Diarylpyrenes vs. diaryltetrahydropyrenes: Crystal structures, fluorescence, and upconversion photochemistry

Article abstract of DOI:10.1016/j.jphotochem.2013.07.018

The synthesis of two series of substituted 2,7-diaryl-4,5,9,10- tetrahydropyrenes (1a-c) and 2,7-diarylpyrenes (2a-c) is reported; the opposing phenyl tips being terminated with ^tBu (a), OCH₃ (b), or F (c) to impart variation in electronic properties. X-ray crystallographic analysis of the six compounds revealed edge-to-face packing predominately due to van der Waals forces in the solid state in all instances. The photophysical properties of these compounds were investigated in solution and in solid state/thin films. Since the 2,7-bis(4-tert-butylphenyl)tetrahydropyrene 1a possesses unity fluorescence quantum efficiency, it was successfully employed as the emitter in a low power upconversion scheme featuring fac-Ir(ppy) ₃ [ppy = 2-phenylpyridine] as the photosensitizer.

Full text of DOI:10.1016/j.jphotochem.2013.07.018

Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Diarylpyrenes vs. diaryltetrahydropyrenes: Crystal structures,
fluorescence, and upconversion photochemistry
Ala’a O. El-Balloulia, Rony S. Khnayzera,b, Jihane C. Khalifec, Alexandr Fonarid,
Kassem M. Hallala, Tatiana V. Timofeevad, Digambara Patraa, Felix N. Castellanob,e
,
Brigitte Wexc,∗, Bilal R. Kaafarania,∗
a Department of Chemistry, American University of Beirut, Beirut 1107-2020, Lebanon
b Department of Chemistry & Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, USA
c Department of Natural Sciences, Lebanese American University, Byblos, Lebanon
d Department of Biology & Chemistry, New Mexico Highlands University, Las-Vegas, NM 87701, USA
e Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2013
Received in revised form 19 July 2013
Accepted 30 July 2013
Available online 7 September 2013
The synthesis of two series of substituted 2,7-diaryl-4,5,9,10-tetrahydropyrenes (1a–c) and 2,7-
diarylpyrenes (2a–c) is reported; the opposing phenyl tips being terminated with tBu (a), OCH3 (b),
or F (c) to impart variation in electronic properties. X-ray crystallographic analysis of the six compounds
revealed edge-to-face packing predominately due to van der Waals forces in the solid state in all instances.
The photophysical properties of these compounds were investigated in solution and in solid state/thin
films. Since the 2,7-bis(4-tert-butylphenyl)tetrahydropyrene 1a possesses unity fluorescence quantum
efficiency, it was successfully employed as the emitter in a low power upconversion scheme featuring
fac-Ir(ppy)3 [ppy = 2-phenylpyridine] as the photosensitizer.
Keywords:
Pyrene
Tetrahydropyrene
OLED
© 2013 Elsevier B.V. All rights reserved.
Upconversion photochemistry
1. Introduction
emitter suffers from photo-dimerization that competes with UC
processes [15]. Aromatic hydrocarbons are sought due to their
ability to create a singlet excited state upon triplet–triplet colli-
4,5,9,10-Tetrahydropyrene and pyrene have attracted consider-
able attention in optoelectronic device applications [1–6]. Within
the field of optoelectronics, photon upconversion (UC) [7–9] is a
promising approach toward transparent, flexible devices for dis-
plays [10] and lighting [11]. In UC, low power density radiation is
energy transferred from a sensitizer to create metastable triplet
states of organic molecules, which annihilate to release upcon-
verted emission. Recently, UC has been employed for solar energy
harvesting applications [12]. Current challenges revolve around
solid state UC systems, theoretical modeling of the photochemical
processes in UC systems as well as the development of efficient
acceptor molecules capable of emitting in the UV region and
subsequently sensitize wide bandgap semiconductors using sub-
bandgap photons [13,14]. Thus far, aromatic hydrocarbons, such
as substituted anthracene, rubrene and perylene have been eval-
uated in UC [7]. However, anthracene which is a widely used UV
sion [16]. The needed extremely low intersystem crossing rates
are found in highly symmetrical fluorescent acceptors [14], while a
steric hindrance is desirable to block excimer formation as shown
for pyrene [13]. Both, 4,5,9,10-tetrahydropyrene (4) [17–20], here-
after referred to as tetrahydropyrene, and pyrene (3) [21–24] have
well characterized photophysical behaviors and thus served as
starting point for the design of potential new materials for UC
applications. The engineering of packing in the solid state ren-
ders favorable electronic properties for materials intended for
OFET-applications [25], while the engineering of an absence of
␲-stacking is important for OLED [26] and UC [13] applications.
To get a better understanding of molecular structure–property
and crystal structure–property relationships, a series of 4,4ꢀ-
disubstituted 2,7-diphenyltetrahydropyrenes (1a–c) and a series
of 4,4ꢀ-disubstituted 2,7-diphenylpyrenes (2a–c) were synthesized.
The phenyl groups on positions 2 and 7 of tetrahydropyrene (4)
and pyrene (3) impart steric factors to prevent ␲–␲ stacking,
while the substituents on 4 and 4ꢀ positions of the phenyl groups
were chosen to impart an additional steric factor (4-tert-butyl),
electron-donating factor (4-methoxy) and electron-withdrawing
factor (4-fluoro).
∗
Corresponding authors. Tel.: +961 3 151451.
E-mail addresses: brigitte.wex@lau.edu.lb (B. Wex), bilal.kaafarani@aub.edu.lb
(B.R. Kaafarani).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.07.018

50
A.O. El-Ballouli et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
Scheme 1. Synthesis of dibromotetrahydropyrene (5) [28] and dibromopyrene (6) [28] from commercially available pyrene (3). Synthesis of substituted 2,7-
diaryltetrahydropyrene and pyrene derivatives 1a–c and 2a–c via Suzuki coupling reactions. Conditions: (i) Raney-Ni, 2 days; Pd/C, H2, 40 psi, 3 days; (ii) Br2 in AcOH;
(iii) Br2 in CS2; (iv) Pd(PPh3)4, K2CO3, toluene, ethanol, water and 18-crown-6 (reflux, 1 day).
2. Results and discussion
considering the 4,4ꢀdi-t-butyl-substitution, yet decreased to 248
and 277 ◦C by the 4,4ꢀ-dimethoxy- and 4,4ꢀ-difluoro-substitution,
respectively. The same trend is observed for the tetrahydropy-
rene series (1a–c) thus melting point engineering with substitution
pattern is expected. High melting temperatures are important in
optoelectronic device applications as a heating of the device during
operations might be observed.
2.1. Synthesis
Pyrene (3) served as starting material for the synthesis
of both series of compounds. Activation by Raney nickel
followed by catalytic hydrogenation using Pd/C furnished
4,5,9,10-tetrahydropyrene
(4)
[27].
2,7-Dibromo-4,5,9,10-
tetrahydropyrene (5) was collected after the reaction of
4
2.3. X-ray structural analysis
with bromine in acetic acid at RT for 1 day [28]. Oxidation of
5 in the presence of bromine and CS2 at RT for 1 day yielded
2,7-dibromopyrene (6), [28] Scheme 1. The target molecules
1a–c and 2a–c were prepared through Suzuki coupling reactions
between the fused ring arylbromides (5, 6) and the corresponding
substituted arylboronic acids (7a–c) in presence of Pd(PPh3)4 and
K2CO3 as base. Specifically, the coupling of 2,7-dibromo-4,5,9,10-
tetrahydropyrene (5) and 4-tert-butylbenzeneboronic acid (7a),
4-methoxybenzeneboronic acid (7b), or 4-fluorobenzeneboronic
acid (7c) yielded 1a–c and the coupling of 2,7-dibromopyrene
(6) and identical boronic acids (7a–c) yielded 2a–c, respectively,
Scheme 1. The desired 2,7-diaryltetrahydropyrenes and pyrenes
(1a–2c) were obtained in 40–97% yield. While the structure of
compound 1b was reported previously as a side product, no
structural characterization data were provided [29]. Thus, these
compounds represent new chromophoric materials and each
molecule was structurally characterized using 1H and 13C NMR,
elemental analysis, and X-ray crystallography.
To further investigate the structure–property relationships in
compounds 1a–2c, X-ray crystal structure analysis was carried out.
Crystal structure determination allowed obtaining detailed infor-
mation on molecular geometry for all synthesized compounds,
as well as, the evaluation of short intermolecular contacts and
type of molecular packing in crystals. Single crystals of 1a–c and
2a were obtained from toluene and of 2a–c were obtained from
CH2Cl2/ethyl acetate. Details of X-ray diffraction experiments and
structures refinement are presented in Supporting Information.
Fundamentally, the two series are differentiated by the central
core, i.e. a non-planar tetrahydropyrene central core (1a–c) or pla-
nar aromatic pyrene central core (2a–c) with same types of aryl
substituents in positions 2 and 7, Fig. 1. X-ray diffraction revealed
a remarkable example of polymorphism for compound 2a grown
from different solvents.
To date, no packing pattern for unsubstituted tetrahydropyrene
(4) is reported. Only the crystal structure of 2-chloro-4,5,9,10-
tetrahydropyrene is available showing rotation of the two benzene
rings around the biphenyl bond of 16.9◦ and a second disor-
dered molecule [33]. In 1a–c, the central C(10) C(16) bonds of
the tetrahydropyrene core show pronounced single bond charac-
ter typical for conjugated systems, Table S4. Four aromatic rings
(see Fig. 2) constitute planar fragments in series 1. The values of
the dihedral angles between two central aromatic rings range from
16.5(1) to 20.8(7)◦ in 1a–c, Table S4. Crystal of 1a contains two inde-
pendent molecules in asymmetric unit (A and B) that differ slightly.
Fitting of all carbon atoms shows root mean square (r.m.s) deviation
2.2. Thermal studies
Differential scanning calorimetry (DSC) was carried out on com-
pounds 1a–2c (see Supporting Information). Tetrahydropyrene
(4) has a melting point of 138–140 ◦C, [30] while pyrene has a
melting point of 148–149 ◦C [31] with an enthalpy of crystalliza-
tion of 17.11 kJ/mol [32]. As indicated in Table S1 in Supporting
Information, the melting points for the 2,7-disubstituted tetrahy-
dropyrene (1a–c) and pyrene derivatives (2a–c) ranged from
215 to 359 ◦C and 248 to 356 ◦C, respectively. Thus, the melting
points were effectively doubled in both series, when compared
to the non-substituted parent molecules tetrahydropyrene 4 and
pyrene 3, respectively. In comparison to 2,7-diphenylpyrene, the
observed melting point of 316 ◦C [22] was increased to 356 ◦C, when
˚
of just 0.011 A. Comparing interplanar angles in the tetrahydropy-
rene series 1, it can be noted that molecules of 1a are slightly closer
to planarity than 1b and 1c.
Based on X-ray crystallographic data, pyrene [34–36] is placed
in one of the four classic packing patterns for aromatic compounds

A.O. El-Ballouli et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
51
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Fig. 1. ORTEP drawing for 1a–c (a-c) and 2aꢀ (d), 2aꢀꢀ (e), 2b–c (f–g) with the numbering scheme in projection on the best plane. Thermal ellipsoids are shown with the 50%
probability level. One symmetry independent molecule of 1a (a) is shown. Only one position for the disordered tert-butyl fragments in 2aꢀ (d) is shown.
Fig. 2. Planes distinguished in geometries of 1a–c (left) and 2a–c (right).
[37], namely the type of sandwich-herringbone packing charac-
terized by a short axis of >8.0 A [37]. A “sandwich” motif consists
approximately 10 and 20 degrees, Table S4 of Supporting Infor-
mation. Presented data allow us to consider these crystals to
be conformational polymorphs [40,41]. All compounds in the
pyrene series (2a–c) are characterized by planar central aro-
matic pyrene core, Fig. 3. Because of the aromaticity, the central
C(10) C(16) bond has a length typical for aromatic systems,
shorter than C(4) C(7) and C(21) C(23) bonds. Comparing dihe-
dral angles between planes in the pyrene series 2a–c, the largest
deviation is observed for 2aꢀ, whereas the lowest one for 2aꢀꢀ.
2,7-Diphenylpyrene crystallizes in an orthorhombic crystal sys-
tem similar to 2aꢀ, wherein 2,7-diphenylpyrene molecules show a
non-planar conformation with a dihedral angle of 33.253(56)◦ pri-
marily ascribed to steric hindrance of the hydrogen atoms between
the phenyl substitutents and the pyrene core. 2,7-Diphenylpyrene,
˚
of two parallel molecules that experience ␲–␲ stacking, wherein
“sandwiches” are arranged in a herringbone motif with interactions
dominated by C H. . .␲ contacts [38], whereas a predominance of
the ␤- and ␥-sandwich-herringbone is found [39]. In our series
of substituted pyrene molecules, compound 2a demonstrates two
crystalline polymorphs obtained from toluene (2aꢀ) and a mix-
ture of dichloromethane and ethyl acetate (2aꢀꢀ). X-ray analysis
revealed that molecular conformations in these two polymorphs
are slightly different. Molecule 2aꢀ is located in general position,
whereas molecule 2aꢀꢀ resides on the inversion center, in asym-
metric unit. In addition, the dihedral angles between central core
and terminal phenyl rings in 2aꢀ are larger than those in 2aꢀꢀ by

52
A.O. El-Ballouli et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
Fig. 3. Edge-to-face molecular arrangement (left) and crystal packing (right) of 1a (a), 1b (b), and 1c (c).
and stability, we conclude that polymorph 2aꢀ should be more
stable at the absolute zero temperature [40,43]. Importantly, in
contrast to 2,7-diphenylpyrene, molecules that have substituents
on 4-position of the phenyl groups (series 2a–c) are held together
in crystal predominately by van der Waals forces. A similar trend is
found in the tetrahydropyrene series 1a–c. Thus, we conclude that
the presence of phenyl groups alone on the pyrene – and extrapo-
lated to the tetrahydropyrene – parent molecules was insufficient
to completely eliminate the intermolecular interactions leading to
packing in the solid state, while additional substitution on the 4-
position of the phenyl groups resulted in the complete removal of
weak intermolecular interactions in the solid state.
˚
however, shows short C—H. . .␲ nonbonding interactions of 2.806 A
˚
and 2.814 A and face-to-edge herringbone packing in one of the two
crystalline layers present [42].
Molecules of the pyrene and tetrahydropyrene series are packed
in a similar edge-to-face fashion. In cases of 1b and 2a–c, the helices
of molecules related by 21 screw axis are formed, along the b-axis
for monoclinic lattice type, and along the a-axis for orthorhombic
lattice. Stacks in 1a (Fig. 3a) are in A-B-A-B sequence (A-B pairs are
related by translations along the b-axis). In 1c, stacks are formed
along the a crystallographic axis where molecules are related via an
inversion center, Fig. 3c. In 1a, stacks are packed in a parquet-like
mode (Fig. 3a) whereas 1b molecules form chains along the c-axis,
Fig. 3b. Finally, 1c stacks are arranged in a brick-like mode, Fig. 3c.
Crystal packings for the pyrene series 2a–c are shown in Fig.
S7 of the Supporting Information. Comparing arrangement of the
stacks in polymorphs 2aꢀ, we note that in 2aꢀ, stacks are arranged
in parquet-like fashion (Fig. S7a), while in polymorph 2aꢀꢀ long
molecular directions are approximately parallel to crystallographic
axis a, Fig. S7b. The former molecular arrangement is somewhat
more efficient as shown by the calculated density values 1.232
vs. 1.203 mg/m3 for 2aꢀ and 2aꢀꢀ, respectively. Following Burger
and Ramberger rule with regard to the relation between density
2.4. Molecular photophysics
The absorption and fluorescence spectra of 1a–c and 2a–c were
recorded in THF solution as well as solid state/thin films and are
displayed in Figs. 4 and 5, respectively. The relevant associated
data are collected in Table 1 and Figs. S9–S14. Tetrahydropyrene
(4) displays an electronic spectrum exhibiting fine structure with
an absorption maximum at 280 nm in isooctane [18]. Substitution
with 4-tert-butylphenyl, 4-methoxyphenyl, or 4-fluorophenyl in

A.O. El-Ballouli et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
53
1.2x105
1.0x105
8.0x104
6.0x104
4.0x104
0.18
1a
1a
1b
1c
2a
2b
2c
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
1b
1c
2a
2b
2c
2.0x104
0.0
260
260
280
300
320
340
360
380
400
280
300
320
340
360
380
400
Wavelength (nm)
Wavelength (nm)
Fig. 4. Absorption spectra of solutions in THF (left) and thin films (right) of 1a–c and 2a–c.
1a
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1b
1c
2a
2b
2c
1a
1b
1c
2a
2b
2c
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 5. Fluorescence spectra of solutions of 1a–c and 2a–c in THF (left, ꢀex = 330 nm, 2 ␮M) and solid state (right, ꢀex = 320 nm).
the 2- and 7-positions results in significant bathochromic shifts
in this absorption maximum of 0.64 eV (47 nm), 0.65 eV (48 nm),
and 0.57 eV (40 nm) in cyclohexane, respectively. In more polar
THF, a broad absorption band and the absence of vibronic fine
structure is observed for the tetrahydropyrenes 1a–c. A remark-
able resemblance of spectral features in both the absorption
and fluorescence spectra and quantum yields between 1a–c and
1,1ꢀ:4ꢀ,1ꢀꢀ:4ꢀꢀ,1ꢀꢀꢀ-quaterphenyl is noted [44,45]. In this related
compound, wherein the central two phenyl units can rotate
freely as opposed rotationally-restricted, diethylene-bridged
2,7-diphenyltetrahydropyrene derivatives, the structureless
absorption and structured emission is attributed to a significantly
steeper torsional potential in the S1 state compared to S0. Using ab
initio quantum chemical methods combined with Franck–Condon
treatment, Heimel et al. [46] related the mirror image symmetry
breakdown to torsional degree of freedom along the quaterphenyl
backbone, which may be correlated to our results on compounds
1a–c. In addition, we investigated the UV–vis spectra of thin films
prepared on quartz glass. In thin film, the absorption maxima
for both 1a and 1b narrowed and blue shifted, while a redshift is
observed for the remaining compounds, Fig. 4 and Table S5. The
photophysical properties of pyrene itself are well-established and
the review by Crawford and coworkers is particularly compre-
hensive [22]. In short, pyrene exhibits four absorption bands with
maxima at 372 (S1 ← S0), 334 (S2 ← S0), 272 (S3 ← S0), and 243 nm
(S4 ← S0) in cyclohexane [21,47]. For the short wavelength
Table 1
Fluorescence quantum yields (˚f)a of 1a–2c in various solventsb and solid state.
Compound
Cy
Hex
Tol
CHCl3
THF
DCM
EtOH
MeOH
ACN
DMF
Solid statec
1a
1b
1c
2a
2b
2c
0.61
0.81
0.48
0.11
0.08
0.04
0.52
0.92
0.55
0.04
0.04
0.02
∼1.00
∼1.00
∼1.00
0.09
0.09
0.07
0.53
0.39
0.55
0.05
0.04
0.04
0.68
0.76
0.81
0.08
0.17
0.07
∼1.00
∼1.00
∼1.00
0.06
0.09
0.05
0.48
0.53
0.18
0.13
0.12
0.05
0.39
0.45
0.24
0.12
0.09
0.05
0.55
0.60
0.19
0.07
0.13
0.08
0.66
0.55
0.56
0.38
0.34
0.09
0.76
0.52
0.45
0.17
0.12
0.08
Legend: Cy = cyclohexane; Hex = n-hexane; Tol = toluene; CHCl3 = chloroform; THF = tetrahydrofuran; DCM = dichloromethane; EtOH = ethanol; MeOH = methanol;
ACN = acetonitrile; DMF = N,N-dimethylformamide.
a
Error on quantum yield measurements is 10%.
Solvents are arranged in increasing order of their dielectric constants.
Error on absolute quantum yield measurements for solid state is 3%.
b
c

54
A.O. El-Ballouli et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
absorption band, substitution with phenyl introduces
a
2,7-dianthryl substituted tetrahydropyrene is reported as 0.52 in
THF [5]. A wide, solvent-dependent fluorescence quantum yield
range is also known for pyrene, 0.65 in cyclohexane and near unity
in acetone [49,52]. This solvent effect on the quantum yields is
conserved in the pyrene derivatives 2a–c, albeit with a significant
decrease in comparison to the parent, Table 1. The fluorescence
quantum yields of 2a–c range from only 0.02 in hexanes to 0.38 in
DMF. Consistent in both series, the quantum yield is lowest for the
compound substituted with the halogen [47], i.e. 4-fluorophenyl
(1c, 2c), Table 1. Important to note is the impressive quantum
yields of unity observed for 1a–c in dichloromethane and toluene.
Saturated carbon-carbon bonds as found in tBu (1a) impart stretch-
ing flexibility or a “loose-bolt effect” on the molecule resulting
in decreased fluorescence quantum yields via radiationless transi-
tions [47]. Curiously, in dichloromethane and toluene, the saturated
bonds in 1a and 1b apparently do not lower the fluorescence
quantum yields via radiationless transitions. Thus, the tetrahy-
dropyrene derivatives (1a–c) are anticipated to be more interesting
in fluorescence-based applications. For comparison, we have deter-
mined the absolute quantum yield of 1a–c and 2a–c in solid state
by using an integrating sphere. A similar trend is observed to
solution-based quantum yields, the quantum yields for the tetrahy-
dropyrene series 1a–c was consistently higher compared to the
pyrene series 2a–c ranging from 0.76 to 0.45 in comparison to 0.17
to 0.08, respectively. This observation correlates well with the com-
plete absence of intermolecular interactions observed in the X-ray
crystal structure for 1a–c, Table 1.
The fluorescence lifetime (ꢂf), and calculated radiative (kr)
and non-radiative (knr) decay rate constants are also collected in
Tables S6 and S7 and Figs. S19–24. The short fluorescence lifetimes
(1–10 ns) for 1a–c were observed with biexponential decay with
a 5–10 fold shorter fluorescence lifetime compared to the pyrene-
based derivatives (2a–c). The calculated values of kr and knr for
these molecules utilized average lifetimes [48]. The lifetime for
1a–c are in good agreement as observed values for disubstituted
tetrahydropyrenes [53], while the pyrene-based derivatives exhibit
monoexponential decay 10–20-fold shorter compared to the fluo-
rescence lifetimes reported for unsubstituted pyrene. Specifically,
pyrene has a solvent-dependent and oxygen-sensitive singlet state
lifetime of 650 ns and 190 ns in cyclohexane and ethanol, respec-
tively [52].
bathochromic shift of 0.84 eV (48 nm) to 291 nm, a significant
increase in extinction coefficient as well as a loss of vibronic
fine structure [42]. A similar increase was observed and was
attributed to overlapping absorptions of the aryl substituents with
the pyrene absorptions [22,42]. The present series of compounds,
when compared to 2,7-diphenylpyrene, carry substitutions with
tert-butyl (2a), methoxy (2b), or fluoro (2c) group on the 4-position
of the phenyl moiety. The short wavelength absorption bands are
shifted to 295, 304, and 290 nm, respectively. The vibronic fine
structure of the 334 nm (S2 ← S0) absorption band of pyrene, in
contrast, remains comparably unaffected, when phenyl groups
are placed in the pyrene 2- and 7-positions, and absorption peaks
are observed at 342, 326, and 313 nm. Thus, only a 0.09 eV (8 nm)
bathochromic shift is induced by phenyl substitution [42]. These
slight shifts in wavelength seem to reflect the electron-donating
(2a,b) or electron-withdrawing effect (2c) of the substituents.
The longest wavelength absorption band and vibronic structures
remain comparably unaffected by the substituents utilized in this
study. A similarly negligible effect on the (S2 ← S0) transition of
pyrene was observed for 2,7-di(C6H4-4-CO2C8H17) substitution
[22].
To investigate the effect of the dielectric continuum on the spec-
troscopic properties of 1a–2c, their absorption and fluorescence
spectra were recorded in a range of solvents (Figs. S9–S14). The
excitation spectra for 1a–c and 2a–c were recorded at wavelengths
corresponding to the position of peak/shoulder in the emission
spectra of each compound (Fig. S16). Overall, the excitation spec-
tra of all compounds resemble their corresponding absorption
spectra thus confirming the absence of excimer formation. The
plots of Stokes shift versus ꢁf (orientation polarizability) and ET30
(solvent polarity scale) were generated. A common feature of com-
pounds (1a–2c) is their minimal solvent effect as noticed by the
near zero slope for plots of Stokes shifts versus ꢁf and ET30, Figs.
S9–S14. In contrast to the broad and structureless absorption spec-
tra, tetrahydropyrene derivatives 1a–c possess two emission bands
containing a shoulder. A vibrationally structured fluorescence spec-
trum suggests a planar excited state configuration as the origin of
emission [48]. Fused, polycyclic aromatic compounds exhibit flu-
orescence spectra rich in vibronic structure, wherein a marked
absence in solvatochromic effect is also well established. These
aromatic hydrocarbons also show a significant Ham effect, i.e. the
pertubation in the relative intensity of corresponding vibrational
bands attributed to both solvent dipole moment and dielectric
constants [26,47,49]. Pyrene and its derivatives have thus found a
multitude of applications as fluorescent probes based solely on this
property [50]. Pyrene exhibits five well-resolved emission bands at
372 (I), 378 (II), 393 (III), 388 (IV) to 393 (V) nm with peak intensity
ratios of III/I ranging from 1.65 to 0.54 in hexane and acetonitrile,
respectively [26,47]. In the present molecules, this rich vibronic
structure is replaced by two unresolved bands having shoulders.
The emission band intensities were computed as a function of the
solvent for compounds 1a–2c, where the emission band with the
maximum intensity is assigned to I, whereas II is the band at higher
wavelength [51]. Accordingly, the peak intensity ratios I/II versus
ꢁf and ET30 for 4-fluorophenyl derivative 1c are reported (see Sup-
porting Information). These plots further confirm that only minimal
solvent effects can be detected. The calculations of I/II for the entire
series 1a–2c are summarized in Table S5. Notably, the newly con-
ceived substituted 2,7-diarylpyrenes no longer exhibit the Ham
effect typically associated with naked pyrene.
2.5. Upconversion photochemistry
Preliminary sensitized upconversion studies were conducted
with tris[fac-2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3) as
sensitizer and 1a as the acceptor/annihilator. The latter was
selected as its absolute fluorescence quantum yield in toluene is
unity, 1.00 0.01. Fig. 6 presents the normalized absorption (solid
lines) of Ir(ppy)3 (red) and 1a (black) along with their normal-
ized emission spectra (dashed lines). In the visible, Ir(ppy)3 can
be selectively excited and its triplet state is properly positioned
for triplet energy transfer to 1a from comparison to pyrene.16
A
Jablonski diagram depicting the complete upconversion process is
presented in Scheme 2. Variable intensity 488 nm (2.54 eV) laser
excitation from an Ar/Kr laser is absorbed exclusively by Ir(ppy)3
to form the singlet excited state of the sensitizer (1Ir(ppy)3*). Inter-
system crossing occurs with unity efficiency yielding the triplet
state of the sensitizer (3Ir(ppy)3*) as previously established.16 This
long lived triplet state (ꢂ = 1.3 ␮s in an argon-degassed toluene
solution, Fig. S18 is energetically poised for triplet–triplet energy
transfer (kq = 1.7 × 109 M−1 s−1, Figs. S19–S20) with 1a, producing
the excited triplet state of this acceptor (31a*), regenerating the
ground state of the Ir(III) sensitizer. Two molecules of 31a* can then
undergo triplet–triplet annihilation yielding the single excited state
of 1a (11a*). Finally, 11a* decays to ground state through singlet
To further study the emission properties as a function of sol-
vent matrix, fluorescence quantum yields (˚f) were measured
in a wide range of solvents. For compounds 1a–c, the quan-
tum yields range from 0.18 (ethanol) to unity (dichloromethane
and toluene). In comparison, the fluorescence quantum yield of

A.O. El-Ballouli et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
55
Fig. 7 (left) presents the upconverted singlet emission emanat-
ing from 1a in the UV subsequent to selective visible excitation of
Ir(ppy)3 measured as a function of monochromatic 488 nm incident
power density. The intensity of the photoluminescence response at
411 nm was subsequently plotted against the 488 nm incident laser
power density generating a quadratic power response, Fig. 7 (right).
In this weak-annihilation kinetic regime, the quadratic upcon-
verted emission intensity dependence on the excitation power
density is easily demonstrated with an inherent slope of 2.0 in the
associated double logarithmic plot (Fig. 7 right inset) [54].
Absorbance of 1a
Emission of 1a
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Absorbance of Ir(ppy)3
Emission of Ir(ppy)3
3. Conclusions
The synthesis and photophysical properties of new series of
substituted 2,7-diaryl-4,5,9,10-tetrahydropyrenes and pyrenes are
reported. The aryl groups carry 4-tert-butyl-, 4-methoxy- and 4-
fluoro groups. Both series exhibit absence of solvatochromic and
Ham effects yet show solvent-dependent quantum yields and
lifetimes. Interestingly, all tetrahydropyrene derivatives exhibit
quantum yields of unity in dichloromethane and toluene. X-ray
crystallographic analysis of all six compounds revealed edge-to-
face packing and the absence of weak intermolecular interactions
in the solid state. These properties combined make the tetrahy-
dropyrene series 1a–c interesting candidates for UC applications.
Visible-to-UV UC studies performed using 1a as acceptors in con-
junction with Ir(ppy)3 sensitizers produced photons exhibiting a
0.8 eV anti-Stokes shift with respect to the incident light at moder-
ate power densities. This example provides proof-of-principle that
this family of compounds bearing solubilizing groups can be poten-
tially useful for implementation in upconversion schemes requiring
the production of UV light.
400
500
600
700
Wavelength (nm)
Fig. 6. Normalized absorption (solid black) and corrected emission (dashed black)
spectra of 1a, and normalized absorption (solid red) and corrected emission (dashed
red) spectra of Ir(ppy)3 in degassed toluene. To collect emission spectra, 1a was
excited using a Xe lamp at 330 2 nm and Ir(ppy)3 at 450 2 nm. All measurements
were performed using optically dilute solutions (abs ∼0.1 at each excitation wave-
length). (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of the article.)
4. Experimental
˚
Chemicals, solvents, standard grade silica gel (60 A, 32–63 ␮m),
and silica gel plates (200 ␮m) were purchased from commercial
sources. Reactions that required anhydrous conditions were carried
out under argon in oven-dried glassware. CDCl3 was the solvent for
NMR and chemical shifts relative to TMS at 0.00 ppm are reported
in parts per million (ppm) on the ı scale.
Scheme 2. Jablonski diagram depicting the upconversion process using Ir(ppy)3
as sensitizer and 1a as the acceptor. Solid lines indicate radiative pathways,
and dashed lines indicate non-radiative pathways. ISC = intersystem crossing,
TTET = triplet–triplet energy transfer and TTA = triplet–triplet annihilation.
A
JASCO V-570 UV-NIR spectrophotometer was used for
fluorescence, emitting photons (3.36 eV) anti-Stokes with respect
to the absorbed photons (2.54 eV). The overall process affords
visible-to-UV frequency upconversion with a net 0.8 eV energy
increase.
measurements of the absorption spectra and the fluorescence
measurements were done using a Jobin-Yvon-Horiba Fluorolog III
spectrofluorimeter. The excitation source was a 100 W xenon lamp,
the detector used was R-928 operating at a voltage of 950 V, and the
1.2x105
1x105
Slope = 1.99
Power Density
1 R2 = 0.998
(mW/cm2)
1.0x105
135
279
0.1
8.0x104
8x104
358
557
6.0x104
884
0.01
0.1
1
Normalized Power Density (a.u.)
4x104
0
4.0x104
2.0x104
0.0
0
200
400
600
800
1000
360
380
400
420
440
460
Power Density (mW/cm2)
Wavelength (nm)
Fig. 7. Upconverted emission from 1a as a function of 488 nm incident laser power density (left) using 24 ␮M of Ir(ppy)3 and 1 mM of 1a in deaerated toluene. The emission
profile was not corrected for the detector response. Upconversion emission intensity from 1a recorded at 411 nm as a function of measured excitation power density (right).
Inset: Double logarithmic plot of the normalized UC intensity versus the normalized incident power density yielding a slope of 2.0.

56
A.O. El-Ballouli et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
slit width was fixed at 5 nm for all measurements, while the excita-
tions were done at 330 nm. Fluorescence lifetimes were measured
using a 282 nm diode laser.
DSC heat/cool/heat experiments were carried out on com-
pounds (1a–2c) with a Q2000 TA Instrument Differential Scanning
Calorimeter while maintaining cooling and heating rates at
10 ◦C/min in all runs. DSC curves represent 2nd heating and 1st
cooling upon heating to a maximum of 450 ◦C and cooling to 0 ◦C.
The area under the DSC curves was integrated using analysis soft-
4.1.3. 2,7-Bis(4-fluorophenyl)tetrahydropyrene (1c)
White solid (53 mg, 45%), m.p. (determined by DSC) 285.5 ◦C. 1
H
NMR (300 MHz, CDCl3): ı 7.62–7.57 (t, J = 8.7 Hz, 4H), 7.29 (s, 4H),
7.16–7.11 (t, J = 8.7 Hz, 4H), 2.99 (s, 8H). 13C NMR (75.5 MHz, CDCl3):
ı 160.8, 139.0, 137.3, 135.8, 129.5, 128.5, 124.7, 115.7, 28.5. Anal.
Calcd. for C28H20F2: C, 85.26; H, 5.11. Found: C, 85.15; H, 5.07.
4.2. General procedure for the synthesis of 2a–c based on a
modified literature procedure for Suzuki coupling [57]
ware in order to find ꢁHmelting and ꢁHcrystallization
.
X-ray diffraction experiments were carried out with a Bruker
APEX II diffractometer, using Mo K␣ radiation (ꢀ = 0.71073 A) at
100 K. The raw data frames were integrated with the SAINT+ pro-
gram using narrow-frame algorithm.
2,7-Dibromopyrene 6 (1.64 g, 4.56 mmol) and boronic acid (7a,
b, or c; 10.9 mmol, 2.4 equivalents) were suspended in toluene
(137 mL), ethanol (13.7 mL) and K2CO3 (2 N solution in water,
23 mL). The mixture was purged with argon for 45 min. Catalytic
amounts of 18-crown-6 and Pd(PPh3)4 were added and the mix-
ture was refluxed for 24 h in the dark. The reaction was stopped
and left to cool. The obtained solid was washed with cold acetone
and was recrystallized from toluene to yield the pure product.
˚
Ir(ppy)3 was synthesized according to a literature procedure
[13]. In all spectroscopic measurements, Ir(ppy)3 solutions were
bubble degassed with Ar for at least 30 min prior to measure-
ment. Absorption spectra were acquired using a Hewlett-Packard
8453 diode array spectrophotometer and static luminescence was
measured using a FL/FS920 Edinburgh Instruments single photon
counting fluorimeter. Excitation for upconversion experiments was
achieved using an Ar/Kr ion laser (Coherent Innova 70C spectrum)
whose 488 nm output was passed through a 400 nm long pass fil-
ter and a 488 nm band pass filter prior to sample excitation. The
excited state lifetime of Ir(ppy)3 in degassed toluene was mea-
sured using a N2 pumped dye laser (MNL100, Lasertechnik Berlin)
with photomultiplier detection as described previously [55]. Abso-
lute quantum yields were acquired using a Hamamatsu absolute
quantum yield spectrometer (Quantaurus-QY C11347) equipped
with a 150 W xenon lamp and multichannel detector (CCD sensor).
The AQYs reported here were averages of several measurements of
solutions with O.D.∼0.05–0.10 at peak wavelength.
4.2.1. 2,7-Bis(4-tert-butylphenyl)pyrene (2a)
Silver solid (3.42 g, 80%), m.p. (determined by DSC) 383.2 ◦C.1H
NMR (300 MHz, CDCl3): ı 8.40 (s, 4H), 8.14 (s, 4H), 7.85 (d, J = 8.4 Hz,
4H), 7.61 (d, J = 8.4 Hz, 4H), 1.40 (s, 18H). 13C NMR (75.5 MHz,
CDCl3): ı 150.5, 138.7, 138.6, 131.5, 127.9, 127.7, 126.0, 123.7, 34.6,
31.4. Anal. Calcd. for C36H34: C, 92.66; H, 7.34. Found: C, 92.55; H,
7.34.
4.2.2. 2,7-Bis(4-methoxyphenyl)pyrene (2b)
Light yellow solid (3.15 g, 97%), m.p. (determined by DSC)
294.7 ◦C.1H NMR (300 MHz, CDCl3): ı 8.36 (s, 4H), 8.13 (s, 4H),
7.85 (d, J = 8.4 Hz, 4H), 7.12 (d, J = 8.4 Hz, 4H), 3.92 (s, 6H). 13C NMR
(75.5 MHz, CDCl3): ı 159.3, 138.5, 134.0, 131.5, 129.0, 127.9, 123.5,
114.5, 55.4. Anal. Calcd. for C30H22O2: C, 86.93; H, 5.35. Found: C,
86.88; H, 5.16.
4,5,9,10-Tetrahydropyrene (4) [27], 2,7-dibromo-4,5,9,10-
tetrahydropyrene (5) [28], and 2,7-dibromopyrene (6) [56] were
synthesized according to literature procedures.
4.2.3. 2,7-Bis(4-fluorophenyl)pyrene (2c)
Silver solid (700 mg, 40%), m.p. (determined by DSC) 302.0 ◦C.
1H NMR (300 MHz, CDCl3): ı 8.36 (s, 4H), 8.15 (s, 4H), 7.85 (m, 4H),
7.25 (m, 4H). 13C NMR (75.5 MHz, CDCl3): ı 138.0, 137.6, 131.5,
129.6, 129.5, 128.0, 123.8, 116.1, 115.8. Anal. Calcd. for C28H16F2:
C, 86.14; H, 4.13. Found: C, 85.86; H, 4.02.
4.1. General procedure for the synthesis of 1a–c based on a
modified literature procedure for Suzuki coupling [57]
2,7-Dibromo-4,5,9,10-tetrahydropyrene 5 (110 mg, 0.30 mmol)
and boronic acid (7a, b, or c; 0.75 mmol, 2.5 equiv.) were suspended
in toluene (11 mL), ethanol (1.1 mL) and K2CO3 (2 N solution in
water, 2.5 mL). The mixture was purged with argon for 45 min. Cat-
alytic amounts of 18-crown-6 and Pd(PPh3)4 were added and the
mixture, which was refluxed for 24 h in the dark. The reaction was
stopped and left to cool. The organic layer was washed with water
and the solvent was removed under vacuum. The obtained solid
was washed with cold acetone and was recrystallized from toluene
to yield the pure product.
Acknowledgments
This work was supported by the Petroleum Research Fund (PRF)
of the American Chemical Society (grant #: 47343-B10). X-ray stud-
ies were supported by NSF grants DMR 0934212 and CHE 0820852.
The authors are grateful for this support. One of the authors, B.W.,
acknowledges support from a Royal Society of Chemistry (RSC)
Research Fund grant. R.S.K. acknowledges support from a McMaster
Research Fellowship.
4.1.1. 2,7-Bis(4-tert-butylphenyl)tetrahydropyrene (1a)
White solid (100 mg, 80%), m.p. (determined by DSC) 392.6 ◦C.
1H NMR (300 MHz, CDCl3): ı 7.57 (d, J = 8.1 Hz, 4H), 7.48 (d,
J = 8.1 Hz, 4H), 7.33 (s, 4H), 2.98 (s, 8H), 1.37 (s, 18H). 13C NMR
(75.5 MHz, CDCl3): ı 150.1, 139.7, 138.3, 135.7, 129.5, 126.6, 125.7,
124.6, 34.5, 31.4, 28.5. Anal. Calcd. for C36H38: C, 91.86; H, 8.14.
Found: C, 91.94; H, 8.07.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jphotochem.
2013.07.018.
References
4.1.2. 2,7-Bis(4-methoxyphenyl)tetrahydropyrene (1b)
White solid (60 mg, 48%), m.p. (determined by DSC) 262.3 ◦C. 1
H
[1] H. Zhang, Y. Wang, K. Shao, Y. Liu, S. Chen, W. Qiu, X. Sun, T. Qi, Y. Ma, G. Yu,
Z. Su, D. Zhu, Novel butterfly pyrene-based organic semiconductors for field
effect transistors, Chem. Commun. (2006) 755–757.
[2] P. Sonar, M.S. Soh, Y.H. Cheng, J.T. Henssler, A. Sellinger, 1,3,6,8-
Tetrasubstituted pyrenes: solution-processable materials for application in
organic electronics, Org. Lett. 12 (2010) 3292–3295.
NMR (300 MHz, CDCl3): ı 7.59 (d, J = 8.7 Hz, 4H), 7.30 (s, 4H), 7.00 (d,
J = 8.7 Hz, 4H), 3.87 (s, 6H), 2.97 (s, 8H). 13C NMR (75.5 MHz, CDCl3):
ı 159.1, 139.4, 135.7, 133.8, 129.2, 128.0, 124.3, 114.2, 55.4, 28.6.
Anal. Calcd. for C30H26O2: C, 86.09; H, 6.26. Found: C, 85.64; H, 6.15.

A.O. El-Ballouli et al. / Journal of Photochemistry and Photobiology A: Chemistry 272 (2013) 49–57
57
[3] X. Feng, W. Pisula, K. Müllen, Large polycyclic aromatic hydrocarbons: synthesis
and discotic organization, Pure Appl. Chem. 81 (2009) 2203–2224.
[4] N. Ueda, B. Natsume, K. Yanagiuchi, Y. Sakata, T. Enoki, G. Saito, H.
Inokuchi, S. Misumi, The synthesis of 2,7-bis(dimethylamino)pyrene and -
tetrahydropyrene and the electrical conductivities of their complexes, Bull.
Chem. Soc. Jpn. 56 (1983) 775–779.
[30] P. Soustek, M. Michl, N. Almonasy, O. Machalicky, M. Dvorak, A. Lycka, The syn-
thesis and fluorescence of N-substituted 1- and 2-aminopyrenes, Dyes Pigm.
78 (2008) 139–147.
[31] R.J. Shozda, E.A. Depp, C.M. Stevens, M.B. Neuworth, Synthesis of p-
hydroxybenzylpyrene and homologs, J. Am. Chem. Soc. 78 (1956)
1716–1718.
[5] K.-S. Kim, S. Jeong, C. Kim, E. Kim, Y. Kwon, H. Kim, B.-D. Choi, Y.S. Han, Synthesis
and electro-optical properties of biphenyl analogues, Synth. Met. 160 (2010)
549–555.
[32] F. Casellato, C. Vecchi, A. Girelli, B. Casu, Differential calorimetric study of poly-
cyclic aromatic hydrocarbons, Thermochim. Acta 6 (1973) 361–368.
[33] C.A. Bear, D. Hall, J.M. Waters, T.N. Waters, Crystal structure of the doubly
bridged biphenyl, 2-chloro-4,5,9,10-tetrahydropyrene, J. Chem. Soc., Perkin
Trans. 2 (1973) 314–318.
[34] B. Marciniak, E. Rozycka-Sokolowska, A. Balinska, W. Pawliuk, Morphology
and X-ray characterization of acenaphthene, fluoranthene, and pyrene crystals
grown by sublimation, Proc. SPIE 4413 (2001) 408–412.
[35] C.S. Frampton, K.S. Knight, N. Shankland, K. Shankland, Single-crystal X-
ray diffraction analysis of pyrene II at 93 K, J. Mol. Struct. 520 (2000)
29–32.
[36] J.M. Robertson, J.G. White, The crystal structure of pyrene. A quantitative X-ray
investigation, J. Chem. Soc. (1947) 358–368.
[6] F. Moggia, C. Videlot-Ackermann, J. Ackermann, P. Raynal, H. Brisset, F. Fages,
Synthesis and thin film electronic properties of two pyrene-substituted olig-
othiophene derivatives, J. Mater. Chem. 16 (2006) 2380–2386.
[7] T.N. Singh-Rachford, F.N. Castellano, Photon upconversion based on sensitized
triplet–triplet annihilation, Coord. Chem. Rev. 254 (2010) 2560–2573.
[8] S. Baluschev, V. Yakutkin, T. Miteva, Y. Avlasevich, S. Chernov, S. Aleshchenkov,
G. Nelles, A. Cheprakov, A. Yasuda, K. Müllen, G. Wegner, Blue-green up-
conversion: noncoherent excitation by NIR light, Angew. Chem. Int. Ed. Engl.
46 (2007) 7693–7696.
[9] A. Monguzzi, R. Tubino, F. Meinardi, Multicomponent polymeric film for red
to green low power sensitized up-conversion, J. Phys. Chem. A 113 (2009)
1171–1174.
[10] A. Rapaport, J. Milliez, M. Bass, A. Cassanho, H. Jenssen, Review of the properties
of up-conversion phosphors for new emissive displays, J. Disp. Technol. 2 (2006)
68–78.
[37] G.R. Desiraju, A. Gavezzotti, From molecular to crystal structure; polynuclear
aromatic hydrocarbons, J. Chem. Soc., Chem. Commun. (1989) 621–623.
[38] J.J. McKinnon, M.A. Spackman, A.S. Mitchell, Novel tools for visualizing and
exploring intermolecular interactions in molecular crystals, Acta Crystallogr.
B. B60 (2004) 627–668.
[11] T.N. Singh-Rachford, R.R. Islangulov, F.N. Castellano, Photochemical upconver-
sion approach to broad-band visible light generation, J. Phys. Chem. A 112
(2008) 3906–3910.
[12] R.S. Khnayzer, J. Blumhoff, J.A. Harrington, A. Haefele, F. Deng, F.N. Castellano,
Upconversion-powered photoelectrochemistry, Chem. Commun. 48 (2012)
209–211.
[39] R. Kiralj, B. Kojic-Prodic, I. Piantanida, M. Zinic, Crystal and molecular structures
of diazapyrenes and a study of ␲–␲ interactions. This paper was presented
orally at The Seventeenth European Crystallographic Meeting, Lisbon, Portugal,
24–28 August 1997, Acta Crystallogr. B 55 (1999) 55–69.
[40] J. Bernstein, Polymorphism in Molecular Crystals, Oxford University Press, New
York, 2002.
[13] W. Zhao, F.N. Castellano, Upconverted emission from pyrene and di-tert-
butylpyrene using Ir(ppy)3 as triplet sensitizer, J. Phys. Chem. A 110 (2006)
11440–11445.
[14] A. Monguzzi, R. Tubino, S. Hoseinkhani, M. Campione, F. Meinardi, Low power,
non-coherent sensitized photon up-conversion: modelling and perspectives,
Phys. Chem. Chem. Phys. 14 (2012) 4322–4332.
[15] R.R. Islangulov, F.N. Castellano, Photochemical upconversion: anthracene
dimerization sensitized to visible light by a RuII chromophore, Angew. Chem.
Int. Ed. 45 (2006) 5957–5959.
[16] A. Köhler, H. Bässler, Triplet states in organic semiconductors, Mater. Sci. Eng.
66 (2009) 71–109.
[41] A. Nangia, Conformational polymorphism in organic crystals, Acc. Chem. Res.
41 (2008) 595–604.
[42] Y. Qiao, J. Zhang, W. Xu, D. Zhu, Novel 2,7-substituted pyrene deriva-
tives: syntheses, solid-state structures, and properties, Tetrahedron 67 (2011)
3395–3405.
[43] A. Burger, R. Ramberger, On the polymorphism of pharmaceuticals and other
molecular crystals. I. Theory of thermodynamic rules, Mikrochim. Acta 2 (1979)
259–271.
[44] B. Milian-Medina, S. Varghese, R. Ragni, H. Boerner, E. Orti, G.M. Farinola, J. Gier-
schner, Excited-state switching by per-fluorination of para-oligophenylenes, J.
Chem. Phys. 135 (2011) 124509.
[17] Y.Y. Cheng, B. Fu¨ ckel, T. Khoury, R.l.G.C.R. Clady, M.J.Y. Tayebjee, N.J. Ekins-
Daukes, M.J. Crossley, T.W. Schmidt, Kinetic analysis of photochemical
upconversion by triplet–triplet annihilation: beyond any spin statistical limit,
J. Phys. Chem. Lett. 1 (2010) 1795–1799.
[45] N.I. Nijegorodov, W.S. Downey, M.B. Danailov, Systematic investigation
of absorption, fluorescence and laser properties of some p- and m-
oligophenylenes, Spectrochim. Acta
783–795.
A Mol. Biomol. Spectrosc. 56 (2000)
[18] K. Mislow, M.A.W. Glass, H.B. Hopps, E. Simon, G.H. Wahl, The stereochem-
istry of doubly bridged biphenyls: synthesis, spectral properties, and optical
stability, J. Am. Chem. Soc. 86 (1964) 1710–1733.
[19] S. Sumalekshmy, K.R. Gopidas, Synthesis and photophysical studies of
donor–acceptor substituted tetrahydropyrenes, New J. Chem. 29 (2005)
325–331.
[20] S. Sumalekshmy, K.R. Gopidas, Intramolecular charge transfer processes in
donor–acceptor substituted vinyltetrahydropyrenes, Photochem. Photobiol.
Sci. 4 (2005) 539–546.
[21] I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.,
Academic, New York, N.Y., 1971.
[46] G. Heimel, M. Daghofer, J. Gierschner, E.J. List, A.C. Grimsdale, K. Mullen, D.
Beljonne, J.L. Bredas, E. Zojer, Breakdown of the mirror image symmetry in the
optical absorption/emission spectra of oligo(para-phenylene)s, J. Chem. Phys.
122 (2005) 54501.
[47] N.J. Turro, V. Ramamurthy, J.C. Scaiano, Modern Molecular Photochemistry of
Organic Molecules, University Science Books, Sausalito, CA, 2010.
[48] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York,
1983.
[49] D.S. Karpovich, G.J. Blanchard, Relating the polarity-dependent fluorescence
response of pyrene to vibronic coupling. Achieving a fundamental understand-
ing of the py polarity scale, J. Phys. Chem. A. 99 (1995) 3951–3958.
[50] C.-Y. Chen, Y. Ito, Y.-C. Chiu, W.-C. Wu, T. Higashihara, M. Ueda, W.-C. Chen,
Design and synthesis of new cationic water-soluble pyrene containing den-
drons for DNA sensory applications, J. Polym. Sci. Part A: Polym. Chem. (2011)
297.
[22] A.G. Crawford, A.D. Dwyer, Z. Liu, A. Steffen, A. Beeby, L.O. Palsson, D.J. Tozer, T.B.
Marder, Experimental and theoretical studies of the photophysical properties
of 2- and 2,7-functionalized pyrene derivatives, J. Am. Chem. Soc. 133 (2011)
13349–13362.
[23] J.B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience, London,
1970.
[51] For instance, for 1c band I is at 378 nm and band II is at 394 nm. Compound 2c
has band I at 410 nm and band II at 432 nm.
[24] J.B. Birks, Organic Molecular Photophysics, Wiley-Interscience, London, 1973.
[25] Z. Bao, J. Locklin, Organic Field-Effect Transistors, CRC Press, Boca Raton, FL,
2007.
[52] M. Montalti, A. Credi, L. Prodi, M.T. Gandolfi, Handbook of Photochemistry, 3rd
ed., CRC Press, Boca Raton, 2006.
[53] S. Sumalekshmy, K.R. Gopidas, Photoinduced intramolecular charge transfer
in donor–acceptor substituted tetrahydropyrenes, J. Phys. Chem. B 108 (2004)
3705–3712.
[54] A. Haefele, J. Blumhoff, R.S. Khnayzer, F.N. Castellano, Getting to the (square)
root of the problem: how to make noncoherent pumped upconversion linear,
J. Phys. Chem. Lett. 3 (2012) 299–303.
[55] D.S. Tyson, F.N. Castellano, J. Phys. Chem. A 103 (1999) 10955–10960.
[56] H. Lee, R.G. Harvey, Synthesis of 2,7-dibromopyrene, J. Org. Chem. 51 (1986)
2847–2848.
[26] J.N. Moorthy, P. Natarajan, P. Venkatakrishnan, D.-F. Huang, T.J. Chow, Steric
inhibition of pi-stacking: 1,3,6,8-tetraarylpyrenes as efficient blue emitters in
organic light emitting diodes (OLEDs), Org. Lett. 9 (2007) 5215–5218.
[27] D.M. Connor, S.D. Allen, D.M. Collard, C.L. Liotta, D.A. Schiraldi, Efficient syn-
thesis of 4,5,9,10-tetrahydropyrene: a useful synthetic intermediate for the
synthesis of 2,7-disubstituted pyrenes, J. Org. Chem. 64 (1999) 6888–6890.
[28] D. Rausch, C. Lambert, Synthesis and spectroscopic properties of a hexapyrenyl-
benzene derivative, Org. Lett. 8 (2006) 5037–5040.
[29] K. Tani, J. Takano, Y. Tohda, M. Ariga, H. Higuchi, S. Misumi, Synthesis and prop-
erties of [2.2](3,5)- and [2.2.2.2](3,3ꢀ,5,5ꢀ)-diphenoquinonophanes, Chem. Lett.
(1991) 1461–1464.
[57] B.K. Shah, D.C. Neckers, J. Shi, E.W. Forsythe, D. Morton, Photophysical proper-
ties of anthanthrene-based tunable blue emitters, J. Phys. Chem. A 109 (2005)
7677–7681.