Photoinduced reversible isomerization of 9H-fluorene into 1H-fluorene by means of hydrogen-atom migration and the lowest electronically excited triplet state studied by matrix-isolation FTIR spectroscopy

Article abstract of DOI:10.1016/j.cplett.2018.11.011

Photoinduced reversible intramolecular hydrogen-atom migration between 9H-fluorene and 1H-fluorene isolated in an Ar matrix is found by FTIR spectroscopy with an aid of DFT calculation. The forward isomerization from 9H-fluorene to 1H-fluorene occurs upon

Full text of DOI:10.1016/j.cplett.2018.11.011

ChemicalPhysicsLetters714(2019)160–165
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Chemical Physics Letters
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Research paper
Photoinduced reversible isomerization of 9H-fluorene into 1H-fluorene by
means of hydrogen-atom migration and the lowest electronically excited
triplet state studied by matrix-isolation FTIR spectroscopy
Takehiro Kumakura, Nobuyuki Akai⁎, Munetaka Nakata
Graduate School of BASE (Bio-Applications and Systems Engineering), Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo 184-8588, Japan
H I G H L I G H T S
9H-Fluorene isomerizes to 1H-fluorene by photoinduced hydrogen-atom migration.
The backward isomerization occurs upon longer-wavelength light irradiation.
The experimental IR spectrum of 9H-fluorene in the T1 state was obtained.
•
•
•
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fluorene
Photoinduced reversible intramolecular hydrogen-atom migration between 9H-fluorene and 1H-fluorene iso-
lated in an Ar matrix is found by FTIR spectroscopy with an aid of DFT calculation. The forward isomerization
from 9H-fluorene to 1H-fluorene occurs upon UV irradiation (λ ≥ 295 nm), while the backward isomerization
occurs upon λ ≥ 320 nm light irradiation. The less stable isomer, 1H-fluorene, is identified by comparison of the
measured IR spectrum with the corresponding simulated spectral pattern obtained at the B3LYP/6-31++G(d,p)
level. In addition, an IR spectrum of 9H-fluorene in the T1 state is measured during UV irradiation (λ ≥ 275 nm).
Photoinduced hydrogen-atom migration
Matrix-isolation infrared spectrum
Electronically excited T1 state
1. Introduction
functional-theory (DFT) calculations. In the present study, the spec-
troscopic detection and characterization of species produced in the
The photoreaction mechanism of polycyclic aromatic hydrocarbons
(PAHs) is an important research subject in general chemistry. Since
most of the PAHs were recognized as notorious pollutants and/or pre-
cursors, their photochemistry has been actively investigated in en-
vironmental chemistry [1–3]. In addition, since the PAHs are included
in a lot of organic molecules as a basic framework, the reactivity of
PAHs has been studied in fundamental organic and physical chemistries
for long time [4]. Various experimental and theoretical researches have
been in progress and published. However, the photochemistry of 9H-
fluorene is not elucidated clearly, although this molecule is not only one
of the typical PAHs but also frequently used for basic materials to yield
organic electroluminescence elements, dyes and pesticides. In 2002,
vacuum UV photolysis of 9H-fluorene isolated in an Ar matrix has been
studied by Szczepanski et al. from the point of view in interstellar
chemistry [5,6], where a neutral ring-opening species, a radical species
due to dehydrogenation from position 9 of 9H-fluorene, and its radical
cation have been spectroscopically detected and confirmed by density-
photolysis of 9H-fluorene in an Ar matrix are initially performed, and
the isomerization from 9H-fluorene to the less stable isomer, 1H-
fluorene shown in Fig. 1, due to intramolecular hydrogen-atom mi-
gration upon UV irradiation is found.
Reports on the photolysis of 9H-fluorene are few so far, but the
electronically excited states of 9H-fluorene are sometimes investigated
by several spectroscopies and theories [7–12] because 9H-fluorene and
its derivatives have interesting π-electron conjugation and are used as
basic materials to yield organic electroluminescence elements [13–15].
The π-electron conjugation between the two benzene rings is compar-
able with that in biphenyl structure [16–19]. Especially, the lowest
electronically excited triplet (T1) state is investigated in detail [16,17];
Buntinx and Poizat reported time-resolved resonance Raman spectra of
the T1 state of 9H-fluorene and its radical cation [16], and Matsunuma
et al. reported time-resolved resonance CARS spectrum in the T1 state of
9H-fluorene and its deuterated species [17]. Although the lifetime of
the T1 state is known to be relatively long, ca. 5 s [20], no IR spectrum
⁎ Corresponding author.
E-mail address: akain@cc.tuat.ac.jp (N. Akai).
https://doi.org/10.1016/j.cplett.2018.11.011
Received 8 August 2018; Received in revised form 4 November 2018; Accepted 7 November 2018
Availableonline08November2018
0009-2614/©2018ElsevierB.V.Allrightsreserved.

T. Kumakura et al.
ChemicalPhysicsLetters714(2019)160–165
H
H
3. Results and discussion
H
H
3.1. Intramolecular hydrogen-atom migration from position 9 to position 1
The matrix-isolated sample of 9H-fluorene was prepared by slow
deposition on the CsI plate for 120 min, and the IR spectrum of 9H-
fluorene was measured, which is essentially same as the previous re-
ports [7,10]. Spectral change of 9H-fluorene in an Ar matrix was ob-
served when the matrix sample was exposed to the light of λ ≥ 295 nm,
but not 320 nm, which is consistent with the reported S1-S0 transition
energy of 9H-fluorene [28,29]. Fig. 2(a) shows a difference spectrum
between spectra measured before and after UV irradiation, λ ≥ 295 nm,
for 60 min. Roughly 10% of 9H-fluorene was changed to photoproducts
when the spectrum changed no longer. In order to classify the photo-
product bands into a few groups, a matrix sample irradiated by
λ ≥ 275 nm light for 70 min was continuously exposed to various wa-
velength lights. It is noted that the bands of the reactant of 9H-fluorene
increase upon λ ≥ 320 nm irradiation (Fig. 2(b)), which decrease in
Fig. 2(a); for example, see the most intense 740 cm−1 band. On the
other hand, most of the bands increasing in Fig. 2(a) decrease in
9H-fluorene
1H-fluorene
Fig. 1. Molecular structures of 9H- and 1H-fluorene.
has been reported except for only three bands of a sample in poly-
ethylene matrix at 77 K [20]. We have established a technique to
measure IR spectra of transient species existing only during irradiation
and reported IR spectra of some aromatic molecules in the T1 states so
far [21–26]. In the present study, the IR spectrum of 9H-fluorene in the
T
1 state is also reported, providing complementary information against
the Raman spectra reported previously.
2. Experimental and calculation methods
Fig. 2(b); for example, see the doublet bands at 752 and 754 cm−1
,
A small amount of 9H-fluorene (Tokyo Chemical Industry Co., Ltd.)
was placed in a glass sample holder located on the pathway of a de-
position system and evaporated at room temperature. The evaporated
sample gas was mixed with much excess Ar gas (Nippon Sanso,
99.9999% purity) in the holder, and deposited on a CsI plate cooled by
a closed-cycle helium refrigerator (CTI Cryogenics, Model M-22) to ca.
15 K.
IR spectra were measured with an FTIR spectrometer (JEOL, JIR-
7000) equipped with an MCT detector. The spectral resolution was
0.5 cm−1, and the number of accumulations was 100. A superhigh-
pressure Hg lamp (USHIO, 500 W) was used as a radiation source to
induce photoreaction through short-wavelength cutoff filters (ASAHI
spectra) and a water filter (10 cm path). See Refs. [21–26] for other
experimental detail.
implying that a reversible transformation occurs between 9H-fluorene
and its isomer in the course of the irradiations. By comparison of the
measured spectrum with simulated spectral patterns of some candidates
for the isomer, 1H-fluorene is found to be the most suitable isomer of
9H-fluorene, as shown in Fig. 2(c); theoretical IR spectra of other
fluorene isomers are shown in Fig. S1 in Supporting information. Jud-
ging from the absorbance of the both fluorene, roughly 30% of 9H-
fluorene changes to 1H-fluorene upon λ ≥ 275 nm, and about 25% of
yielded 1H-fluorene reconverts to 9H-fluorene upon λ ≥ 320 nm, sug-
gesting that photochemical equilibrium between the both isomers de-
pends on irradiation wavelength. The observed and calculated wave-
numbers of 1H-fluorene are summarized in Table
1 with their
vibrational assignments. The characterized vibration modes of 1H-
fluorene appear at 752/754 cm−1 doublet, 818 cm−1, 1418 cm−1 and
1453/1457 cm−1 doublet, which are assigned to theoretical values of
754 cm−1 (C-H out-of-plane bending), 823 cm−1 (C9-H out-of-plane
bending), 1413 cm−1 (CH2 scissoring) and 1453 cm−1 (aromatic-ring
stretching), respectively. No vibrational spectrum of 1H-fluorene has
been reported so far, to our knowledge. Two bands at 834 and
Theoretical energies, optimized geometries, electronic transition
energies, and predicted IR spectra were obtained by the DFT and time-
dependent DFT calculations using B3LYP hybrid functional with a basis
set of 6-31++G(d,p) on the Gaussian 09 W program package [27].
Fig. 2. Difference IR spectra between spectra
measured before and after the first irradiation
(λ ≥ 295 nm for 60 min) (a), measured before and
after the second irradiation (λ ≥ 325 nm for
90 min) continuously after λ ≥ 275 nm for 70 min
irradiation (b), and the simulated IR spectrum
composed of 9H-fluorene (positive bands) and 1H-
fluorene (negative bands) calculated at the
B3LYP/6-31++G(d,p) level (c). The bands
marked with “+” are due to unknown products.
161

T. Kumakura et al.
ChemicalPhysicsLetters714(2019)160–165
Table 1
Observed and calculated wavenumbers (in cm−1) of 1H-fluorene in the region
between 600 and 3100 cm−1
.
Obsd.
Calcd.a
Sym. Assignmentb
−1
−1
∼
∼
Abs.
IR Int./km
mol−1
ν /cm
ν /cm
630
652
707
747
752/754
815
818
866
931
1008
1012
1020
1133
1153/1155 21.3
1189
1202
37.6
60.2
12.7
36.2
90.5
23.1
35.3
8.1
13.1
21.3
21.7
26.7
7.7
624
660
711
748
754
812
823
870
917
11.8
36.0
8.2
37.8
30.8
7.8
18.3
5.6
7.0
A’
β (ring)
A”
A”
A”
A”
A’
A”
A”
A’
γ(C2,3,4-H), CH2 rock.
γ(C2,3,4-H)
γ(C5,6,7,8-H)
γ(C5,6,7,8-H)
ν(C4a-C9a)
γ(C9-H)
γ(C9-H),γ(C5,6,7,8-H)
ν (C1-C2)
Combination?
ν (C3-C4)
ν (C3-C4)
1010
1022
1134
1154
1191
1203
1280
1354
8.8
4.7
1.5
6.6
11.6
10.3
1.9
A’
A’
A’
A’
A’
A’
A’
A’
β(C9-H)
β(H-C6-C7-H)
ν(C4b-C8a-C9)
ν(C8a-C9), β(C5-H)
ν(C2-C1-C9a-C4a)
ν(C4a-C9a), ν(C4b-C8a)
Combination?
ν(C4a-C9a),ν(C8a-
C9),ν(C6-C7)
16.7
23.1
1276/1278 14.0
1343
1352
1364
67.0
20.4
14.0
19.1
Fig. 3. Numbering of carbon atoms and optimized structure of 1H-fluorene.
Blue, red and black-colored lines have single, double bond and aromatic
characters, respectively. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
1372
5.8
A’
1398/1403 21.7
1418 11.8
1453/1457 100.0 1453
1402
1413
2.6
A’
A’
A’
A’
A’
A’
A’
β(H-C2-C3-H)
14.2
72.4
4.0
25.4
9.9
CH2 scissor.
ν(aromatic ring)
β(H-C6-C7-H)
ν(C9-C9a),ν(C6-C7)
ν(C2-C3-C4-C4a)
ν(C4b-C8a),ν(C6-
C7),ν(C9-C9a)
ν(aromatic ring)
1466
26.2
1468
1572
1584
1599
1H-fluorene absorbs longer-wavelength light than 9H-fluorene. This
expectation is consistent with the experimental results that the back-
ward isomerization from 1H-fluorene to 9H-fluorene occurs by
λ ≥ 320 nm irradiation (Fig. 2(b)) and the forward isomerization from
9H-fluorene to 1H-fluorene occurs by λ ≥ 295 nm irradiation
(Fig. 2(a)).
1562/1564 19.5
1568
1587
76.5
19.5
16.0
1609
2843
2852
3007
35.3
2.3
9.0
1616
2877
2889
3045
3056
3062
3070
3075
3084
14.7
20.3
6.1
A’
A’
A”
A’
A’
A’
A’
A’
A’
νsym(CH2)
νasym(CH2)
9H-fluorene is more stable than 1H-fluorene by ca. 117 kJ mol−1
,
2.5
5.5
ν(C-H)
and the barrier height from 9H-fluorene to 1H-fluorene is estimated to
be ca. 271 kJ mol−1 in the electronic ground state at the B3LYP/6-31+
+G(d,p) level with zero-point energy correction. The barrier height
inhibits thermal isomerization between the two isomers in low-tem-
perature Ar matrices. Another possible isomer is 3H-fluorene, the re-
lative energy of which is estimated to be ca. 104 kJ mol−1 against 9H-
fluorene, meaning that 3H-fluorene is more stable than 1H-fluorene.
However, no IR bands assigned to 3H-fluorene were detected in the
present experiment. We assume that the hydrogen-atom migration to
produce 3H-fluorene is difficult, because hydrogen-atom migration
easily occurs between next nearest carbon atoms from C1 to C9 by
photo-induced dissociation-association (PIDA) mechanism as described
below. Relative energies of other possible isomers, 2H– and 4H-
fluorene, are estimated to be 174 and 188 kJ mol−1, respectively, which
are much higher than that of 9H-fluorene (see Table S5 in Supporting
information).
The reversible isomerization between 9H-fluorene and1H-fluorene
is interesting because intramolecular hydrogen-atom migration occurs
between two carbon atoms. This is exceptionally rare case, to our
knowledge, beside the 2,4-shift at cyclohexadiene-1-thione produced
from thiophenol [31], to our knowledge, although photoinduced re-
versible intramolecular hydrogen-atom migration between a carbon
atom and a hetero atom such as O or N or between hetero atoms is well-
known and investigated by matrix-isolation spectroscopy with quantum
chemical calculations; for example, hydroxypyridines (2(1H)-pyr-
idones) [32,33], cytosines [34,35], acetylacetones [36,37] and indole
[38].
18.9
29.8
35.9
32.0
13.4
ν(C2-H),ν(C4-H)
ν(C5,6,7,8-H)
ν(C2,3,4-H)
ν(C5,6,7,8-H)
ν(C9-H)
3048
3054
3057
38.9
24.9
19.0
a
Obtained at the B3LYP/6-31++G(d,p) level. The calculated wavenumbers
are adjusted by scaling factors of 0.98 below 2000 cm−1 and 0.96 above
2000 cm−1
. Calculated wavenumbers with IR intensity less than 5.5 are
omitted. All calculation results are provided in supporting information.
b
Dominantly vibrational modes are shown in. Symbols of γ, β and ν re-
present out-of-plane bending, in-plane bending and stretching modes, respec-
tively. Numbering of atom is defined in Fig. 3.
884 cm−1 marked with “+” in Fig. 2 are due to unknown products, not
assigned to 1H-fluorene.
The optimized structure of 1H-fluorene is shown in Fig. 3; the
symmetry of 1H-fluorene is changed to Cs symmetry from C2v symmetry
of 9H-fluorene. The hydrogen-atom migration largely affects not only
the right six-membered ring, which is changed from a benzene ring to a
1,3-cyclohexadiene ring with CH2 group, but also the central five-
membered ring; the bond lengths of C9-C9a and C8a-C9 are shortened
to 1.362 and 1.465 Å, respectively, from 1.516 Å of the corresponding
bond of 9H-fluorene. It may be assumed that the delocalized π-electron
system of 1H-fluorene expands on the right six-membered ring and the
left benzene ring through the C9a-C9-C8a fragment, leading to the
conclusion that the lowest electronic transition energy of 1H-fluorene is
lower than that of 9H-fluorene [30]. Actually, the first, the second, and
the third electronic transitions of 1H-fluorene are estimated by the
time-dependent DFT calculation to be 452 nm (its estimated oscillator
strength f = 0.049), 361 nm (f = 0.266) and 284 nm (f = 0.061), re-
spectively, while the corresponding values of 9H-fluorene are 276 nm
(f = 0.162), 266 nm (f = 0.287) and 257 nm (f = 0.007), implying that
The irradiation energies for the forward and the backward reactions
are ca. 405 kJ mol−1 (295 nm) and 375 kJ mol−1 (320 nm), respec-
tively, which are sufficiently higher than the estimated dissociation
energies for the C9-H bond of 9H-fluorene and the C1-H bond of 1H-
162

T. Kumakura et al.
ChemicalPhysicsLetters714(2019)160–165
Fig. 4. Matrix-isolation IR spectrum of 9H-
fluorene, (a), and IR difference spectra of 9H-
fluorene in an Ar matrix: UV pump (λ ≥ 275 nm)
source on minus source off, (b), where IR bands of
transient species and photoproducts increase
while those of 9H-fluorene in the S0 state de-
creases; and after UV excitation for 7.5 min minus
before, (c), where IR bands of photoproducts and
9H-fluorene in the S0 state increase and decrease,
respectively.
Fig. 5. An expanded difference IR spectrum of
Fig. 4b, (a), compared with theoretical spectrum of
9H-fluorene in the T1 state (positive bands) and S0
state (negative bands), (b), and theoretical spec-
trum of 1H-fluorene in the T1 state, (c), obtained at
the B3LYP/6-31++G(d,p) level. The bands
marked with asterisk “*” are assigned to 1H-
fluorene as a photoproduct.
fluorene, ca. 326 kJ mol−1 (367 nm) and 210 kJ mol−1 (570 nm), re-
spectively, at the B3LYP/6–31++G(d,p) level. Szczepanski et al. stu-
died the vacuum UV photolysis of 9H-fluorene in an Ar matrix at 12 K
and identified the dehydrogenated fluorene radical and its radical ca-
tion, and neutral ring-opening species as photoproducts, but 1H-
fluorene was not recognized at all [5]. On the other hand, no infrared
bands of dehydrogenated fluorene radical, which is expected to be an
intermediate, were observed in the present study. This discrepancy may
be explained by difference on the irradiation photon energy. In their
previous experiment [5], the dissociated hydrogen atom freely migrates
from a matrix cage by excess energy of vacuum UV light to form H2
molecules, and the counterpart radicals can be kept in the matrix cage.
In the present experiment, the dissociated hydrogen atom is kept in a
matrix cage to recombine with the counterpart radical immediately,
because the difference between irradiation energy and dissociation
energy is too small to go out of the matrix cage.
distributions of C1, C3 and C9 are calculated to be 0.20, 0.10 and 0.66,
respectively, at the B3LYP/6-31++G(d,p) level, it is expected that the
hydrogen atom dissociated from C9 (or C1) is immediately recombined
to C9 or close C1, while the hydrogen-atom migration from C1 to C3
across C2 is more difficult.
3.2. IR spectrum of transient species photoproduced from 9H-fluorene
By analogy with the previously reported molecules [21–26], the T1
lifetime of 9H-fluorene is expected to be prolonged in an Ar matrix at
ca. 15 K. Actually, blue phosphorescence was observed for ca. 20 s after
stopping UV light for excitation. Fig. 4 shows a transient spectrum
which is spectrum measured “during” UV excitation (λ ≥ 275 nm)
minus before and a difference spectrum between spectra measured
before and “after” the light irradiation. The bands increasing in the both
difference spectra (Fig. 4(b) and (c)) are due to stable photoproducts,
and the bands increasing only during the light excitation (Fig. 4(b) but
not 4(c)) are due to transient species produced from 9H-fluorene, which
are expected to be assigned to 9H-fluorene in the T1 state. To confirm
the expectation, theoretical spectrum of the T1 state is predicted at the
B3LYP/6-31++G(d,p) level, which is known to be the most popular
This reversible tautomerization between 9H- and 1H-fluorene could
be explained by PIDA mechanism proposed for photoinduced tauto-
merization reactions in the cryogenic matrix [39]. The intermediate
fluorene radical yielded by dissociation of hydrogen atom upon light
irradiation has a reactive carbon atom. Since Mulliken spin density
163

T. Kumakura et al.
ChemicalPhysicsLetters714(2019)160–165
Table 2
Observed and calculated wavenumbers (in cm−1) of 9H-fluorene in the T1 state
in the region between 590 and 3100 cm−1
.
Obsd.
Calcd.b
Sym. Assignmentc
−1
−1
a
−1
∼
∼
∼
Abs.
IR Int./
km
ν /cm
ν /cm
ν /cm
mol−1
592
681
768
769
853
41.3 593
595
679
756
760
858
30.1
103.2
7.1
4.5
10.7
B1
B1
B1
B2
B1
γ(C2,4,5,7-H)
γ(C(all)-H)
100
11.5
9.3
680
γ(C(all)-H)
β(aromatic ring)
γ(C1-H), γ(C8-H), CH2
rock.
10.4
910
11.5
896
3.8
B2
ν(C1-C2-C3),ν(C6-C7-
C8)
944
977
1045
1078
1220
1314
4.5
6.3
3.0
3.0
3.3
13.8
956
970
1042
1077
1219
1313
3.5
9.3
4.9
2.2
3.5
11.5
B1
B2
A1
A1
A1
A1
CH2 rock.
β(C1,4,5,8-H)
ν(C2-C3), ν(C6-C7)
β(C2-H), β(7-H)
β(C(1,4,5,8)-H)
ν(C1-C2-C3), ν(C6-C7-
C8), β(C2-H), β(C7-H)
ν(aromatic ring),
β(C2-H), β(C7-H)
β(H-C3-C4-H)
CH2 scissor.
ν(C3-C4), ν(C5-C6)
ν(C1-C9a), ν(8-C8a)
Combination
Combination
Combination
1338
7.1
1323
1.5
B2
1415
1424
1486
1531
1687
1718
1740
1854
2827
2832
2907
33.5
15.2
4.1
5.2
2.6
7.4
2.2
5.9
4.5
1411
1441
1471
1532
8.2
3.6
3.3
B2
A1
B2
B2
1476
18.2
Combination
Combination
Fig. 6. Numbering of carbon atoms and optimized structure of 9H-fluorene in
the T1 state. Blue and red lines represent longer and shorter bonds than the
corresponding values in the S0 state, respectively. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web
version of this article.)
2.6
13.8
2896
2918
3049
3050
3053
3054
3075
3082
31.4
9.1
7.6
16.2
5.4
19.1
23.1
42.8
A1
B1
ν
ν
sym(CH2)
asym(CH2)
3046
3066
3075
7.1
8.2
5.6
A1
A1
B2
ν(C1,3,4,5,6,8-H)
ν(C2,4,5,7-H)
ν(C2,7-H)
reproduced by the theory. Some bands appearing in the region between
1600 and 1900 cm−1 could be assigned to overtones or combination
modes relating to the intense 592 and 681 cm−1 bands.
a
Measured in polyethylene matrix at 77 K [20].
The optimized structure of 9H-fluorene in the T1 state is shown in
Fig. 6, which keeps C2v symmetry even in the T1 state like the S0 state.
As mentioned in the previous reports [16,17], the structural change in
the T1 state is similar to biphenyl, where the two benzene rings changes
to quasi quinoid rings. The lengthening and shortening bonds are co-
lored by blue and red lines, respectively, in Fig. 6. The bond lengths of
C1-C2 (1.433 Å), C2-C3 (1.424 Å) and C4-C4a (1.454 Å) in the T1 state
are longer than the corresponding values in the S0 state, 1.401, 1.401
and 1.398 Å, respectively. On the other hand, the bond lengths of C3-C4
(1.374 Å), C1-C9a (1.365 Å) and C4a-C4b (1.381 Å) in the T1 state are
shorter than 1.398, 1.392 and 1.470 Å in the S0 state, respectively.
Other optimized geometrical parameters are summarized in supporting
information.
b
Obtained at the B3LYP/6-31++G(d,p) level. The calculated wavenumbers
are adjusted by scaling factors of 0.98 below 2000 cm−1 and 0.96 above
2000 cm−1
Calculated wavenumbers withIR intensity less than 3.0 are
.
omitted. All calculation results are in supporting information.
c
Dominantly vibrational modes are shown in. Symbols of γ, β and ν re-
present out-of-plane bending, in-plane bending and stretching modes, respec-
tively. Numbering of atom is defined in Fig. 6.
DFT functional [26], and compared with the transient spectrum in the
region between 580 and 1780 cm−1 in Fig. 5. It is found that the si-
mulated spectrum satisfactorily reproduces the observed one. One may
concern the existence of 1H-fluorene in the T1 state. However, the
predicted spectrum, Fig. 5(c), poorly reproduces the experimental one,
Fig. 5(a). After UV (λ ≥ 275 nm) irradiation to yield 1H-fluorene as
much as possible, we tried to detect bands due to any transient species
of 1H-fluorene during light irradiation, but no transient bands were
obtained.
4. Summary
Photoreaction of 9H-fluorene isolated in an Ar matrix was in-
vestigated by FTIR spectroscopy with an aid of the DFT calculation.
Photoinduced isomerization from 9H-fluorene, which is the most stable
fluorene isomer, to 1H-fluorene, which is the third stable isomer, upon
UV irradiation (λ ≥ 295 nm) was found. The backward isomerization
also occurred upon λ ≥ 320 nm light irradiation, suggesting that the
both tautomers isomerize in photochemical equilibrium. The vibra-
tional assignments for 1H-fluoreneare performed for the first time, to
our knowledge. The time-dependent DFT calculation showed that 1H-
fluorene absorbs longer-wavelength light than 9H-fluorene, which was
consistent with our experimental results.
We have observed at least 25 bands of species in the T1 state besides
the three bands at 593, 680 and 1476 cm−1 reported by Krumschmidt
and Kryschi [20]. The observed and calculated wavenumbers are
summarized in Table 2 with their vibrational assignments, where the
theoretical wavenumbers are adjusted by scaling factors of 0.98 below
2000 cm−1 and 0.96 above 2000 cm−1. They are consistent within
17 cm−1 in the IR region below 1600 cm−1 each other, although the
relative absorbance of the bands at 944, 1424, 2832 and 2907 cm−1
,
which are assigned to the characteristic CH2 rocking (956 cm−1), CH2
scissoring (1441 cm−1), CH2 symmetric stretching (2896 cm−1) and
CH2 asymmetric stretching (2918 cm−1) modes, respectively, is not
An IR spectrum of 9H-fluorene in the lowest electronically excited
164

T. Kumakura et al.
ChemicalPhysicsLetters714(2019)160–165
triplet (T1) state was measured during UV irradiation, which was con-
firmed by the comparison between the observed spectrum and calcu-
lated spectral pattern obtained at the B3LYP/6-31++G(d,p) level. It
was found that the C-C bond length in the center five-membered ring is
shorter in the T1 state than in the S0 state, implying that π-conjugation
expands between the five-membered ring and benzene rings.
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calculations of naphthalene in the T1 state, J. Mol. Struct. 475 (1999) 253–260.
[22] M. Nakata, S. Kudoh, M. Takayanagi, T. Ishibashi, C. Kato, Lowest excited triplet
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Acknowledgements
[23] N. Akai, S. Kudoh, M. Nakata, Lowest excited triplet states of 1,2- and 1,4-dicya-
nobenzenes by low-temperature matrix-isolation infrared spectroscopy and density-
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This work was supported by JSPS Grant-in-Aid for Scientific
Research; Grant numbers 24550012 and K18 K05026.
Appendix A. Supplementary material
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structure of 2,3-dicyanonaphthalene in the T1 state, Chem. Phys. Lett. 392 (2004)
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Supplementary data to this article can be found online at https://
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naphthene in the lowest electronically excited triplet (T1) state, J. Mol. Struct. 1172
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