Charge instability of symmetry broken dipolar states in quadrupolar and octupolar triphenylamine derivatives

Article abstract of DOI:10.1039/c3cc47514j

The quadrupolar and octupolar cyano triphenylamines shows symmetry broken dipolar charge transfer state, however, its stability can be controlled by the rotation of N-C bond of amino and phenylene moiety. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc47514j

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Charge instability of symmetry broken dipolar
states in quadrupolar and octupolar
triphenylamine derivatives†
Shanmugam Easwaramoorthi,*ab Pichandi Thamaraiselvi,a
Kumaraguru Duraimurugan,c Arockiam Jesin Beneto,c Ayyanar Siva*c and
Balachandran Unni Naira
Cite this: Chem. Commun., 2014,
50, 6902
Received 1st October 2013,
Accepted 6th April 2014
DOI: 10.1039/c3cc47514j
www.rsc.org/chemcomm
The quadrupolar and octupolar cyano triphenylamines shows
While the CT interaction of simple D–p–A motif can be
symmetry broken dipolar charge transfer state, however, its stability explained by the solvatochromism in fluorescence accompanied
can be controlled by the rotation of N–C bond of amino and by large changes in dipole moments, that of multipolar systems
phenylene moiety.
might either originate from the cumulative behaviour of whole
molecules with zero dipole moment change or symmetry broken
Photoinduced intramolecular charge-transfer (ICT) interactions, dipolar ICT state from the subchromophoric, single arm
one of the primary photochemical processes has been of groups.8 Thus formed states may not have the same stability
great interest as the magnitude of ICT has a significant influ- because the donor–acceptor groups which are not part of the CT
ence on the functions and efficiencies of molecular systems in state would destabilize by competing to become part of the CT
the fields of photovoltaics,1 non-linear optics,2 fluorescent state. Further, most of the molecules studied so far are based on
sensors,3 and organic light emitting diodes (OLED).4 Recently, the vibronic coupling between the branches and no system
it has been found that molecules having multiple donors (D) based on the rotational motion of the bond has been reported.
or acceptors (A) such as D–D–p–A,5 D–p–A–A,6 D(p–A)2 or Hence, we have chosen the triphenylamine derivatives shown
A(p–D)2 (quadrupolar), and A(p–D)3 or D(p–A)3 (octupolar)7 in Chart 1 with one (mCN), two (dCN), and three (tCN) cyanide
motif, show higher efficiency in solar cells and multi-photon groups substituted at the para positions, which have, respectively,
absorbing properties when compared to the single-arm parent the dipolar, quadrupolar, and octupolar configurations to
compound with D–p–A motif. The intramolecular charge trans- understand the CT interactions.
fer interactions and subsequent charge redistribution upon
The cyano substituted triphenylamines, 4-(diphenylamino)
excitation has been identified as one of the important factors benzonitrile (mCN), 4,40-(phenylazanediyl)dibenzonitrile (dCN),
responsible for the enhanced properties of multipolar chromo- and 4,40,400-nitrilotribenzonitrile (tCN) were synthesized and
phores. Most of the work reported so far has focused on the characterized using IR, H, 13C-NMR (see ESI,† Fig. S1–S8) and
1
structure–property relationship with respect to the dipolar, single crystal X-ray diffraction methods. The single crystal X-ray
quadrupolar, and octupolar nature to the nonlinear optical structure of mCN and tCN depicted in Fig. 1 shows a propeller
properties. No systematic photophysical studies to understand shaped structure with no significant structural differences with
the nature and stability of the CT states or magnitude of respect to the number of cyano groups (ESI,† Table S1). The
CT interactions has been reported so far to the best of our absorption and fluorescence spectra of mCN, dCN, and tCN
knowledge.
dissolved in toluene solvent are given in Fig. 2. While two
a Chemical Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai,
India. E-mail: moorthi@clri.res.in; Fax: +91-44-24411630
b CSIR-Network of Institutes for Solar Energy (NISE), India
c Department of Inorganic Chemistry, Madurai Kamaraj University, Madurai, India.
E-mail: drasiva@gmail.com
† Electronic supplementary information (ESI) available: Synthesis, characteriza-
tion, MO diagrams, UV-visible, fluorescence spectral details. CCDC 963776 and
963777. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc47514j
Chart 1
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between TPA and DMABN could be; the lone pair of central
nitrogen atom in TPA is delocalized through conjugation with
phenylene moieties, at the expense of ICT interactions,11 in
DMABN the N-methyl groups facilitate the CT interaction by an
inductive electron-donating effect with phenylene moiety. The
Stokes shift values for mCN, dCN, and tCN was calculated to be
4040, 3230, and 2750 cmꢀ1, respectively. Being with the same
molecular skeleton, the larger difference in the Stokes shift values
reflects that the magnitude of charge transfer from TPA, donor to
cyanide acceptor decreases when more than one cyanide group is
present. While the simple donor–acceptor systems form a dipolar,
D+Aꢀ charge separated state, quadrupolar and octupolar system
shows, respectively, either symmetric, non-solvatochromic, quad-
rupolar (AdꢀD+Adꢀ) and octupolar (AdꢀD+(Adꢀ)Adꢀ) CT state or
symmetry broken, solvatochromic, dipolar CT state.12 As can be
seen from Fig. 2, dCN and tCN also show solvatochromism in
fluorescence similar to mCN. The observed fluorescence might
have originated from the symmetry broken dipolar CT state which
is localized on one of the donor–acceptor pairs (Scheme 1). A
quantitative measure for CT efficiency, the slope of the Lippert–
Fig. 1 X-ray crystal structure of (a) mCN and (b) tCN.
Mataga plot13 was calculated to be 15 400, 5300, and 9600 cmꢀ1
,
respectively, for mCN, dCN, and tCN (ESI,† Fig. S12). Interestingly,
the excited state dipole moments (ESI,† Table S5) were calculated
to be 8.1, 15.0, 11.0 D, respectively, for mCN, dCN, and tCN.
However, the difference between the ground and excited state
dipole moments follows the order mCN 4 tCN 4 dCN, which
suggests that all of the derivatives show symmetry broken intra-
molecular charge transfer interactions which are in fact strongly
Fig. 2 (a) UV-visible absorption (solid line) and fluorescence spectra
(dotted line) of cyanotriphenylamine derivatives in toluene. Fluorescence
spectra of (b) mCN, (c) dCN, and (d) tCN in different solvents.
absorption peaks were observed for mCN (302 (s), 331 nm) and influenced by the number of donor–acceptor pairs.
dCN (313(s), 334 nm) that of tCN shows an intense, single The fluorescence decay profiles measured using time correlated
absorption peak at 336 nm, having the molar absorptivity at least single-photon counting (TSCPC) technique by exciting the sample
two times higher than that of mCN and dCN. This feature is at 350 nm, B200 ps pulsed light are given in Fig. 3 and Fig. S13
ascribed to the degeneracy of the S1 state of tCN and also to the (ESI,† Table S6). For all TPA derivatives, longer lifetimes were
increased number of cyano groups that alters the electronic observed in polar solvents. Specifically, the fluorescence lifetime of
energy levels of TPA core through p-conjugation. Further insight mCN has increased at least by B20 times while going from
about the absorption spectrum has been gained from density nonpolar, hexane (0.6 ns) to polar, DMSO (10 ns). This feature
functional theory calculations using Gaussian 09 programme.9a suggests that the polar CT state might have been stabilized by the
The lowest energy electronic transition for all the TPA derivatives polar solvents. The radiative and non-radiative rate constants
are found to originate from the highest occupied molecular calculated from the fluorescence quantum yields and lifetime
orbital (HOMO) to the lowest unoccupied molecular orbital suggests that the radiative rate constant (Kr) becomes enhanced
(LUMO) (ESI,† Fig. S9–S11, Tables S2 and S3). Furthermore, by a factor of 8, 4, and 5 times, respectively, for mCN, dCN and tCN
electron density redistribution occurs to a significant extent
during electronic transitions from HOMO to LUMO, which
suggests the possibility of ICT interactions. To confirm, we have
calculated natural transition orbitals (NTOs) of first excited
state9b where a pronounced electron density shift can be found
between the phenylene moieties, which ascertains the CT nature
of the first excited singlet state (ESI,† Table S4). The fluorescence
spectrum of mCN, dCN, and tCN in toluene has been observed at
405, 387, and 369 nm, respectively. Interestingly, the mono cyanide
substituted TPA shows red shifted fluorescence than the di- and
tri-cyanide, despite additional p-conjugation between the cyano
and phenylene moieties. Notably, the backbone of mCN closely
resembles N,N-dimethylaminobenzonitrile (DMABN), a classical
example for ICT molecules whose well-studied dual fluorescence
behaviours are still highly debated.10 Nonetheless, the differences
Scheme 1 Potential energy diagram of charge resonance structures for
octupolar molecule.
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involving one of the cyanophenylene groups, and then it is
relaxed to the S0 state through radiative decay. In this case, the
lifetime of the CT state is controlled by the rate at which the
phenyl group rotates (tunnelling); the higher the rotations
the lower the CT state stability and vice versa. For instance,
dCN and tCN show nearly two times longer fluorescence
lifetime in acetonitrile and DMSO when compared to non-
polar solvent, toluene and ethyl acetate. This feature is
ascribed to the stabilization of the CT state, which sub-
sequently prevents the destabilization process by hindering the
inter-conversion between the charge resonance structures by means
of phenylene rotation. In order to confirm this hypothesis we have
obtained the optimized geometry of the first excited state were one
of the N–C bonds has more double bond character for dCN and tCN
when compared to the ground state (ESI,† Tables S7 and S8). Thus,
based on the above facts we can conclude that although the
quadrupolar and octupolar molecules form the symmetry broken
CT state, the lifetime of the thus formed state is significantly
influenced by the remaining acceptor moieties that are not part of
the CT state.
In summary, the quadrupolar and octupolar molecules with
triphenylamine as donor and cyano group as an acceptor form
an emissive, long-lived, symmetry broken intramolecular
charge transfer state similar to the dipolar molecule. The
radiative rate constant of the CT state becomes highly sensitive
to the number of donor–acceptor pairs. The formation of the
charge transfer state can be manipulated by controlling the
rotational motion of the phenylene moiety.
We thank CSIR-TAPSUN (NWP-54) and the DST-fast track
(FT/CS-150/2011) programme for financial support. PT thanks
DST for INSPIRE fellowship. We thank NCUFP and Dr V.
Subramanian, respectively, for TCSPC and computational
facilities. AS acknowledges the financial support from UGC
(41-215/2012 (SR)) and CSIR (01(2540)/11/EMR-II), India.
Fig. 3 Fluorescence decay profiles of (a) mCN and (b) dCN in different
solvents monitored at their respective emission maximum wavelength.
in DMSO when compared to toluene (ESI,† Table S6). On the other
hand the nonradiative rate constant (Knr) remains almost constant
irrespective of the solvent. Thus, based on the above facts we can
conclude that although dCN and tCN form symmetry broken
dipolar CT state, its lifetime is shorter than that of mCN. Symmetry
broken CT state for quadrupolar and octupolar systems can be
explained, respectively, by two and three, isoenergetic charge
resonance structures. The inter-conversion between the CT states
is driven by the phenyl ring rotation around N–C bond of amine
and phenyl group. Effect of phenylene rotation on the fluorescence
behaviour was probed by increasing the viscosity of the solution
using polystyrene, where higher viscosity is expected to hinder the
N–C bond rotation. It was found, as shown in Fig. 4 that the
fluorescence intensity decreased with increased percentage of
polystyrene (ESI,† Fig. S14). Indeed, up to two fold decreased
fluorescence was noted for mCN, however, that of dCN and tCN
was measured to be B1.4 fold when compared to neat toluene.
This observed feature underscores the importance of phenyl
rotation in the CT state formation.
The schematic electronic energy level diagram of cyanotriphenyl-
amine (tCN) is given in Scheme 1. According to Prof. Painelli’s phase
diagram for quadrupolar and octupolar molecules,14 the cyanotri-
phenylamines belongs to the class I, where the nondipolar ground
state undergoes symmetry breaking at the excited state having two
and three equivalent minima, respectively, for quadrupolar and
octupolar chromophores with equal and opposite dipole moments.
The initial photo-excitation of the TPA derivatives populates the
higher excited state which is then relaxed to form a long-lived CT
state. The CT state formation involves the rotation of the phenyl
ring with the subsequent formation of quinonoid type structure
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