A Tethered Tolane: Twisting the Excited State

Article abstract of DOI:10.1002/chem.201804095

The synthesis of a doubly bridged tolane is reported. The target is obtained in a five-step synthesis, starting from commercially available 2-amino-meta-xylene by a combination of a Sandmeyer reaction, radical bromination, and Stille-type coupling, followed by double ring closing. The doubly tethered tolane is crystalline; the two phenyl rings are highly twisted with respect to each other both in solution and in the solid state. Optical spectroscopy and quantum chemical calculations show that the doubly bridged tolane is twisted not only in the ground state, but also in the excited state, leading to emission from the twisted state in solution and in the solid state. Strong phosphorescence is observed at cryogenic temperatures.

Full text of DOI:10.1002/chem.201804095

A Journal of
Accepted Article
Title: A Tethered Tolane: Twist the Excited State
Authors: Yuri Kozhemyakin, Maximilian Krämer, Frank Rominger,
Andreas Dreuw, and Uwe Heiko Bunz
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201804095
Link to VoR: http://dx.doi.org/10.1002/chem.201804095
Supported by

10.1002/chem.201804095
Chemistry - A European Journal
COMMUNICATION
A Tethered Tolane: Twisting the Excited State
Yury Kozhemyakin,[a] Maximilian Krämer,[b] Frank Rominger,[a] Andreas Dreuw,*[b] und
Uwe H. F. Bunz*[a]
and quantum chemical calculations of 1.[6] The absorption
spectrum of 1 is blue-shifted in comparison to that of 1a, while the
emission spectra of 1 and 1a are superimposable (for optical
spectra of 1a see Supporting Information). Only for samples of 1
that are cooled to 77 K one observes a blue-shifted emission and
long-living phosphorescence.
Abstract: We report the synthesis of a doubly bridged tolane. The
target is obtained in a five-step synthesis, starting from commercially
available 2-amino-meta-xylene by a combination of a Sandmeyer
reaction, radical bromination and Stille-type coupling, followed by
double ring closing. The doubly tethered tolane 7 is crystalline; the
two phenyl rings are highly twisted with respect to each other both in
solution and in the solid state. Optical spectroscopy and quantum
chemical calculations show that doubly bridged 7 is twisted not only
in the ground state but also in the excited state, leading to emission
from the twisted state in solution and in the solid state. Strong
phosphorescence is observed at cryogenic temperatures.
Here we describe the synthesis and the properties of a doubly
bridged tolanophane, 7, twisted in its ground and excited states,
due to the presence of two molecular tethers.
Diphenylacetylene, tolane, is an important fundamental chromo-
phore in organic and polymer chemistry.[ 1 ] Linear and cyclic
oligomers as well as tolane-based polymers display optical
properties which render this class of molecules attractive,
particularly in sensor-type applications.[2] The parents´ optical
properties are defined by its planarity in the solid state. In solution
tolane features a minute rotational barrier (<1kcal/mol). Despite
this, tolane absorbs and emits only from the planar conformation,
even in solution. Oligomers and polymers of tolane, optical
properties strongly suggest, however, twisted phenyl rings.
Scheme 1. Synthesis of a doubly bridged permanently twisted tolanophane 7.
Starting from 2 a Sandmeyer reaction furnishes 3.[8] Double
radical bromination (via 4[9]) and hydrolysis to 5[10] gives the open
tolane 6 after Stille coupling.[11] The last step closes both auxiliary
cycles, using malonyl chloride to give the target 7 in moderate
yield. 7 is crystalline so a single crystal structure analysis was
performed.
One possibility to explore the photophysics of twisted tolane is to
suppress planarization either through steric hindrance (difficult)
or,[3] or forcing the tolane by a molecular tether into the desired
twisted conformation. Tethers successfully enforce torsion angles
between the two phenyl rings of tolane.[4] Length and composition
of the tether are critical, or otherwise the planar form is stabilized.
Since the first papers by Finney and Crisp and Bubner, tethering[5]
is a tool for the engineering of the twist angle in tolanes.[6] Yet,
only in 2013 we published a privileged tether structure, which sets
the torsion close to 90°.[7] The tolanophane 1 is twisted in its
ground state in solution, but planar in the excited state, despite
tethering. This behavior is illuminated by the optical properties
[a]
[b]
Prof. U. Bunz, Dr. Y. Kozhemyakin, Dr. F. Rominger,
Organisch-Chemisches Institut, Ruprecht-Karls-Universität
Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, FRG
E-Mail: uwe.bunz@oci.uni-heidelberg.de
Prof. A. Dreuw, Dr. M. Krämer,
Interdisziplinäres Zentrum für Wissenschaftliches Rechnen,
Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 205,
69120 Heidelberg, FRG
Figure 1. Single X-ray crystal structure of 7. The torsion angle between both
phenyl rings is = 74.4° in the solid state.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/chem.201804095
Chemistry - A European Journal
COMMUNICATION
optimized, starting from the geometry of the X-ray crystal structure.
DFT/CAM-B3LYP gives a torsion angle of 73.1°. Even though a
search of the potential hypersurface shows the existence of
further local minima, conformational isomers of 7, all highly
energetic and at least 10 kcal/mol above the energy of the twisted
geometry of 7 in the crystal structure. We excluded these other
conformers; the twisted one is the only one present in solution.
Figure 2. Absorption- (top) and emission spectra (bottom) of 1 and 7 in solution
(n-hexane, solid line, excitation wavelength: 1 at 280 nm, 7 at 265 nm).
Emission spectra in cryogenic matrices at 77 K in EPA (diethyl ether :
isopentane : ethanol 5:5:2, dotted line, excitation wavelength: 1 at 264 nm, 7 at
265 nm).
Figure 3. Frontier molecular orbitals of 7 (DFT/CAM-B3LYP/6-31G*) in the
optimized ground state structure.
The vertical excitation energies of 7 were calculated for the
optimized twisted structure (TDDFT/CAM-B3LYP, gas phase,
Table 1). The S2-state displays huge oscillator strength and
therefore dominates the absorption spectrum. The computed
vertical excitation energy is 4.92 eV, corresponding to a
wavelength of 256 nm, in reasonable agreement within the
expected error of the applied methodology of ±0.2 eV (n-hexane,
Figure 2). The S2 state represents the HOMO->LUMO excitation.
The S1 state is almost isoenergetic with excitation energy of 4.84
eV but with vanishing oscillator strength, representing an
electronic excitation from HOMO-1 into LUMO. This state is not
observed in the experimental spectra.
Figure 1 displays the molecular structure of 7 in the single crystal.
Bond length and bond angles are inconspicuous, the acetylene
bridge linear – a consequence of the double symmetrical tethers
that exact a symmetrical twisted conformation. The torsion of the
phenyl rings is  = 74.4°. If one assumes that the -overlap is
proportional to (cos 2 of the torsion angle, the -overlap is only
7% of that of the planar conformation, to be neglected. The
absorption and emission spectra of 7 and 1 are shown in Figure
2. The absorption spectra of 1 and 7 are similar, but the spectrum
of 7 is a bit red-shifted. Significant differences, however, exist in
the emission spectra. 1 shows at room temperature the typical red
shift that accompanies planarization, (max.em at 312 nm), the
emission of 7 is blue shifted but structured with a max.em at 289
nm (4.3 eV) and a second emission band at 310 nm (4.0 eV). The
emissive quantum yield (n-hexane, 0.01) is low but easily
measurable. The emissive life time is 0.2 ns fairly short; planar
and singly tethered tolanes have emissive lifetimes (n-hexane) of
0.4 ns.[4a,b]
Table 1: Relative energies of the relevant excited states for the photochemistry
of 7 at the identified local minima in the electronic ground state S0 (twisted (tw),
planar (pl)), in the S1 state (tw), trans-bent (tb), pl) and the T1 state (tw, pl), the
corresponding maximum absorption/emission wavelengths (λmax) as well as the
twisted angle computed at TDDFT/CAM-B3LYP level of theory.
S0
S1
T1
tw
pl
tw
tb
pl
tw
pl
E(S0)
[eV]
E(S1)
[eV]
E(S2)
[eV]
E(T1)
[eV]
0.0
0.42
0.45
1.29
0.74
0.47
0.83
Upon decrease of the temperature, we find a slight blue shift of
the emission of 1 and the appearance of strong phosphorescence.
This behavior is also observed for 7. The phosphorescence of 7
displays two dominant bands at 425 and 450 nm (2.75 and 2.9
eV), but the residual low temperature fluorescence is identical to
the fluorescence of 7 at room temperature; 7 must be twisted in
ground and excited states and does not planarize – contrary to 1
at room temperature in the excited state.
4.84
(0.00)
4.92
(0.27)
3.45
4.97
(0.67)
5.33
(0.06)
3.49
4.39
(0.00)
4.54
(0.24)
3.22
4.27
(0.01)
5.42
(0.09)
3.60
4.54
(0.72)
4.98
(0.09)
3.13
4.63
(0.12)
4.69
(0.31)
3.06
4.57
(0.80)
5.02
(0.06)
3.11
λmax
[nm]
φ [deg]
256a
273a
315b
416b
326b
479c
66.4
544c
73.1
20.4
70.4
67.5
-16.6
-13.6
acorresponds to the maximum absorption wavelength
bcorresponds to the maximum fluorescence wavelength
ccorresponds to the maximum phosphorescence wavelength
To support the interpretation of the optical spectra we performed
quantum chemical calculations. The geometry of 7 was DFT-
This article is protected by copyright. All rights reserved.

10.1002/chem.201804095
Chemistry - A European Journal
COMMUNICATION
2.59 eV, a phosphorescence wavelength of 479 nm, in good
agreement with the experimental data.
In conclusion we have prepared a doubly bridged tolane 7, and
investigated its single crystal structure, spectroscopy, and
quantum chemistry. We demonstrated for the first time that two
tethers fixate the forcibly twisted tolane geometry in ground and
excited states, with the excited state being twisted trans-bent. The
blue-shifted absorption and emission of 7 fashions the doubly
bridged tolane into an attractive building block, to be introduced
into conjugated polymers to achieve band-gap engineering
towards the blue or alternatively to achieve enforced
phosphorescence.
Experimental
All quantum chemical calculations were performed with Q-Chem 5.0, the
electronic ground state was calculated with DFT and the electronically
excited state was calculated using time dependent DFT (TDDFT) with
CAM-B3LYP and 6-31G* basis set respectively. In earlier investigations of
tolane and a bridged tolane 1 this methodology was found to be precise.
Figure 4. Optimized potential energy curves (TDDFT/CAM-B3LYP) of the S1
state along the twist angle of the phenyl rings for the identified local minima:
trans-bent
S1twisted, S1
und S1planar. The insets show the optimized structures from
above (left) and from the front (right).
6: A published procedure was adapted.[11] Bis(trimethylstannyl)acetylene
(1.00 g, 2.84 mmol, 0.5 eq) and Pd(PPh3)4 (0.657 g, 0.569 mmol, 0.1 eq)
were added to a degassed solution of 5 (1.24 g, 5.69 mmol, 1.0 eq) in
anhydrous 1,4-dioxane (140 mL) followed by stirring for 20 h at 110 °C.
Upon cooling, Celite was added and volatiles were removed in vacuo.
Chromatographic purification (silica gel; CH2Cl2/methanol 10:1, v/v)
yielded a brownish solid. Yield: 509 mg (1.71 mmol, 60%). Analytical
sample was prepared by recrystallization from methanol.
The geometry optimization of the electronically excited singlet
state, S2, starting from the equilibrium geometry of the ground
state, leads directly into a twisted minimum (S1twisted), structurally
similar to the ground state. The twist angle decreases to 70.4°.
Geometry optimization transforms the S2-state into the S1-state
with a vertical fluorescence energy of 3.94 eV (315 nm), in
excellent agreement with the fluorescence spectrum of 7.
The S1-potential surface shows further local minima
corresponding to stable conformational isomers, a trans-bent
isomer (S1trans-bent) and a planar isomer (S1planar). The planar
minimum (-16.6°, 0.15 eV) is energetically localized above that of
the twisted minimum (Table 1) and can only be reached via a very
high energy barrier (Figure 4), so we exclude the population of the
minimum after optical excitation. The twisted trans-bent minimum
is 0.12 eV lower in energy than the twisted one and can be
reached over a barrier of 0.2 eV (Figure 4). The radiationless
transition of the planar trans-bent structure is the main
deactivation channel, which leads to the fluorescence quenching
of 7; it explains the low emissive quantum yield of 0.01. Only the
part of the excited molecules fluoresce that are trapped in the
twisted minimum. As the oscillator strength of the twisted trans-
bent minimum is small we find impulse deactivation leading to
intersystem crossing into the triplet manifold, as shown for the
unsubstituted tolane.[1a] Contrary to the unsubstituted tolane and
to 1, in case of 7 the central dihedral is still 67.5°. Here also the
double tether hinders the planarization through massive steric
interactions.
Rf=0.23 (CH2Cl2/methanol 10:1, v/v); m.p. 208°C (decomp.); 1H NMR
(DMSO-d6, 600 MHz): δ=4.77 (d; J 5.8 Hz; 8H; ArCH2), 5.38 (t; J 5.8 Hz;
4H; OH), 7.41–7.48 (m; 6H, HAr) ppm; 13C NMR (DMSO-d6, 150 MHz):
δ=61.6, 94.0, 116.9, 124.3, 128.3, 143.9 ppm; IR (neat): ν=̃ 3267, 2818,
1462, 1417, 1363, 1327, 1232, 1161, 1067, 1043, 780, 668 cm–1; HRMS
(DART+) m/z: [M+NH4]+ calcd for C18H22NO4: 316.1543; found: 316.1544,
correct isotope distribution; elemental analysis calcd (%) for C18H18O4: C
72.47, H 6.08; found: C 72.07, H 6.36.
7: A solution of malonyl dichloride (171 μL, 1.756 mmol, 4 eq) in dry CH2Cl2
(6 mL) was added in 1.5 h to a suspension of the tetra-ol 6 (131.0 mg,
0.439 mmol, 1 eq) in dry CH2Cl2 (11 mL) under nitrogen at RT. The
reaction mixture was stirred for 24 h filtered through silica gel and flushed
with ethyl acetate (100 mL). Chromatography (silica gel; CHCl3/petroleum
ether 1:1, v/v) gives 7 as colorless material in 15% (30.0 mg, 69.1 μmol).
Rf=0.38 (petroleum ether/ethyl acetate 1:1, v/v); m.p. 302°C (decomp.); 1H
NMR (CDCl3, 400 MHz): δ = 3.39 (s; 4H; COCH2CO), 4.91 (d; J 11.3 Hz;
2H; ArCH2), 5.43 (d; J 11.3 Hz; 2H; ArCH2), 7.36–7.45 (m; 6H; HAr) ppm;
13C NMR (CDCl3, 150 MHz): δ=41.7, 67.4, 92.4, 125.3, 129.0, 131.5, 137.2,
166.1 ppm; IR: ν=̃ 2954, 1724, 1472, 1373, 1274, 1211, 1138, 999, 984,
To describe the phosphorescence by theory, the geometry of the
lowest energy triplet, T1 was optimized. As in the singlet state, the
triplet is also twisted. The central dihedral displays an angle of
66.4°. For T1 we found an additional, planar minimum, however,
it is energetically not accessible. Both triplet structures are – by
inspection – almost indistinguishable from the twisted and planar
structures of the S1 state, while a trans-bent minimum does not
exist. The observed phosphorescence emanates therefore from
this twisted minimum. In the twisted T1 state an unpaired electron
occupies the HOMO and one the LUMO of the singlet ground
state. In the equilibrium geometry of T1 the excitation energy is
795 cm–1; HRMS (DART+) m/z: [M+NH4]+ calcd for C24H22NO8: 452.1340;
found: 452.1334, correct isotope distribution; elemental analysis calcd (%)
C24H18O8: C 66.36, H 4.18; found C 65.90, H 4.28; X-ray analysis:
C24H18O8, Mw=434.40, colorless crystal was harvested after slow
evaporation of a CDCl3-solution of 7. Size 0.203 × 0.097 × 0.076 mm3,
monoclinic, space group C2/c, Z=8, a=29.675(2) Å, b=10.1444(7) Å, c =
17.3726(12) Å, α = 90°, β=115.2430(19)°, γ = 90°, V = 4730.3(6) Å3, ρ =
1.220 g/cm3, T =200(2) K, Θmax = 25.390°, Radiation MoKα, λ=0.71073 Å,
15163 reflections measured, 4345 independent reflections, (Rint=0.0606),
2537 observed reflections (I>2σ(I)); R1(F) = 0.051, wR(F2) = 0.098 for the
observed data. CCDC 1858258 contains additional crystallographic
This article is protected by copyright. All rights reserved.

10.1002/chem.201804095
Chemistry - A European Journal
COMMUNICATION
[4] a) Y. Kozhemyakin, A. Kretzschmar, M. Krämer, F. Rominger, A.
Dreuw, U. H. F. Bunz, Chem. Eur. J. 2017, 23, 9908-9918. b) S.
Menning, M. Krämer, A. Duckworth, F. Rominger, A. Beeby, A. Dreuw, U.
H. F. Bunz, J. Org. Chem. 2014, 79, 6571-6578; c) G. Brizius, K.
Billingsley, M. D. Smith, U. H. F. Bunz, Org. Lett. 2003, 5, 3951-3954.
information for this publication. Data can be obtained from Cambridge
Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/.
All synthetic details and analytical data are found in the Supporting
Information.
[5] a) G. T. Crisp, T. P. Bubner, Tetrahedron 1997, 53, 11881-11898; b)
S. A. McFarland, N. S. Finney, J. Am. Chem. Soc. 2002, 124, 1178-1179.
Acknowldegement
We thank the DFG for generous financial support (DFG project number
256288978, BU771/12-1, UHF B. und A D.).
[6] a) For specially twisted biphenyls see: A Mishchenko, D. Vonlanthen,
V. Meded, M. Burkle, C. Li, I. V. Pobelov, A. Bagrets, J. K. Viljas, F.
Pauly, F. Evers, M. Mayor, T. Wandlowski, Nano Lett. 2010, 10, 156-163.
b) D. Vonlanthen, A. Mishchenko, M. Elbing, M. Neuburger, T.
Wandlowski, M. Mayor, Angew. Chem. Int. Ed. 2009, 48, 8886-8890. c)
J. Rotzler, D. Vonlanthen, A. Barsella, A. Boeglin, A. Fort, M. Mayor, Eur.
J. Org. Chem. 2010, 1096-1110. d) J. Rotzler, H. Gsellinger, M.
Neuburger, D. Vonlanthen, D. Häussinger, M. Mayor, Org. Biomol.
Chem. 2011, 9, 86-91.
References: Tolane • Twisting • Tolanophane • Bridged Chromophore •
REFERENCES
[1] a) M. Krämer, U. H. F. Bunz, A. Dreuw, J. Phys. Chem. A 2017, 121,
946-953 and therein cited literature. b) J. M. Seminario, A. G. Zacarias, J.
M. Tour, J. Am. Chem. Soc. 2000, 122, 3015-3020 and therein cited
literature.
[7] S. Menning, M. Krämer, B. A. Coombs, F. Rominger, A. Beeby, A.
Dreuw, U. H. F. Bunz, J. Am. Chem. Soc. 2013, 135, 2160-2163.
[8] G. W. Kabalka, A. R. Mereddy, Organometallics 2004, 23, 4519-4521.
[2] a) U. H. F. Bunz, Chem. Rev. 2000, 100, 1605-1644; b) U. H. F. Bunz,
Macromol. Rapid Commun. 2009, 30, 772-805; c) U. H. F. Bunz, K.
Seehafer, M. Bender, M. Porz, Chem. Soc. Rev. 2015, 44, 4322-4336. d)
U. H. F. Bunz, J. M. Imhof, R. K. Bly, C. G. Bangcuyo, L. Rozanski, D. A.
V. Bout, Macromolecules 2005, 38, 5892-5896; e) J. Kim, T. M. Swager,
Nature 2001, 411, 1030-1034; f) T. Miteva, L. Palmer, L. Kloppenburg, D.
Neher, U. H. F. Bunz, Macromolecules 2000, 33, 652-654. g) M. I. Sluch,
A. Godt, U. H. F. Bunz, M. A. Berg, J. Am. Chem. Soc. 2001, 123, 6447-
6448; h) L. T. Liu, D. Yaron, M.I. Sluch, M. A. Berg, J. Phys. Chem. B
2006, 110, 18844-18852.
[9] A. Casitas, M. Canta, M. Solа, M. Costas, X. Ribas, J. Am. Chem.
Soc. 2011, 133, 19386-19392.
[10] a) D. Hellwinkel, W. Krapp, Chem. Ber. 1977, 110, 693-702; b) L.
Ye, D. Ding, Y. Feng, D. Xie, P. Wu, H. Guo, Q. Meng, H. Zhou,
Tetrahedron 2009, 65, 8738-8744.
[11] A. Ricci, M. Chiarini, M. T. Apicella, D. Compagnone, M. Del Carlo,
C. Lo Sterzo, L. Prodi, S. Bonacchi, D. Villamaina, N. Zaccheroni,
Tetrahedron Lett. 2013, 54, 303-307.
[3] a) S. Toyota, Chem. Rev. 2010, 110, 5398-5424; b) O. S. Miljanic, S.
D. Han, D. Holmes, G. R. Schaller, K. P. C. Vollhardt, Chem. Commun.
2005, 2606-2608.
This article is protected by copyright. All rights reserved.

10.1002/chem.201804095
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Let’s twist again: Two tethers fix the
two phenyl rings of the target
structure, a tolane, twisted - even in
the electronically excited state. The
double bridging is necessary because
a single linker is not sufficient for
holding the two phenyl rings
Y. Kozhemyakin, M. Krämer, F.
Rominger, A. Dreuw,*
U. H. F. Bunz*
Page No. – Page No.
A Tethered Tolane: Twisting the
Excited State
perpendicular to each other in the
excited state.
1
2
3
4
5
M. Krämer, U. H. F. Bunz, A. Dreuw, J. Phys. Chem
U. H. F. Bunz, Che m. Rev. 00 100, 1605-1644; c) U. H. F. Bunz, Macromol. Rapid Commun.
a) S. Toyota, Chem. Re v. 010 110, 5398-5424 b) O. S. Miljanic, S. D. Han, D. Holmes, G. R. Schaller, K. P. C. Vollhardt, Che m. Commun.
a) Y. Kozhemyakin, A. Kretzschmar, M. Krämer, F. Romi nger, A. Dreuw, U. H. F. Bunz, Che m. Eur. J. 2017 23, 9908-9918. b) S. Menni ng, M. Krämer, A. Duckworth, F. Romi nger, A. Beeby, A. Dreuw, U. H. F. Bunz, J. Org. Che m. 2014, 79, 6571-6578; c) G. Brizi us, K. Billingsley, M. D. Smith, U. H. F. Bunz, Org. Le tt. 2003, 5, 3951-3954.
a) G. T. Crisp, T. P. Bubner, Tetrahedron 997 53, 11881-11898 b) S. A. McFarland, S. Finney, J. Am. Chem. Soc. 02 124, 1178-1179.
.
A
201
7
,
121, 946-953. J. M. Seminario, A. G. Zacarias, J. M. Tour, J. Am. Chem
09 30, 772-805; U. H. F. Bunz K. Seehafer, M. Bender, M. Porz,
005 2606-2608.
. Soc. 2000, 122, 3015-3020.
2
2
0
,
2
0
,
,
Chem. Soc. Rev. 2015, 44, 4322-4336.
,
;
2
,
,
1
,
;
N
.
2
0
,
6
7
S. Menning, M. Krämer, B. A. Coombs, F. Romi nger, A. Beeby, A. Dreuw, U. H. F. Bunz, J. Am. Che m. Soc. 2013, 135, 2160-2163.
This article is protected by copyright. All rights reserved.