP/6-31G* level, it was found that the PA bridges
are planar to the acceptor unit in all cases. In the case of
CPADBPs, the Cz donors were tilted from the PADBP
plane. The frontier orbitals of CPADBPs (Figure 2) reveal
HOMOs localized on both CPA units and the LUMOs on
the DBP acceptor. This points toward CT character for
HOMOfLUMO transitions. The calculated ground-state
(GS) dipoles resulted are low for both dipolar compounds
(CPADBP27, μS0 = 0.20 D; CPADBP36, μS0 = 1.92 D) as
expected from their symmetry.The calculated difference
between excited and ground state dipole moments via
TDDFT is significant for both compounds, especially for
the case of CPADBP36 (CPADBP27, Δμ = 12.9 D;
CPADBP36, Δμ = 38.1 D). This anticipates that if the
molecule experiences small internal reorganization ener-
gies, the S1 state energy will be dependent on the solvent
reorganization. TDDFT also reveals that the extension of
the πꢀsystem negligibly affects the nꢀπ* transitions while
decreasing the energy gap of the πꢀπ* transitions. This is
more evident when incorporating Cz units. For both
PADBP variants, the S3rS0 transition is mapped as an
The PL lifetimes of DBPDs, as well as the ΦF values in
toluene and THF solutions at room temperature, are
reported in Table 2. The PL of DBPDs followed mono-
exponential decay in toluene, while both Cz-containing
variants presented biexponential decay in THF in the
nanosecond time regime. Given that solvatochromic be-
havior from DBPDs is important for the cases of 2,
7-DBPDs and CPADBP36, as well as the observation that
PADBP27 presents monoexponential decay in THF, there
could be an equilibration between LE and CT states in the
case of dipolar DBPDs. The shorter lifetime component is
minor for CPADBP27, contrary to its 3,6-variant in THF.
On the other hand, the rapid NRD found for PADBP36
might be explained in terms of a very fast intersystem
crossing (ISC) possibly mediated by an upper nꢀπ* state.
This is based on the negligible solvatochromic effects
observed in this system (i.e., no CT occurs in this
compound) plus the fact that the rigidity of the DBPD
acceptor implies minuscule vibrational coupling between
states.
The ionization potential of Cz (IP = 5.3 eV)14 and
electron affinity of Pz (EA = 3.1 eV)4 have been reported
in acetonitrile (εS = 37). Assuming solvent separated ion-
pair (SSIP) formation, analysis of the driving force for
(15) Kavarnos, G. J. Fundamentals of Photoinduced Electron Trans-
fer; VCH: New London, CT, 1993.
(16) Rehm, D.; Weller, A. Z. Isr. J. Chem. 1970, 8, 259.
(17) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Decker: New York, 1993.
(14) Tsuchida, A.; Yamamoto, M.; Nishijima, Y. J. Phys. Chem.
1984, 88, 5064.
3306
Org. Lett., Vol. 13, No. 13, 2011

nꢀπ* with energies relatively close to the S1rS0 transition,
which is πꢀπ* (ΔE ∼ 7.1ꢀ14.5 kcal/mol range). However,
while CPADBPs possess nꢀπ* transitions of the same
energy as those from PADBPs, there are many πꢀπ*
transitions of lower energy gap in between the πꢀπ* (S1)
and the nꢀπ* (Sn) states.
based on our DFT data (vide supra). The appearance of
plane polarized phosphorescence had been attributed to an
out-of-plane ring deformation mode21 implying the parti-
cipation of an nꢀπ* intermediate state.22 Later studies via
ps laser flash photolysis (LFP) labeled this ISC transition as
S1(nꢀπ*)fT1(πꢀπ*).23 However, assigning S1 as nꢀπ*
state seems inappropriate if considering the negligible
UVꢀvis solvatochromic shifts of DBP24 and DBPDs. Davis
assigned the S1 of DBP as πꢀπ* in the early 1970s after study-
ing the photoreduction of DBP with tri-n-butylstannane.25
Such an assignment was supported by others,24,26ꢀ28 and on
the basis of our observations, we find it appropriate for our
systems. Interestingly, many mechanisms seem to be actively
competing in all dipolar DBPDs, wherein polar media CT
deactivation is favored in all cases.
In summary, novel dipolar compounds based on Cz
donors and a DBP acceptor were synthesized and their
photophysics characterized. The compounds possess small
electronic communication between DꢀA units in the
ground state. The S1 state of dipolar DBP derivatives
deactivate mostly nonradiatively via charge transfer under
polar conditions.
Acknowledgment. We thank Prof. Massimo Olivucci
from the Laboratory for Computational Photochemistry
and Photobiology at BGSU for support with theoretical
computations and Prof. F. N. Castellano and Dr. F.
€
Spanig from the Center for Photochemical Sciences at
BGSU for support with voltammetry and photolumines-
cence measurements. DFT calculations were perfor-
med with resources from of the Ohio Supercomputer
Center. L.A.E. thanks the McMaster Endowment for a
fellowship. This work was funded by the Endowment
Fund of the Center for Photochemical Sciences from
BGSU.
Figure 2. Frontier orbitals of optimized dipolar DBPDs com-
puted at the B3LYP/6-31G* DFT level (isodensity = 0.03).
Initial studies on DBP by Hochtrasser focused on
oscillator strengths for the nꢀπ* transition in single crys-
tals at low temperatures (4.1 and 77 K).18 While DBP’s
lowest nꢀπ* and πꢀπ* states are close in energy, the nꢀπ*
transition possibly localizes in the region of the positive
hole at the nitrogen. This is based on the spectral simila-
rities between phenazine (Pz) and DBP. The negligible
fluorescence from Pz highlights effective ISC (kISC ∼1012
Supporting Information Available. Synthetic proce-
dures, compound characterizations, and 1H and 13C
NMR data. Cartesian coordinates of DBPD-optimized
geometries, TDDFT data, and orbital maps. Cyclic and
differential pulse voltammograms. UVꢀvis absorption
and PL spectra in solvents of varied polarity. Lip-
pertꢀMataga plot. This material is available free of
charge via the Internet at http://pubs.acs.org.
s
ꢀ1).19,20 This is also apparent in DBP (ΦF ∼ 1%)19 and
could consequently apply to some DBPDs studied herein
(18) Hochstrasser, R. M. J. Chem. Phys. 1962, 36, 1808.
(19) Kleinerman, M.; Azarraga, L.; McGlynn, S. P. J. Chem. Phys.
1962, 37, 1825.
(24) Dey, D.; Bose, A.; Bhattacharyya, D.; Basu, S.; Maity, S. S.;
Ghosh, S. J. Phys. Chem. A 2007, 111, 10500.
(20) Lin, H.-B.; Topp, M. Chem. Phys. 1979, 36, 365.
(21) Lim, E. C.; Li, R.; Li, Y. H. J. Chem. Phys. 1969, 50, 4925.
(22) Turro, N. J. Modern Molecular Photochemistry; University
Science Books: Sausalito, 1991.
(23) Dobrinevskii, S. F.; Karypov, A. V.; Maier, G. V.; Tikhomirov,
S. A.; Tolstorozhev, G. B. J. Appl. Spectrosc. 1991, 55, 694.
(25) Davis, G. A. Tetrahedron Lett. 1971, 12, 3045.
(26) Dubey, R. K.; Kumpulainen, T.; Efimov, A.; Tkachenko, N. V.;
Lemmetyinen, H. Eur. J. Org. Chem. 2010, 3428.
(27) Dey, D.; Bose, A.; Chakraborty, M.; Basu, S. J. Phys. Chem. A
2007, 111, 878.
(28) Gautrot, J. E.; Hodge, P. Polymer 2007, 48, 7065.
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