Synthesis and Photoisomerization of Substituted Dibenzofulvene Molecular Rotors

Article abstract of DOI:10.1002/chem.201600854

The synthesis, spectral and structural characterization, and photoisomerization of a family of 2-substituted dibenzofulvene molecular actuators based on (2,2,2-triphenylethylidene)fluorene (TEF) are reported. The 2-substituted species investigated are nitro (NTEF), cyano (CTEF), and iodo (ITEF). X-ray structures of these three compounds and three intermediates were determined to assign alkene configuration and investigate the effects of the 2-substituents on steric gearing. The addition–elimination reaction of Z-9 with trityl anion to form Z-10 proceeded with complete retention of configuration. Rates of photoisomerization were measured at irradiation wavelengths between 266–355 nm in acetonitrile/dioxane solutions at room temperature. Photoisomerization quantum yields (φ) were calculated by means of a mathematical model that accounts for a certain degree of photodecomposition in the cases of CTEF and ITEF. Quantum yields vary significantly with substituent, having maximum values of φ=0.26 for NTEF, 0.39 for CTEF, and 0.50 for ITEF. NTEF is photochemically robust and has a large quantum yield for photoisomerization in the near-UV, making it a particularly promising drive rotor moiety for light-powered molecular devices.

Full text of DOI:10.1002/chem.201600854

DOI: 10.1002/chem.201600854
Full Paper
&
Isomerization
Synthesis and Photoisomerization of Substituted Dibenzofulvene
Molecular Rotors
Stephanie C. Everhart,[a] Udaya K. Jayasundara,[a] HyunJong Kim,[a] Rolando Procfflpez-
Schtirbu,[a, b] Wayne A. Stanbery,[a] Clay H. Mishler,[a] Brian J. Frost,[a] Joseph I. Cline,[a] and
Thomas W. Bell*[a]
Abstract: The synthesis, spectral and structural characteriza-
tion, and photoisomerization of a family of 2-substituted di-
benzofulvene molecular actuators based on (2,2,2-triphe-
nylethylidene)fluorene (TEF) are reported. The 2-substituted
species investigated are nitro (NTEF), cyano (CTEF), and iodo
(ITEF). X-ray structures of these three compounds and three
intermediates were determined to assign alkene configura-
tion and investigate the effects of the 2-substituents on
steric gearing. The addition–elimination reaction of Z-9 with
trityl anion to form Z-10 proceeded with complete retention
of configuration. Rates of photoisomerization were mea-
sured at irradiation wavelengths between 266–355 nm in
acetonitrile/dioxane solutions at room temperature. Photo-
isomerization quantum yields (f) were calculated by means
of a mathematical model that accounts for a certain degree
of photodecomposition in the cases of CTEF and ITEF. Quan-
tum yields vary significantly with substituent, having maxi-
mum values of f=0.26 for NTEF, 0.39 for CTEF, and 0.50 for
ITEF. NTEF is photochemically robust and has a large quan-
tum yield for photoisomerization in the near-UV, making it
a particularly promising drive rotor moiety for light-powered
molecular devices.
Introduction
A prototype for well-understood, photoactivated molecular
devices is the cis–trans photoisomerization of stilbene, shown
in Figure 1. Stilbene offers high photoisomerization quantum
yields, typically ranging from 0.3–0.5.[8] However, a potential
Currently there is keen interest in developing molecular devi-
ces that function as switches, actuators, shuttles, and
motors.[1–3] A wide variety of actuation methods have been
demonstrated, including electrochemically[2,4] or thermally[5]
driven reactions, as well as photoactivated[1a,3,6,7] processes. In
many cases, two or more methods are combined to drive
a multistep process.[3g,l,6d,f,m,7] Photoactivated devices are of par-
ticular interest because they potentially offer a high degree of
positional control and much faster cycling rates than other
methods. Of particular importance in this regard is the work of
Feringa et al.[3a,d,7] who developed a diverse family of rotary
molecular motors that operate by photoactivation of an aro-
matic alkene and thermal relaxation of the resulting, strained
photoisomer.
Figure 1. Structures of stilbene, fulvene, and dibenzofulvene (9-methylene-
fluorene, with numbering system for positions of substituents).
practical difficulty in many applications is that the cis and trans
isomers of stilbene have significantly different geometries and
absorption spectra. For example, to achieve fast, continuous
rotary motion, two different excitation wavelengths would be
required. Herein we consider the family of substituted dibenzo-
fulvene actuators based on (2,2,2-triphenylethylidene)fluorene
(TEF) shown in Figure 2. In these molecules a triphenylmethane
(trityl) substituent is attached to the exocyclic C=C bond. This
unique structure is a prototype for designed molecular motors,
in which the dibenzofulvene rotor is sterically geared into
a chiral trityl stator to produce directional rotation about the
CꢀC bond between the rotor and stator.[1c] Photoisomerization
about the torsional angle q interconverts the E and Z isomers
shown in Figure 2 and serves to drive directional rotation
about the adjacent single bond in the molecular motor design.
[a] Dr. S. C. Everhart, Dr. U. K. Jayasundara, Dr. H. Kim, R. Procfflpez-Schtirbu,
Dr. W. A. Stanbery, C. H. Mishler, Prof. B. J. Frost, Prof. J. I. Cline,
Prof. T. W. Bell
Department of Chemistry and Program in Chemical Physics
University of Nevada, Reno NV 89557-0216 (USA)
E-mail: twb@unr.edu
[b] R. Procfflpez-Schtirbu
Coordinador Sección Química General, Escuela de Química
Universidad de Costa Rica, P.O. Box 11501-2060 (Costa Rica)
The ORCID identification number(s) for the author(s) of this article and Sup-
porting Information including a table summarizing X-ray crystallographic
data collection/solution/refinement parameters and synthetic procedures/
characterization data for compounds 7–9, Z-10, E-10, and Z-11 can be
found under: http://dx.doi.org/10.1002/chem.201600854.
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Figure 2. Structures of previously reported (TEF and TTEF) and new 2-substi-
tuted dibenzofulvene rotors (NTEF, CTEF, and ITEF). Note: polarized photo-
isomerization of NTEF in a polymer matrix has been reported.[9]
Scheme 1. Synthesis of E- and Z-TTEF.[11]
In 2-substituted TEF derivatives, the dibenzofulvene chromo-
phore is preserved in both stereoisomers, which accordingly
have very similar absorption spectra. The dibenzofulvene rotor
moiety also allows for rotational symmetry about its photoiso-
merizable exocyclic double bond, facilitating its incorporation
as a drive unit in rotary molecular devices. Theoretical studies
on the fulvene parent chromophore, shown in Figure 1, pre-
dicted an extremely low photoisomerization quantum yield.[10]
However, our previous experimental studies on the dibenzoful-
vene derivative TTEF, shown in Figure 2, revealed photoisome-
rization quantum yields of 0.04–0.09.[11] Dibenzofulvene moie-
ties have been used in many of Feringa’s second-generation
molecular motors,[3d,7d–o,q–s] but these chromophores bear some
resemblance to stilbene in that the photoactive C=C bond is
conjugated to arene rings at both ends. In contrast, the diben-
zofulvene chromophores in the 9-(2,2,2-triphenylethylidene)-
fluorenes shown in Figure 2 are not conjugated to the trityl
phenyl rings. Herein we show that substituents on the diben-
zofulvene chromophore can be used to tune both the absorp-
tion spectra and photoisomerization efficiencies of this family
of compounds for potential application in light-driven molecu-
lar devices and that NTEF is of particular interest for this pur-
pose.
triphenylethylidene)fluorene (ITEF), the structures of which are
shown in Figure 2. NTEF was selected for initial synthesis
because it was expected to have the longest wavelength
UV/Vis absorption. Unfortunately, the direct approach shown
in Scheme 1 could not be applied to the synthesis of NTEF
because 2-nitrofluorenone[12] was found to have very low solu-
bility in solvents suitable for the Wittig reaction step. There-
fore, the alternate synthetic approach shown in Scheme 2 was
pursued. This route involves four additional steps: reduction of
the nitro group, protection of the resulting amine as a 2,5-di-
methyl-1H-pyrrol-1-yl substituent, and later the deprotection
and oxidation to the nitro group. This seven-step synthesis is
considerably longer than the straightforward, three-step
approach (Scheme 1), but it also produces two additional di-
benzofulvenes (10 and 11) as potential candidates for photo-
isomerization studies.
Results
Synthesis
Current synthetic studies were modeled on the previously
reported synthesis of (E)- and (Z)-2-tert-butyl-9-(2,2,2-tripheny-
lethylidene)fluorene (TTEF) by the route shown in Scheme 1
(R=tBu).[11] Thus, 2-tert-butylfluorene 1 was converted into the
corresponding fluorenone 2 by aerobic oxidation in the pres-
ence of base. A Wittig reaction of 2 with bromomethylenetri-
phenylphosphorane gave the corresponding 9-bromomethy-
lene derivative 3 as a mixture of the two stereoisomers (E and
Z). Substitution of the bromide using an addition–elimination
reaction with trityllithium gave both isomers of the target
compound 4=TTEF, after separation by chromatography and
crystallization.
Scheme 2. Synthesis of Z-NTEF; a) Zn, CaCl2, EtOH, reflux; b) 2,5-hexane-
dione, benzene, AcOH, reflux; c) air, BnNMe3OH, pyridine; d) BrCH2PPh3Br/
NaHMDS, THF, ꢀ608C to rt; e) Ph3CLi, THF,ꢀ788C to rt; f) HONH3Cl, Et3N,
nPrOH, reflux; (g) NaBO3, AcOH, 608C.
The key target compounds of the current synthesis are
2-nitro-9-(2,2,2-triphenylethylidene)fluorene (NTEF), 2-cyano-9-
(2,2,2-triphenylethylidene)fluorene (CTEF), and 2-iodo-9-(2,2,2-
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Whereas 2-aminofluorene 6 is commercially available, it can
also be produced economically by the previously reported
reduction of 2-nitrofluorene 5, which was prepared on large
scale by the nitration of fluorene.[13] The amino group of 6 was
protected by Paal–Knorr condensation with 2,5-hexanedione,[14]
affording 7 in high yield. Oxidation of 7 to the corresponding
fluorenone 8 was carried out by treatment with air in the pres-
ence of base. A Wittig reaction of 8 with bromomethylenetri-
phenylphosphorane proceeded efficiently, from which 9-bro-
momethylene derivative 9 was isolated as a 5:4 (Z/E) mixture
by column chromatography. Fractional crystallization then
gave pure Z-9 in 54% yield. The configuration of this inter-
mediate was assigned by X-ray crystallography; the solid-state
structure of Z-9 is displayed in Figure 3. As observed for other
Table 1. Crystallographically determined bond angles of the photoactive
double bond in the dibenzofulvene rotors.[a]
Compound
a1
a2
a3
E-TTEF (mol 1)[c]
E-TTEF (mol 2)[c]
Z-TTEF (mol 1)[c]
Z-TTEF (mol 2)[c]
Z-NTEF
E-ITEF
Z-ITEF
Z-10
Z-11 (mol 1)
Z-11 (mol 2)
TEF (mol 1)[d]
TEF (mol 2)[d]
133.2(5)
133.6(5)
134.1(4)
130.8(3)
122.5(2)
131.4(1)
134.8(7)
131.7(1)
134.6(1)
134.2(1)
134.7(3)
133.5(3)
133.7(5)
134.8(5)
133.7(4)
132.6(3)
129.7(2)
132.8(1)
134.3(7)
133.3(1)
134.9(1)
133.7(1)
134.5(3)
134.3(3)
121.8(5)
120.8(5)
121.7(4)
122.5(3)
124.3(2)
122.1(1)
121.5(7)
121.8(1)
120.5(1)
121.1(1)
120.8(3)
121.1(3)
[a] Angles corresponding to C13-C14-C15 for a1, C14-C13-C12 for a2, and
C14-C13-C1 for a3. [b] DMP=2,5-dimethyl-1H-pyrrolyl. [c] mol=molecule;
Ref. [11]. [d] Ref. [15].
troscopy or gas chromatography. Contrary to the effect of
bromine in Z-9, the trityl group in Z-10 produces an upfield
shift of the 1H NMR resonance of the 1-position proton
(d=6.10 ppm) and a weaker shielding effect is observed for
the 3- and 4-position protons of the 2,5-dimethyl-1H-pyrrol-1-
yl group. Attempts to isolate E-10 from a reaction of E/Z-9
with trityllithium gave a sample of E-10 that was pure enough
for spectroscopic comparison with the Z isomer.
As the protected intermediate was found to have very low
solubility in ethanol,[14] even at reflux, the deprotection of the
amino group in Z-10 was accomplished by reaction of the 2,5-
dimethylpyrrole moiety with hydroxylamine hydrochloride in
hot n-propanol. From this, amino derivative Z-11 was obtained
in good yield, and an X-ray structure again confirmed the
double bond configuration (see Figure 3 and Table 1). Finally,
the amino group of Z-11 was cleanly oxidized to nitro using
sodium perborate in hot acetic acid.[16] This route gave pure
samples of nitro target compound Z-NTEF for photoisomeriza-
tion studies, and full details of the molecular structure were
revealed by X-ray crystallography (see Figure 3 and Table 1).
The synthetic route shown in Scheme 2 gave pure Z-NTEF,
but it did not readily produce pure samples of E-NTEF because
of separation difficulties. So, the alternate route displayed in
Scheme 3 was devised. Not only is this a shorter approach to
the nitro derivatives (NTEF), it was also designed to produce
samples of the E and Z isomers of the target CTEF and ITEF
analogues. Iodoarenes are readily converted into nitriles and
amines, and the Z isomer of amine 11 was easily oxidized to
Z-NTEF using the synthesis shown in Scheme 2.
Figure 3. Solid-state structures of synthetic intermediates Z-9, Z-10, and
Z-11, and target compounds Z-NTEF, E-ITEF, and Z-ITEF.
9-bromomethylenefluorenes,[11,15] steric compression by the
1
bromine atom causes the H NMR resonance of the aromatic
proton in the 1-position of the fluorene ring system to shift
downfield (d=8.43 ppm for Z-9). The remaining 1H NMR
signals of Z-9 were assigned by 2D-NMR spectroscopy.
Isolation and unambiguous assignment of the configuration
of Z-9 provided an opportunity to probe the stereoselectivity
of the subsequent addition–elimination reaction. A reaction of
Z-9 with trityllithium in THF gave only Z-10, as determined by
X-ray crystallography (Figure 3; Table 1). None of the E isomer
could be detected in the crude product by either NMR spec-
The starting material for the synthesis shown in Scheme 3,
2-iodofluorene 12, was prepared in 77% yield by iodination of
fluorene in acetic acid.[17] The corresponding fluorenone 13[18]
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Scheme 3. Synthesis of E-NTEF; a) air, BnNMe3OH, pyridine; b) BrCH2PPh3Br/
NaHMDS, THF, ꢀ608C to rt; c) Ph3CLi, THF, ꢀ788C to rt; d) CuCN, DMF,
reflux; (e) Ph2C=NH, Pd2(dba)3, BINAP (2,2’-bis(diphenylphosphino)-1,1’-bi-
naphthyl), NaOtBu, [18]crown-6; f) NaBO3, AcOH, 608C.
was then obtained from 12 by air oxidation.[19] A Wittig reac-
tion of 13 gave bromomethylene derivative 14 as a mixture of
E and Z isomers and a reaction of this mixture with trityllithium
gave target compound ITEF. Pure samples of E-ITEF and Z-ITEF
were then obtained by crystallization of the resulting mixture
of stereoisomers and unambiguously identified by X-ray crys-
tallography (see Figure 3 and Table 1). The E/Z mixture was
converted into CTEF by a Rosenmund–von Braun reaction[20]
with copper cyanide in DMF[21] and both E-CTEF and Z-CTEF
were isolated for photoisomerization studies. These stereoiso-
mers were assigned by means of the observed shielding effect
of the trityl group on the NMR resonances of the adjacent pro-
tons on the fluorene ring system. To obtain E-NTEF, ITEF was
converted as a mixture of stereoisomers to the corresponding
amine 11 by Pd-catalyzed coupling with benzophenone imine,
followed by hydrolysis.[22] Pure E-11 was obtained by crystalli-
zation of the resulting E/Z mixture, then it was oxidized to the
desired E-NTEF target compound. This completed preparation
of unambiguously assigned samples of both stereoisomers of
all three target compounds, namely NTEF, CTEF, and ITEF.
Figure 4. Absorption spectra of E (blue trace) and Z (red trace) isomers of
NTEF, CTEF, and ITEF in 15:85 (v/v) 1,4-dioxane/acetonitrile.
studies were performed in the current work. Neither NTEF nor
ITEF in solution showed signs of thermal isomerization at room
temperature over the course of months. Solutions of CTEF
showed evidence of isomerization at room temperature that
became detectable after 1–2 weeks. Crystalline CTEF showed
no evidence of thermal isomerization at room temperature.
Compounds in solution were irradiated at various wave-
lengths in the UV region of 266–390 nm. Over the course of
irradiation, 40 mL aliquots of the sample were extracted at time
intervals to follow the progress of photoisomerization by
means of chromatographic analysis, using either GC-MS or
HPLC. Figure 5 shows typical chromatograms of aliquots
removed from rotor molecule solutions at selected times
during their irradiation. For the experiments shown in Figure 5,
the initial solutions contained only the Z isomer of the diben-
zofulvene rotor molecule. As the UV irradiation progresses, the
Z isomer peak decreases and the peak corresponding to the E
isomer increases. Dividing the peak area of a given isomer by
the sum of the total integrated signals gives the mole fractions
fE and fZ describing the isomeric distribution of the rotor mole-
cule in solution. Figure 6 shows the evolution of these mole
fractions as a function of the average number of absorbed
photons per rotor molecule for experiments starting with both
pure E and pure Z isomers. (The average number of absorbed
photons is proportional to time as described in Equation (3)
below.) Note that fE and fZ reach the same photostationary
ratios at long irradiation times, regardless of the choice of
starting stereoisomer.
Photoisomerization
Our previous studies of the photoisomerization of TTEF were
performed in acetonitrile. A mixed solvent of 15% 1,4-dioxane
and 85% acetonitrile (v/v) was used in the current studies
investigating NTEF, CTEF, and ITEF. The 1,4-dioxane cosolvent
was used to increase the solubility of these compounds in
acetonitrile to allow for comparison with our previous experi-
ments on TTEF under similar solvent conditions. Figure 4
shows the near-UV molar absorptivities of the E and Z isomers
of NTEF, CTEF, and ITEF in the mixed solvents obtained from
a series of absorption spectra measured at concentrations
between 10ꢀ6 and 10ꢀ5 m.
Thermal isomerization data was previously reported for TTEF
at elevated temperatures;[11] however, no thermal isomerization
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Figure 6. Photoisomerization progress quantified by isomer fraction fE and fZ
for NTEF, CTEF, and ITEF during 355 nm irradiation, plotted as a function of
the average number of photons absorbed per rotor molecule. Photodecom-
Figure 5. Chromatograms showing isomerization progress of NTEF (top
panel) during 266 nm irradiation as measured by GC-MS, and for CTEF
(middle panel) and ITEF (bottom panel) during 310 nm irradiation as mea-
sured by HPLC. HPLC chromatograms were detected by UV absorption at
wavelengths where both isomers have equal absorptivity. In each case the
initial solution contained only the Z isomer, and individual chromatograms
are labeled according to the irradiation time (in units of average photons
absorbed per rotor molecule; see Equation (3)). Peaks assigned to the rotor
molecule isomers Z and E are labeled. The peaks labeled D are assigned to
unidentified photodecomposition products.
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position is ignored in this analysis. The solid traces and symbols and
correspond to experiments in which only the Z isomer was initially present.
The dashed traces and crossed symbols ꢂ and correspond to experiments
„
in which only the E isomer was initially present. The solid and dashed traces
are obtained by least squares optimization of fEZ and fZE in a fit to the kinet-
ic model represented by Equations (1), (2), and (4).
photons absorbed per molecule, x(t). For an optically dense
sample, in which all photons are absorbed, it is:[11]
Photoisomerization kinetic model
vIEIlIt
hc
ðCVNAÞꢀ1
ð3Þ
xðtÞ ¼
The photoisomerization reaction in this family of rotor mole-
cules can be expressed as:
where ll, El, ul, h, and c are the laser wavelength, pulse energy,
laser pulse repetition frequency, Planck’s constant, and the
speed of light, respectively. The number of rotor molecules in
the sample solution is CVNA, where C is the solution concentra-
tion, V is the volume (taking into account the volume decrease
due to the removal of aliquots for chromatographic analysis),
and NA is Avogadro’s number. Equation (2), expressed in terms
of x and the isomer mole fractions fE and fZ, becomes:
kEZ
E
Z
H
ð1Þ
G
kZE
where kEZ and kZE are the effective rate constants that depend
upon the irradiation intensity.
The corresponding rate equation is:
d½Eꢁ
ð2Þ
¼ kZE½Zꢁ ꢀ kEZ ½Eꢁ
dfE ꢀZEeZfZ ꢀ ꢀEZ eEfE
dt
ð4Þ
¼
dx
eZfZ þ eEfE
The rate constants kEZ and kZE include the photon flux, absorp-
tion cross section of the molecule, and the photoisomerization
quantum yield. To express Equation (2) in a way that is more
directly related to the photoisomerization quantum yields, we
transform the irradiation time, t, into the average number of
where eE and eZ are the measured molar absorptivities of the E
and Z isomer, respectively, at the irradiation wavelengths
shown in Table 2; fEZ and fZE are the absolute photoisomeriza-
tion quantum yields for E!Z and Z!E, respectively. Kinetic
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dfZ ꢀEZ eEfE ꢀ ꢀZEeZfZ ꢀ ꢀDeZfZ
Table 2. Molar absorptivities, photostationary isomeric ratios fE:fZ (t=1),
and fitted photoisomerization quantum yields for rotor molecules at irra-
diation wavelengths l.[a]
ð9Þ
¼
dx
eZfZ þ eEfE þ eDfD
[c]
Cmpnd
TTEF[b]
l
eE[c]
eZ
fE:fZ (t=1) fEZ
fZE
dfD
dx
2ꢀDðeEfE þ eZfZ Þ
eZfZ þ eEfE þ eDfD
[nm]
ð10Þ
¼
266
280
320
–
–
–
–
–
–
0.59:0.41
0.44:0.55
0.56:0.43
0.38:0.62
0.38:0.62
0.40:0.60
0.33:0.67
0.48:0.52
0.53:0.47
0.47:0.53
0.62:0.38
0.49:0.51
0.52:0.48
0.52:0.48
0.07ꢃ0.05 0.10ꢃ0.02
0.05ꢃ0.01 0.04ꢃ0.01
0.07ꢃ0.01 0.09ꢃ0.01
0.25ꢃ0.05 0.17ꢃ0.01
0.26ꢃ0.03 0.17ꢃ0.03
0.31ꢃ0.03 0.21ꢃ0.03
0.21ꢃ0.05 0.12ꢃ0.01
0.21ꢃ0.03 0.20ꢃ0.03
0.35ꢃ0.02 0.42ꢃ0.03
0.34ꢃ0.02 0.29ꢃ0.02
0.31ꢃ0.03 0.33ꢃ0.03
0.50ꢃ0.04 0.46ꢃ0.04
0.49ꢃ0.02 0.49ꢃ0.02
0.55ꢃ0.04 0.45ꢃ0.04
where fD is the photodecomposition quantum yield and eD is
the molar absorptivity of the decomposition product D. The
quantity fD represents the mole fraction of D such that fE +fZ +
fD =1; fD was measured experimentally as the sum of the areas
of all decomposition chromatogram peaks (see Figure 5) divi-
ded by the sum of all chromatogram peaks D, Z, and E. The
detection wavelength for the HPLC chromatographic analysis
was chosen such that both E and Z isomers have the same ab-
sorptivity at that wavelength; however, the absorption spec-
trum of the decomposition product is unknown. Therefore, the
accuracy of fD is dependent upon the relation between the
molar absorptivities of the photoproducts, eD and the values eE
and eZ at the HPLC detection wavelength.
NTEF
266 17200 17700
290 29400 30500
340 23500 23800
355 17700 18100
390
3600
5400
CTEF
ITEF
266 28100 28100
310 16400 14200
355
1440
1550
266 28200 28600
310 15300 16200
355
480
640
[a] Quantum yields are obtained by fits to experimental kinetics data
using the kinetic model represented by Equations (1), (2), and (4), in
which photodecomposition is ignored. [b] Isomer fractions fE and fz deter-
mined by 1H NMR spectroscopy.[11][c] [Lmolꢀ1 cmꢀ1]. For TTEF, molar ab-
sorbtivities reported for lmax values.[11]
In kinetic modeling that accounts for photodecomposition,
the value of eD was arbitrarily chosen to be equal to the aver-
age of eE and eZ at both the photoisomerization and HPLC
modeling of forward and reverse reactions of the measured
E!Z photoisomerization data is accomplished by numerical
integration of Equation (4) to obtain fE(x) with the constraint
that fZ(x)=1ꢀfE(x). Optimization of fEZ and fZE in a fit of fE(x) to
the experimental data by nonlinear least-squares fitting gives
the traces shown in Figure 6, and the corresponding quantum
yields are reported in Table 2.
Irradiation of CTEF and ITEF gave rise to minor photo-
products that appear at shorter elution times in the chromato-
grams in Figure 5. In the case of CTEF, the photoproducts
appear as two peaks. The ratio of the product peaks areas in
relation to each other as a function of irradiation time shows
a trend similar to that of the E and Z isomers of the parent
CTEF. Irradiation of ITEF led to the development of one major
and numerous minor photodecomposition peaks. Given their
tiny quantities, no attempt was made to isolate or characterize
these photoproducts. In an attempt to account for photo-
decomposition mathematically, Equation (1) was modified by
introducing a photodecomposition product, D, into the previ-
ous kinetic model:
ꢀEZ
E
E
Z
Z
H
ð5Þ
ð6Þ
ð7Þ
G
ꢀZE
ꢀD
D
D
ƒ!
ꢀD
ƒ!
Figure 7. Photoisomerization progress quantified by isomer fraction fE and fZ
for irradiation of NTEF (355 nm), CTEF (266 nm), and ITEF (266 nm), plotted
as a function of the average number of photons absorbed per rotor mole-
cule. Photodecomposition is accounted for in this analysis. The solid traces
Equation (4) now becomes:
&
*
and symbols and correspond to experiments in which only the Z isomer
was initially present. The dashed traces and crossed symbols ꢂ and corre-
„
spond to experiments in which only the E isomer was initially present. The
solid and dashed traces are obtained by least squares optimization of fEZ,
fZE, and fD in a fit to the kinetic model represented by Equations (5)–(10).
dfE ꢀZEeZfZ ꢀ ꢀEZ eEfE ꢀ ꢀDeEfE
ð8Þ
¼
dx
eZ fZ þ eEfE þ eDfD
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detection wavelengths. Equations (8), (9), and (10) were inte-
grated numerically and fEZ, fZE, and fD were optimized in a
simultaneous fit to the measured fE and fZ, shown in Figure 7.
The decomposition is manifest in the negative slope at long
irradiation times of what was previously represented as a pho-
tostationary plateau in Figure 6. This photodecomposition de-
cline in both fE and fZ is most noticeable for ITEF. There was no
measurable decomposition for NTEF. Again, the final values of
fE and fZ are the same, regardless of the identity of the starting
isomer. Table 3 gives the optimized values of fEZ, fZE, and fD
X-ray structures
The solid-state structures of the new compounds were ana-
lyzed carefully to understand the effects of substituents on
bond angles and steric gearing between the trityl group and
the dibenzofulvene rotor, which is relevant to possible use of
substituted dibenzofulvenes in designed molecular motors.[1c]
Table 1 gives selected bond angle data for the crystallographi-
cally determined structures of three of the key target com-
pounds, the Z nitro compound (Z-NTEF) and both stereoiso-
mers of the iodo analogue (E-ITEF and Z-ITEF). Also provided in
Table 1 are corresponding data for intermediates Z-10, and
Z-11, as well as for the unsubstituted parent compound TEF
(9-(2,2,2-triphenylethylidene)fluorene).[15]
Table 3. Optimized photoisomerization and photodecomposition quan-
tum yields of CTEF and ITEF for the model described by Equations (5)–
(10) accounting for photodecomposition.
Not only do the crystal structures of these compounds,
along with other spectroscopic data, unambiguously prove the
E/Z configurations of the compounds used in photoisomeriza-
tion experiments, they also show the effects of substituents on
ground-state structures. Of particular interest are the bond
angles about the photoactive, exocyclic C=C double bond. The
previously reported structure of TEF[15] showed an unusual dis-
tribution of strain for an overcrowded alkene. No significant
twisting of the C=C double bond was observed. Instead, steric
gearing between the trityl group and the fluorene moiety
causes in-plane distortion, as shown by bond angles a1, a2,
and a3 listed in Table 1. In particular, bond angles a1 and a2
are 133–1358; a3, corresponding to a2, but on the side oppo-
site the trityl group, is only 121–1228 (range for the two sym-
metry-nonequivalent molecules, denoted mol 1 and mol 2, in
the asymmetric unit).
Compound
CTEF
l [nm]
fEZ
fZE
fD ”102
266
310
355
266
310
355
0.40ꢃ0.01
0.31ꢃ0.01
0.37ꢃ0.04
0.48ꢃ0.06
0.49ꢃ0.01
0.55ꢃ0.09
0.45ꢃ0.01
0.32ꢃ0.01
0.40ꢃ0.04
0.44ꢃ0.06
0.50ꢃ0.01
0.45ꢃ0.07
0.32ꢃ0.03
0.189ꢃ0.005
0.180ꢃ0.001
4.5ꢃ0.6
ITEF
0.98ꢃ0.02
0.13ꢃ0.09
after decomposition has been taken into account, and can be
compared to the quantum yields in Table 2, which ignore
photodecomposition. With the exception of ITEF at 266 nm
excitation, fD is two orders of magnitude smaller than fEZ and
fZE. The inclusion of the additional photodecomposition quan-
tum yield fD into the kinetic model resulted in fitted photoiso-
merization quantum yields fEZ and fZE for ITEF to increase by
no more than 0.02, and those for CTEF to increase an average
of 0.05.
The previously reported structures of (E)- and (Z)-2-tert-
butyl-9-(2,2,2-triphenylethylidene)fluorene (TTEF)[11] show no
significant changes in C=C double bond angles relative to the
unsubstituted parent compound TEF, as seen in Table 1. Angles
a1 and a2 on the same side as the trityl group range from
131–1358 and a3 is 121–1238. This indicates that there is no
significant steric repulsion between the bulky trityl and tert-
butyl groups. The corresponding bond angles fall in the same
ranges for the 2-iodo (E- and Z-ITEF), 2-(2,5-dimethyl-1H-pyrrol-
1-yl) 10, and 2-amino 11 analogues, as also shown in Table 1.
The exception is the 2-nitro analogue Z-NTEF, which has
decreased bond angles for a1 and a2 (both ca. 1238) and an
increased a3 angle (1248). This may be due either an electronic
perturbation of sp2 carbon geometry in the dibenzofulvene
p-system, or to an attractive, dipole-induced dipole interaction
between the nitroaromatic dipole and the trityl group. Not
shown Table 1 are the corresponding double bond angles for
9-bromomethylene compound Z-14 (a1 =1258, a2 =1308, a3 =
1228), that suggest decreased steric interaction between the
bromo group and the fluorene moiety, relative to the corre-
sponding trityl–fluorene interactions in all the other structures
discussed herein.
Discussion
Synthesis
The key target molecule NTEF was synthesized by the two dif-
ferent routes shown in Scheme 2 and 3. In principle, both
routes produce both the E and Z stereoisomers, but for practi-
cal reasons involving separation procedures (crystallization and
chromatography), the route in Scheme 2 more readily yielded
Z-NTEF, whereas the shorter route in Scheme 3 fortuitously
gave more E-NTEF. Iodo target compound ITEF, an intermediate
in Scheme 3, was also converted into cyano target compound
CTEF in one step and good yield.
An important finding of these synthetic studies is that
nucleophilic substitution of bromine in 9-bromomethylene-
fluorenes by trityllithium is highly stereoselective and gives
complete retention of configuration, as demonstrated clearly
in the conversion of Z-9 into Z-10 (Scheme 2). This is consis-
tent with rate-determining attack by trityl anion to produce
the intermediate fluorenyl anion, that very rapidly eliminates
bromide to form Z-10. The retention product is formed via the
least-motion geometry for the fast elimination step in this
addition–elimination (add–E1) process.[23]
Photoisomerization
Photoisomerization quantum yields for the well-studied
stilbene molecule are relatively independent of substitution,
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varying only by about ten% from typical values of fEZ ꢄ0.5
and fZE ꢄ0.3.[8] For the dibenzofulvene derivatives studied
herein, changing the substituent has a larger effect on the
photoisomerization efficiency, as seen in Table 2. The com-
pounds show a wide range of quantum yields with TTEF
having a photoisomerization quantum yield of about 0.07[11]
and increasing in the order of TTEF <NTEF <CTEF <ITEF,
where ITEF has a quantum yield of about 0.50. There is no con-
sistent wavelength dependence of the quantum yields for
lex > 266 nm. The maximum quantum yields (average of fEZ
and fZE) are 0.26 for NTEF at 340 nm, 0.43 for CTEF at 266 nm
(corrected for photodecomposition), and 0.50 for ITEF at
355 nm. NTEF favors the Z isomer in the photostationary state
at all the excitation wavelengths investigated, whereas the
others show nearly a 1:1 photostationary isomer ratio.
tion quantum yield will be small. Bearpark et al.[10] performed
S1 excited state dynamics calculations on fulvene, that predict-
ed a very low photoisomerization quantum yield, likely meas-
urable only upon excitation at the 0–0 vibrational origin.
Subsequent to our earlier experimental study that showed
significant quantum yields for the dibenzofulvene derivative
TTEF,[11] S1–S0 transitions in fulvene were examined in greater
detail by Bearpark[24] and Sumita and Saito.[25] Although
Figure 8 depicts the potential energy surfaces only along the
exocyclic double bond torsion angle q, the S1/S0 conical inter-
section is actually a high-dimensional space in the nuclear co-
ordinates. Theoretical studies by Bearpark examined the multi-
dimensional surfaces for fulvene S1 and S0 to determine the ful-
vene S1/S0 surface crossing dynamics.[24] Figure 1 of refer-
ence [24a] and Figure 2(b) of reference [24c] depict the S1 and
S0 surfaces along the nuclear coordinates of the exocyclic
double bond torsion angle, q, and the “bond inversion” or
“delocalization” coordinate in which the single and double
bond character of the CꢀC bonds of fulvene is interchanged.
Reference [24a] shows that the q-torsion and bond inversion
coordinates are the two involved in the S1–S0 conical intersec-
tion. The global energy minimum in the surface crossing seam
for fulvene is located at q=638,[24,25] so that crossing to the S0
surface at this point results in a return to the original configu-
ration at q=08,[25] with no isomerization. Trajectory calculations
on the fulvene S1 and S0 surfaces performed by Sumita and
Saito show that crossing between 808 <q <08 is required for
relaxation along the S0 surface to result in complete 1808 rota-
tion about the double bond.[25] For fulvene, the topography of
the S1 and S0 surfaces leads to a lowest energy S1/S0 curve
crossing at q=638, making photoisomerization inefficient.
As our previous studies of a dibenzofulvene derivative
showed significant photoisomerization quantum yields,[11]
Sumita and Saito theorized that the addition of benzene rings
to fulvene may affect the S1/S0 degeneracy space in such
a way as to favor surface crossing at q closer to 908 before
relaxing along the S0 surface.[25] The addition of the benzene
moieties to fulvene is at the location of two CꢀC bonds
involved in the bond inversion coordinate. It is reasonable to
expect that addition of the highly aromatic benzene moieties
to form dibenzofulvene will cause motion along the bond
inversion coordinate to be more energetically costly. This
modification of the fulvene surfaces could shift the energetical-
ly accessible region of the conical intersection seam to angles
closer to 908, promoting relaxation dynamics that successfully
achieve isomerization.
A rigorous theoretical understanding of the mechanism of
photoisomerization for the compounds studied herein is
beyond the scope of this study; here we speculate on impor-
tant key processes. As a reference point for this discussion,
Figure 8 provides an idealized, schematic representation of the
Figure 8. Schematic singlet potential energy surfaces plotted along the q
photoisomerization coordinate (defined in Figure 2). Photoisomerization via
the singlet mechanism occurs by 1) excitation from the ground singlet state,
So, to the first excited singlet state, S1, 2) structural relaxation on the S1 sur-
face, followed by intersystem crossing to the So surface at the conical inter-
section, and relaxation on the So surface either 3a) returning to the initial
isomer with no net change in q or 3b) successful photoisomerization with q
changing by 1808.
potential energy surfaces likely to be important in the photo-
isomerization dynamics. Figure 8 depicts the energy along the
q torsion coordinate defined in Figure 2, which is the reaction
coordinate for photoisomerization. The qualitative surfaces in
Figure 8 are analogous to those believed important in stilbene,
and extending the stilbene photoisomerization picture to di-
benzofulvene, the singlet mechanism consists of excitation
from the ground singlet state, S0, to the first excited singlet
state, S1, followed by structural relaxation along q on S1. The
system crosses to the S0 surface at a conical intersection at
some critical torsion angle (shown as qꢄ908 in Figure 8). Sub-
sequent structural relaxation on the S0 surface results in either
a return to the original configuration at q=0, or to q=1808
for a successful isomerization event. Within this dynamical pic-
ture, if the minimum energy conical intersection of S1 and So
occurs at q significantly less than 908, or if the S1 excited state
dynamics do not efficiently convey the system to a conical in-
tersection configuration near q=908, then the photoisomeriza-
Our experimentally measured photoisomerization quantum
yield increases in the order TTEF <NTEF <CTEF <ITEF, indicat-
ing that the substituent identity also affects the location of the
conical intersections of the S1 and S0 surfaces. The triplet
mechanism for the photoisomerization of dibenzofulvene, or
even fulvene, is not as well understood as the singlet mecha-
nism, and therefore not much can be conjectured herein. That
ITEF shows the highest photoisomerization quantum yields
measured in this family (fEZ ꢄfZE ꢄ0.5) suggests the iodo sub-
stituent could enable access to triplet surface photoisomeriza-
tion pathways that augment those available in the singlet
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metry (MALDI-MS) recorded with no matrix and analyzed by time
of flight (TOF), or electrospray mass spectrometry (ESI-MS) with
positive ion detection. Samples for microanalysis were dried under
vacuum (0.5–1.0 mm) at 65–808C for 48–72 h.
mechanism. Considering that the photodecomposition quan-
tum yield is also increased for ITEF, the mechanisms made
available by the iodo substituent may include other pathways
that lead to photodecomposition rather than photoisomeriza-
tion. The photolability of iodine in iodoaryl compounds is well
known.[26] Whereas the photoisomerization quantum yields for
CTEF and ITEF are larger than for NTEF, the latter is deemed
most useful for practical applications. NTEF absorbs more
strongly in the near-UV, has a substantial photoisomerization
quantum yield (f =0.1–0.3), and no photodecomposition was
detected at the wavelengths studied. NTEF has also been
shown to undergo angular distribution hole-burning in a thin
polymer matrix when irradiated with polarized laser light.[9]
(Z)-2-Nitro-9-(2,2,2-triphenylethylidene)fluorene (Z-NTEF):[16]
A
solution of NaBO3·4H2O (0.55 g, 3.59 mmol) in AcOH, (15 mL) was
stirred at 608C and a hot solution of Z-11 (0.30 g, 0.69 mmol) in
AcOH (20 mL) was added dropwise over 1.5 h. The reaction mix-
ture was stirred for 2 h at 608C then concentrated to dryness by
rotary evaporation. The residue was partitioned between 15 mL of
CH2Cl2 and 50 mL of H2O, then the aqueous layer was separated
and extracted with CH2Cl2 (3”10 mL). The combined organic solu-
tions were dried over Na2SO4 and concentrated by rotary evapora-
tion. The resulting brown solid was dried in vacuo, then recrystal-
lized from a (1:1:1, v/v/v) mixture of EtOH/cyclohexane/CH2Cl2
(20 mL). Filtration and drying at ca. 0.1 mm over P2O5 gave Z-NTEF
(0.21 g, 65%) as yellow–brown crystals, m.p.> 3008C (dec).
1H NMR (300 MHz, CD2Cl2): d=8.06 (dd, 3J(H,H)=8.4, 4J(H,H)=
1.8 Hz, 1H, H3), 8.03 (s, 1H, C=CH), 7.87 (d, 3J(H,H)=8.0 Hz, 1H,
Conclusion
We have synthesized and studied a family of dibenzofulvene
compounds that efficiently function as light-driven molecular
3
3
H8), 7.80 (d, J(H,H)=8.0 Hz, 1H, H5), 7.75 (d, J(H,H)=8.4 Hz, 1H,
4
1
H4), 7.45 (m, 2H, H6,7), 7.37 (d, J(H,H)=1.1 Hz, 1H, H1), 7.28 ppm
actuators. X-ray structures and H NMR data have been used to
(m, 15H, ArH); 13C NMR (75 MHz, CD2Cl2): d=146.9, 143.0, 130.7,
130.6, 129.1, 128.9, 128.8, 127.4, 124.0, 121.4, 121.0, 119.7 ppm;
GC-MS: m/z (%) 465 (59) [M+], 388 (11), 370 (16), 341 (43), 270 (20),
252 (28), 207 (16), 165 (100); IR (KBr): Ç=3052, 1597, 1518, 1335,
701 cmꢀ1; UV/Vis (CH3CN): lmax(e)=296 nm (1,500 molꢀ1 m3 cmꢀ1);
fluorescence (CH3CN, lex338): lem =376, 392 nm; elemental analysis
calcd (%) for C39H31NO2·H2O: C 84.32, H 5.04, N 2.98; found: C 84.27,
H 5.32, N 2.94.
assign all alkene configurations of intermediates and products.
The X-ray data show small geometric effects of 2-substituents,
suggesting negligible differences in steric gearing between the
trityl group and the neighboring aromatic ring. The addition–
elimination reaction of Z-9 with the trityl anion to form Z-10
has been shown to proceed with complete retention of config-
uration, consistent with a least-motion reaction mechanism.
Rates of photoisomerization were measured at irradiation
(E/Z)-9-Bromomethylene-2-iodofluorene (E/Z-14):[27] Bromo-
wavelengths between 266–355 nm in acetonitrile/dioxane sol- methyltriphenylphosphonium bromide (17.8 g, 40.7 mmol) was
dried by gentle heating at ca. 0.1 mm Hg. The flask was cooled
under a nitrogen atmosphere, anhydrous THF (200 mL) was added,
and the resulting suspension was cooled to ꢀ608C (CHCl3/dry-ice
bath) and stirred for 10 min before a 2m solution of NaHMDS in
THF (20 mL, 40 mmol) was added. After 50 min a solution of 2-io-
dofluorenone 13 (10.4 g, 34 mmol)[18] in anhydrous THF (100 mL)
was added. The reaction mixture was allowed to warm to room
temperature overnight then water (500 mL) was added. The result-
ing mixture was extracted with CH2Cl2 (2”500 mL) then the com-
bined organic solutions were dried over Na2SO4. Rotary evapora-
tion gave a brown solid (13 g) consisting of a 1.3:1 mixture of Z-14
utions at room temperature. Photoisomerization quantum
yields f were calculated by means of a mathematical model
that accounts for a small degree of photodecomposition in the
cases of CTEF and ITEF. The isomerization quantum yield,
absorption spectrum, and photodurability of the compound
are dependent on the substituent. While ITEF gives the largest
f, it also displays the highest rate of photodecomposition. The
compound NTEF demonstrates high photostability, a broad ab-
sorption spectrum encompassing the near-visible region, and
photoisomerization quantum yields in the range of 0.1–0.3,
making it an exceptional candidate as a light-powered molecu-
lar actuator. With appropriate functionalization, these mole-
cules could be readily incorporated into molecular devices and
motors as drive and control elements.
1
and E-14 (by H NMR analysis). Purification by flash chromatogra-
phy, eluting with 7:3 (v/v) hexane/CH2Cl2, gave 1:1 E-14/Z-14 (by
1H NMR analysis) as light yellow crystals (7.8 g, 61%), m.p. 85–
878C. 1H NMR (300 MHz, CDCl3): d=8.91 (s, 0.5H, Z-H1), 8.56 (d,
3J(H,H)=7.3 Hz, 0.5H, E-H8) ), 7.3–7.9 ppm (m, 7H, C=CH, ArH) ;
13C NMR (126 MHz, CDCl3): d=138.2, 138.1, 137.9, 137.3, 134.5,
129.6, 129.4, 128.8, 127.80, 127.76, 125.7, 121.4, 121.3, 120.1, 119.9,
107.1, 107.0, 92.0 ppm; GC-MS: m/z (%) 384 (100) [M+2], 382 (89)
[M+], 257 (3), 255 (3), 176 (43); elemental analysis calcd (%) for
C14H8BrI: C 43.90, H 2.10; found: C 44.21, H 2.48.
Experimental Section
Synthesis and characterization
General methods: All reactions were carried out under dry nitro- (E)- and (Z)-2-Iodo-9-(2,2,2-triphenylethylidene)fluorene (E-ITEF
gen using commercially available, high-purity solvents, unless and Z-ITEF): Triphenylmethane (6.81 g, 22.5 mmol) was dried by
noted otherwise. Anhydrous THF and toluene were obtained by gentle heating at ca. 0.1 mm Hg. The flask was cooled under a ni-
distillation from sodium benzophenone ketyl. Pyridine was heated trogen atmosphere, anhydrous THF (200 mL) was added, and the
at reflux over CaH2, distilled, and stored over 4 Š molecular sieves resulting suspension was cooled to ꢀ788C. After 5 min, a 1.6m so-
under nitrogen. Flash chromatography was conducted on alumina lution of n-butyllithium in hexane (12 mL, 19 mmol) was added.
(80–200 mesh). NMR spectra were recorded with TMS as an inter- The resulting solution was stirred for 5 min, the cooling bath was
nal reference. Melting points were measured in open glass capillary replaced with an ice bath, the reaction mixture was stirred at 08C
tubes and are uncorrected. Mass spectra were recorded by gas for 80 min, then the cooling bath was replaced with a dry ice/ace-
chromatography-mass spectrometry (GC-MS) with electron impact tone bath. A solution of E/Z-14 (7.12 g, 18.6 mmol) in anhydrous
ionization (70 eV), matrix assisted laser desorption mass spectro- THF (20 mL) was added to the stirred solution. The reaction mix-
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ture was allowed to warm to room temperature overnight then
water (400 mL) was added. The mixture was extracted with
dichloromethane (2”300 mL). The combined organic solutions
were dried (Na2SO4) and concentrated to dryness to give a brown
gum. Purification by flash chromatography eluting with 9:1 (v/v)
hexane/CH2Cl2 gave of an off-white solid (9.94 g, 98%). A solution
of part of this mixture (6 g) in boiling toluene (70 mL) was concen-
trated to 50 mL and cooled to room temperature. Hexane (5 mL)
was added and the solution was allowed to stand at room temper-
ature for 2 min then filtered. A solution of the residue in hot tolu-
ene (25 mL) was cooled and stored for 6 h at ꢀ208C, yielding pure
E-ITEF (0.6 g) as white needles, m.p. 2628C. 1H NMR (300 MHz,
CD2Cl2): d=8.08 (d, 4J(H,H)=1.4 Hz, 1H, H1), 7.76 (s, 1H, C=CH),
7.69 (d, 3J(H,H)=7.8 Hz, 1H, H3), 7.63 (d, 3J(H,H)=7 Hz, 1H, H4),
7.46 (d, 3J(H,H)=8 Hz, 1H, H5), 7.2–7.3 (m, 15H, ArH), 7.17 (t,
120.4, 120.2, 119.7, 110.2, 62.0 ppm; IR (KBr): Ç=3078, 3047, 2218,
1281, 1184, 1035, 896, 728, 697, 630, 538 cmꢀ1; MALDI-MS: m/z=
447 [M+1], 446 [M+], 445; UV/Vis (dioxane/CH3CN, 1:3, v/v);
lmax(e)=327 (15000), 312 (17000), 294 (18000), 269 nm
(31000 molꢀ1 m3 cmꢀ1); fluorescence (CH3CN, lex266): lem =442 nm;
elemental analysis calcd (%) for C34H23N·2/3 H2O: C 89.25, H 5.36, N
3.06; found: C 89.28, H 5.80, N 3.17. Evaporation of the mother
liquor from crystallization of E-CTEF gave a residue that was dis-
solved in boiling toluene (2 mL) then hexane (1 mL) was added.
The resulting solid was collected by filtration, washed with hexane
(2”0.5 mL), and dried under vacuum to give an off-white powder
(0.1 g). To a solution of this solid in boiling chloroform (1 mL)
hexane (1 mL) was added. The resulting off-white precipitate was
collected by filtration, washed with hexane (1 mL), and dried over-
night (ca. 0.1 mm Hg, 288C) to give pure Z-CTEF (72 mg) as an off-
3
1
3J(H,H)=8 Hz, 1H, H6), 6.71 (t, J(H,H)=8 Hz, 1H, H7), 6.46 ppm (d,
white powder, m.p. 263–2658C. H NMR (400 MHz, CD2Cl2): d=7.97
3J(H,H)=8 Hz, 1H, H8); 13C NMR (126 MHz, CD2Cl2): d=146.7, 142.8,
141.0, 140.5, 138.5, 137.5, 136.9, 135.1, 130.3, 129.4, 128.4, 128.28,
128.25, 126.78, 126.77, 121.2, 119.4, 92.0, 61.9 ppm; IR (KBr): Ç=
3088, 3058, 3022, 1962, 1885, 1593, 1491, 1445, 1260, 1035, 1004,
830, 764, 702 cmꢀ1; GC-MS: m/z (%) 547 (30) [M+1], 546 (100) [M+
], 545 (14), 420 (17), 342 (27), 166 (27); UV/Vis (CH3CN): lmax(e)=
(s, 1H, C=CH), 7.84 (d, 3J(H,H)=8 Hz, 1H, H7), 7.78 (d, 3J(H,H)=
8 Hz, 1H, H6), 7.73 (d, 3J(H,H)=8 Hz, 1H, H3), 7.41–7.47 (m, 3H,
H4,5,8), 7.25–7.33 (m, 15H, ArH), 6.52 ppm (s, 1H, H1); 13C NMR
(100 MHz, CD2Cl2): d=146.5, 145.7, 142.1, 141.6, 137.3, 136.8, 135.9,
132.0, 131.5, 130.3, 128.9, 128.7, 128.6, 128.5, 127.2, 120.7, 120.5,
119.9, 109.5, 62.2 ppm; IR (KBr) Ç=2914, 2848, 1189, 1148, 1086,
1035, 901, 769, 728, 702, 661 cmꢀ1; MALDI-MS: m/z=447 [M+1],
446 [M+], 445; UV/Vis (dioxane/CH3CN, 1:3, v/v): lmax(e)=329
(11000), 314 (14000), 293 (18000), 267 (30000) 259 nm
320
(13000),
306
(15000),
292
(16000),
266 nm
(26000 molꢀ1 m3 cmꢀ1); elemental analysis calcd (%) for C33H23I:
C 72.53, H 4.24; found: C 72.78, H 4.61. The mother liquor from the
first crystallization was concentrated to dryness by rotary evapora-
(23000 molꢀ1 m3 cmꢀ1); fluorescence (CH3CN, lex330 nm): lem
=
1
tion. A solution of this solid (1:1 E-ITEF/Z-ITEF by HNMR analysis)
440 nm; elemental analysis calcd (%) for C34H23N: C 91.65, H 5.20,
N 3.14; found: C 91.80, H 4.82, N 3.30.
in boiling toluene (5 mL) was cooled, stored at ꢀ208C, and filtered.
A solution of the residue in boiling toluene (2 mL) was cooled,
stored at ꢀ208C, and filtered. Evaporation of the filtrate gave a mix-
(E)-2-Amino-9-(2,2,2-triphenylethylidene)fluorene
(E-11):[22]
[18]Crown-6 (0.51 g, 5.3 mmol) was dried at ca. 0.1 mm Hg over-
night. Under a nitrogen atmosphere Pd2(dba)3 (0.19 g, 0.21 mmol),
BINAP (0.39 g, 0.63 mmol), 2:1 E-ITEF/Z-ITEF (2.1 g, 3.8 mmol), and
0.51 g (5.3 mmol) of NaOEt were dried at ca. 0.1 mm Hg for 30 min
in a second flask before being stored under nitrogen. A solution of
the [18]crown-6 in anhydrous THF (3 mL) was added to the second
flask, followed by benzophenone imine (1 mL, 8.8 mmol), then the
reaction mixture was stirred at room temperature for 72 h. The
reaction mixture was diluted with dichloromethane (20 mL) and fil-
tered. The filtrate was concentrated to dryness, giving a brown
gum. A solution of the crude product in THF (20 mL) was stirred
for 20 min with 2m aqueous HCl (1 mL), diluted with water, then
extracted with EtOAc (2”50 mL). The combined extracts were
shaken with 20% aqueous NaHCO3solution (30 mL) and the layers
were separated. The organic layer was evaporated, giving a yellow
solid (1.56 g, 63%) after drying at ca. 0.1 mm Hg. The product was
purified by flash chromatography, eluting with hexane/CH2Cl2, 7:3
(v/v), to give a yellow solid that was recrystallized from chloroform,
washed with hexane, and dried under vacuum (ca. 0.1 mm Hg,
288C) to give pure E-11 as a yellow solid (0.45 g), m.p. 262–
2658C.1H NMR (400 MHz, CD2Cl2): d=7.78 (d, 3J(H,H)=8 Hz, 1H,
H4), 7.64 (s, 1H, C=CH), 7.50 (d, 3J(H,H)=8 Hz, 1H, H5), 7.44 (t,
1
ture (0.73 g) of 7:5 Z-ITEF/E-ITEF, by H NMR analysis. A solution of
this solid in toluene (1 mL) was stored at ꢀ208C and filtered.
Drying under vacuum (ca. 0.1 mm, 288C) gave pure Z-ITEF (90 mg)
1
as yellow crystals, m.p. 2678C. H NMR (300 MHz, CD2Cl2): d=7.88
(s, 1H, C=CH), 7.78 (d, 3J(H,H)=8 Hz, 1H, H4), 7.70 (d, 3J(H,H)=
3
8 Hz, 1H, H5), 7.52 (d, J(H,H)=8 Hz, 1H, H8), 7.24–7.42 (m, 18H,
ArH,H3,6,7), 6.72 ppm (s, 1H, H1); 13C NMR (126 MHz, CD2Cl2): d=
146.7, 141.3, 140.7, 140.4, 138.0, 137.4, 137.2, 137.1, 137.0, 130.3,
128.4, 128.3, 127.7, 127.0, 120.8, 120.2, 119.7, 91.5, 62.0 ppm; IR
(KBr): Ç=3109, 3058, 3017, 1598, 1491, 1271, 1153, 1035, 999, 907,
810, 753, 728, 697 cmꢀ1; GC-MS: m/z (%) 547 (30) [M+1], 546 (100)
[M+], 420 (17), 342 (27), 166 (27); UV/Vis (CH3CN): lmax(e)=310
(13000), 292 (15000), 264 nm (26000 molꢀ1 m3 cmꢀ1); elemental
analysis calcd (%) for C33H23I: C 72.53, H 4.24; found: C 72.90,
H 3.90.
(E)-
and
(Z)-2-Cyano-9-(2,2,2-triphenylethylidene)fluorene
(E-CTEF and Z-CTEF):[21] A mixture of 2:1 E-ITEF/Z-ITEF (2.1 g,
3.8 mmol), CuCN (0.52 g, 5.8 mmol), and DMF (35 mL) was stirred
and heated under reflux for 4 h. A solution of ferric chloride
(FeCl3·6H2O, 5.0 g, 19 mmol) in 2.5 N aq HCl (30 mL) was added.
The reaction mixture was stirred at 1008C for 1 h then poured into
water (100 mL) and filtered. The residue was washed with water
(2”50 mL) and dried to give an off-white solid, that was purified
by flash chromatography, eluting with EtOAc, yielding a pale
yellow solid (1.25 g, 73%) of 5:3 E-CTEF/Z-CTEF, by 1H NMR analysis.
A solution of this solid in toluene (20 mL) was cooled to ꢀ208C to
give pure E-CTEF (0.2 g) as an off-white powder, mp 262–2638C.
1H NMR (300 MHz, CD2Cl2): d=8.08 (s, 1H, H1), 7.91 (s, 1H, C=CH)
3
3J(H,H)=8 Hz, 1H, H6), 7.19–7.45 (m, 15H, PhH), 7.12 (t, J(H,H)=
3
8 Hz, 1H, H7), 6.63 (m, 2H, H1,3), 6.40 ppm (d, J(H,H)=8 Hz, 1H,
H8); 13C NMR (126 MHz, CD2Cl2): d=147.0, 139.1, 138.3, 135.7,
130.3, 129.6, 129.5, 128.4, 128.3, 128.22, 128.16, 128.1, 126.7, 125.6,
119.5, 118.8, 113.9, 61.7 ppm; IR (KBr): Ç=3420, 3344, 3324, 3058,
3027, 1956, 1619, 1593, 1485, 1454, 1291, 1034, 810, 764, 707 cmꢀ1
;
MS (ESI): m/z (%) 465 (100) [M+]; elemental analysis calcd (%) for
C33H25N: C 91.00, H 5.79, N 3.20; found: C 91.40, H 5.49, N 2.95.
3
3
7.82 (d, J(H,H)=8 Hz, 1H, H3), 7.74 (d, J(H,H)=8 Hz, 1H, H4) 7.67
(d, 3J(H,H)=8 Hz, 1H, H5), 7.24–7.34 (m, 16H, ArH, H6), 6.82 (t,
3J(H,H)=8 Hz, 1H, H7), 6.53 ppm (d, 3J(H,H)=8 Hz, 1H, H8);
13C NMR (100 MHz, CDCl3): d=146.5, 142.7, 141.8, 141.2, 140.2,
136.9, 136.2, 131.8, 130.3, 128.7, 128.4, 128.3, 127.9, 126.9, 124.1,
(E)-2-Nitro-9-(2,2,2-triphenylethylidene)fluorene (E-NTEF):[16]
A
solution of NaBO3·4H2O (1.2 g, 7.9 mmol) in AcOH (10 mL) was
stirred at 608C as a solution of E-11 (0.25 g, 0.57 mmol) in hot
AcOH (10 mL) was added dropwise over 1.5 h. The reaction mix-
&
&
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Full Paper
ture was stirred for 2 h at 608C then poured into water (100 mL).
The resulting solid was collected by filtration, washed with water
(2”50 mL), and dried under vacuum. A solution of the crude prod-
uct in CH2Cl2 (200 mL) was filtered through alumina (10 g), eluting
with EtOAc (200 mL). Rotary evaporation gave a yellow solid
(0.20 g, 75%), that was dissolved in CHCl3 (1 mL) then hexane
(1 mL) was added to give of pure E-NTEF (0.12 g, 56%) as yellow
Chromatographic analysis
The isomeric composition of NTEF as a function of irradiation time
was measured using a gas chromatograph (Varian CP-3800) with
a tandem mass spectrometer detector (Saturn 2200) with front in-
jector temperature of 2908C, oven temperature starting at 808C
ramping up to 2908C at 158 per minute, and a flow rate of
6 mLminꢀ1. Peak identities were verified by mass spectrometry and
by comparison with the chromatographic spectra of samples with
known isomer purity. Photoisomerization of CTEF and ITEF was
measured using an HPLC (Waters 1525) with a reverse-phase
column (Symmetry C18 5 mm). A flow rate of 1 mLminꢀ1 of 9:1 (v/
v) methanol/water was sufficient to separate the E and Z isomers
of CTEF, where they were detected by absorption spectroscopy at
320 nm. ITEF was analyzed using a 100% methanol mobile phase,
at a flow rate of 1 mLminꢀ1, and a detection wavelength of
298 nm. In each case, the detection wavelength was chosen such
that the molar absorptivities of the E and Z isomers were equiva-
lent. Isomer peaks were verified by comparison with the chromato-
grams of samples with known isomer purity.
1
crystals, m.p. 2548C. H NMR (400 MHz, CD2Cl2): d=8.62 (s, 1H, H1),
3
3
8.27 (d, J(H,H)=8 Hz, 1H, H3), 8.0 (s, 1H, C=CH), 7.85 (d, J(H,H)=
3
8 Hz, 1H, H4), 7.77 (d, J(H,H)=8 Hz, 1H, H5), 7.26–7.30 (m, 16H,
3
3
PhH,H6), 6.84 (t, J(H,H)=8 Hz,1H, H7), 6.56 ppm (d, J(H,H)=8 Hz,
1H, H8); 13C NMR (125 MHz, CD2Cl2): d=146.5, 142.3, 139.7, 137.1,
137.0, 132.6, 130.4, 130.3, 130.2, 128.8, 128.6, 128.5, 128.4, 126.9,
123.7, 120.7, 119.8, 115.8, 62.0 ppm; IR (KBr): Ç=3119, 3063, 3017,
1952, 1885, 1808, 1593, 1516, 1445, 1419, 1143, 907, 820, 759, 733,
697 cmꢀ1; MALDI-MS: m/z=466 [M+1], 465, [M+], 464; UV/Vis
(dioxane/CH3CN, 1:3, v/v): lmax(e)=334 (24000), 298 (29000),
289 nm (29000 molꢀ1 m3 cmꢀ1); fluorescence (dioxane/CH3CN, 1:3
(v/v), lex266): lem =471 nm; elemental analysis calcd (%) for
C33H23NO2·1= H2O: C 83.52, H 5.10, N 2.95; found: C 83.19, H 5.27,
2
N 2.90.
Acknowledgements
Crystallographic structure determination
This research was supported by the National Science Founda-
tion (no. CHE-0210549). CHM acknowledges a National Science
Foundation Summer REU Fellowship (no. CHE-0552816).
X-ray crystallographic data was collected at 100(1) K on a Bruker
APEX CCD diffractometer with Mo K radiation (0.71073 Š) and a de-
tector-to-crystal distance of 4.94 cm. Data collection was optimized
utilizing the APEX2 software with a 0.58 rotation about between
frames, and exposure times of 10 or 20 s per frame. Data integra-
tion, correction for Lorentz and polarization effects, and final cell
refinement were performed using SAINTPLUS, and corrected for
absorption using SADABS. The structures were solved using direct
methods followed by successive least squares refinement on F2
using the SHELXTL 6.10 software package.[28] All nonhydrogen
atoms were refined anisotropically and hydrogen atoms placed in
Keywords: alkenes · aromatic compounds · isomerization ·
molecular switches · photophysics
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CCDC 784492 (E-ITEF),
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contain the supplementary crystallographic data for this paper.
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Irradiation
Photoisomerization excitation wavelengths used were generated
either as harmonics of a pulsed Nd:YAG laser (10 Hz repetition
rate) or from a Nd:YAG-pumped, frequency-doubled dye laser.
Light pulses ca. 5 ns in duration of energies 0.3–0.5 mJ pulseꢀ1 irra-
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Received: February 23, 2016
Published online on && &&, 0000
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Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
12
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
A useful twist: 2-Substituted dibenzo-
fulvene molecular actuators based on
(2,2,2-triphenylethylidene)fluorene are
reported. The 2-nitro compound is
photochemically robust and has a large
quantum yield for photoisomerization
in the near-UV, making it a promising
drive rotor moiety for light-powered
molecular devices. The 2-cyano and
2-iodo analogues (E and Z isomers of
the latter shown in picture) have higher
quantum yields but absorb light at
shorter wavelengths and undergo
minor competing photodecomposition.
&
Isomerization
S. C. Everhart, U. K. Jayasundara, H. Kim,
R. Procfflpez-Schtirbu, W. A. Stanbery,
C. H. Mishler, B. J. Frost, J. I. Cline,
T. W. Bell*
&& – &&
Synthesis and Photoisomerization of
Substituted Dibenzofulvene Molecular
Rotors
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
13
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ