Spiroconjugated intramolecular charge-transfer emission in non-typical spiroconjugated molecules: The effect of molecular structure upon the excited-state configuration

Article abstract of DOI:10.1002/cphc.201201095

A set of terfluorenes and terfluorene-like molecules with different pendant substitutions or side groups were designed and synthesized, their photophysical properties and the excited-state geometries were studied. Dual fluorescence emissions were observed in compounds with rigid pendant groups bearing electron-donating N atoms. According to our earlier studies, in this set of terfluorenes, the blue emission is from the local π-π* transition, while the long-wavelength emission is attributed to a spiroconjugation-like through-space charge-transfer process. Herein, we probe further into how the molecular structures (referring to the side groups, the type of linkage between central fluorene and the 2,2′-azanediyldiethanol units, and - most importantly - the amount of pendant groups), as well as the excited-state geometries, affect the charge-transfer process of these terfluorenes or terfluorene-like compounds. 9-(9,9,9′′,9′′-tetrahexyl- 9H,9′H,9′′H-[2,2′:7′,2′′-terfluoren] -9′-yl)-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolone (TFPJH), with only one julolidine pendant group, was particularly synthesized, which exhibits complete perpendicular conformation between julolidine and the central fluorene unit in the excited state, thus typical spiroconjugation could be achieved. Notably, its photophysical behaviors resemble those of TFPJ with two pendant julolidines. This study proves that spiroconjugation does happen in these terfluorene derivatives, although their structures are not in line with the typical orthogonal π fragments. The spiroconjugation charge-transfer emission closely relates to the electron-donating N atoms on the pendant groups, and to the rigid connection between the central fluorene and the N atoms, whereas the amount of pendant groups and the nature of the side chromophores have little effect. These findings may shed light on the understanding of the through-space charge-transfer properties and the emission color tuning of fluorene derivatives. The relationship between molecular structure and spiroconjugation intramolecular charge-transfer emission is investigated. It is demonstrated that the latter is closely related to both the electron-donating N atoms on the pendant groups and the rigid connection between the central fluorene and the N atoms. In contrast, the amount of pendant groups and the nature of the side chromophores have little effect on the emission. Copyright

Full text of DOI:10.1002/cphc.201201095

CHEMPHYSCHEM
ARTICLES
DOI: 10.1002/cphc.201201095
Spiroconjugated Intramolecular Charge-Transfer Emission
in Non-Typical Spiroconjugated Molecules: The Effect of
Molecular Structure upon the Excited-State Configuration
Linna Zhu,[a, b] Cheng Zhong,*[a] Cui Liu,[a] Zhongyin Liu,[a] Jingui Qin,[a] and Chuluo Yang*[a]
A set of terfluorenes and terfluorene-like molecules with differ-
ent pendant substitutions or side groups were designed and
synthesized, their photophysical properties and the excited-
state geometries were studied. Dual fluorescence emissions
were observed in compounds with rigid pendant groups bear-
ing electron-donating N atoms. According to our earlier stud-
ies, in this set of terfluorenes, the blue emission is from the
local p–p* transition, while the long-wavelength emission is at-
tributed to a spiroconjugation-like through-space charge-trans-
fer process. Herein, we probe further into how the molecular
structures (referring to the side groups, the type of linkage be-
tween central fluorene and the 2,2’-azanediyldiethanol units,
and—most importantly—the amount of pendant groups), as
well as the excited-state geometries, affect the charge-transfer
process of these terfluorenes or terfluorene-like compounds.
9-(9,9,9’’,9’’-tetrahexyl-9H,9’H,9’’H-[2,2’:7’,2’’-terfluoren]-9’-yl)-
1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolone (TFPJH), with
only one julolidine pendant group, was particularly synthe-
sized, which exhibits complete “perpendicular” conformation
between julolidine and the central fluorene unit in the excited
state, thus typical spiroconjugation could be achieved. Notably,
its photophysical behaviors resemble those of TFPJ with two
pendant julolidines. This study proves that spiroconjugation
does happen in these terfluorene derivatives, although their
structures are not in line with the typical orthogonal p frag-
ments. The spiroconjugation charge-transfer emission closely
relates to the electron-donating N atoms on the pendant
groups, and to the rigid connection between the central fluo-
rene and the N atoms, whereas the amount of pendant groups
and the nature of the side chromophores have little effect.
These findings may shed light on the understanding of the
through-space charge-transfer properties and the emission
color tuning of fluorene derivatives.
1. Introduction
Dual fluorescence emissions originating from two different
emitting states in one molecule, for example, one locally excit-
ed state and one formed in an intramolecular charge-transfer
state, have been intensively studied in the past several
years.[1–3] The mostly studied dual-emission systems usually in-
clude an ICT (intramolecular charge transfer),[4–8] a TICT (twisted
intramolecular charge transfer), and the formation of an exci-
mer/exciplex system.[9–12] In the above-mentioned courses, ICT
and TICT are the most common CT emission characters, and
could be considered as through-bond charge-transfer process-
es, whereas the formation of the exciplex is a through-space
charge transfer, in which the donor and the acceptor moiety
are generally linked by a flexible non-conjugated chain.[13–15]
Apart from these systems, there is also a special through-
space charge transfer, that is, the spiroconjugation, depicting
two mutually perpendicular p-systems (donor and acceptor)
linked by a spirojunction. An interaction that allows for elec-
tronic coupling between the two parts will occur, provided
that the molecular orbitals of the mutually perpendicular frag-
ments are antisymmetric with respect to the two perpendicular
bisecting planes.[16–20] The interactions between these orbitals
thus lead to charge delocalization spanning the entire mole-
cule, and to the formation of two new spiroconjugated orbi-
tals, a new HOMO (corresponding to the bonding combination
of the subsystem orbitals) with lower energy, and a new LUMO
(corresponding to the higher-energy antibonding combination)
with higher energy (Scheme 1). This kind of through-space in-
teraction is controlled by the energy and symmetry of these
frontier orbitals. However, spiroconjugation caused fluores-
cence emission is rarely reported to date.
Earlier in 2008, we first reported a novel terfluorene, TFOH,
with 4-(bis(2-hydroxyethyl)amino)phenyl as side chains, exhibit-
ing dual fluorescence emissions in polar solvents.[21] Though its
photophysical behavior is similar to the TICT character, the
emission cannot be assigned to a TICT mechanism because
the conjugation between fluorene and the pendent 4-(bis(2-
hydroxyethyl)amino)phenyl group is interrupted by the sp3 hy-
bridization of the C9 atom in the central fluorene unit. To fur-
ther investigate the emission mechanism, then we specially
synthesized an analogue of TFOH, namely, TFPJ, with the
amino nitrogen immobilized in the planar position of the juloli-
[a] Dr. L. Zhu, Dr. C. Zhong, C. Liu, Z. Liu, Prof. Dr. J. Qin, Prof. Dr. C. Yang
Department of Chemistry
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials
Wuhan University, Wuhan 430072 (P.R. China)
[b] Dr. L. Zhu
College of Chemistry and Chemical Engineering
Hubei University, Wuhan 430062 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201201095.
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 982 – 989 982

CHEMPHYSCHEM
ARTICLES
www.chemphyschem.org
ies and the LUMO in the fluorene moieties is demonstrated to
be of spiroconjugation-like nature, from both experimental re-
sults and theoretical studies.
The purpose of this work is to probe deeper into the photo-
physical processes occurring in these molecules to study the
structure–property relationship by structure modification and
exploitation, and thereby to investigate the scope and general-
ity of the emission mechanism. In this article, we design and
synthesize a set of terfluorenes and terfluorene-like molecules
with systematically varied side groups (using phenyl rings and
its derivatives), or with different spacer groups between fluo-
rene and the 2,2’-azanediyldiethanol pendant groups (hexyl
chains are introduced instead of rigid phenyl rings, Scheme 3).
The influences of the structure modifications on the emission
behaviors are discussed, including the effect of the side
groups, and of the bridge ligand between the central fluorene
and the 2,2’-azanediyldiethanol units, as well as the amount of
the pendant groups. Notably, for comparison with TFPJ, TFPJH
(with only one julolidine substitution) was particularly synthe-
sized, taking into account that the small steric effect on the C9
position of the central fluorene should bring enough space for
structure adjustment in the excited state, which might lead to
a “real” perpendicular configuration of the pendant julolidine
and the central fluorene group, and as a result, a typical spiro-
conjugated conformation could be achieved in TFPJH. Corre-
spondingly, the photophysical properties, together with the ex-
cited-state geometries of these compounds, were investigated.
By comparison with the previously reported results on terfluor-
enes, showing spiroconjugation-like charge-transfer emission,
the intrinsic interaction between pendant substitutions and
fluorene moieties in the excited states could be well under-
stood. More importantly, this special charge-transfer emission
mechanism could be finally determined by both experimental
results and theoretical calculations.
Scheme 1. Molecular orbitals responsible for the spiroconjugation effect.
Scheme 2. Molecular structures of TFP, TFOH, TFAc, TFE, and TFPJ.
dine groups (Scheme 2). In this case, the charge-transfer emis-
sion should be totally inhibited if the terfluorenes went
through a TICT mechanism. Unexpectedly, the photophysical
properties of TFPJ still resembled those of TFOH, though the
rotation of the CꢀN single bond is fully restricted in TFPJ,
which thereby rules out the TICT mechanism. At the same
time, we synthesized two analogues of TFOH, that is, TFAc
(with 4-(acetamido)phenyl as pendent groups) and TFE [with
4-(bis(2-acetoxyethyl)amino)phenyl as pendent groups], that
are distinguished from the electron-donating abilities of the
pendant groups (Scheme 2).[22] From their fluorescence spectra,
no charge-transfer emission can be observed in TFP or TFAc
(with no N atoms on the pendants, or quite weak electron-do-
nating pendant groups), whereas dual fluorescence emission
and a bathochromic shift of the long-wavelength emission can
2. Computational Methodology
The geometries of the ground states were optimized at the
PBE0[23] def2-SVP[24] and PBE0 6-31G(d) levels for comparison.
Excited-state geometry optimizations were performed at the
TD/PBE0 def2-SVP level. Then, the vertical excitation energies
were calculated by TD/PBE0 def2-SVP and TD/PBE0 6-311+
G(d) for comparison. Calculations using the def2-SVP and 6-
31G(d), 6-311+G(d) basis sets were performed with the Turbo-
mole 6.0 Package[25] and the Gaussian 03 program,[26] respec-
tively.
be observed in TFE, with relatively weak electron-donating 3. General Information
aminophenyl groups. It turned out that the stronger the elec-
3.1. Methods
tron-donating ability, the larger the bathochromic shift.
Common ICT behaviors were gradually excluded, and finally
a process related to the spiroconjugation-like mechanism was
put forward. Obviously, the terfluorenes are not structurally in
accordance with the typical spiroconjugated molecules bearing
orthogonal donor and acceptor fragments; however, the inter-
action between the HOMO in the pendant aminophenyl moiet-
The 1H NMR and 13C NMR spectra were recorded on
a MERCURYVX 300 spectrometer in CDCl3 using tetramethylsi-
lane as an internal reference. Elemental analyses of carbon, hy-
drogen, and nitrogen were performed on a Carlorerba-1106
microanalyzer. MALDI-TOF was measured on a Voyager-DE STR
MALDI TOF instrument, using a-cyano-4-hydroxyclinnamic acid
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 982 – 989 983

CHEMPHYSCHEM
ARTICLES
www.chemphyschem.org
Scheme 3. Chemical structures of the compounds studied in this article.
3.3. Synthesis of TFHOH
(CHCA) as a matrix. EI Mass was recorded on a Finnigan TRACE
mass spectrometer. UV/Vis absorption spectra were recorded
on a Shimadzu UV-2550 recording spectrophotometer. PL spec-
tra were recorded on a Hitachi F-4500 fluorescence spectro-
photometer. The PL quantum yields were measured from the
dilute solution of the terfluorenes (ca. 10ꢀ6 molLꢀ1) by an abso-
lute method using an Edinburgh Instruments system (FLS920)
integrating sphere excited with a Xe lamp. Fluorescence
decays were measured at room temperature by a single-
photon-counting spectrometer from Edinburgh Instruments
(FLS920) with a hydrogen-filled pulse lamp as the excitation
source. The data were analyzed by iterative convolution of the
luminescence-decay profile with the instrument response func-
tion using the software package provided by Edinburgh Instru-
ments. The goodness of the nonlinear least-squares fit was
judged by the reduced c2 value (<1.3 in all cases). The concen-
tration of all the compounds in the UV/Vis absorption, fluores-
cence emission, and lifetime experiments was 5ꢁ10ꢀ6 m.
An amount of M1 (150 mg, 0.13 mmol) was added to a stirred
mixture of diethanolamine (0.5 mL) and Na2CO3 (42.4 mg,
0.4 mmol) in acetonitrile (10 mL). The mixture was then re-
fluxed for 48 h with stirring under argon atmosphere. After re-
action, the mixture was cooled to room temperature and fil-
tered. The solvent was removed under reduced pressure. The
residue was dissolved in CHCl3 and washed with brine. The or-
ganic phase was dried over anhydrous Na2SO4. The compound
TFHOH was gained as a clear oil after column chromatography
using CH3OH/EA (1:5, VVꢀ1) as eluent. Yield: 60 mg, 38%.
1H NMR (300 MHz, CDCl3): d=7.84–7.66 (m, 16H, phenyl H),
7.37–7.34 (m, 4H, phenyl H), 3.53 (t, J=6.0 Hz, 8H, CH2OH),
2.56 (t, J=6.0 Hz, 8H, NCH2CH2), 2.40 (br, 4H, CH2CH2N), 2.08–
2.02 (m, 12H), 1.13–1.09 (m, 40H), 0.88–0.75 ppm (m, 20H).
13C NMR (75 MHz, CDCl3): d=153.53, 152.87, 142.65, 142.41,
142.25, 141.91, 128.98, 128.75, 127.95, 124.86,123.24, 121.96,
121.85, 121.67, 61.43, 57.90, 57.11, 56.55, 42.30, 33.40, 31.85,
31.61, 28.90, 28.68, 25.71, 24.49, 15.96 ppm. Anal. Calcd. for
C83H116N2O4 (%): C, 82.67; H, 9.70; N, 2.32; Found: C 82.73, H
9.44, N 2.21. MALDI-TOF-MS: m/z 1204.1 (M+).
3.2. Materials
3.4. Synthesis of PFN
Phenylboronic acid and tetrakis-(triphenylphosphine)palladium
were purchased from Acros. Other materials were used directly
without further purification unless otherwise stated. The com-
pounds 4,4’-(2,7-dibromo-9H-fluorene-9,9-diyl)bis(N,N-diethyl-
aniline) and 4-tert-butyl-2,6-dimethylphenylboronic acid were
synthesized according to literature procedures.[27–29] The syn-
thesis of 2,7-dibromo-9-(1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]qui-
nolin-9-yl)-9H-fluoren-9-ol (M2) has been reported else-
where.[22] The synthetic procedures for the intermediates M1
and M3 are described in the Supporting Information (SI).Sy-
nthetic routes for the final compounds are shown in Scheme 4.
A mixture of 4,4’-(2,7-dibromo-9H-fluorene-9,9-diyl)bis(N,N’-di-
ethylaniline) (200 mg, 0.33 mmol), phenylboronic acid (119 mg,
0.98 mmol), Pd(PPh3)4 (22.5 mg, 0.02 mmol), and 2m K2CO3
aqueous solution (2 mL) in THF (6 mL) was stirred at 758C for
24 h. After reaction, the organic layer was extracted by CH2Cl2,
and was washed with saturated brine, then dried over anhy-
drous Na2SO4. The solvent was removed under reduced pres-
sure, and the compound PFN was gained as a white solid after
column chromatography using EA/PE (1:10, V/V) as eluent.
1
Yield: 0.13 g, 63%. H NMR (300 MHz, CDCl3): d=7.80 (d, J=
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 982 – 989 984

CHEMPHYSCHEM
ARTICLES
www.chemphyschem.org
Scheme 4. Synthetic routes for the compounds studied in this article.
8.1 Hz, 2H, fluorene H), 7.65 (s, 2H, fluorene H), 7.60-7.56 (m,
6H), 7.43-7.38 (m, 6H), 7.30-7.26 (m, 6H), 7.14 (d, J=8.0 Hz,
4H, pendant Phenyl H), 6.53 (d, J=8.0 Hz, 4H, pendant Phenyl
H), 3.27 (q, J=12.0 Hz, 8H, CH2), 1.10 ppm (t, J=6 Hz, 12H,
CH3). 13C NMR (75 MHz, CDCl3): d=152.67, 145.29, 140.56,
139.28, 137.82, 131.44, 128.15, 127.58, 126.16, 125.93, 125.09,
123.90, 119.19, 110.21, 62.99, 43.12, 11.65, 11.62 ppm. Anal.
Calcd. for C45H44N2 (%): C, 88.19; H, 7.24; N, 4.57; Found: C
88.73, H 7.44, N 4.21. EI-MS: m/z 613.5 (M+ +1).
ed brine, then dried over anhydrous Na2SO4. The solvent was
removed under reduced pressure, and the compound DMFN
was gained as a white solid after column chromatography
1
using EA/PE (1:20, VVꢀ1) as eluent. Yield: 0.33 g, 85%. H NMR
(300 MHz, CDCl3): d=7.77 (d, J=8.0 Hz, 2H, fluorene H), 7.14-
7.10 (m, 8H, fluorene H), 7.05 (d, J=8.0 Hz, 4H, phenyl H), 6.46
(d, J=8.0 Hz, 4H, phenyl H), 3.25 (q, J=6.0 Hz, 8H, CH2CH3),
2.06 (s, 12H, CH3), 1.34 (s, 18H, C(CH3)3), 1.08 ppm (t, J=6 Hz,
12H, CH2CH3). 13C NMR (75 MHz, CDCl3): d=152.91, 149.49,
146.17, 139.75, 139.41, 138.21, 135.69, 133.04, 129.04, 127.99,
127.58, 124.24, 119.59, 111.68, 63.87, 44.28, 34.26, 31.40, 21.33,
12.47 ppm. Anal. Calcd. for C57H68N2 (%): C, 87.64; H, 8.77; N,
3.59; Found: C, 87.71, H 8.66, N 3.32. ESI-MS: m/z 782 (M+ +1).
3.5. Synthesis of DMFN
A mixture of 4,4’-(2,7-dibromo-9H-fluorene-9,9-diyl)bis(N,N-di-
ethylaniline) (308 mg, 0.5 mmol), 4-tert-butyl-2,6-dimethylphe-
nylboronic acid (309 mg, 1.5 mmol), Pd(PPh3)4 (35 mg,
0.03 mmol), and 2mK2CO3 aqueous solution (2.5 mL) in THF
(7.5 mL) was stirred at 758C for 24 h. After reaction, the organ-
ic layer was extracted by CH2Cl2, and was washed with saturat-
3.6. Synthesis of TFPJH
A mixture of M3 (300 mg, 0.61 mmol), 9,9’-dihexyl-9H-fluoren-
2-ylboronic acid (693 mg, 1.83 mmol), Pd(PPh3)4 (46 mg,
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 982 – 989 985

CHEMPHYSCHEM
ARTICLES
www.chemphyschem.org
0.04 mmol) in THF (12 mL) and 2m K2CO3 aqueous solution
(4 mL) was stirred at 758C for 24 h. After reaction, the organic
layer was extracted with CHCl3, and washed with saturated
brine, then dried over anhydrous Na2SO4. The crude product
was purified by column chromatography on silica gel using
EtOAc/PE (1:15, v/v) as eluent to give a light-yellow powder.
and no prominent solvatochromic effect appears (Figure 2a),
representing a typical fluorene emission which is similar to
that of TFP and TFAc, with no electron-donating pendant
phenyl groups (see supporting information). TFPJH, bearing
only one julolidine group, exhibits a quite similar photolumi-
nescence behavior to that of TFPJ, with two julolidine groups,
namely, a dual fluorescence emission, and the long-wavelength
emission shows a significant bathochromic shift when the sol-
vent polarity increases (Figure 2b). Yet, for PFN and DMFN,
with shorter conjugation lengths, the maximum emission
bands gradually shift to longer wavelengths when the solvent
polarity increases, indicating the existence of intramolecular
charge-transfer process. Overall, both PFN and DMFN exhibit
similar emission behaviors, although in PFN, two side phenyl
rings can freely rotate around the single bond with respect to
the central fluorene ring. Meanwhile, the rotation is mostly re-
stricted in DMFN owing to the steric effect of the methyl sub-
stitutions on the side phenyl rings. The phenomena described
above indicate that charge transfer apparently occurs in TFPJH,
PFN and DMFN, but not in TFHOH. A common structural fea-
ture in TFPJH, PFN and DMFN is the electron-donating amino-
phenyl group directly connected to the C9 position of the cen-
tral fluorene unit, whereas for TFHOH, the presence of hexyl
chains between fluorene and the pendant N atoms totally ex-
cludes the possibility of charge-transfer emission,in spite of the
two electron-donating bis(2-hydroxyethyl)amino groups.
Therefore, the phenomena above imply that the rigid phenyl
pendant groups, as well as the electron-donating units, are
quite essential for the charge-transfer emission.
1
Yield: 0.24 g, 40%. H NMR (300 MHz, CDCl3): d=7.90 (d, J=
6.0 Hz, 2H, fluorene H), 7.73-7.58 (m, 12H, fluorene H), 7.36-
7.30 (m, 6H, fluorene H), 6.68 (s, 2H, julolidine H), 5.07 (s, 1H,
fluorene H), 3.15 (br, 4H, julolidine CH2), 2.75 (br, 4H, julolidine
CH2), 2.00-1.98 (m, 12H), 1.12-1.05 (m, 32H), 0.76-0.72 ppm (br,
12H). 13C NMR (75 MHz, CDCl3): d=151.67, 151.15, 140.91,
140.69, 140.11, 139.95, 127.25, 126.99, 126.29, 124.24, 123.11,
121.54, 120.46, 120.14, 119.96, 55.39, 54.11, 40.66, 40.61, 31.70,
29.93, 23.98, 22.81, 14.25 ppm. Anal. Calcd. for C75H87N (%): C,
89.86; H, 8.75; N, 1.40; Found: C 90.00, H 9.15, N 1.31. MALDI-
TOF-MS: m/z 1002.4 (M+).
4. Results and Discussion
4.1. Photophysical Properties
Figure 1 shows the UV/Vis absorption and fluorescence spectra
of the compounds in THF. Both TFHOH and TFPJH show ab-
sorption peaks at approximately 350 nm, ascribed to the p–p*
transition from the terfluorene moiety. For PFN and DMFN,
with side phenyl substitutions, the absorptions locate at short-
er wavelengths (namely, 313 and 277 nm) due to the shorter
conjugation length in the two molecules. In addition, the maxi-
mum absorption of these compounds has almost no shifts
when the solvent polarity increases (see SI for details), suggest-
ing a small intramolecular charge-transfer character in the
ground state, as had been observed for the other terfluorenes
previously studied.
Table 1 summarizes the photophysical data of these com-
pounds in solvents with different polarities. Obviously, TFHOH
is strongly emissive, despite the solvent polarity—and with
very high quantum efficiency (>90%), yet quite short excited-
state lifetime, which represents the typical strong blue emis-
sion of terfluorene. This behavior is almost the same as that
observed for TFP and TFAc. In contrast, the emission efficiency
of the other compounds studied herein gradually decreases
with increasing solvent polarity, which is consistent with their
emission behavior in solution. In addition, their emission
decays are in accordance with those of the previously reported
terfluorenes TFOH and TFPJ, that is, apart from a short lifetime,
there is also a longer lifetime corresponding to the charge-
transfer emission band. This indicates that relaxation of the ex-
cited state may lead to a lower energy emission.
The photoluminescence behaviors of the studied com-
pounds in THF show remarkable differences. TFHOH, with
a hexyl spacer, exhibits a strong blue emission around 410 nm,
Concerning the nature of the charge-transfer emission,
when we first found this phenomenon in TFOH, we tentatively
attributed it to an analogue of the TICT mechanism, that is,
the rotation of a pendant CꢀN bond was supposed to cause
the TICT state. But this speculation was overthrown upon the
synthesis and investigation of TFPJ, in which the rotation of
the CꢀN bond in the pendant groups is completely restricted.
After that, we also excluded the through-space D–A dyad
mechanism by measuring the fluorescence spectra of the ter-
fluorene-bearing hexyl chains (5 mm) in the presence of juloli-
dine in THF solution. As a result, the blue emission of the fluo-
rene fluorophore is greatly quenched upon addition of juloli-
dine, and meanwhile, a new weak emission band around
Figure 1. UV/Vis absorption and normalized fluorescence emission spectra
of the compounds in THF solution, with a concentration of 5 mm. The excita-
tion wavelength for PFN and DMFN is 320 nm, and that for TFHOH and
TFPJH is 350 nm.
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 982 – 989 986

CHEMPHYSCHEM
ARTICLES
www.chemphyschem.org
Figure 2. Normalized fluorescence emission spectra of a) TFHOH, b) TFPJH, c) PFN, and d) DMFN in different solvents. The concentrations of the compounds
are 5 mm. The excitation wavelength for PFN and DMFN is 320 nm, and that for TFHOH and TFPJH is 350 nm.
Table 1. Fluorescence data for TFHOH, TFPJH, PFN, and DMFN in different solvents.
Tol
CH2Cl2
THF
DMF
lem [nm] F[a] [%] t[b] [ns] [%][c] lem [nm] F [%] t [ns] [%][c] lem [nm] F [%] t [ns] [%][c] lem [nm] F [%] t [ns] [%][c]
TFHOH 412
97
92
0.5
5.0
0.6
8.0
0.7
12.0
1.3
8.1
0.7
8.5
0.2
4.7
99.2
0.8
91.4
8.6
13.5
86.5
70.5
29.5
5.7
414
94
56
0.5
4.3
0.8
9.3
1.5
23.8
1.1
12.8
0.7
18.3
1.9
99
1
415
98
34
0.5
3.5
0.7
5.6
2.4
12.7
1.3
10.4
0.8
12.8
1.1
8.7
0.5
7.7
97
3
417
98
25
0.5
4.2
0.7
7.6
3.0
10.9
1.3
3.6
2.3
23.7
1.2
11.9
0.8
19
99
1
TFPJH
398
460
398
512
94.5
5.5
11.6
88.4
75
25
2.7
97.3
43.4
56.6
6.7
398
530
95.6
4.4
14.3
85.7
47
398
570
89.0
11.0
46.4
53.6
99.6
0.4
PFN
350
434
21
21
363
470
24
12
355
485
21
17
356
520
13
18
53
5.2
94.8
52
48
12
4.3
94.3
11
89
95.7
84.3
15.7
5.2
DMFN
396
351
426
370
421
355
465
12.6
0.4
7.6
96.3
88
94.8
[a] F denotes the fluorescence quantum efficiency. [b] The fluorescence decays were recorded at a concentration of 5 mm in different solvents. t represents
the fluorescence lifetime (there are two lifetimes at each emission wavelength). [c] % denotes the contribution of each component.
570 nm becomes evident, attributed to the exciplex emission.
These findings indicated that the strong green emission of
TFPJ in THF solution (536 nm) was not from the exciplex.
Common ICT processes are gradually excluded from our exper-
imental results.
According to our previous studies on terfluorene derivatives
with spiroconjugated emission,[22] the existence of spiroconju-
gated charge transfer in this series of compounds could simply
be determined by their photophysical behaviors. TFPJH, PFN
and DMFN all exhibit a distinct charge-transfer character, with
the common structure of pendant electron-donating amino-
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 982 – 989 987

CHEMPHYSCHEM
ARTICLES
www.chemphyschem.org
Table 2. Calculated values of the excited states of TFPJH, PFN and DMFN. The absorption and fluorescence properties in CH2Cl2 solution (experimental re-
sults) are also included.[a]
Geometry
Transition
Orbitals involved
f
(Calcd.)
(Exptl.) [nm]
Dipole [debye]
2.89
PFN
S0-S1
S0-S2
S0-S3
S0-S3
S0-S1
S0-S1
S0-S2
S0-S7
S0-S7
S0-S1
S0-S1
S0-S2
S0-S2
S0-S1
H!L
0.0025
0.0645
1.0911
1.229
0.0268
0.0028
0.0347
0.5756
1.291
0.0254
0.0674
2.1368
2.2093
0.0453
375.39
365.75
316.08
371.51
454.53
337.36
329.86
275.02
401.73
411.02
389.82
351.59
418.85
487.85
–
–
314
377
475
–
Ground state
H-1!L
H-2!L
H-2!L
H!L
Relaxed S3
Relaxed S1
4.81
17.24
DMFN
TFPJH
H!L
Ground state
H-1!L
H-2!L
H-2!L
H!L
–
2.84
276
402
425
–
348
398
512
Relaxed S7
Relaxed S1
8.09
16.86
H!L
Ground state
2.2366
H-1!L
L!H-1
L!H
Relaxed S2
Relaxed S1
4.6728
19.5379
[a] In this table, the dipole moment is given in debye, and the vertical excitation energy in nm. The oscillator strength is that in the ground state (absorp-
tion). H and L represent the HOMO and LUMO orbitals, respectively.
fragment. The relaxed S2 state thus has two subsequent pathways.
phenyl groups that could be assigned to a spiroconjugation-
One is the vertical transition from S2 to S0 (f=2.2093, lmax
=
like charge transfer. The charge-transfer emission is regardless
of the side chromophores, which only act by changing the
whole conjugation length of the compound (the emission
wavelengths of PFN and DMFN are shorter than those of the
other terfluorenes). Regarding the hexyl linkage, the strong
blue emission of TFHOH, irrespective of the solvent polarity,
provides further evidence that the phenyl pendants are neces-
sary to offer proper orbitals that may interact with the orbitals
of the central fluorene moiety, thus leading to charge transfer.
The above phenomena bring us to the following conclusions:
1) the existence of electron-donating N atoms in company
with the pendant phenyl rings guarantees the orbitals needed
for the charge-transfer process; 2) the side chromophores
mainly affect the conjugation length, and the rotation of side
chromophores will not lead to charge transfer; 3) one or two
pendant amino-phenyl groups can both result in charge-trans-
fer emission.
418.85 nm), correlating well with the experimentally observed blue
emission (lmax =398 nm); the other one is the internal conversion
from S2 to S1 (see Table 2 for detail). In the relaxed S1 state of
TFPJH, the calculated vertical transition from LUMO to HOMO (f=
0.0453, lmax =487.85 nm) is in good agreement with the experi-
mentally observed green emission (lmax =512 nm) in CH2Cl2 solu-
tion. Additionally, the calculated S1 excited-state dipole moment
(19.5379 D) is much larger than the ground state (2.2366 D), ac-
counting for the large solvatochromic effect of the lower energy
emission. The evidence that the HOMO orbitals are delocalized
over the central fluorene unit in the S1 relaxed geometry suggests
enhanced spiroconjugation between julolidine and terfluorene in
the S1 state. This is because that this HOMO orbital can be consid-
erd as anti-bonding combination between HOMO of julolidine and
the HOMO of terfluorene. So in the ground state the spiroconjuga-
tion is disfavored. When an electron is excited from this orbital to
LUMO in the S1 state, the weakened anti-bonding interaction leads
to energy in favor of spiroconjugation. This kind of CT excited-
state-enhanced-spiroconjugation character is the same as we have
observed from the other terfluorene derivatives (TFOH, TFPJ and
TFE). Similar phenomena are also observed in PFN and DMFN, their
Kohn–Sham frontier orbitals are presented in the supporting infor-
mation, and their electronic properties are summarized in Table 2.
Calculation Details
DFT calculations were performed to investigate the electronic
structures of these compounds, to identify the charge-transfer
character in the excited state. Taking the terfluorene TFPJH as an
example, its absorptions could be clearly assigned as follows: First-
ly, the HOMO orbital is located on the julolidine moiety, the LUMO
orbital is distributed on the terfluorene moiety. There is little over-
lap between the two orbitals in the ground state, and the electron
transition from HOMO to LUMO is quite weak, which is in accord-
ance with the little absorption changes. On the other hand, the
HOMO-1 is mainly contributed from HOMO in terfluorene, and the
transition from HOMO-1 to LUMO (f=2.1368, lmax =351.59 nm)
We specially note that in TFPJH, there is a small steric effect on the
C9 position of the central fluorene ring, due to the single julolidine
pendant group. In the excited state, the julolidine group could be
in an orthogonal relationship with the central fluorene plane, im-
plying that a typical spiroconjugated conformation is achieved in
the excited state of TFPJH (Figure 3). It turns out that the photo-
physical properties of TFPJH are consistent with those of TFPJ
bearing two julolidines, which directly reveals that spiroconjuga-
tion does exist in this set of terfluorene derivatives, whether their
structures are in line with the typical orthogonal p fragments or
not.[30,31] Moreover, the spiroconjugated charge-transfer emission
has little to do with the amount and conformation of pendant
groups.
matches well with the absorption maximum of TFPJH (lmax
=
348 nm) measured in dichloromethane.
When this molecule is excited to the S2 state, the vibrational relax-
ation firstly makes the excited-state molecule back to the 0 vibra-
tional level of the S2 singlet state. In the relaxed S2 excited state,
the dihedral angle between the side fluorenes and the central fluo-
rene is reduced to get better planarity of the whole terfluorene
We had reported that the electron-donating ability of the pendant
groups directly affects the charge-transfer energy. The stronger the
electron-donating ability of the pendant nitrogen atom, the lower
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 982 – 989 988

CHEMPHYSCHEM
ARTICLES
www.chemphyschem.org
Scholars and Innovative Research
Team in University (IRT1030). L.Z.
thanks the National Natural Sci-
ence Foundation of China (No.
51203046). C.Z. thanks the Na-
tional Natural Science Foundation
of China (No. 51203121).
Keywords: charge-transfer
emission
·
chromophores
·
molecular structure · pendant
groups
·
spiroconjugated
molecules
Figure 3. Kohn–Sham frontier orbitals of TFPJH. S0, S1 and S2 mean ground state, first singlet excited state, and
second excited state, respectively. EDD indicates the differential density for the corresponding excited state, in
which the blue part represents a negative value (electron-density decrease) and orange represents a positive
value (electron-density increase).
[1] Y. V. Il’ichev, W. Kꢂhnle, K. A. Za-
chariasse, J. Phys. Chem. A 1998,
102, 5670.
[2] W. Schuddeboom, S. A. Jonker,
J. M. Warman, U. Leinhos, W.
the energy of the charge-transfer emission. Hereby we further dis-
close the influence of the molecular structures and the excited-
state conformations on the emission behaviors. It turns out that
the rigid pendant groups, as well as the electron-donating N
atoms, are necessary for the charge-transfer emission, whereas the
side substitutions have no effect on the charge-transfer emission.
We especially investigated a complete “spiroconjugation” configu-
ration in TFPJH with a small steric effect on the pendant groups,
and a similar charge-transfer emission was observed compared to
other terfluorenes we had studied earlier. The calculations confirm
that spiroconjugation does exist in those compounds with elec-
tron-donating aminophenyl pendants directly connected to the C9
position of the central fluorene unit, although their chemical struc-
tures are not in line with the typical orthogonal conformations.
Kuehnle, K. A. Zachariasse, J. Phys. Chem. 1992, 96, 10809.
[3] S. Cogan, S. Zilberg, Y. Haas, J. Am. Chem. Soc. 2006, 128, 3335.
[4] D. Srikun, E. W. Miller, D. W. Domaille, C. J. Chang, J. Am. Chem. Soc.
2008, 130, 4596.
[5] J. Wang, X. Qian, J. Cui, J. Org. Chem. 2006, 71, 4308.
[6] G. Jones, W. R. Jackson, S. Kanoktanaporn, W. R. Bergmark, Photochem.
Photobiol. 1985, 42, 477.
[7] M. Zhu, M. Yuan, X. Liu, J. Xu, J. Lv, C. Huang, H. Liu, Y. Li, S. Wang, D.
Zhu, Org. Lett. 2008, 10, 1481.
[8] F. B. Dias, S. Pollock, G. Hedley, L.-O. Pꢃlsson, A. Monkman, J. Phys.
Chem. B. 2006, 110, 19329.
[9] B. J. Hrnjez, P. T. Yazdi, M. A. Fox, K. P. Johnston, J. Am. Chem. Soc. 1989,
111, 1915.
´
´
[10] J. Dobkowski, J. Wꢄjcik, W. Kozminski, R. Kolos, J. Waluk, J. Michl, J. Am.
Chem. Soc. 2002, 124, 2406.
[11] S. M. King, R. Matheson, F. B. Dias, A. P. Monkman, J. Phys. Chem. B 2008,
112, 8010.
[12] L. I. Bosch, M. F. Mahon, T. D. James, Tetrahedron Lett. 2004, 45, 2859.
[13] H. Wu, D. Zhang, L. Su, K. Ohkubo, C. Zhang, S. Yin, L. Mao, Z. Shuai, S.
Fukuzumi, D. Zhu, J. Am. Chem. Soc. 2007, 129, 6839.
[14] J. Karpiuk, J. Phys. Chem. A 2004, 108, 11183.
[15] L. He, F. Xiong, S. Li, Q. Gan, G. Zhang, Y. Li, B. Zhang, B. Chen, G. Yang,
J. Phys. Chem. B 2004, 108, 7092.
5. Conclusions
In summary, a series of compounds containing aminophenyl as
pendant groups and fluorene derivatives as main chains were
synthesized through the Suzuki coupling reaction. Their photo-
physical properties were studied. By combining experiments
with DFT studies of the excited-state properties, we are able to
conclude that terfluorenes with pendant phenyl rings linked to
the C9 position of the central fluorene, together with electron-
donating N atoms, provide sufficient orbital and energy for an
unusual intramolecular charge transfer, substantiated as the
spiroconjugation interaction. This study has expanded the
theory that spiroconjugation only existed in systems with or-
thogonal p-fragments, and has made a contribution to the ap-
plication of the through-space electronic effect. From another
point of view, fluorescence emission could be facilely tuned by
structure modification of the system, and might find use in
other electrical and optical fields, for example, as chemical or
biological sensors. We are now working on the development
of their application as fluorescent sensors.[32]
[16] V. Minkin, Chem. Rev. 2004, 104, 2751.
[17] H. E. Simmons, T. Fukunaga, J. Am. Chem. Soc. 1967, 89, 5208.
[18] R. Hoffmann, A. Imamura, G. D. Zeiss, J. Am. Chem. Soc. 1967, 89, 5215.
[19] P. Maslak, A. Chopra, C. Moylan, R. Wortmann, S. Lebus, A. L. Rheingold,
G. P. A. Yap, J. Am. Chem. Soc. 1996, 118, 1471.
[20] M. Abe, W. Adam, W. T. Borden, M. Hattori, D. A. Hrovat, M. Nojima, J.
Wirz, J. Am. Chem. Soc. 2004, 126, 574.
[21] L. Zhu, C. Yang, J. Qin, Chem. Commun. 2008, 6303.
[22] L. Zhu, C. Zhong, Z. Liu, C. Yang, J. Qin, Chem. Commun. 2010, 46, 6666.
[23] J. P. Perdew, M. Ernzerhof, K. Burke, J. Chem. Phys. 1996, 105, 9982.
[24] A. Schꢃfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571.
[25] R. Ahlrichs, M. Bar, M. Haser, H. Horn, C. Kolmel, Chem. Phys. Lett. 1989,
162, 165.
[26] L. Laaksonen, J. Mol. Graph. 1992, 10, 33.
[27] L. Zhu, C. Yang, W. Zhang, J. Qin, Polymer 2008, 49, 217.
[28] P. R. Ashton, K. D. M. Harris, B. M. Kariuki, D. Philp, J. M. A. Robinson, N.
Spencer, J. Chem. Soc. Perkin Trans. 2 2001, 2166.
[29] D. Vak, B. Lim, S.-H. Lee, D.-Y. Li, Org. Lett. 2005, 7, 4229.
[30] R. G. Hicks, L. ꢅhrstrçm, G. W. Patenaude, Inorg. Chem. 2001, 40, 1865.
[31] P. Naumov, P. Yu, K. Sakurai, J. Phys. Chem. A 2008, 112, 5810.
[32] L. Zhu, J. Qin, C. Yang, Chem. Commun. 2010, 46, 8755–8757.
Acknowledgements
Received: December 28, 2012
Revised: January 22, 2013
Published online on February 21, 2013
C.Y. thanks the National Science Fund for Distinguished Young
Scholars of China (No. 51125013), the program for Changjiang
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPhysChem 2013, 14, 982 – 989 989