Shedding Light on the Origin of Solid-State Luminescence Enhancement in Butterfly Molecules

Article abstract of DOI:10.1002/chem.202002920

Different molecular strategies have been carefully evaluated to produce solid-state luminescence enhancement (SLE) in compounds that show dark states in solution. A set of α-phenylstyrylarene derivatives with a butterfly shape have been designed and synthesised, for the first time, with the aim of improving the solid-state fluorescence emission of their parent styrylarene compounds. Although these butterfly molecules are not fluorescent in solution, one of them (1,2,4,5-tetra(α-phenylstyryl)benzene) exhibits a fluorescence quantum yield as high as 68 % in a drop-cast sample and 31 % in its crystalline form. In contrast, 1,3,5-tris(α-phenylstyryl)benzene and 4,6-bis(α-phenylstyryl)pyrimidine do not show SLE. A range of fluorescence spectroscopy experiments and DFT calculations were carried out to unravel the origin of different photophysical behaviour of these compounds in the solid state. The results indicate that a rational strategy to control the SLE effect in luminogens depends on a delicate balance between molecular properties and inter-/intramolecular interactions in the solid state.

Full text of DOI:10.1002/chem.202002920

Accepted Article
Accepted Article
Title: Shedding light on the origin of the-solid state luminescence
enhancement in butterfly molecules
Authors: Antonio Sánchez-Ruiz, Julián Rodríguez-López, Andrés
Garzón-Ruiz, Sonia B. Jiménez-Pulido, Nuria A. Illán-Cabeza,
Amparo Navarro, and Joaquin C. Garcia Martinez
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202002920
Link to VoR: https://doi.org/10.1002/chem.202002920
01/2020

10.1002/chem.202002920
Chemistry - A European Journal
Shedding light on the origin of the-solid state luminescence enhancement in butterfly
molecules
Antonio Sánchez-Ruiz, Julián Rodríguez-López, Andrés Garzón-Ruiz, Sonia B. Jiménez-
Pulido, Nuria A. Illán-Cabeza, Amparo Navarro,* Joaquín C. García-Martínez,*
Dr. Antonio Sánchez-Ruiz, Dr. Joaquín C. García-Martínez
Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Farmacia,
Universidad de Castilla-La Mancha, C/ Josꢀ María Sꢁnchez Ibaꢂez s/n, 02008 Albacete, Spain.
Centro Regional de Investigaciones Biomédicas (CRIB), C/ Almansa s/n, 02008 Albacete,
Spain.
E-mail: joaquinc.garcia@uclm.es
Dr. Julián Rodríguez-López
Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Ciencias y
Tecnologías Químicas, Universidad de Castilla-La Mancha, Avenida Camilo José Cela 10,
13071 Ciudad Real, Spain.
Dr. Andrés Garzón-Ruiz
Departamento de Química Física, Facultad de Farmacia, Universidad de Castilla-La Mancha,
C/ José María Sánchez Ibañez s/n, 02008 Albacete, Spain.
Dr. Sonia B. Jiménez-Pulido, Dr. Nuria A. Illán-Cabeza,
Departamento de Química Inorgánica y Orgánica, Facultad de Ciencias Experimentales,
Universidad de Jaén, Campus Las Lagunillas, 23071 Jaꢀn, Spain.
Dr. Amparo Navarro
Departamento de Química Física y Analítica, Facultad de Ciencias Experimentales,
Universidad de Jaén, Campus Las Lagunillas, 23071 Jaén, Spain.
E-mail: anavarro@ujaen.es
Keywords: phenylstyrylarene derivatives, π-π stacking, solid-state luminescence enhancement,
short-range exciton coupling, fluorescence.
Different molecular strategies have been carefully evaluated to produce Solid-State
Luminescence Enhancement (SLE) in compounds that show dark states in solution. A set of α-
phenylstyrylarene derivatives with a butterfly-shape has been designed and synthesised for the
first time with the aim of improving the solid-state fluorescence emission of their parent
styrylarene compounds. Although these butterfly molecules are not fluorescent in solution, one
of them (1,2,4,5-tetra(α-phenylstyryl)benzene) exhibited a fluorescence quantum yield as high
1
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Chemistry - A European Journal
as 68% in a drop cast sample and 31% in its crystalline form. In contrast, 1,3,5-tris(α-
phenylstyryl)benzene and 4,6-bis(α-phenylstyryl)pyrimidine did not show SLE. A range of
fluorescence spectroscopy experiments and Density Functional Theory calculations were
carried out to unravel the origin of the different photophysical behaviour of these compounds
in the solid state. The results indicate that the rational strategy to control the SLE effect in
luminogens depends on a delicate balance between molecular properties and inter-
/intramolecular interactions in the solid state.
1. Introduction
Solid-State Luminescence Enhancement (SLE) is an amazing phenomenon that has
attracted a great deal of attention due to its potential use in the development of new functional
materials in which the luminescence properties depend on a range of factors beyond the
chemical formula.[1,2] Aggregation-Induced Emission (AIE) is also used as a synonym of SLE
although the concept of ‘molecular aggregation’ is more frequently associated with solution
phases.[2] Considerable effort is currently being made to obtain SLE luminogens and to unravel
the mechanisms responsible for fluorescence ‘on/off’ switching between solution and solid state,
such as exciton coupling effects,[3-7] restriction of intramolecular motions (RIM),[8-10] blocking
of Z/E-photoisomerisation,[11] and restricted access to conical intersections (RACI),[2,12,13]
amongst others. The control of the supramolecular structure is therefore key to obtaining SLE
luminogens because short- and long-range intermolecular interactions determine solid-state
photophysical properties. Accordingly, the type of solid phase (powder, crystal, drop cast, etc.),
the procedure used to obtain it (drop casting, spin coating, etc.) and some external physical
stimuli (mechanoluminescence) are also critical aspects to be considered in the design of light-
emitting solid materials.[14-16]
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A number of bulky chemical groups with sterically congested structures, such as
tetraphenylethylene (TPE), cis,cis-1,2,3,4-tetraphenylbutadiene, tetraphenylpyrazine (TPP)
and hexaphenylsilole (HPS), are commonly used to obtain SLE luminogens.[8-10,14,17-19] In
general, the incorporation of these moieties has a huge impact on the fluorescence emission
wavelength and quantum yield. In the electronically excited state of these compounds, the
molecular motions of their phenyl rotors and carbon skeleton vibrations consume the excess
energy and result in dark states in solution. However, the intermolecular interactions of these
bulky moieties with neighbouring molecules can restrict their molecular motions and yield
bright states in solid state. The understanding of the so-called RIM mechanism has allowed the
development of a large family of SLE luminogens containing TPE, HPS or other sterically
congested groups with twisted phenyl rings. [8-10,14,17-19] Although the RIM mechanism has been
successfully employed to explain the fluorescence enhancement observed in a large variety of
luminogens, a thorough description of the SLE effect should involve a global study of the
potential energy surface.[12,13] In this sense, other mechanisms such as RACI or the blocking of
Z/E-photoisomerisation have also been associated with the fluorescence enhancement observed
in the solid state or in molecular aggregates formed in solution.[2,11-13,20,21] The main drawback
of this global treatment is the considerable increase in the computational cost associated with
calculations of the electronic excited states.[12,13] Intermolecular forces are also important
factors that determine the photophysical properties of a bulk material. For instance, many
conventional fluorophores have planar and π-conjugated structures that favour the
establishment of π-π stacking interactions in the solid state, which are typically associated with
fluorescence quenching effects. Nevertheless, this is not a general rule because there are diverse
examples of planar π-conjugated molecules that exhibit SLE due to different mechanisms such
as exciton coupling.[6,7] Diverse SLE luminogens with sterically congested structures, such as
butterfly-like or propeller-like shapes, have been designed to reduce intermolecular stacking
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interactions.[9,22-24] In these cases, the effect of the intramolecular interactions on the
photophysical properties of the material could gain importance and should also be considered.
Styrylarenes are a well-known family of compounds[25-30] but only a few studies on their solid-
state photophysical properties have been reported to date.[2,11,20,31] Interestingly, SLE and AIE
(in solvent mixtures) have been reported for some compounds belonging to this family with
two and three styryl branches (compounds 4, 5 and some 1,4-di(styryl)benzenes) but, to our
knowledge, fluorescence enhancement has not previously been reported for
tetra(styryl)benzenes in the solid state.[2,11,19,20] In compound 6 some rocking movements occur
in the phenyl rings (associated with high Huang–Rhys factors) and these are not totally
restricted in the solid state and hence SLE is not observed.[11] The new molecular structures
designed here (1–3) contain the styrylarene core (in blue in Scheme 1) but also incorporate new
phenyl rotors to generate triphenylethylene moieties (in red in Scheme 1). The presence of
several triphenylethylene moieties in the same molecule leads to butterfly-like structures that
resemble some TPE derivatives for which SLE has been reported.[10] The modification of the
molecular structure of π-conjugated fluorophores by introducing additional molecular rotors,
e.g., phenyl rings, has proven to be a suitable strategy to produce luminogens with high
fluorescence quantum yields in the solid state by tuning the balance between the loss of
planarity of the molecule, the restriction of intramolecular movements and hindering of π-π
stacking interactions.[8-10] Different spectroscopic experiments have been conducted to
investigate the solid-state photophysical properties and to analyse the modifications of these
photophysical properties with respect to those reported for the parent styrylarene compounds
(4, 6 and 7). In addition, Density Functional Theory (DFT) provided valuable information to
interpret the mechanisms responsible for SLE in this type of butterfly molecule.
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2. Results and discussion
2.1. Synthesis and molecular structure.
The synthesis of compounds 1 and 2 was based on the Horner-Wadsworth-Emmons
(HWE) reaction for the formation of double bonds, a methodology extensively developed by
some of us since 1999 with slight modifications.[25] The main advantage of this methodology is
that the HWE reaction usually requires an easy workup in a catalyst-free approach. However,
compound 3 was prepared according to the procedure reported by Gramage-Doria et al.[32]
through a general ruthenium(II)-catalysed methodology for the arylation of alkenylic C–H
bonds. More detailed characterisation data for the compounds are provided in the Supporting
Information (SI).
Good quality crystals suitable for single crystal X-ray diffraction analysis were obtained
for compounds 2 and 3 by vapour diffusion from CHCl3/EtOH and CHCl3/CH3CN solvent
systems, respectively. A view of the molecular structure of 2 and 3 is shown in Figure 1, 2 and
3. For compound 2, two crystallographically independent molecules are observed, i.e., 2a
(centrosymmetric) and 2b (see Figure 1). In both cases, there are no classical hydrogen bonds
or strong - stacking interactions, as evidenced by the large intermolecular distances (ca. 5.0–
5.9 Å). Nevertheless, the highly twisted conformation of the molecules, especially in 2b,
promotes a larger number of inter- and intramolecular interactions by C–H··· hydrogen bonds.
For instance, 2b exhibits three C–H··· intramolecular interactions that could explain its
overcrowded arrangement in relation to 2a (see Figure 2).
Although short-range - stacking interactions were not found for compound 2, the
presence of parallel stacking - and C–H··· interactions at longer distances contributes to the
three-dimensional structure observed in the crystal in a similar way to that observed in a large
number of proteins (see the view of the 3D structure of 2 along the c axis in Figure S1).[33]
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The crystal packing in compound 3 is due to three types of interactions, i.e., a non-
classical hydrogen bond between C31–H31···N2 atoms that promotes a dimeric arrangement
(Figure 3A) and - stacking (Figure 3B) and C–H··· (Figure 3C) interactions. In addition,
molecule 3 exhibits one intramolecular interaction between a hydrogen of the pyrimidine ring
and the closest phenyl ring of the -phenylstyryl branch (see Figure S2).
2.2 Photophysics in solution.
The experimental UV-vis absorption spectra of the studied compounds were recorded
in THF solution (see Figure 4). Intense bands were observed at 313 and 331 nm for compounds
1 and 2, respectively. The UV-vis spectrum of compound 3 shows a less prominent band centred
at 337 nm and a weak peak at around 300 nm. These bands were assigned by performing full
geometry optimisations to predict the most stable conformations in THF solution and
calculating the vertical electronic transitions at the TD-M06-2X/6-31G* level of theory.
Molecular structures in which the diphenylethylene branches are pointing outward in relation
to the central benzene/pyrimidine plane were modelled for all of the compounds in solution (see
Figure 5; some selected geometrical parameters calculated for these compounds are also
collected in Tables S1–S4). The geometries computed for 2 and 3 in solution closely resemble
their X-ray crystal structures (Figure S3 and Tables S2-S4 for a comparison of some
geometrical parameters calculated in solution with the experimental data in solid state). For
compound 2, in which two crystallographically independent molecules were observed, our
theoretical calculations predicted that conformation 2b is 7.8 kcal/mol more stable than 2a in
THF solution at 298.15 K. The largest deviations between the calculated and the experimental
structures were found for 2b, in which two dihedral angles belonging to the same α-phenylstyryl
branch are markedly modified, with variations of 22 and 49 on going from solution to solid
state, |∆τs-c|. In molecule 2a, two dihedral angles also belonging to the same α-phenylstyryl
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branch showed significant variations (|∆τs-c| of 18 and 20). Smaller differences were observed
between the calculated structure of 3 and its crystal structure (|∆τs-c| < 10). These results can
be related to the more overcrowded crystal structures of 2a and 2b when compared to 3.
풄풂풍풄
풂풃풔
In general, the wavelengths calculated for the lowest-lying electronic transitions (흀
)
풆풙풑
풆풙풑
match well with the experimental absorption maxima (흀풂풃풔 ), with ∆흀 = 흀풄풂풂풃풍풔풄 − 흀풂풃풔
≤
|
|
ퟐퟏ 풏풎 in all cases (see Table 1). As reported for other 1,3,5-tris(styryl)benzenes,[11,20,34] some
degeneracy was observed in the frontier molecular orbitals of compound 1 (see Figure 5). In
consequence, two electronic transitions (S0→S1 and S0→S2) with close energy and oscillator
strength were found for this compound. Three electronic transitions (S0→S1, S0→S2 and
S0→S3) involving the HOMO-1 to LUMO+1 frontier orbitals were assigned to the broad
absorption band of compound 2 centred at 331 nm. It is worth noting that while the frontier
orbitals are completely delocalised over the whole structure of 2a, they become localised along
two -phenylstyryl branches in diagonal positions in 2b. This result could mean that molecule
2a would act as a chromophore as a whole, while only two -phenylstyryl branches in 2b would
be involved in the photoexcitation. A similar observation, namely the planarization of two
diagonal styryl branches upon photoexcitation, was previously reported for the parent
compound 6.[11] In the case of compound 3, the lowest-lying electronic transition was assigned
to a HOMO→LUMO transition (contribution of 89%) and these molecular orbitals were fully
delocalised.
Fluorescence emission spectra recorded for the studied compounds in THF solution are
shown in Figure 4. An unstructured band centred at 424 nm was found for compound 1. In the
case of 2, a broad emission band with a weak vibronic structure was recorded, i.e., two peaks
at 467 nm and 488 nm can be distinguished. This result is in contrast to the clear vibronic
structure observed in the fluorescence emission spectra of the parent styrylarene compounds 4
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and 6.[11] A vibronic structure is also clearly discerned in the emission spectrum of 3, with two
peaks at 400 and 421 nm. The calculated emission energies for the S1→S0 transition are
collected in Table 2. The best match is found for compound 2 while the larger discrepancies
for compounds 1 and 3 are within the range of expected deviations.[24,35,36] In order to get some
insight in this issue, the molecular orbitals of the first excited state have been represented in the
Figure S4. As can be seen, while the molecular orbitals in the ground state of compound 3 are
delocalised on all the molecule, there is a significant redistribution of the charge in the excited
state. Thus, we could postulate that M06-2X overestimates the intramolecular charge transfer
in the excited state (and the energetic stabilization of the S1 state in a polar solvent) in the case
of compound 3 justifying the large deviation between the experimental a calculated emissions.
It is worth noting that the presence of additional phenyl rotors in compounds 1–3 produces a
substantial drop in the fluorescence quantum yield (ΦF) in solution when compared to their
parent styrylarene compounds, i.e., the ΦF values measured for compounds 1–3 in THF solution
was < 1%, in contrast to the values reported for 4 and 6 (60% and 57%) (see Table 2).[20,11]
A
low quantum yield (albeit higher than that recorded for 3) was also obtained for compound 7
(6%) by Achelle et al.[29] In addition, the oscillator strength, f, calculated for the lowest-lying
electronic transition of compound 3 (0.63) is lower than those calculated for equivalent
transitions in 1, 2a and 2b (≥ 0.82). Hence, a lower radiative rate constant (kr) is expected for 3
than for 1 and 2 on considering the Strickler–Berg relation, which establishes that kr is directly
related to f.[37,38] This issue is key because the fluorescence quantum yield of a compound
depends on the balance between the radiative and non-radiative (knr) rate constants. SLE is a
phenomenon that originates from restrictions of the non-radiative pathways in the solid state,
which in turn lead to kr becoming comparable or higher than knr. Therefore, although non-
radiative pathways were partially blocked, a low value of kr could prevent the appearance of
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the SLE phenomenon. This issue will be considered again in the analysis of the photophysical
properties of 3 in the solid state.
Different comparative analyses of the molecular properties were performed in order to
gain insights into the low luminescence experienced by compounds 1–3 upon photoexcitation
(see Figure 6 and Tables S1–S4). Compounds 1 and 3 undergo large changes in one -
phenylstyryl branch on varying diverse dihedral angles, |τS1-S0|, with values up to almost 40°.
In the case of molecules 2a and 2b, the main changes occurred in the dihedral angles belonging
to two -phenylstyryl branches located along a diagonal axis (|τS1-S0| ≤ 25°). The rotational
barriers calculated for the phenyl rotors indicate that these rings can be twisted with a relatively
low energy consumption (for instance, ~2 kcal mol–1 or less for torsions of 30°, see Figure 6).
This fact, which is probably due to the poor conjugation of the -phenylstyryl moieties, could
help to dissipate the excess energy from the photoexcitation processes and yield dark states in
solution. It is worth mentioning that although there is an increase in the co-planarity between
the central arene ring and some ethylene moieties upon photoexcitation (see Figure S5), the
peripheral phenyl rotors remain twisted with respect to the central unit. This contrasts with the
behaviour of compounds 4 and 6, in which a planarization of one styryl branch (in 4) or two
(diagonal) styryl branches (in 6) was observed upon photoexcitation.[11]
The reorganisation energy (λ) and Huang–Rhys (HR) factors are also key parameters
that provide valuable information on the dynamics of the electronic relaxation of excited states.
The decomposition of the reorganisation energy into contributions from each normal mode, i,
enable us to determine the main vibrations involved in the electronic relaxations.
풏
∑
흀 = 풊=ퟏ 흀풊
(1)
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Graphical representations of 흀풊 vs. normal mode wavenumber (흎풊) calculated for the
studied compounds are shown in Figures 7 and S9. Several vibrational modes in the low energy
region (< 200 cm–1) make a significant contribution to the total reorganisation energy of the
three compounds, as already reported for molecules such as TPE and HPS.[39,40] These out-of-
plane modes correspond to twisting or wagging vibrations, which can help to dissipate the
absorbed energy and lead to dark states in solution (see Figure S6). C–C stretching modes in
the high energy region (1600–1750 cm–1) could also contribute to the non-radiative deactivation.
The dimensionless HR factors account for the displacement of the equilibrium positions of the
nuclei upon photoexcitation and quantify the strength of the electron-vibrational coupling.
These factors can be calculated from the reorganisation energy of each vibrational mode
according to
흀
풊
푺풊 =
(2)
ℏ흎
풊
All of the compounds show large HR factors associated with the normal mode in the
low energy region (see Figure 7 and S7 and Table S5). Some HR factors calculated for
compounds 1 and 3 are particularly high, e.g., those associated with wagging and twisting
modes of the phenyl rotors of 1 at 20 cm–1 (Si = 34) and 34 cm–1 (Si = 13), which are comparable
to the HR factor values reported for related compounds such as cis,cis-1,2,3,4-tetraphenyl-1,3-
butadiene and 1,1,4,4-tetraphenyl-butadiene.[41] It must be remembered that the largest
structural changes upon photoexcitation were also calculated for 1 and 3 while less extensive
modifications were found for the overcrowded structures of 2a and 2b. Although somewhat
lower HR factor values were obtained for 2a and 2b, a large number of significantly high HR
factors (Si > 5) was obtained for these compounds in comparison with the tris(α-phenylstyryl)
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derivative. In general, these results highlight the high efficiency of the non-radiative
deactivation pathways in compounds 1–3 and are consistent with the low fluorescence quantum
yields measured for all of the compounds in solution.
2.3 Photophysics in the solid state
The emission and excitation spectra obtained for compounds 1–3 in powder form are
shown in Figure 8. More detailed information on the fluorescence excitation and emission
wavelength maxima, and quantum yields of the studied compounds (and the related compounds
4–6) in different solid forms (crystal, drop cast and powder) are collected in Table 3. The
difference in optical properties observed between the dropcast, powder and crystal could be a
consequence of the existence of different morphologies between crystalline and amorphous
zones in the different solid states. At present, we are carrying out studies to help understand this
fact better.
Compound 2 shows green fluorescence emission with CIE chromaticity coordinates of
(0.11, 0.49), in contrast with the blue emission of 1 and 3 (see Figure 8c and Table 3). There
was an increase in the fluorescence quantum yield for compound 2 from < 1% in THF solution
to 31% in the crystalline state or 68% in a drop cast sample when the solid sample was prepared
by drop casting method (from a 5 mM solution in 1:1 mixture of acetonitrile:THF). On the
contrary, SLE effect was not observed for compounds 1 and 3. This result could be due to the
absence of close π-π stacking interactions observed in the crystal structure of compound 2. This
type of intermolecular interaction, which is commonly associated with fluorescence quenching
effects, is generally hampered in bulky molecular structures such as propeller-shaped and
butterfly-shaped molecules.[19] In addition, the SLE effect observed for compound 2 can be
related to the large number of inter- and intramolecular interactions by C–H··· hydrogen bonds
that were found in the crystal structure (see Figure 2 and S1). Recently, Yu et al. demonstrated
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that C–H··· intramolecular interactions restrict the molecular motions and lead to an SLE
effect in bis(α-phenylstyryl)benzene derivatives.[42] Similar conclusions have been reported for
luminescent molecules in which C–H··· intermolecular interactions are established in the
crystal structure.[43,44] In contrast, the fluorescence quantum yield reported for the parent
compound 6 decreases from THF solution (57%) to the solid state (1.7%).[11] Transmission
electron micrographs showed narrow lattice fringes (with a periodicity of tenths of nanometers)
for this compound in the solid state. Discotic conjugated molecules tend to form columnar
mesophases through π-π stacking interactions and these lattice fringes are generally associated
with the intracolumnar periodicity. Thus, a shorter intracolumnar periodicity was found for the
supramolecular structure of compound 6 (with d-spacing of 0.21 nm) than in 4 and 5 (with d-
spacing of 0.88−0.92 nm and 0.70 nm, respectively).[11,20] Accordingly, noticeable spectral
differences were found for compound 6 in the solid state (흀풆풆풎풙풑 = 547 nm, in powder), with
respect to THF solution (흀풆풆풎풙풑 = 461 nm).[11] On the contrary, closer emission spectra in the
solid state and solution were obtained for compound 2 (흀풆풆풎풙풑 = 509 nm, in the crystal; 흀풆풙풑
=
풆풎
467, 488 nm in THF solution) than for 6. This fact illustrates the role of those intermolecular
interactions in the distinct photophysical behaviour of compounds 2 and 6 in the solid state.
Unfortunately, a thorough analysis on the inter- and intramolecular interactions could
not be carry out for compound 1 since its crystal structure was not obtained. Nevertheless, it
must be highlighted some spectral differences and similarities between 1 and its parent
compound 4. As mentioned, the fluorescence quantum yield of compound 4 in THF solution
(60%) is significantly higher than the value of ΦF measured for 1 (< 1%).[20] In the solid state,
no significant variations of ΦF were found for both compounds (< 1% for 1; 50-51% for 4) with
respect to THF solution.[20] In the case of compound 1, the fluorescence emission spectra in the
solid state (흀풆풆풎풙풑= 421 nm in powder) is very close to that in THF solution (흀풆풆풙풎풑= 424 nm). For
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compound 4, the spectral differences found between the solid state and solution are small but
slightly larger than for 1 (흀풆풆풎풙풑 = 448 nm, in powder; 흀풆풆풎풙풑 = 399, 420 nm in THF solution). This
fact suggests weak electronic coupling between neighbouring molecules in the solid state of
compound 1. In a similar way to this compound, weak fluorescence emission was also observed
for 3 in the solid state. Within the series of compounds 1 – 3, the lowest radiative rate constant,
kr, in solution is expected for 3, as discussed in the previous section. In the solid state, the non-
radiative rate constant, knr, could also increase due to the - stacking intermolecular
interactions observed in its crystal structure (see Figure 3). kr could also be affected by -
stacking intermolecular interactions but the low fluorescence quantum yield measured in the
solid state indicates that the raditative decay is not an efficient mechanism in comparison with
the non-radiative deactivation.
In addition to the analysis based on the restriction of intramolecular movements,
a parallel study was performed on the effect that intermolecular interactions have on the
photophysical properties of the crystal considering the short-range exciton coupling. According
to Spano and Hestand,[6,7] the charge-transfer produced by wave function overlap between
neighbouring molecules with short intermolecular distances creates an effective short-range
exciton coupling that can induce J- or H-like behaviour, which resembles the photophysical
effects of the classical Coulombic exciton-coupling produced in molecular aggregates. This
photophysical behaviour can be estimated from the sign of the charge transfer intermolecular
coupling, JCT α – (thte), where thte is the product of the hole and electron transfer integrals
between neighbouring chromophores. H-type behaviour is expected when the sign of the
product thte is negative and, hence, JCT > 0, while J-behaviour occurs when thte > 0 and JCT < 0.
The values of the electron and hole transfer integrals are provided in Table 4 along with the
product thte calculated for different dimers extracted from the crystal X-ray structure. The dimer
bb corresponds to two neighbouring molecules of 2b while ab is formed by a molecule of 2a
13
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and a neighbouring molecule of 2b (Figure S8). A, B and C are dimers of the molecule 3 and
are shown in Figure 3. According to the results collected in Table 4, a significantly high value
of the product thte was only found for dimer C.[6,7] Therefore, a short-range exciton coupling
could be produced in that dimer and this could lead to H-type behaviour in the crystal of
compound 3. This result is also consistent with the low fluorescence quantum yield measured
for 3 in the solid state, since a lowering of the radiative rate constant is associated with H-type
behaviour.
3. Conclusions
A set of butterfly-shaped α-phenylstyrylarenes (1–3) have been synthesised for the first
time with the aim of improving the solid-state fluorescence emission of their parent styrylarene
compounds (4, 6 and 7). All of the α-phenylstyrylarene derivatives showed poor fluorescence
emission in solution but the fluorescence quantum yield increased markedly for compound 2 in
the solid form (ΦF = 31% in the crystalline state and ΦF = 68% in a drop cast sample). SLE was
only observed for compound 2 and this was associated with the lack of close π-π stacking
interactions in the crystal structure as well as the existence of diverse inter- and intramolecular
C–H··· interactions, which restrict the molecular motions and lead to a fluorescence
enhancement. In contrast, in compound 3 - stacking intermolecular interactions are
established in the crystal and these are typically associated with fluorescence quenching effects.
In addition, a short-range exciton coupling could be produced in the crystal of compound 3 and
this led to an H-type behaviour and lowering of the radiative rate constant. Both results are
consistent with the low fluorescence quantum yield measured for 3 in the solid state. An in-
depth study on the origin of the poor fluorescence emission of compound 1 in the solid form
could not be carried out because the crystal structure was not available.
14
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In this work we have tried to shed light on the origin of the SLE phenomenon in butterfly
molecules. The inclusion of bulky chemical groups with sterically congested structures, such
as TPE, TPP and HPS, or triphenylethylene in our case, is an interesting strategy to obtain
luminescence enhancement in the solid state, but this outcome is not guaranteed. Only the
thorough study of the effects of diverse phenomena such as RIM, exciton coupling, RACI and
photoisomerisation, amongst others, on the radiative and non-radiative constants allow
conclusions to be drawn about the origin of SLE phenomena. The analysis of inter- and
intramolecular interactions in the crystal structure can assist in this task and is an essential
strategy for the rational design of highly luminescent compounds.
4. Experimental Section
Synthesis. Compounds 1 and 2 were prepared following a standard methodology by the HWE
reaction between the commercial benzophenone and the corresponding phosphonate core in the
presence of potassium tert-butoxide (see SI, Scheme S1). Compound 3 was synthesised
according to the procedure reported by Gramage-Doria et al.[32] Specific details concerning
preparation of 1, 2 and 3 and their characterisation are provided in the SI (Figure S9–S12).
Crystallography. Details of the crystallographic data collection and refinement parameters are
given in Table S6. The measurements were performed on a Bruker-Apex-II CCD
diffractometer with graphite monochromated Mo-K (= 0.71073 Å) radiation at 100 K. The
structures were solved by direct methods and refined using SHELXL-2016/4 software
employing full-matrix least-squares methods on F2.[45] Lorentz, polarisation and multiscan
absorption corrections were applied with SADABS.[46] All non-H atoms were refined
anisotropically; some hydrogen atoms were located and refined isotropically and others were
placed in idealised positions and treated using riding models. All calculations were carried out
with PLATON[47] and graphics were drawn with MERCURY.[48] The CIF files have been
15
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Chemistry - A European Journal
deposited at the Cambridge Crystallographic Data Centre and allocated the deposition numbers
CCDC 1971518 (2) and 1971517 (3). Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Spectroscopy. Unless otherwise stated, spectra were acquired in THF at 20 °C with a sample
concentration of 1 μM. Quartz cuvettes (Hellma Analytics) of 10 mm were employed for all
absorption and emission measurements on liquid samples. UV-Vis absorption spectra were
acquired on a V-750 (Jasco) spectrophotometer at a scan rate of 600 nm min‒1. A Peltier
accessory was employed to control the temperature of the spectrophotometer measuring cell.
Fluorescence emission spectra in solution were acquired on an FS5 (Edinburgh Instruments)
spectrofluorometer equipped with a 150 W Xe lamp as light source and a PMT (photomultiplier
tube) detector (R928P model). The temperature was controlled using a Peltier accessory
(temperature-controlled cuvette holder, TLC 50, Quantum Northwest). Fluorescence emission
spectra in the solid state were recorded on the FS5 spectrofluorometer using an integrating
sphere. Fluorescence quantum yield and chromaticity calculations were carried out using the
F980 Software from Edinburgh Instruments.
Computational Details. The theoretical calculations were performed using the Density
Functional Theory (DFT) on the Gaussian 09 package (version D.01)[49] at the M06-2X/6-31G*
level of theory.[50] This functional has proven to be suitable for the calculation of photophysical
properties of related styrylbenzenes.[11,20,27,34] Full geometry optimisations of the ground (S0)
and first excited (S1) states were carried and vibrational frequencies were calculated to check
the absence of imaginary frequencies. The initial molecular geometries were those from X-ray
diffraction of compounds 2 and 3. In the absence of experimental geometry data, a
conformational analysis was carried out on compound 1. The effect of the solvent was included
by the polarisable continuum model (PCM) as implemented in the Gaussian package.[51-53]
16
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Vertical electronic transitions and geometry optimisation of the excited electronic states were
computed using Time-Dependent DFT calculations (TD-M06-2X/6-31G*) in solution. The
fluorescence emission energy from the first excited state was calculated at the same level of
theory as Eem(S1) = ES1(GS1) − ES0(GS1), where ES1(GS1) is the energy of the S1 state in its
equilibrium geometry (GS1), in the state-specific solvation approach,[54] while ES0(GS1)
corresponds to the energy of the S0 state at the S1 state geometry (GS1) and with the static
solvation from the excited state.[55] The contribution of each normal mode to the reorganisation
energy was calculated using the programme DUSHIN developed by Reimers.[56] The electronic
coupling was calculated at the M06-2X/6-31G* level for dimers extracted from the crystal
structure. The calculations were carried out using the projective method implemented in the J-
from-g03 programme.[57,58]
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
The authors would like to thank the Ministerio de Economía y Competitividad/Agencia Estatal
de Investigación/FEDER (Spain) (Project No. CTQ2017-84561-P) for supporting this research
and the Universidad de Castilla-La Mancha for additional support of the research group (Grant
Nos. GI20163441, GI20173955, 2019-GRIN-27152).The authors thank to ‘Centro de Servicios
de Informꢁtica y Redes de Comunicaciones’ (CSIRC) (Universidad de Granada, Spain) for
providing the computing time. The authors also thanks the ‘Consejería de Economía y
Conocimiento. Junta de Andalucía’ (FQM-337) and ‘Acción 1_PIUJA’ (Universidad de Jaꢀn,
Spain).
17
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Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
References
[1] Y. Chen, J. W. Y. Lam, R. T. K. Kwok, B. Liu, B. Z. Tang, Mater. Horiz. 2019, 6,
428−433.
[2] J. Shi, L. E. Aguilar Suarez, S.-J. Yoon, S. Varghese, C. Serpa, S. Y. Park, L. Lüer, D.
Roca-Sanjuán, B. Milián-Medina, J. Gierschner, J. Phys. Chem. C 2017, 121,
23166−23183.
[3] E. G. McRae, M. Kasha, J. Chem. Phys. 1958, 28, 721−722.
[4] M. Kasha, Radiat. Res. 1963, 20, 55−71.
[5] M. Kasha, H. R. Rawls, M. Ashraf El-Bayoumi, Pure Appl. Chem. 1965, 11, 371−392.
[6] N. J. Hestand, F. C. Spano, Acc. Chem. Res. 2017, 50, 341−350.
[7] N. J. Hestand, F. C. Spano, Chem. Rev. 2018, 118, 7069−7163.
[8] J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam, B. Z. Tang, Chem. Rev. 2015
,
115, 11718–11940.
[9] Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev. 2011, 40, 5361−5388.
[10] Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun. 2009, 4332−4353.
[11] R. Domínguez, M. Moral, M. P. Fernández-Liencres, T. Peña-Ruiz, J. Tolosa, J.
Canales-Vázquez, J. C. García-Martínez, A. Navarro, A. Garzón-Ruiz, Chem. Eur. J.
2020, 26, 3373−3384.
[12] R. Crespo-Otero, Q. Li, L. Blancafort, Chem. Asian J. 2019, 14, 700−714.
[13] X.-L. Peng, S. Ruiz-Barragán, Z.-S. Li, Q.-S. Li, L. Blancafort, J. Mater. Chem. C
2016, 4, 2802−2810.
18
This article is protected by copyright. All rights reserved.

10.1002/chem.202002920
Chemistry - A European Journal
[14] M. Jiang, X. Gu, R. T. K. Kwok, Y. Li, H. H. Y. Sung, X. Zheng, Y. Zhang, J. W. Y.
Lam, I. D. Williams, X. Huang, K. S. Wong, B. Z. Tang, Adv. Funct. Mater. 2017, 28,
1704589.
[15] Y. Yu, C. Wang, Y. Wei, Y. Fan, J. Yang, J. Wang, M. Han, Q. Li, Z. Li, Adv. Optical
Mater. 2019, 7, 1900505.
[16] N. Zerby, O. Malka, S. Bhattacharya, N. N. Kadamannil, M. Baranov, R. Jelinek, Adv.
Optical Mater. 2019, 7, 1900232.
[17] J. Chen, B. Xu, X. Ouyang, B. Z. Tang, Y. Cao, J. Phys. Chem. A 2004, 108,
7522−7526.
[18] M. Chen, L. Li, H. Nie, J. Tong, L. Yan, B. Xu, J. Z. Sun, W. Tian, Z. Zhao, A. Qin,
B. Z. Tang, Chem. Sci. 2015, 6, 1932−1937.
[19] Z. Zhao, B. He, B. Z. Tang, Chem. Sci. 2015, 6, 5347–5365.
[20] A. Garzón, M. P. Fernández-Liencres, M. Moral, T. Peña-Ruiz, A. Navarro, J. Tolosa,
J. Canales-Vázquez, D. Hermida-Merino, I. Bravo, J. Albaladejo, J. C. García-Martínez,
J. Phys. Chem. C 2017, 121, 4720−4733.
[21] A. Garzón, A. Navarro, D. López, J. Perles, E. M. García-Frutos, J. Phys. Chem. C
2017, 121, 27071−27081.
[22] X. Yin, J. Z. Low, K. J. Fallon, D. W. Paley, L. M. Campos, Chem. Sci. 2019, 10,
10733−10739.
[23] H. Tong, Y. Hong, Y. Dong, Y. Ren, M. Häussler, J. W. Y. Lam, K. S. Wong, B. Z.
Tang, J. Phys. Chem. B 2007, 111, 8, 2000−2007.
[24] M. P. Fernꢁndez-Liencres, T. Peꢂa-Ruiz, J. M. Granadino-Roldꢁn, M. Moral, A.
Valenzuela-Pereira, A. Garzón-Ruiz, A. Navarro, J. Phys. Chem. C 2018, 122,
12002−12014.
19
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10.1002/chem.202002920
Chemistry - A European Journal
[25] J. C. García-Martínez, E. Díez-Barra, J. Rodríguez-López, Curr. Org. Synth. 2008, 5,
267−290.
[26] E. Díez-Barra, J. C. García-Martínez, S. Merino, R. del Rey, J. Rodríguez-López, P.
Sꢁnchez-Verdꢃ, J. Tejeda, J. Org. Chem. 2001, 66, 5664−5670.
[27] M. Moral, R. Domínguez, M. P. Fernández-Liencres, A. Garzón-Ruiz, J. C. García-
Martínez, A. Navarro, J. Chem. Phys. 2019, 150, 064309.
[28] P. Ehlers, A. Hakobyan, A. Neubauer, S. Lochbrunner, P. Langer, Adv. Synth. Catal.
2013, 355, 1849−1858.
[29] S. Achelle, I. Nouira, B. Pfaffinger, Y. Ramondenc, N. Plé, J. Rodríguez-López, J.
Org. Chem. 2009, 74, 3711−3717.
[30] H. Zhang, F. Rominger, U. H. F. Bunz, J. Freudenberg, Chem. Eur. J. 2019, 25,
11218–11222.
[31] C. Coya, A. de Andrés, R. Gómez, C. Seoane, J. L. Segura, J. Lumines 2008, 128,
761–764.
[32] R. Gramage-Doria, S. Achelle, C. Bruneau, F. Robin-le Guen, V. Dorcet, T. Roisnel,
J. Org. Chem. 2018, 83, 1462–1477.
[33] M. Nishio, Y. Umezawa, J. Fantini, M.S. Weiss, P. Chakrabarti, Phys. Chem. Chem.
Phys. 2014, 16, 12648–12683.
[34] M. Moral, J. Tolosa, J. Canales-Vꢁzquez, J. Carlos Sancho-García, A. Garzón-Ruiz,
J. C. García-Martínez, J. Phys. Chem. C 2019, 123, 11179–11188.
[35] T. Zhang, H. Ma,Y. Niu, W. Li, D.Wang, Q. Peng, Z. Shuai, W. Liang, J. Phys. Chem.
C
2015, 119, 5040–5047.
[36] Q. Wu, T. Zhang, Q. Peng, D. Wang, Z.Shuai, Phys. Chem. Chem. Phys. 2014, 16,
5545–5552.
[37] J. P. Vikesland, S. J. Strickler, J. Chem. Phys. 1974, 60, 660–663.
20
This article is protected by copyright. All rights reserved.

10.1002/chem.202002920
Chemistry - A European Journal
[38] M. Fecková, P. le Poul, F. Robin-le Guen, T. Roisnel, O. Pytela, M. Klikar, F. Bureš,
S. Achelle, J. Org. Chem. 2018, 83, 11712–11726.
[39] N. L. C. Leung, N. Xie, W. Yuan, Y. Liu, Q. Wu, Q. Peng, Q. Miao, J. W. Y. Lam, B.
Z. Tang, Chem. Eur. J. 2014, 20, 15349–15353.
[40] T. Zhang, Y. Jiang, Y. Niu, D. Wang, Q. Peng, Z. Shuai, J. Phys. Chem. A 2014, 118,
9094–9104.
[41] Q. Peng, Y. Yi, Z. Shuai, J. Shao, J. Am. Chem. Soc. 2007, 129, 9333–9339.
[42] B. Yu, D. Liu, Y. Wang, T. Zhang, Y.-M. Zhang, M. Li, S. X.-A. Zhang, Phys. Chem.
Chem. Phys. 2018, 20, 23851–23855.
[43] Z. Xie, T. Yu, J. Chen, E. Ubba, L. Wang, Z. Mao, T. Su, Y. Zhang, M. P. Aldred, Z.
Chi, Chem. Sci. 2018, 9, 5787–5794.
[44] M. Islam, Z. Hu, Q. Wang, C. Redshaw, X. Feng, Mater. Chem. Front. 2019, 3, 762–
781.
[45] Sheldrick GM. SHELXL-2016/4. University of Göttingen. Germany. 2016.
[46] Sheldrick GM. SADABS 2.10. 2003.
[47] Spek AL. PLATON. A multipurpose crystallographic tool. Utrecht University.
Utrecht. The Netherlands. 2003.
[48] Cambridge Crystallographic Data Centre. MERCURY 3.9. Cambridge. England.
2001-2016.
[49] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato,
A. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J.
V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini,
F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G.
Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R.
21
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Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd,
E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K.
Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam,
M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O.
Farkas, J. B. Foresman, D. J. Fox, Gaussian. Inc. Wallingford CT, 2009.
[50] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 2008, 120, 215–241.
[51] M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem. 2003, 24, 669–681.
[52] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 2005, 105, 2999–3094.
[53] R. Cammi, S. Corni, B. Mennucci, J. Tomasi, J. Chem. Phys. 2005, 122, 104513.
[54] R. Improta, J. Chem. Phys. 2006, 125, 054103.
[55] G. Scalmani, M. J. Frisch, B. Mennucci, J. Tomasi, R. Cammi, V. Barone. J. Chem.
Phys. 2006, 124, 094107.
[56] J. R. Reimers, J. Chem. Phys. 2001, 115, 9103–9109.
[57] B. Baumeier, J. Kirkpatrick, D. Andrienko, Phys. Chem. Chem. Phys. 2010, 12,
11103−11113.
[58] J. Kirkpatrick, Int. J. Quantum Chem. 2008, 108, 51−56.
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2
Scheme 1. Chemical structures of synthesized compounds (1–3) along with some related
compounds (4–7). As an example, the styrylarene core of compound 1 is shown in blue while
a triphenylethylene subunit is highlighted in red.
Figure 1. Distribution in the crystal structure of compound 2 and geometrical structures (view
through the central ring) of the two conformers found (2a and 2b).
23
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Figure 2. Intramolecular interactions in molecule 2b.
Figure 3. Detail views of the C-H··N (A), - stacking (B) and C-H··· (C) intermolecular
interactions in compound 3.
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0.06
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
0.04
0.02
0.00
300
400
500
600
Wavelength (nm)
Figure 4. Absorption spectra and fluorescence emission spectra of compounds 1, 2, and 3 in
THF (sample concentrations were 1 μM).
0
LUMO +1
1
-1
LUMO
-2
-3
2a
1
2a
2b
3
-4
-5
-6
-7
-8
2b
HOMO
HOMO-1
3
25
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Figure 5. Energy levels and shapes of frontier molecular orbitals calculated for 1, 2a, 2b and 3
(isocontour plots (0.02 au)).
a
8
C -C -C -C
10 26
6
3
150
100
50
C -C -C -C
6
3
9
30
6
4
2
0
0
-50
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
0
10
20
30
40
50

Rotational angle (deg)
12
10
8
C
-C -C -C
23 20
12
4
b
150
100
50
C
-C -C -C
4 8 9
12
6
4
0
2
-50
0
1
2
3
4
5
6
7
8
9
10
0
10
20
30
40
50

Rotational angle (deg)
Figure 6. Left: Selected dihedral angles calculated for the ground (grey) and excited state
(black) in THF solution for compounds 2a (a) and 3 (b). The number of the dihedral angle (τ)
is represented in the ordinate axis while its value is shown in the abscissa axis. The dihedral
angle numbering is shown in Tables S2-S4. The vertical discontinuous lines are used to delimit
the dihedral angles of each -phenylstyryl branch in the molecule. The arrows represent the
largest changes upon excitation. Right: Relative energy barrier as a function of the rotational
angle of phenyl rotors calculated for compounds 2a (a) and 3 (b).
26
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Figure 7. (a) Reorganization energy and (b) Huang-Rhys factors versus normal mode
wavenumbers calculated for 2b. (c) Atomic displacements of the two vibrational modes for
which the highest Huang-Rhys factors were obtained.
27
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Figure 8. Compounds 1, 2 and 3 in powder: Excitation (a) and fluorescence emission (b)
spectra. CIE1976 chromaticity diagrams (c) and picture under ultraviolet light (d).
28
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Table 1. Absorption maximum wavelength (휆푒푎푥푏푝푠), logarithm of the molar absorption
coefficient (ε), wavelengths calculated for the vertical electronic transitions (휆푐푎푎푏푙푠푐 ),
oscillator strengths (f) and main components of the transitions (% contribution).
Experiments and calculations were carried out in THF solution.
풆풙풑
풂풃풔
풄풂풍풄
흀
log(ε)
[mM−1 cm−1]
흀
풂풃풔
Compd
f
Transition
Contribution (>20%)
[nm(eV)]
[nm(eV)]
292(4.25)
291(4.26)
347(3.58)
332(3.73)
321(3.87)
335(3.70)
304(4.08)
297(4.18)
320(3.88)
282(4.40)
‒
0.844
0.822
1.316
0.805
0.643
0.957
0.174
0.836
0.633
0.405
‒
S0→
S0→
S0→
S0→
S0→
S0→
S0→
S0→
S0→
S0→
‒
S1
S2
S1
S2
S3
S1
S2
S3
S1
S2
H-1 L(28); H→L+1(24)
→
1
313(3.96)
4.76
H
→
→
L(34)
L(84)
H
2a
331(3.75)
331(3.75)
4.71
H-1→L(88)
H
H
H
→
L+1(83)
L(84)
→
→
2b
3
4.71
L+1(77)
H-1
→
L(74)
L(89)
L(68); H→L+1(29)
337(3.68)
~300(4.13)
329 (3.77)
356 (3.48)
350 (3.54)
4.37
4.37
4.99
4.77
4.99
H→
H-1→
4 a)
5 b)
6 b)
‒
‒
‒
‒
‒
‒
‒
‒
‒
a) From reference [20]. a) From reference [11]
Table 2. Fluorescence excitation and emission
푒푥푝
푒푥
wavelength maxima (휆
and 휆푒푒푥푚푝 ) along with
the wavelength calculated for the S1→S0 transition
(휆푐푒푎푚푙푐). Experiments and calculations were carried
out in THF solution.
푒푥푝
푒푥
푒푥푝
푒푚
푐푎푙푐
푒푚
Compound
휆
[nm(eV)]
휆
[nm(eV)] ΦF [%]
휆
[nm(eV)]
1
318(3.89)
340(3.65)
424(2.92)
467(2.65)
488(2.54)
467(2.65)
488(2.54)
400(3.10)
421(2.94)
399 (3.11)
420 (2.95)
439 (2.82)
461 (2.69)
< 1
< 1
516(2.40)
502(2.47)
2a
2b
3
340(3.65)
329(3.77)
‒
< 1
< 1
60
477(2.60)
510(2.43)
‒
4 a)
5 b)
6 b)
‒
‒
< 1
57
‒
‒
a) From reference [20]. a) From reference [11]
29
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