Strong Dipolar Effects on an Octupolar Luminiscent Chromophore: Implications on their Linear and Nonlinear Optical Properties

Article abstract of DOI:10.1021/acs.jpca.6b02805

Design parameters derived from structure-property relationships play a very important role in the development of efficient molecular-based functional materials with optical properties. Here, we report on the linear and nonlinear optical properties of a fluorene-derived dipolar system (DS) and its octupolar analogue (OS), in which donor and acceptor groups are connected by a phenylacetylene linkage, as a strategy to increase the number of delocalized electrons in the π-conjugated system. The optical nonlinear response was analyzed in detail by experimental and theoretical methods, showing that, in the octupolar system OS, the dipolar effects induced a strong two-photon absorption process whose magnitude is as large as 2210 GM at infrared wavelengths. Solvatochromism studies were implemented to obtain further insight on the charge transfer process. We found that the triple bond plays a fundamental role in the linear and nonlinear optical responses. The strong solvatochromism behavior in DS and OS was analyzed by using four empirical solvent scales, namely Lippert-Mataga, Kamlet-Taft, Catalán, and the recently proposed scale of Laurence et al., finding consistent results of strong solvent polarizability and viscosity dependence. Finally, the role of the acceptor groups was further studied by synthesizing the analogous compound 2DS, having no acceptor group.

Full text of DOI:10.1021/acs.jpca.6b02805

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Article
Strong Dipolar Effects on an Octupolar Luminiscent Chromophore:
Implications on Their Linear and Nonlinear Optical Properties
Arturo Jiménez-Sánchez, Itzel Isunza-Manrique, Gabriel Ramos-Ortiz, Jesús
RODRÍGUEZ Rodríguez-Romero, Norberto Farfan, and Rosa Santillan
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b02805 • Publication Date (Web): 09 Jun 2016
Downloaded from http://pubs.acs.org on June 17, 2016
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Strong Dipolar Effects on an Octupolar Luminiscent
Chromophore: Implications on their Linear and
Nonlinear Optical Properties
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Arturo JiménezꢀSánchez,†,* Itzel IsunzaꢀManrique,§ Gabriel RamosꢀOrtiz,§ Jesús Rodríguezꢀ
Romero,† Norberto Farfán† and Rosa Santillan‡
† Facultad de Química, Universidad Nacional Autónoma de México, Cd. Universitaria. Ciudad
de México No. 04510, México. arturojs@cinvestav.mx. Phone: +52ꢀ55ꢀ56223731.
‡ Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN,
CINVESTAV, Apdo., Postal 14ꢀ740, Ciudad de México, 07000, México.
§ Centro de Investigaciones en Óptica, CIO, Apdo., Postal 1ꢀ948, 37000 León Gto, México.
ABSTRACT
Design parameters derived from structureꢀproperty relationship play a very important role in the
development of efficient molecularꢀbased functional materials with optical properties. Here, we
report on the linear and nonꢀlinear optical properties of a fluoreneꢀderived dipolar system (DS)
and its octupolar analogue (OS) in which donor and acceptor groups are connected by a
phenylacetylene linkage as strategy to increase the number of delocalized electrons in the πꢀ
conjugated system. The optical nonlinear response was analyzed in detail by experimental and
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theoretical methods, showing that in the octupolar system OS, the dipolar effects induced a
strong twoꢀphoton absorption process whose magnitude is as large as 2210 GM at infrared
wavelengths. Solvatochromism studies were implemented to obtain further insight on the charge
transfer process. We found that the triple bond plays a fundamental role in the linear and nonꢀ
linear optical responses. The strong solvatochromism behaviour in DS and OS was analyzed by
using four empirical solvent scales, namely LippertꢀMataga, KamletꢀTaft, Catalán and the
recently proposed scale of Laurence et al., finding consistent results of strong solvent
polarizability and viscosity dependence. Finally, the role of the acceptor groups was further
studied by synthesizing the analogous compound 2DS having no acceptor group.
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INTRODUCTION
During the last decades, the nonꢀlinear optical process of twoꢀphoton absorption (TPA) has
become an area of intensive research owing to its application in optical limiting, 3ꢀD
microfabrication, photodynamic therapy, highꢀdensity optical storage, holographic data storage,
frequencyꢀupconversion/twoꢀphoton pumped lasing,1ꢀ3 and bioimaging.4ꢀ6 Much effort has been
made to design and synthesize molecules with high TPA efficiency.7 Three main design
strategies can be adopted to enhance the TPA response, a) increasing the strength of the acceptor
(A) and donor (D) groups;8 b) increasing the number of delocalized electrons in the πꢀconjugated
system (the πꢀbridge);9ꢀ10 c) produce an efficient ICT transition induced by the effective
HOMO−LUMO separation.11ꢀ12 Coe proposed to categorize the electronic systems on the basis
of the part of the molecule that is altered during the commutation reaction, namely, the donor
part (type I), the acceptor part (type II), or the conjugated bridge (type III).13 In this regards,
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fluoreneꢀderived TPA compounds have been designed under strategies a and c and types I and II
through functionalization with strong D groups on the fluorene moiety.14ꢀ16 However, no
fluoreneꢀderived dipolar–octupolar TPA fluorophores under strategy b or type III has been
reported yet. This strategy is of interest for several reasons. First, the transition moments are
related to the chromophore length over which the charge is displaced (polarized) during a
transition, such that when the fluorene (D) moiety is oriented along the longitudinal axis of the
chromophore, the D to A length is increased without incorporating flexibility to the system, thus
increasing the TPA response. Second, a proper πꢀbridge, like the phenylꢀacetylene linkage
should also promote solubility and viscosityꢀdependent properties for biological applications or
bioimaging. Third, since no steric hindrance is present, several A groups can be used without
loss of conformational planarity. Fourth, these compounds can have different commutation
processes, including cisꢀtrans photoisomerization, intramolecular proton/charge transfer or
photochromic reactions.17ꢀ19
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In this work, we explored the implications of the phenylꢀacetylene πꢀbridge on the linear and
nonꢀlinear optical properties of a fluoreneꢀderived octupolar compound, namely OS. Of our
particular interest is to analyze the dipolar contribution on the octupolar response. Thus, we
implemented a systematic study to demonstrate the importance of pure dipolar contribution by
synthetizing the compound DS which represents the exact dipolar fragment integrated in
compound OS. Moreover, the role of benzonitrile and triazine as A groups on the phenylꢀ
acetylene linkage was further studied by synthesizing the analogous compound having no
acceptor moiety, namely 2DS. Results show that TPA response increases dramatically going
from DS to OS with an important contribution of dipolar response. Moreover, as a consequence
of specific solvent interactions occurring in DS and OS a strong positive solvatochromism was
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studied by four empirical solvent scales, namely LippertꢀMataga, KamletꢀTaft, Catalán and the
recently proposed scale of Laurence et al. finding consistent results of solvent polarizability and
viscosity dependence due to the strong dipolar charge transfer, which is in agreement with the Xꢀ
ray structure analysis of DS. These optical responses were experimentally and theoretically
analyzed and characterized.
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RESULTS AND DISCUSSION
Synthesis and characterization. The fluorene dipolar (DS) and octupolar (OS) systems, as
well as compound 2DS were prepared according to Scheme 1. Compounds 1–3 were synthetized
according to reported methodologies.20ꢀ21 Subsequent treatment of compound 3 with 4ꢀ
(bromo)benzonitrile under Sonogashira crossꢀcoupling conditions by using Pd(Cl)2(PPh3)2 as
catalyst gave fluorophore DS with excellent yield. On the other hand, the oneꢀstep synthesis of
fluorophore OS was found to be highly difficult by using the wellꢀknown nitrile
cyclotrimerization in triflic acid conditions.22 Further, we used a triple Sonogashira crossꢀ
coupling methodology between 3 and trisꢀ4ꢀbromophenyꢀ1,3,5ꢀtriazine, which also resulted
unsuccessful due to the formation of predominant monoꢀ and diꢀcoupling products. However, the
later methodology was implemented with trisꢀ4ꢀiodophenyꢀ1,3,5ꢀtriazine (4) obtaining OS in a
good yield. We also applied a recently described nitrileꢀcyclotrimerization by using pure ZnCl2
as catalyst.23 Then, compound DS and ZnCl2 in a 3 : 1 molar ratio were filled into an ampoule
under nitrogen and sealed under vacuum and heating at 300 °C for 48 hours, thus obtaining OS
in 20% yield.
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C8H17C8H17
C8H17C8H17
1ꢀBromooctane
KOH / DMSO
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6
Me3SiC CH
Si(Me)3
Br
Br
Pd(CH3CO2)2, CuI,
PPh3, DIPA
2
1
7
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9
MeOH / Et2O
K2CO3
CN
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C8H17
C8H17
C8H17C8H17
C8H17
C8H17
CN
Br
Pd(Cl)2(PPh3)2, CuI
TEA
C8H17
C8H17
CuI
TEA
3
2DS
DS
I
C8H17
C8H17
CN
I
Pd(Cl)2(PPh3)2
CuI / TEA
CF3SO2OH
N
N
N
ZnCl2
I
I
4
N
N
N
C8H17
C8H17
C8H17
C8H17
OS
Scheme 1. Synthesis of compounds DS, 2DS and OS.
Xꢀray structure analysis of DS. DS grew up in the triclinic Pꢀ1 space group having four
molecules per unit cell. The molecule is a πꢀelectronic system involving the fluoreneꢀbenzonitrile
fragment having an acetylene linkage. It is noteworthy that the acetylene linkage of the two
molecules in the unit cell present a strong torsion angle deviation from the planar conformation
for one molecule, while the other molecule is completely planar. In fact, a torsion angle of 38.4°
was found, the overlapped structure diagram is shown in Figure 1. Also, the bond length from
C14 to C15 (C≡C) is 1.202(3) Å and 1.195(3) Å for the twisted and planar structure,
respectively. Complete crystal structure information is summarized in Table S1, Supporting
Information. Further analysis of the DS Xꢀray structure showed that this conformational changes
lead to a significant difference in the C=N bond length, to know, the bond length from C20 to N1
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(C=N) is 1.147(3) Å and 1.120(2) Å for the twisted and planar structure, respectively. These
results highlight the importance of a triple bond rotation on the electronic structure of DS since
the intramolecular charge transfer (ICT) is strongly affected by rotation. It is worth mentioning
that nꢀoctyl chains play an important role in the crystal packing for the molecules since these
aliphatic chains are completely perpendicular to the plane of the fluorophore whose arrangement
induces alignment of the counterpart, see crystal packing diagram in Figure S1, Supporting
Information. Both, the length and conformation of linear aliphatic chains substituted at the 9,9’
positions of fluorene have shown a determinant role in the solidꢀstate optoelectronic properties
for conducting polymer applications.24 Further crystal packing analysis revealed that DS is not
governed by close contacts, having only two close contacts located in the alkyl chains, this is an
important consideration for crystal packing.25
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Figure 1. Overlap diagram of the Xꢀray structure for DS.
Quantum chemical calculations. Quantum chemical calculations were used to investigate the
nature of oneꢀphoton absorption (OPA) and to estimate TPA crossꢀsection for the compounds
under study. The molecular geometry and vibrational frequencies for DS and OS in the ground
(GS) and firstꢀexcited (ES) state were optimized by using Gaussian 09,26 with the global hybrid
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exchange correlation functional B3LYP,27 and charge distribution parameters were estimated
with the parameterꢀfree hybrid PBE0 functional.28 Vertical excited state energies were computed
at TDꢀCAMꢀB3LYP/631+G(d,p)/PCM level of theory.29 Bulk solvent effects were considered by
means of the Integral Equation Formalism Polarizable Continuum Model (IEFꢀPCM) in the
linear response formalism and in equilibrium regime.30ꢀ31 All calculations were performed
considering DMSO as solvent. The TPA optical properties were calculated by means of response
theory,32 as implemented in DALTON.33
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The calculated transition energies, static and transition dipole moments were fed into a sumꢀ
over states (SOS) approach in order to obtain the TPA transition matrix elements (Sα,β) which can
be extracted from the single residue of response function given by the equation:34



g
ꢀ
α i i ꢀβ
f
g
ꢀ
β i i ꢀα
ωf / 2
f
= Σ
(1)
Sα
+
,
β
ωi ωf / 2
−
ωi
−

i


∈
ωi
ωf
where α, β (x,y,z)
and
are the excitation frequency for the intermediate i and final
f
twoꢀphoton states, respectively; µ is the dipole operator at the given direction. The SOS
approach assumes that only few states dominate the response, which provides a good description
near the photon resonances.35 Moreover, all singleꢀparticle states are assumed to have a common
energy broadening parameter in damped response theory.36 Then, the TPA crossꢀsection in
atomic units δTPA is represented as follows:
δ
TPA = FδF +GδG + HδH
(2)
where δF Sαα Sββ δG
Sαβ Sαβ δH
Sαβ Sβα
= Σ
= Σ
= Σ
α
,
β
α
,
β
α ,β
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and F = G = H are coefficients dependent on polarization of the light, which are equal to 2 for
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linear polarized light. For example, for an excitation by a linearly polarized single beam of light,
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δ
TPA is evaluated by the following expression in atomic units (a.u.):37
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ꢀꢁꢂꢃ = 6ꢄꢅꢆꢇꢆ + ꢅꢈꢇꢈ + ꢅꢉꢇꢉꢊ + 8ꢄꢅꢆꢇꢈ + ꢅꢆꢇꢉ + ꢅꢈꢇꢉꢊ + 4(ꢅꢆꢆꢅꢈꢈ + ꢅꢈꢈꢅꢉꢉ + ꢅꢆꢆꢅꢉꢉ)
(3)
Then this formalism was used to calculate the TPA crossꢀsection σTPA in GM ([1 GM =10ꢀ50
cm4 s moleculeꢀ1 photonꢀ1]) units as a means of the twoꢀphoton transition amplitude between the
electronic groundꢀstate (S0) and the two lowest electronic exited states (S1 and S2). The
calculated OPA and TPA properties of DS and OS in DMSO are shown in Table 1.
Table 1. Calculated OPA and TPA photophysical properties for DS and OS in DMSO.
a
b
b
Compound
λOPA
365
374
EOPA
µge (D)c
7.2
16.5
ETPA
µee’ (D)d
e
f
ꢋꢍꢌꢎꢏ
0.077
σTPA
28.816
575.500 0.495
DS
OS
3.39
3.31
1.55
1.51
2.1
7.5
a OPA absorption wavelength in nm.
b OPA or TPA energy in eV.
c Static ground to first excited state dipole moment in Debye.
d First excited state to the TPA state transition dipole moment in Debye.
e TPA crossꢀsection in GM evaluated at 820 nm.
f Intrinsic TPA crossꢀsection at 820 nm, in GM, to account for the NLO efficiency.
On the other hand, although TDꢀDFT provides a good description in the determination of
molecular optical parameters given the accurate description of ground and excited state
geometries, commonly TDꢀDFT turns out to describe an excited state in terms of several single
electronic excitations from an occupied to a virtual orbital. Fortunately, the various contributions
to the electronic excitation can be clarified by a Natural Transition Orbital (NTO) analysis,38
which provides a compact orbital representation of the electronic transition through a single
configuration of a hole and electron interaction. Therefore, the photoinduced electron transfer
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(PET) process is not depicted by a simple change in the elementary molecular orbital occupancy,
but in a holeꢀelectron distribution.
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Figure 2 illustrates the NTO diagram for molecules DS and OS. It must be noted that DS can
be represented as a planar dipolar structure having a CS symmetry. Thus, the electronic transition
from the ground (|g>) to the first excited (|e1>) state can be represented as A’ → A”. For DS the
hole is slightly more localized along the fluorene fragment with a contribution of the ethynyl
group, while the electron is slightly more located in the benzonitrile unit with a small
contribution of the fluorene unit which confers a subtle dipolar polarization due to the charge
displacement from the fluorene to the benzonitrile moiety. This implies an intramolecular charge
transfer (ICT) character for the A’ → A” transition, corresponding to the Highest Occupied
Natural Transition Orbital and the Lowest Unoccupied Natural Transition Orbital (HONTO –
LUNTO) levels (w = 0.7) responsible for the TPA response. The parameters for this electronic
transition are shown in Figure S2, Supporting Information. TDꢀDFT predicted a σTPA of 28.816
GM for this transition at 820 nm (see Table 1). Although this computed TPA response is
relatively low, the result is interesting since dipolar molecules (nonꢀcentrosymmetric) normally
exhibits very small or null σTPA values.39
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b) OS
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electron
electron
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a) DS
|E’1>
|E’1>
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|E’2>
|E’2>
electron
|A’1>
|A’1>
|f>
A”
hole
hole
hv
electron
electron
hv
|E’1>
|E’1>
|E’2>
|E’2>
|g>
A’
|A’1>
|A’1>
hole
hole
hole
Figure 2. TPA energy level diagrams with natural transition orbitals for the first excited state
(A” for DS and E’1 and E’2 for OS) computed at PBE0/6ꢀ31+G(d,p)/IEFꢀPCMꢀDMSO. (a)
ground A’ and excited A” electronic states of DS in CS symmetry and, b) ground |A’1> and
degenerated excited states |E’1> y |E’2> of OS associated with four transitions: in the upper
scheme the contribution is purely octupolar, whereas in the lower scheme the contribution is
dipolar in nature.
In the case of the octupolar OS molecule, the generated transition moment is the result of the
specific interaction between the first photon with HONTO – LUNTO levels and the second
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photon with HONTOꢀ1 – LUNTO levels. Here, the TPA crossꢀsection obtained by TDꢀDFT
method for OS was 575.5 GM at 820 nm ( Table 1). Figure 2b shows the charge displacements
obtained by TDꢀDFT associated to ꢁgi y ꢁgf for OS in which the first excited state is doubly
degenerated; it also shows that the oscillator strength values for these two states are much higher
than those of other states, so the TPA cross section can be calculated considering only three
states (ground |g>, and two excited |e1> and |e2> states). In this case, for the C3h (or C3) point
group, the ground state function corresponds to the A' (or A for C3) representation, whereas the
two degenerated excited states correspond to E' (or E for C3) representations. Thus, the ground
state and the two degenerate excited states are symmetry allowed for the TPA process. As shown
in Figure 2b (upper scheme) which refers to the first excited state, the hole is uniformly spread
along the two fluorene branches, while the electron is strongly localized towards the triazine
center with practically no distribution on fluorene. This transition corresponds to the HONTO –
LUNTO levels (w = 0.65) and involves a strong charge transfer. However, in the corresponding
HONTOꢀ1 – LUNTO+1 transition (Figure 2b, lower scheme), the hole is distributed only over
one fluorene branch and the electron is strongly located towards the triazine nucleus, conferring
a dipolar charge transfer character to this transition, comparatively lower than the HONTO –
LUNTO. Consequently, it can be established that the charge displacement occurring from the
fluorene branches to the triazine ring completely breaks the symmetry of the charge transfer
states.40 Then, the HOMOꢀ1, HOMO and LUMO levels contribute to the TPA response.
Interestingly, a similar threeꢀlevel model (the ground state and first two excited states to
represent all the susceptibilities) was shown to predict the experimental nonlinear spectrum of a
similar octupolar molecule. Here, the threeꢀlevel model also applies since the nonlinear response
for OS is large, a result which is certainly attributed to the inclusion of the triazine ring.41
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For a better understanding of their optical properties, we analyzed the charge transfer nature of
DS and OS. PBE0 functional was used for the ground state geometry calculation and the first
excited state was also computed at CIS level keeping the 6ꢀ31+G(d) basis set and PCMꢀDMSO
for the two geometries. We studied the charge transfer excitation by means of the recently
proposed spatial extent index.42 For DS, the obtained fraction of electron charge transferred upon
deꢀexcitation from the local excited (LE) state was qCT = 0.41 at a DCT = 7.34 Å spatial distance
from the donor centroid to the acceptor centroid, having a static dipole moment difference
between ground and excited state of 14.36 D. Figure S3 shows the graphical representation of
DCT, and excess of electron density centroids (C+ (r)/C– (r)) as defined in ref (43). The H index,
[C+ (r)/C– (r)]/2, for the Donor – Acceptor axis is 3.01, thus being 4.33 Å lower than the CT
excitation length, resulting in no overlap between the Donor – Acceptor centroids, making the
ICT process efficient. In the case of compound OS the obtained qCT value was 0.51 which results
larger compared to the dipolar molecule, with a DCT = 5.02 Å spatial distance in the involved
branches with a ꢁµge of 12.2 D, the H index of OS is 4.58 resulting to be just 0.45 Å lower than
the CT excitation length. These results suggest that although DS redistributes a smaller charge
density upon photoꢀexcitation, the dipolar polarization is highly efficient. On the other hand, the
octupolar analog experiences a strong competition between its dipolar branches during the ICT
process which results in a less separated location of the Donor – Acceptor centroids of charge.
This was further confirmed by the electron density difference upon photoꢀexcitation, Figure S4,
Supporting Information.
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Finally, we scaled the computed TPA crossꢀsection with the “effective number of electrons
(Neff)” to allow a fair comparison of the TPA merit. It was shown that the scaling law is
approximately independent of the energy function, so the TPA crossꢀsection scales as ꢐꢑꢇꢒꢒ and
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the quantity for making a comparison between DS and OS must be given by the intrinsic TPA
5
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crossꢀsection, defined as:44,45
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ꢓꢇꢔꢕꢖ = ꢓꢇ/ꢐꢑꢇꢒꢒ
(4)
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Thus, the NLO efficiency was assessed in terms of the intrinsic TPA crossꢀsection, ꢓꢇꢔꢕꢖ, as
defined in Equation 4. The effective number of electrons contributing to the nonlinear response
is calculated by geometrically weighting the number of electrons in each conjugated path of the
molecule:46
ꢐꢑꢒꢒ
=
ꢔ ꢐꢇ
(5)
∑
ꢗ
ꢔ
where Ni is the number of electrons in the ith conjugated part of the molecule. Then, Neff values
of 19.4 and 34.1 resulted for DS and OS, respectively. Table 1 lists the obtained ꢓꢇꢔꢕꢖ values. As
it can be seen, the intrinsic TPA crossꢀsection for the octupolar molecule scales ca. six times the
value for the dipolar molecule. It is important to notice that the conjugation length for both, DS
and OS is practically the same, suggesting that the molecular length has no important effect on
the intrinsic nonlinearities and that the triazine core allows to increase the effective number of
electrons by Neff(OS)/Neff(DS) = 1.75 times, making the octupolar threeꢀbranched system highly
efficient.
Experimental determination of TPA cross section
In order to characterize the TPA of DS and OS compounds, we measured their σTPA by the
twoꢀphoton excited fluorescence (TPEF) technique.47 Figure 3 presents the spectrum of σTPA in
the wavelength range 660 nm – 840 nm for the case of OS in a THF solution. To calculate σTPA
,
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the quantum yield of fluorescence (Φe) had to be measured, resulting in 0.17 and 0.26, for DS
and OS respectively. The maximum σTPA resulted in 2210 GM at the 740 nm but significant
nonlinearities higher than 500 GM are obtained between 680 nm and 800 nm. In contrast, for the
case of DS, the TPEF signal was very weak and below the limit of detection of our apparatus for
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most of the wavelengths. For DS a detectable signal was obtained only at 740 nm with a σTPA
<
100 GM. These experimental results for DS and OS are in very good agreement with the results
obtained by TDꢀDFT formalism in the previous section, as shown in Figure 3. The good
agreement between the σTPA calculated by TDꢀDFT and the experimental measurements supports
the prediction that in the case of the octupolar system there is an important dipolar contribution
influencing the TPA process. This observation has previously been addressed by Beljonne,
et.al.48 for the interplay and extent of electronic coupling among the three dipolar arms in
octupolar molecules by means of molecular exciton theory. Cho, et.al. have also offered an
elementary description by means of a three chargeꢀtransfer state model.49
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Figure 3. TPA crossꢀsection values for OS obtained by TPEF experiments (circles). The dotted
line is a guide for the eye. The figure also presents the values obtained by TDꢀDFT formalism
(squares).
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Solvatochromism.
One photon absorption and emission spectra. The OPA absorption and emission spectra of DS
and OS were taken in different solvents. The UVꢀVis and fluorescence spectra obtained in
acetonitrile (ACN) at 5 x 10ꢀ6 M (exciting in the lowꢀenergy absorption band at an absorbance of
0.1 A.U.) is presented in Figure 4. The absorption band is assigned to the π → π* electronic
transition with a CT contribution. For the case of DS a vibrational structure is clearly observed.
The maximum of the main absorption band of DS is attributed to the 0–0 vibrational band
coming from an intense S0 → S1 electronic transition in fluorene moiety (located at 353 nm).
Then, a shoulder band blueꢀshifted to 330 nm was observed, which was attributed to the 0–1
vibrational band of the same electronic transition. In DS such vibrational structure is also evident
in the emission spectra, with the 0–0 and 0–1 vibrational bands at 360 nm and 466 nm,
respectively. The vibrational structure of DS is not seen in polarꢀaprotic and ꢀprotic solvents.
Interestingly, due to the high rigidity of DS, Stokes shifts in nonꢀpolar solvents are very small.
OS exhibited a maximum of absorption in 363 nm and an emission band with a maximum at 455
nm. In contrast to DS, in OS the vibrational structure is not evident in the emission spectra (in
ACN), and the Stokes shift is more pronounced (1803 cmꢀ1 and 5570 cmꢀ1, respectively). The
molar extinction coefficient (ε, in molꢀ1 cmꢀ1) at λmax for DS and OS in acetonitrile was 6240 and
21,760, respectively, which means a ca. 3.5ꢀfold increase in light attenuation for the octupolar
molecule.
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(a)
5
6
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8
(b)
1.0
0.5
0.0
1.0
0.5
0.0
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250
300
350
400
450
500
550
600
250 300 350 400 450 500 550 600 650 700 750
Wavelength (nm)
Wavelength (nm)
Figure 4. Normalized absorption (blue) and fluorescence (red) spectra obtained in acetonitrile
for (a) DS, λ(max)Abs = 353 nm, λ(max)Em = 358 nm; and (b) OS, λ(max)Abs = 363 nm, λ(max)Em = 455
nm.
A summary of the main linear and nonꢀlinear optical properties for DS and OS are presented in
Table 2. Here, a very good agreement between the electrochemical (by cyclic voltammetry) and
optical band gaps was found.
Table 2. Absorption wavelength (molar extinction coefficient in Mꢀ1cmꢀ1), emission wavelength,
fluorescence quantum yield, Stokes shift, optical band gap, and experimental transition moment
form ground to excited state for DS and OS in THF.
Stokes shift
b
Compound
λmax(ε)a
λemission
Φec
Eopt (eV)d Eelec (eV)e
µge (D)f
(cmꢀ1)
2900
4770
352nm (101420),
334nm (101230)
392 nm
439 nm
0.17
0.26
3.35
3.04
3.27
3.01
6.05
DS
OS
363nm (432000),
13.25
a OPA absorption and b emission wavelength measured at 5x10ꢀ6 M.
c Fluorescence quantum yield relative to Rhodamine (1 in THF).
d Eopt optical energy band gap is obtained from the intercept of absorption spectra.
e Eelec electrochemical energy band gap obtained by cyclic voltammetry in DMSO (see SI).
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f The experimental S0 to S1 transition moment were obtained by computing the area under the
corresponding peak using the equation
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9
ꢀg2e = (1500ln(10)hcn/2
π
2NA) (
ε
(v)/v)dv = 9.18358x10−39 ×n (
ε
(v)/v)dv , where n is the
∫
∫
refractive index (for THF = 1.4050),
)] plot.
(ε
(v)/v)dv is the area under the curve of a [(ε(
v
)/
v
),(
v
∫
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Specific solvent effects. A detailed study was performed in order to understand the nature of
the charge transfer states (CT states). As it is was discussed before, charge transfer mechanisms
influenced the TPA response. The ICT properties were studied by a combined solvatochromic
analysis of LippertꢀMataga, Catalan, KamletꢀTaft and the recently proposed solvent interaction
scale by Laurence, et al.50 All these methods combined allow obtaining quantitative evidence of
the static dipole moment change between S1 and S0 and the effect of acidity, basicity, polarity
and polarizability solvent parameters in the photophysical properties. Figure 5 shows the
absorption and fluorescence emission spectra of DS and OS in different solvents and Table 3
presents a summary of the main results from this data. Analysis of these spectra revealed that the
absorption λmax has a nonꢀmonotonic pattern as a function of solvent polarity, and a strong
broadening effect from nonꢀpolar to polar solvents is observed, particularly for viscous solvents.
In fact, the full width at half height maximum of the maximum absorption band (FWHMabs)
increases from 4120 cmꢀ1 in hexane to 5900 cmꢀ1 in DMSO for DS and from 5190 cmꢀ1 in hexane
to 10270 cmꢀ1 in DMSO for OS. In addition, both molecules experience strong Stokes shift
modification, varying from just 860 cmꢀ1 in hexane to 4897 cmꢀ1 in ethylene glycol for DS, and
from 2461 cmꢀ1 in hexane to 6890 cmꢀ1 in DMSO for OS. Thus, larger Stokes shift were
observed for OS, compared with DS. This observation is important for two main reasons: first, a
large Stokes shift modulation implies the lowest probability of having reabsorption of the
fluorescence emission.51 Second, the photoꢀexcitation process induces significant structural
changes in the excited state of the fluoroꢀsolvatochromes.
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1.6x105
1.4x105
1.2x105
1.0x105
8.0x104
6.0x104
4.0x104
Hexane
Hexane
Dioxane
Toluene
Chloroform
Tetrahydrofurane
Dichloromethane
Octanol
iꢀPropanol
Acetone
1.0
0.5
0.0
Dioxane
Toluene
Chloroform
Tetrahydrofurane
Dichloromethane
Octanol
iꢀPropanol
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Acetone
Methanol
Methanol
Acetonitrile
Dimethylformamide
Ethylene Glycol
Dimethyl Sulfoxide
Acetonitrile
Dimethylformamide
Ethylene Glycol
Dimethyl Sulfoxide
2.0x104
0.0
350
400
450
500
550
600
650
325
350
375
400
425
450
Wavelength (nm)
Wavelength (nm)
3.0x105
2.5x105
2.0x105
1.5x105
1.0x105
Hexane
Dioxane
Toluene
1.0
Hexane
Dioxane
Toluene
Chloroform
Chloroform
Tetrahydrofurane
Dichloromethane
Octanol
Tetrahydrofurane
Dichloromethane
Octanol
iꢀPropanol
Acetone
iꢀPropanol
0.5
0.0
Acetone
Acetonitrile
Acetonitrile
Dimethylformamide
Ethylene Glycol
Dimethyl Sulfoxide
Dimethylformamide
Ethylene Glycol
Dimethyl Sulfoxide
5.0x104
0.0
400 450 500 550 600 650 700 750 800
Wavelength (nm)
350
400
450
500
550
600
Wavelength (nm)
Figure 5. Absorption (left) and fluorescence spectra (right) of 10 µM DS (above) and 5µM OS
(below) in different solvents. The fluorescence spectra were taken at a 10ꢀfold dilution of the
initial concentrations to avoid inner filter effects.
Table 3. Photophysical properties of DS and OS in different media.a
DS
λe
(nm) (cmꢁ1) (nm) (cmꢁ1)
352 28409.09 363 27548.209 860.881 360 27777.778 395 25316.456 2461.32
OS
Solvent
λa
⊽
⊽
e
⊽
a
–
⊽
e
λa
⊽
λe
⊽
e
⊽ –⊽
a e
a
a
εb
n
ꢀf
(cmꢁ1) (nm) (cmꢁ1) (nm) (cmꢁ1)
(cmꢁ1)
Hexane
Dioxane
Toluene
2.02 1.4235
0
2.22 1.4224 0.021 353 28328.61 375 26666.67 1661.94 352 28409.09 409 24449.878 3959.21
2.38 1.4961 0.013 355 28169.01 369 27100.27 1068.74 361 27700.83 402 24875.62 2825.21
Chloroform 4.81 1.4459 0.148 356 28089.887 391 25575.447 2514.44 364 27472.527 433 23094.688 4377.84
THF
7.52 1.4050 0.210 352 28409.09 392 25510.20 2898.89 363 27548.209 439 22779.043 4769.17
8.93 1.4242 0.217 354 28248.587 395 25316.455 2932.132 363 27548.209 453 22075.055 5473.15
10.30 1.4295 0.225 353 28328.61 394 25380.71 2947.9 363 27548.209 441 22675.737 4872.47
20.18 1.3776 0.277 350 28571.428 399 25062.656 3508.772 361 27700.83 455 21978.02 5722.81
21.01 1.3588 0.285 349 28653.295 403 24813.896 3839.399 360 27777.778 466 21459.227 6318.55
DCM
Octanol
2ꢀPrOH
Acetone
MeOH
33.00 1.3288 0.308 349 28653.295 410 24390.24 4263.055 ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
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ACN
DMF
EG
36.64 1.3442 0.305 353 28328.61 377 26525.2 1803.41 363 27548.209 455 21978.02 5570.19
36.70 1.4305 0.274 352 28409.09 409 24449.878 3959.212 364 27472.527 433 23094.688 4377.84
41.40 1.4320 0.276 362 27624.31 440 22727.27 4897.04 370 27027.027 491 20366.60 6660.43
46.68 1.4793 0.263 355 28169.01 416 24038.46 4130.55 368 27173.91 493 20283.976 6889.93
DMSO
7
8
9
a
Dielectric constant, (ε), refractive index, (n); Orientation polarizability, (ꢂf); absorption
a); emission wavelength, (λe); emission
e). Solvent notation: THF (tetrahydrofuran), DCM (dichloromethane), ACN
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wavelength, (λa); absorption wavenumber, (
wavenumber, (
(acetonitrile), DMF (dimethylformamide), EG (ethylene glycol), DMSO (dimethyl sulfoxide).
taken from Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL,
2002.
⊽
⊽
b
The effect of solvent interactions on the absorption and emission spectra of DS and OS was
first investigated in terms of the Lippert−Mataga approximation. The Lippert theory stablish that
the UVꢀVis absorption and fluorescence variations are a consequence of specific solvent effects,
i.e. hydrogen bond, CT and acidꢀbase interactions, preferential solvation, and solventꢀsolute
hydrogen bonding.52,53 Then, LippertꢀMataga expression relates Stokes shift vs. a solvent polarity
parameter, Eq. 6:
2
(
ꢀe
−
ꢀ g
)
2
(6)
ꢁ
ν
= υa
−
υe
=
(ꢁf
)
+ C
hc
a3
where
⊽
a and
⊽
e are wavenumbers (in cm–1) for the absorption and emission bands, h is Planck's
constant, c is the speed of light, a is the Onsager radius, and ꢁꢁ = ꢁeꢀꢁg is the dipole moment
difference between the ground and excited states. The solvent polarity ꢁf is defined in terms of
the dielectric constant (ε) and the refractive index (n) of the solvent as (Eq. 7).
2


ε
−1
n −1
2




ꢁf = f (
ε
) − f (n ) =
−
2
2ε
+1 2n +1


(7)
Although the absorption bands shown in Figure 5 do not display significant shifts as a function
of solvent nature, the emission bands evidence a strong redꢀshifting of ca. 110 nm and 150 nm
for DS and OS, respectively, as the solvent polarity increases, which suggests that the dipole
moment is larger in the excitedꢀstate than in the groundꢀstate. The Lippert plot shows a strong
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nonꢀlinear behavior from the nonꢀpolar (ꢁf = 0.0 – 0.15), the polarꢀaprotic (ꢁf = 0.2 – 0.3) and
the polarꢀprotic (ꢁf = 0.2–0.31), Figure S6. The nonꢀlinear trend in the nonꢀpolar region is
explained by the fact that there is a poor solvation around the fluorophore with almost no
reorientation of nonꢀpolar solvent dipoles. Moreover, the nonꢀlinear behavior suggests that
specific solvent interactions such as hydrogen bond and ICT, and/or localꢀviscosity effects could
be present. To get more experimental clue of the ICT process of DS and OS, the ꢁꢂ values were
obtained from the equation (6). Thus, considering the slope in the nonꢀpolar to polar region
(excluding the highly viscous ethylene glycol) gives a ꢁꢂ of 12.5 D (slope = 8900 ± 750, R2 =
0.95) for DS and a ꢁꢂ of 23.1 D (slope = 10800 ± 1030, R2 = 0.94) for OS. Here, the Onsager
radius of 5.59 Å and 7.91 Å for DS and OS, respectively, were obtained by DFT. The obtained
ꢁꢂ values are comparable to those reported in the literature (3ꢀ20 D) for other solvatochromic
fluorophores,54 and are an indication of the strong charge reorganization upon photoꢀexcitation.
In addition, the Catalán, KamletꢀTaft and Laurence empirical scales also account for solvent
polarizability and polarity as the dominant solvent parameters, and this solvent dependence
becomes more important in the excited state. An exhaustive solvatochromic analysis is presented
in the Supplementary Information. Thus, from the four solvatochromic analyses it is clear that in
the case of viscous solvents (octanol, 2ꢀpropanol, EG and DMSO) the fluorophores presented a
specific effect such that the phenylacetylene linkage could present a localꢀviscosity dependence;
on the other hand, the ICT character turns out to be crucial to give strong differences between DS
and OS and it is not related to the electron donor fragment of these two compounds, but to the
electronic structure arrangement. Moreover, the role of the electron acceptor groups resulted to
be of fundamental importance, these two topics will be discussed in the next sections.
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Solvent viscosity studies. To further evaluate the influence of the πꢀbridge on the ICT process,
we studied the solvent viscosity effects in DS and OS. As it is known, solvent viscosity
influences the rate of the nonꢀradiative deactivation processes coming from intramolecular
rotation in the fluorophore,55 in this case as a consequence of the triple bond rotation. In this
regard, Förster and Hoffmann described a relation between solvent viscosity (η) and fluorescence
quantum yield (Φe).56 Accordingly, the relationship should follow Eq. (8):
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Φꢑ = ꢘꢙꢚ
Eq. (8)
where z and α are constants. A plot of log Φe vs. log η should yield a straight line with a slope α.
Inset of Figure 6 shows a linear FörsterꢀHoffmann relation, suggesting a considerable
influence on solvent viscosity for both molecules, that is, upon increasing viscosity from nꢀ
hexanol to glycerine the ICT process is stabilized by the solvent and a strong emission arises
from a geometrically more planar structure.57
Figure 6. Fluorescence emission of DS (left) and OS (right) in solvents of different viscosity: nꢀ
hexanol (black), DMF (red), iꢀpropanol (cyan), glycerin at 298 K (green) and glycerin at 265 K
(blue). The spectra are normalized to the maximum emission in glycerine. Inset: doubleꢀ
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logarithmic plot of the solvent viscosity (η) vs. fluorescent quantum yield (Φe) according to the
Förster–Hoffmann equation.
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On the other hand, it is interesting to note that for both fluorophores the emission wavelength
in glycerine at 298 K and 265 K does not vary, which is in agreement with twistedꢀ
intramolecular charge transfer (TICT) rotors reported in the literature, where emission emerges
from an energetically invariable local excited state.58,59
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The role of the acceptor groups on the phenylꢀacetylene linkage. Since the phenylꢀ
acetylene linkage resulted critical to the linear and TPA responses, we decided to explore the
importance of the benzonitrile and triazine acceptor groups in the molecular systems. Then, a
SonogashiraꢀGlaser coupling reaction with ethynylꢀfluorene yielded compound 2DS (Scheme 1
and Figure 7), a compound where the ICT process (in this case quadrupolar type interactions)
should be negligible due to the absence of an acceptor group. However, the electronic structure
characteristics of 2DS resulted interesting. The role of the bisꢀacetylene bridge induce the
electron redistribution to slightly behave as a quadrupolar system (D–A–D, where A is
represented by C≡C=C≡C linkage), according to the hole – electron distribution, Figure 7.
Indeed, the HONTO – LUNTO electronic transition governs the UVꢀVis absorption spectrum,
the HONTO level is distributed along the entire molecule, while the LUNTO level is slightly
more distributed toward the C≡C=C≡C linkage suggesting a small quadrupolar effect, Figure 7.
This finding is interesting since the pure πꢀbridge can allow the molecule to present some level
of TPA response of the D–A–D type. According to these theoretical results, one implication
arises, the fluorene molecule acts as a strong electron donor group when it is connected to an
acetylene group. On the other hand, the linear optical properties show no fluorescence maximum
shifts as a function of solvent polarity. Furthermore, another important characteristic of 2DS is
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that the molecule should exhibit some TPA activity. To demonstrate the above, a solvatochromic
analysis and a TPA crossꢀsection determination for 2DS were carried out. The UVꢀVis
absorption and emission data is presented in Figure S7, while the solvatochromic studies through
LipperꢀMataga, Catalán, KamletꢀTaft and Laurence are summarized in Table S2 and Table S3,
SI. As expected, no bathochromic shift was observed in the emission spectra and the dipole
moment difference between excited and ground states is not significant. However, the TPA
crossꢀsection for 2DS was determined following the same methodology as in DS and OS, finding
a σTPA value of 7.09 GM at 740 nm.
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C8H17
C8H17
C8H17
C8H17
Figure 7. Above: molecular structure of compound 2DS, having no acceptor group. Bellow: hole
(left)–electron (right) pair corresponding to the 1A’ → 2A’ transition, considering a C2h
symmetry (NTO eigenvalue w = 0.7)
CONCLUSION
Both, experimental and theoretical analysis demonstrated that compound OS exhibited
interesting TPA properties with unusual strong dipolar contribution. The role of pure dipolar
contribution was analyzed in detail through molecule DS which represents the exact dipolar
fragment integrated in the octupolar compound. Particularly, it can be concluded that:
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The experimental (optical and electrochemical) and the computational (NTO) band gaps
obtained for DS and OS as well as their corresponding σTPA values were in good agreement.
Importantly, the NTO analysis revealed the existence of dipolar and octupolar contributions to
the TPA optical response of OS associated to the ground |A’1> and degenerated excited states
|E’1> and |E’2>.
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Both, DS and OS display strong solvatochromic effects, ranging from 860 cmꢀ1 in hexane to
4897 cmꢀ1 in ethylene glycol for DS and, from 2461 cmꢀ1 in hexane to 6890 cmꢀ1 in DMSO for
OS. Then, it was inferred that significant charge transfer processes occur in the excited states. A
dipole moment difference of 12.5 D for DS and 23.1 D for OS were obtained by the Lippertꢀ
Mataga relation. According to the Kamlet−Taft, Catalán and Laurence solvent scales, the
solvatochromic features of DS and OS are mostly governed by solvent polarity and
polarizability. DS and OS exhibited a noticeable localꢀviscosity dependence and the results
demonstrated a twistedꢀintramolecular charge transfer (TICT) behavior, where emission emerges
from an energetically invariable local excited state. Xꢀray structure analysis also revealed an
important twisting in the acetylene bridge. On the other hand, for 2DS no TPA response was
observed but a stronger solvent viscosity dependence was evidenced.
Finally, the present work demonstrates that the effect of the πꢀbridge takes place when a very
weak if any electronꢀdonor group is attached to the fluorene blue emitter fragment. A significant
dipolar contribution to the octupolar system was observed. Furthermore, the present molecular
systems were found to be suitable for viscosimetric analysis through OPA response detection.
EXPERIMENTAL SECTION
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Materials and physical measurements. All materials and solvents are commercially available.
Infrared spectra were obtained on a FTIR Varian Spectrometer ATR. Melting points were
obtained in an Electrothermalꢀ9200. 1H and 13C NMR spectra, were taken in a JEOL eclipse
ECA +500 spectrometer, the chemical shifts in ppm are relative to (CH3)4Si for 1H and 13C. High
resolution TOFꢀMS mass spectra were recorded on an Agilent G1969A spectrometer. Solution
and solid state UVꢀVis spectra were acquired in a Perkin Elmer LAMBDA 900ꢀ2S
spectrophotometer. Emission spectra in solution and solid state were acquired on a Varian Cary
Eclipse fluorescence spectrometer. 1H and 13C NMR spectra and full chemical characterization
are shown in the Supplementary Information file.
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Quantum yield and twoꢀphoton excited fluorescence (TPEF) experiments. The fluorescence
quantum yield of samples at low concentrations was determined by using the integrating sphere
method. The excitation was provided by a diode laser operating at 375 nm. Twoꢀphoton
absorption crossꢀsection (σTPA) of samples was measured through the twoꢀphoton excited
fluorescence (TPEF) technique using a train of pulses (~90 fs, repetition rate of 1 kHz) delivered
by an optical parametric amplifier (TOPAS, Light Conversion) pumped by a Ti:Sapphire
Regenerative Amplifier. The laser beam from the optical parametric amplifier was focused into a
quartz cell of 1 cmꢀpath length (containing the solutions under test) by using a short focalꢀlength
lens. The TPEF signal was collected in a perpendicular direction with respect to the excitation
beam and focused into the input slit of a monochromator (Acton Research, SpectraProꢀ2500) and
detected at the exit port with a PMT (Hamamatsu RT400Uꢀ02).60 For quantum yield
determination and TPEF experiments rhodamine 6G was used as reference. The values of σTPA
for this reference have been fully characterized for a wide range of wavelengths.
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ASSOCIATED CONTENT
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Supporting Information. Crystal structure details for DS (CCDC Reference No. 1443732), the
synthesis and full chemical characterization, UVꢀVis/fluorescence spectra and solvatochromic
analysis, Lippert plots, and TDꢀDFT (NTO) calculations, spatial extent in chargeꢀtransfer
excitations and density difference plots. This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding author
* arturojs@cinvestav.mx
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors acknowledge the financial support of CONACyT (grant 215708) and PAPIIT
216616 and also supercomputing Xiuhcoatl cluster administration of Cinvestav and Mistli cluster
of UNAM for the computational resources and the Laboratory of Ultrafast Optics at the Center
for Research in Optics (CIO) for the facilities provided for this work.
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