Preparation and photophysical properties of fluorescent difluoroboronated β-diketones having phenanthrene moieties studied by emission and transient absorption measurements

Article abstract of DOI:10.1039/c5pp00454c

Six difluoroboronated β-diketones having the phenanthrene skeleton (Phe@Ar) are prepared. Based on the measurements of the fluorescence quantum yields, lifetimes and transient absorption, the photophysical features of Phe@Ar are studied in comparison with those of difluoroboronated diketones having phenyl, naphthyl and anthryl moieties. β-Diketones having 1-, 2-, 3- and 9-phenanthryl moieties (PheDKAr) were prepared as the precursor to Phe@Ar. 1-Acetylphenanthrene was synthesized by the photocyclization method as the key building block of PheDKAr having the 1-phenanthryl moiety. The counter aromatic moieties (Ar) of the prepared PheDKAr are varied with phenyl, furyl and thienyl rings (Ar = Ph, F and T, respectively) to investigate the effects of π-conjugation on the fluorescence properties. The prepared Phe@Ars are fluorescent with appreciable fluorescence quantum yields which depend on the substitution position of the phenanthrene moiety. 3-Phe@Ph having the 3-phenanthryl moiety provides the largest fluorescence quantum yield (0.81) in acetonitrile among the Phe@Ars whereas 2-Phe@Ph having the 2-phenanthryl moiety shows the smallest fluorescence quantum yield (0.07) in acetonitrile. All the Phe@Ars show fluorescence also in the solid state, and the fluorescence spectra and quantum yields were determined. Transient absorption measurement using laser flash photolysis of the Phe@Ars revealed the triplet formation. DFT and TD-DFT calculations of Phe@Ars rationalize the dependency of the fluorescence quantum yields on the substitution position of the phenanthrene skeleton in terms of difference in the oscillator strength for the HOMO-LUMO transition.

Full text of DOI:10.1039/c5pp00454c

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Preparation and photophysical properties of fluorescent
difluoroboronated βꢀdiketones having the phenanthrene
moieties studied by emission and transient absorption
measurements
Michitaka Mamiya,† Yurie Suwa ,† Hideki Okamoto¶ and Minoru Yamaji,§,*
† Education Program of Materials and Bioscience, Graduate School of Science and
Engineering, Gunma University, Kiryu, Gunma 376ꢀ8515, Japan
¶ Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Sciences and
Technology, Okayama University, Okayama 700ꢀ8530, Japan
§ Division of Molecular Science, Graduate School of Science and Engineering, Gunma
University, Kiryu, Gunma 376ꢀ8515, Japan
Abstract
Six difluoroboronated βꢀdiketones having the phenanthrene skeleton (Phe@Ar) are prepared.
Based on the measurements of the fluorescence quantum yields, lifetimes and transient
absorption, the photophysical features of Phe@Ar are studied in comparing with those of
difluoroboronated diketones having the phenyl, naphthyl and anthryl moieties. βꢀDiketones
having 1ꢀ, 2ꢀ, 3ꢀ and 9ꢀphenanthryl moieties (PheDKAr) were prepared as the precursor to
Phe@Ar. 1ꢀAcetylphenanthrene was synthesized by photocyclization method as the key
building block of PheDKAr having the 1ꢀphenanthryl moiety. The counter aromatic moieties
(Ar) of the prepared PheDKAr are varied with phenyl, furyl and thienyl rings (Ar = Ph, F and
T, respectively) to investigate the effects of the πꢀconjugation on the fluorescence properties.
The prepared Phe@Ars are fluorescent with appreciable fluorescence quantum yields which
depend on the substitution position of the phenanthrene moiety. 3ꢀPhe@Ph having 3ꢀ
1

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phenanthryl moiety provides the largest fluorescence quantum yield (0.81) in acetonitrile
among the Phe@Ars whereas 2ꢀPhe@Ph having 2ꢀphenanthryl moiety shows the smallest
fluorescence quantum yield (0.07) in acetonitrile. All the Phe@Ars show fluorescence also in
the solid state, and the fluorescence spectra and quantum yields were determined. Transient
absorption measurement using laser flash photolysis of the Phe@Ars revealed the triplet
formation. DFT and TDꢀDFT calculations of Phe@Ars rationalize the dependency of the
fluorescence quantum yields on the substitution position of the phenanthrene skeleton in
terms of difference in the oscillator strength for the HOMOꢀLUMO transition.
1. Introduction
Ketoꢀenol tautomerizations of aromatic 1,3ꢀdiketones typically proceed in solution.1ꢀ3 1,3ꢀ
Dibenzoylmethane (DBM), a representative of the 1,3ꢀdicarbonyl compounds, takes an enol
form as the molecular configuration at room temperature.4ꢀ8 Most DBM derivatives are
nonfluorescent in solution due to efficient nonradiative processes from the excited singlet
states. On the other hand, aromatic difluoroboronated βꢀdiketones (BF2DKs) are known to
show intense fluorescence in solution and the solid state.9ꢀ11 BF2DKs of the DBM skeleton as
the ligand have been paid much attention not only as fluorophores but also as photochemical
reagents,12ꢀ17 twoꢀphoton absorbing molecules,18, 19 semiconductors,20, 21 nearꢀIR probes,22, 23
conjugated polymers,24ꢀ30 mechanochromic luminescence materials,31 photochromic
molecules,23 and oxygen32 and mechanical sensors.33 Variation of the arene systems using the
naphthyl and anthryl moieties as the BF2DK chromophores (see Chart 1) is known to
modulate the emission properties.18, 31, 34
2

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Chart 1. Molecular structures of BF2DKs used in this study.
The fluorescence quantum yields of Ph@Ph, 2ꢀNp@Ph and 2ꢀAnt@Ph in dichloromethane
(DCM) are less than 0.5.34 It is of our interest to prepare highly fluorescent BF2DK by
modifying the chromophore.
Fusedꢀring arenes can be divided into two major groups from the molecular shapes. One is
acenes representative of anthracene, tetracene and pentacene having linearly aligned benzene
rings. The other group is phenacenes, such as phenanthrene, chrysene, picene, fulminene, etc.
bearing benzene rings fused in a zigzag array. Large phenacenes had not been subjected for
application to functional materials because of the difficulties in the organic synthesis
processes until our recent finding of a facile preparation method.35 We have reported a facile
synthetic process for preparing phenacenes using photochemical reactions of diarylethanes
using aromatic carbonyls as the photosensitizer (photosensitized cyclization method),35 with
which we prepared chrysene ([4]phenacene), picene ([5]phenacene) and fulminene
([6]phenacene).36 Photophysical features of phenacenes are found to be quite different from
those of corresponding acenes.36 As increasing the number of the fused benzene rings, the
3

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absorption and fluorescence bands of acenes are subjected to large red shifts while those of
phenacenes do not substantially shift to the longer wavelength regions. These spectroscopic
behaviors can be understood by considering the energies of the highest occupied molecular
orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of these arenes. For
acenes, with an increase of the number of the fused benzene rings, the LUMO energy level
significantly decreases while that of the HOMO increases. These features in the state energies
of acenes lead to an aggressive red shift of the absorption bands. By contrast, for phenacenes,
both the energy levels of the HOMO and LUMO show slight and parallel decreases with an
increase of the number of the benzene rings, resulting in gentle red shifts in the absorption
and fluorescence bands in the solid state.37
From the viewpoint of condensed aromatic compounds of three benzene rings, anthracene
of the acene family contrasts with phenanthrene of the phenacene family. Emission features
of BF2DK having the anthracene skeleton such as 2ꢀAnt@Ph are reported34 whereas those of
BF2DK having the phenanthrene skeleton have been unveiled. Thus, it would be of interest to
synthesize and investigate emission spectral feature of BF2DK having the phenanthrene
skeleton to reveal the effects of the difference in the benzeneꢀring fusion manner on the
emission properties. Unfortunately, the photosensitized cyclization method cannot be
applicable to synthesizing the phenanthrene skeleton.35 It is, thus, desired to explore an
alternative route for construction of the functionalized phenanthrene frameworks. Besides the
photosensitization method, photocyclization of stilbenes is an excellent procedure for
phenacene synthesis (the Mallory method).38, 39 In the present research, we have prepared
BF2DK having the phenanthrene moieties substituted at various positions (see Chart 1). In the
course of preparing one of these BF2DKs, the Mallory method was employed for preparing
the phenanthrene component. The emission features of the prepared BF2DK are investigated
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based on fluorescence and transient absorption measurements with the aid of theoretical
computations.
2. Experimental
Analytical Instruments. NMR spectra were recorded on NMR System 400 and 600 MHz
spectrometers. Melting points were obtained with YANACO micro melting point apparatus.
Materials. 2ꢀ, 3ꢀand 9ꢀAcetylphenanthrenes, 2ꢀacetonaphthone, 2ꢀacetylanthracene and
dibenzoylmethane were commercially purchased. 1ꢀAcetylphenanthrene (1ꢀAcPhe) was
synthesized according to the procedures described in the Electronic Supplementary
Information (ESI). Acetonitrile (ACN) for spectral measurements was purified by distillation.
Chloroform (CF, spectroscopic grade) and ethanol (EtOH, spectroscopic grade) were used
without further purification for the spectral measurements.
General procedures for preparing the diketones and BF2DKs. The used diketones were
prepared by Claisen condensation between appropriate aromatic acetyl compounds and
aromatic methyl ethers. The prepared diketone compound was refluxed in benzene for 1 h in
the presence of boron trifluoride diethyl etherate. The desired product was filtrated and
washed with benzene. Details of the synthetic procedures for each compound and the
analytical data are shown in the ESI.
Absorption, emission and transient absorption measurements. Absorption spectra were
recorded on a Uꢀbest 50 (JASCO) spectrophotometer whereas emission spectra were on a
Hitachi Fꢀ4010 fluorescence spectrophotometer. The fluorescence quantum yields and
fluorescence spectra of the samples in the solid states were obtained with a Hamamatsu
Photonics C9920ꢀ02 absolute PL quantum yields measurement system. Fluorescence
lifetimes (τf) were determined with a Hamamatsu Photonics TAU timeꢀcorrelated singleꢀ
photon counting fluorimeter system. Samples in a quartz cell with a 1 cm path length were
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prepared in the dark, and Arꢀpurged when necessary. Third harmonics (355 nm) from a
Nd:YAG laser system (12 mJ / pulse, LotisꢀTII, LTꢀ2137) and 400 nm (7 mJ / pulse) laser
pulses from a Ti:Sapphire laser system (LotisꢀTII, LTꢀ2211) pumped with second harmonics
(532 nm) from the Nd:YAG laser system were used as the excitation laser light sources. The
details of the detection system for the time profiles of the transient absorption have been
reported elsewhere.40 Transient absorption spectra were obtained using a Unisoku USPꢀ
T1000ꢀMLT system, which provides a transient absorption spectrum with one laser pulse.
The obtained transient spectral data were analyzed using the leastꢀsquares bestꢀfitting
method.
Theoretical calculations. The calculation was carried out at the DFT level, using the
Gaussian 09 software package.41 The geometries of the BF2DKs were fully optimized by
using the 6ꢀ31G(d) basis set at the B3LYP method in vacuum and by considering a dielectric
constant of CF in the CPCM model. Atom coordinates for the optimized geometries are
deposited in the ESI. TDꢀDFT calculations were performed at TD B3LYP/6ꢀ31G(d) level
using the optimized geometries.
3. Result and Discussion
Absorption and emission measurements. Figure 1 shows absorption and emission spectra
of the BF2DKs in CF prepared in the resent work. The absorption and fluorescence spectra in
acetonitrile (ACN) are deposited in the ESI as Figure S1.
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Figure 1. Absorption (black) and fluorescence (blue) spectra in CF
at 295 K and phosphorescence spectra (red) in EtOH at 77 K
obtained for Ph@Ph (a), 2ꢀNp@Ph (b), 2ꢀAnt@Ph (c), 1ꢀPhe@Ph
(d), 2ꢀPhe@Ph (e), 3ꢀPhe@Ph (f), 9ꢀPhe@Ph (g), 3ꢀPhe@F (h) and
3ꢀPhe@T (i). The emission spectra are not corrected.
The absorption spectra of the BF2DKs have large molar absorption coefficients (20000–
50000 dm3 mol−1 cm−1, see Table 1), which is typical for π−π* transitions. All the compounds
provided fluorescence and phosphorescence in solution. The maximum wavelengths (λflu) of
fluorescence are added in Table 1. The fluorescence spectrum of Ph@Ph has vibrational
structures while others show structureless fluorescence bands. The fluorescence spectra in
ACN show Stokes shifts larger than those in chloroform (CF), and the λflus in ACN
substantially undergo redꢀshifts compared with those in CF. From these observations, it is
inferred that fluorescence character of the BF2DKs except Ph@Ph is of charge transfer (CT).
Based on the quantum yields (Φf) and lifetimes (τf) of fluorescence determined in CF and
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ACN, the fluorescence rates (kf), quantum yields (Φnr) and rates (knr) for nonradiative
processes were evaluated by eqs 1–3, respectively.
kf = Φfτf−1 (1)
Φnr = 1 − Φf (2)
knr = Φnrτf−1 (3)
The obtained photophysical parameters are listed in Table 1 along with the absorptivities (ε).
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Table 1. Photophysical parameters of BF2DKs obtained in CF.
ε
a
a,b
a,d
e
΄ΝΥΒΓΠ
ΔΚΣ
/ nm
λflu
/ nm
τf
kf
knr
ET
ꢀ
a
a, c
Compound / dm3 mol−1 cmꢀ1
(λabs / nm)
Φf
Φnr
τT a / ꢁs
Φ fPowder
0.38
/ ns
/ 107 s−1
/ 107 s−1
/ kcal mol−1
58.6
Ph@Ph
2ꢀNp@Ph
2ꢀAnt@Ph
1ꢀPhe@Ph
2ꢀPhe@Ph
3ꢀPhe@Ph
9ꢀPhe@Ph
3ꢀPhe@F
3ꢀPhe@T
396
(398)
458
0.21
(0.11)
0.31
0.54
(0.32)
2.5
38.9
(34.4)
12.4
0.79
146.3
(278.1)
27.6
470
25.1
(8.1)
3.7
43000 （365）
30300 (398)
29900 (393)
22600 (392)
54200 (405)
40000 (417)
26400 （402）
48200 (430)
49100 (432)
(0.89)
0.69
52.5
56.3
52.7
55.4
52.4
50.5
51.4
51.3
532
715
535
530
600
536
586
600
0.25
0.01
0.20
0.20
0.24
0.12
0.13
0.33
(507)
571
(0.67)
0.80
(10.8)
11.9
(2.0)
5.7
(6.2)
6.7
(0.33)
0.20
(3.1)
1.7
(8.1)
n/d
(626)
506
(0.08)
0.79
(4.0)
13.9
(0.92)
0.21
(46.0)
3.7
(n/d)
6.2
(550)
499
(0.25)
0.10
(4.2)
7.5
(6.0)
1.3
(0.75)
0.90
(17.9)
12.0
(8.1)
14.7
(13.8)
5.8
(562)
475
(0.07)
0.76
(12.7)
2.6
(0.55)
29.2
(0.93)
0.24
(7.3)
9.2
(516)
471
(0.83)
0.55
(5.0)
3.2
(16.6)
17.2
(0.17)
0.45
(3.4)
14.1
(4.0)
9.4
(508)
475
(0.26)
0.52
(4.2)
1.7
(6.2)
30.6
(0.74)
0.48
(17.6)
28.2
(11.7)
6.3
(516)
478
(0.73)
0.50
(3.3)
1.6
(22.1)
31.3
(0.27)
0.50
(8.2)
31.3
(11.5)
13.3
(11.7)
(519)
(0.76)
(4.1)
(18.5)
(0.24)
(5.9)
a) Data in parentheses were obtained in ACN. b) Determined by the equation, kf = Φf τfꢀ1. c) Determined by the equation, Φnr = 1 − Φf. d)
Determined by the equation, knr = Φnr τf−1. e) Determined from the 0ꢀ0 origin of the phosphorescence spectrum obtained in EtOH at 77 K.
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The studied compounds emit intense fluorescence (Φf ≥ 0.07) in CF and ACN. Interestingly,
the fluorescence quantum yields and lifetimes values of 2ꢀAnt@Ph are dependent on the
solvent. On going from CF to ACN, the Φf value varies from 0.80 to 0.08 while the τf is
shortened from 11.9 to 2.0 ns. The obtained Φf values for Ph@Ph (0.21), 2ꢀNp@Ph (0.31)
and 2ꢀAnt@Ph (0.80) in CF are worth comparing with those reported (0.26, 0.50 and 0.30,
respectively) in DCM determined by the method using the fluorescence quantum yield of
anthracene in ethanol (EtOH) as a standard.34
Figure 2 visualizes the dependency of the photophysical parameters on the substitution
position and the solvent for nꢀPhe@Ph (n = 1, 2, 3 and 9).
Figure 2. Plots of fluorescence quantum yields, Φf (a),
fluorescence lifetimes, τf (b), fluorescence rates, kf (c) and
nonradiative rates, knr (s) obtained for nꢀPhe@Ph (n = 1, 2, 3
and 9) in CF (●) and ACN (△).
It is found that the Φf values of nꢀPhe@Phs strongly depend on the substitution position of
the phenanthrene moiety. 3ꢀPhe@Ph provides large Φf values (ca. 0.8) irrespective of the
solvent while the Φf values for 2ꢀPhe@Ph in both the solvents are the smallest among nꢀ
Phe@Phs. The Φf values of 1ꢀ and 9ꢀPhe@Phs substantially depend on the solvent. On going
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from CF to ACN, the Φf values decrease to the half or a third. The τf values of nꢀPhe@Phs
are shorter than 10 ns except for that in ACN for 2ꢀPhe@Ph. Thus, according to eq 1, the kf
values show similar dependencies on the solvent and the substitution position to those seen
for the Φf values. The dependency of the kf value on the substitution position will be
rationalized later by using theoretical computations of the state energies. According to eq. 2,
we have obtained the definite values of Φnr, indicating the presence of internal conversion
(IC) from the lowest excited singlet state (S1) to the ground state (S0) and intersystem
crossing (ISC) from the S1 to the lowest triplet state (T1). The governing process of
deactivating the S1 state will be revealed by transient absorption measurements using laser
photolysis techniques, which will be mentioned later.
The counter chromophore of the phenanthrene moiety in 3ꢀPhe@Ph was altered from the
phenyl (Ph) to the furan (F) and thiophene (T) rings to investigate the effect of the πꢀ
conjugated systems on the fluorescence properties. The determined photophysical data are
listed in Table 1. Figure 3 shows the dependency of the photophysical parameters on the
counter chromophores and the solvent for 3ꢀPhe@Ar (Ar = Ph, F and T).
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Figure 3. Plots of fluorescence quantum yields, Φf (a), fluorescence
lifetimes, τf (b), fluorescence rates, kf (c) and nonradiative rates, knr (s)
obtained for 3ꢀPhe@Ar (Ar = Ph, F and T) in CF (●) and ACN (△).
The Φf values of 3ꢀPhe@F and 3ꢀPhe@T are smaller than those of 3ꢀPhe@Ph in CF and
ACN, respectively. The kf values resemble among these three complexes, indicating the
transition processes from the S1 to S0 states are not affected by the counter chromophores.
Conversely, the knr values of 3ꢀPhe@F and 3ꢀPhe@T are larger than that of 3ꢀPhe@Ph in CF
while those in ACN are similar to each other. Considering the relationship among Φf, kf and
knr, i.e., Φf = kf (kf +knr)−1, it is understandable that the Φf values of 3ꢀPhe@F and 3ꢀPhe@T
are smaller than those of 3ꢀPhe@Ph in ACN.
Figure 4 shows fluorescence spectra in the solid state (powder) of the used BF2DKs.
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Figure 4. Fluorescence spectra obtained in the solid state (powder) at 295 K
obtained for Ph@Ph (a), 2ꢀNp@Ph (b), 2ꢀAnt@Ph (c), 1ꢀPhe@Ph (d), 2ꢀPhe@Ph
(e), 3ꢀPhe@Ph (f), 9ꢀPhe@Ph (g), 3ꢀPhe@F (h) and 3ꢀPhe@T (i).
΄ΝΥΒΓΠ
ΔΚΣ
It is noteworthy that all the fluorescence maxima (ꢀ
) in the solid state, listed in Table
1, are redꢀshifted compared with those in solution. It is typically observed that the
wavelength of the emission band in solution differs from that in the solid state.42, 43 The
fluorescence quantum yields (Φ fPowder ) in the solid state are determined, and also listed in
Table 1. The Φ fPowder values of Ph@Ph and 2ꢀPhe@Ph are greater than those (Φf) in solution
while those of the other complexes are smaller than the Φf values in solution. Especially, 2ꢀ
Ant@Ph shows a drastically reduced fluorescence quantum yield in the solid state compared
to that in CF.
Transient absorption measurements. We have carried out laser flash photolysis of the
studied BF2DKs in CF to investigate the deactivation processes from the S1 state. Figure 5
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shows transient absorption spectra obtained upon 355 and 400 nm laser pulsing in the CF
solution of the BF2DKs.
Figure 5. Transient absorption spectra obtained at 200 ns
upon laser pulsing in the Arꢀpurged CF solution of Ph@Ph
(a), 2ꢀNp@Ph (b), 2ꢀAnt@Ph (c), 1ꢀPhe@Ph (d), 2ꢀ
Phe@Ph (e), 3ꢀPhe@Ph (f), 9ꢀPhe@Ph (g), 3ꢀPhe@F (h)
and 3ꢀPhe@T (i) at 295 K. A 355 nm Laser pulse was used
for Ph@Ph whereas 400 nm laser pulses were employed for
shining the other BF2DKs.
Transient absorption spectra except for 2ꢀAnt@Ph were obtained in the wavelength region,
420–820 nm in Arꢀpurged CF. The intensities of the transient absorption signals decreased
with decay lifetimes (τT) in the microsecond time domain, which are tabulated in Table 1. In
the presence of the dissolved oxygen, the decay lifetimes were shortened, indicating that the
transient signals are ascribable to the tripletꢀtriplet (TꢀT) absorption. The TꢀT absorption
spectra in ACN are deposited in the ESI as Figure S2, and the τT values in ACN are listed in
Table 1. The shapes of the TꢀT absorption spectra in ACN were similar to those in CF. These
observations indicate that the triplet states except for 2ꢀAnt@Ph have little CT character in
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solution in contrast to the S1 states. Observation of the triplet state in solution indicates that
ISC from the S1 to the T1 state competes with the fluorescence process. As with 2ꢀAnt@Ph,
we have evaluated the Φnr values being 0.20 in CF and 0.92 in ACN. The absence of the TꢀT
absorption spectra in solution demonstrates the nonradiative process from the S1 state is due
to IC to the ground state.
DFT Calculations. To obtain an insight into the photophysical properties of the BF2DKs,
DFT and TDꢀDFT calculations were carried out at the B3LYP/6ꢀ31G(d) level.41 The
calculated results are summarized in Table 2.
Table 2. Calculated photophysical properties of BF2DKs in vacuum.a
HOMO
/ eV
LUMO
/ eV
Configuration
coefficient e
S0 → S1
Transitionsb
π,π*
Compound
1ꢀPhe@Ph
λtr c / nm
f d
ꢀ2.69
(ꢀ2.68)
ꢀ2.75
(ꢀ2.71)
ꢀ2.74
(ꢀ2.71)
ꢀ2.74
(ꢀ2.73)
ꢀ2.71
ꢀ1.26
(ꢀ1.32)
ꢀ1.31
(ꢀ1.35)
ꢀ1.26
(ꢀ1.35)
ꢀ1.29
(ꢀ1.32)
ꢀ1.31
434
(461)
425
(453)
403
(446)
425
(441)
422
0.1241
(0.2513)
0.0083
(0.0141)
0.5056
(0.7468)
0.2387
(0.3977)
0.6000
0.70360
(0.70349)
0.69630
(0.69919)
0.70068
(0.70248)
0.69770
(0.69981)
0.70152
2ꢀPhe@Ph
3ꢀPhe@Ph
9ꢀPhe@Ph
3ꢀPhe@F
3ꢀPhe@T
π,π*
π,π*
π,π*
π,π*
π,π*
(ꢀ2.69)
ꢀ2.72
(ꢀ1.35)
ꢀ1.33
(452)
426
(0.8352)
0.5734
(0.70401)
0.70245
(ꢀ2.70)
(ꢀ1.36)
(455)
(0.8018)
(0.70325)
a Data in parentheses are calculated results considering the dielectric constant of CF.
b Character of the transitions to the excited singlet states with the lowest excitation energies.
c Wavelength estimated from transition energy.
d Oscillator strength for the S0 → S1 transition.
e For transition from HOMO to LUMO.
Figure 6 shows calculated molecular orbital diagrams for nꢀPhe@Ar showing the highest
occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) in
vacuum.
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Figure 6. The HOMO and LUMO surfaces for nꢀPhe@Ar in vacuum.
In the optimized structures of Phe@Ar, the πꢀelectrons in the HOMOs are mainly localized
over the phenanthrene moieties while those in the LUMOs are mainly delocalized over the βꢀ
diketone backbone with small distribution to the two aryl groups. Therefore, the S0 → S1
transitions of nꢀPhe@Ar mainly contributed from the HOMO → LUMO configurations were
characterized to be of π,π* with CT character. The HOMO energy levels of nꢀPhe@Ar as well
as the LUMO energy levels are not varied. This origin is also derived from the localized
HOMO and LUMO distributions on the phenanthrene and βꢀdiketone moieties, respectively.
The counter chromophores, Ph, F and T have similar influence to the electronic distribution
in the excited states, resulting in the Φf and kf values of 3ꢀPhe@Ar being little influenced by
them (see Figures 3a and c). The wavelengths (λtr) estimated from the transition energies are
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located in the wavelength region, 400–435 nm, and redꢀshifted compared to the experimental
λabs values of the lowest energy absorption bands. The estimated oscillator strengths (f) for
the S0 → S1 transitions depend on the substituted positions on the phenanthrene skeleton. It is
noteworthy that the f value for 2ꢀPhe@Ph which shows the smallest Φf value is the smallest
among the six BF2DKs. With the TDꢀDFT results of 2ꢀPhe@Ph, we have the f value being
0.9291 for the transition from the HOMOꢀ1 to the LUMO, which corresponds to the allowed
S0 → S2 excitation. This transition may contribute to the large absorptivity of 2ꢀPhe@Ph. In
contrast, the f values for 3ꢀPhe@Ar which show relatively large Φf value among the studied
compounds are also relatively large. Considering the difference in the calculated f values
between 2ꢀ and 3ꢀPhe@Phs, the substitution position on the phenanthrene moiety may
substantially affect the dipole moments for the S0 → S1 transition in Phe@Ar. The
fluorescence rate, kf is related to the oscillator strength, f via the StricklerꢀBerg equation.44
The dependence of Φf and kf on the substitution positions of the phenanthrene moiety
obtained in the present work can be rationalized in terms of that of the f values.
4. Conclusion
We have prepared a series of Phe@Ar having the phenanthrene moieties substituted at
various positions, and investigated the photophysical features in solution and in the solid state
along with those of Ph@Ph, 2ꢀNp@Ph and 2ꢀAnt@Ph. Phe@Ars show the absorption bands
in the wavelength regions shorter than 470 nm in CF, and the fluorescence appears at 470–
500 nm in solution and at 530–600 nm in the solid state. From the viewpoint of BF2DKs
having chromophores of three fused benzene rings, the spectroscopic features are quite
different from those of 2ꢀAnt@Ph. The Φf and kf values of Phe@Ar in solution are found to
17

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strongly depend on the substitution position of the phenanthrene moiety. 3ꢀPhe@Ar shows
large fluorescence potentials in solution compared to other nꢀPhe@Phs independent of the
solvent polarity. The fluorescence abilities of 3ꢀPhe@Ar seem to be subjected to little
influence of the counter chromophores (Ar). These characteristics are explained with the
results of the TDꢀDFT calculations. The dependency of the substitution position is
understood in terms of the difference in the oscillator strength (f) of Phe@Ar for the π−π*
transitions between the HOMO and LUMO, which are related to the allowed transition
moments on the phenanthrene moiety. The visualized LUMO surface of Phe@Ar shows a
small amount of electronic distributions on the phenanthrene and the counter chromophores
whereas the HOMO is distributed mainly on the phenanthrene moiety. Laser photolysis
studies of Phe@Ar revealed that the competitive process of fluorescence in the S1 state of
Phe@Ar is ISC to the T1 state in solution.
Electronic Supplementary Information (ESI) available: The precise synthesis procedures
and analytical data for the compounds used in this work, absorption and fluorescence spectra
in ACN, transient absorption spectra in ACN, results of DFT calculation including tables of
atom coordinates for the optimized geometries, and 1H and 13C NMR spectra of 1ꢀAcPhe,
PheDKAr and Phe@Ar are provided.
Acknowledgment
This work has been supported by a GrantꢀinꢀAid for Scientific Research (26288032) from the
Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japanese
Government. MY thanks Cosmetrogy Foundation for the financial support. Prof. Fumito Tani
at Kyusyu University is acknowledged for performing the HRMS spectrometry of the new
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compounds under Cooperative Research Program of the Network Joint Research Center for
Materials and Devices.
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F
F
B
9
O
O
1
Ar
2
3
S
O
Ar =
,
,