Photoisomerization of all-cis-1,6-diphenyl-1,3,5-hexatriene in the solid state and in solution: A simultaneous three-bond twist process

Article abstract of DOI:10.1002/anie.200902724

Disrotatory bicycle pedals: Irradiation of the title compound in the solid state gives the all-trans isomer directly in a crystal-to-crystal reaction. This threefold cis-trans photoisomerization is proposed to proceed by a two-stage mechanism that is consistent with two simultaneous bicyclepedal processes occurring in disrotatory fashion about the central bond.

Full text of DOI:10.1002/anie.200902724

Communications
DOI: 10.1002/anie.200902724
Trienes
Photoisomerization of All-cis-1,6-diphenyl-1,3,5-hexatriene in the
Solid State and in Solution: A Simultaneous Three-Bond Twist
Process**
Jack Saltiel,* Dimitrios Papadimitriou, Tallapragada S. R. Krishna, Zhen-Nian Huang,
Govindarahan Krishnamoorthy, Somchoke Laohhasurayotin, and Ronald J. Clark
The photophysical[1] and photochemical[2] behaviors of all-
trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) have attracted
a great deal of attention because, as the first member of the
family of vinylogous a,w-diphenylpolyenes with a 2Ag lowest
excited state, ttt-DPH serves as a polyene model. Medium-
imposed constraints enhance the torsional barriers of olefins
in the lowest excited singlet state[3] and inhibit cis–trans
photoisomerization.[4] Two mechanisms involving concerted
rotation about two bonds in the S1 state, which were initially
postulated to explain retinyl photoisomerization within the
protein environments in rhodopsin and bacteriorhodopsin,
have been applied generally to account for photoisomeriza-
tion in volume-confining media: The bicycle-pedal mecha-
nism (BP) involves simultaneous rotation in the S1 state about
two polyene double bonds,[5] and the hula–twist mechanism
(HT) involves simultaneous rotation about a double bond and
an adjacent essential single bond (equivalent to a 1808
translocation of one CH unit).[6] Our recent work on the
cis,cis-1,4-diphenyl- and cis,cis-1,4-di(o-tolyl)-1,3-butadienes
(cc-DPB and cc-DTB) revealed the BP process as the only
photoisomerization pathway in the solid state at room
temperature[7] and as a significant pathway competing with
the usual one-bond-twist pathway in the soft isopentane glass
at 77 K.[8] Recognizing that the BP process accounts for the
interconversion of ctt-DPH and tct-DPH isomers in solution[2]
and for the one-way ctc-DPH!ttt-DPH photoisomerization
of symmetrical p,p’-disubstituted cis,trans,cis-1,6-diphenyl-
1,3,5-hexatriene derivatives in the solid state,[9] we reasoned
that the analogous extended BP process in the hitherto
unknown all-cis DPH might give tct-DPH in the solid state.
Observations reported herein show that instead, irradiation of
ccc-DPH in the solid state gives ttt-DPH directly in a crystal-
to-crystal reaction. To our knowledge this is the first
observation of simultaneous three-bond cis–trans photoiso-
merization.
cis-1,6-Diphenylhex-3-en-1,5-diyne was prepared from
phenylethyne and cis-1,2-dichloroethene as previously de-
scribed[10] and hydrogenated over the Lindlar palladium
catalyst to afford ccc-DPH as an oil (Scheme 1). Purification
Scheme 1. Synthesis of ccc-DPH.
by chromatography on alumina, followed by crystallization
from hexane yielded white crystals of ccc-DPH (m.p. 718C).
1
The compound was characterized mainly by H NMR spec-
troscopy[11] and X-ray crystallography.[12] The synthesis, pu-
rification, and handling of ccc-DPH were carried out under
red light to avoid photoisomerization. The other DPH
isomers are present in the photostationary state and were
previously described.[13]
Powder X-ray diffraction patterns were measured before
and after irradiation (l = 400 nm) of samples of ccc-DPH
(ꢀ5 mg) that had been ground to a powder with a mortar and
pestle. The X-ray diffraction data were collected with a
Rigaku X-ray diffractometer Ultima III (scan angle 7–318,
176 KW, 0.018 resolution, 1.5418 ꢀ, slits 0.5 nm, 600 s for data
collection, and calibration to silicon). Fluorescence spectra
and 1H NMR spectra of the powdered samples were recorded
following the powder X-ray diffraction measurements for
different irradiation intervals. A Bruker Smart Apex diffrac-
tometer was used for X-ray crystallography.[12] For the
crystallographic data in CIF electronic format and the data
analysis see the Supporting Information.
[*] Prof. J. Saltiel, D. Papadimitriou, Dr. T. S. R. Krishna, Dr. Z.-N. Huang,
Dr. S. Laohhasurayotin, Prof. R. J. Clark
Department of Chemistry and Biochemistry
Florida State University
Tallahassee, FL 32306-4390 (USA)
Fax: (+1)850-644-8281
E-mail: saltiel@chem.fsu.edu
Homepage: https://www.chem.fsu.edu/~jsaltiel/
Dr. G. Krishnamoorthy
The X-ray structure of ccc-DPH (Figure 1) shows that the
planes of the phenyl groups form an angle of roughly 728; the
two planes are twisted in opposite directions, each forming an
angle of 368 with the hexatriene moiety, which is almost
planar (the largest deviations from planarity in the triene are
dihedral angles of 11.48 between the central and terminal
Department of Chemistry, Indian Institute of Technology
Guwahati, Guwahati 781039 (India)
[**] J.S. thanks the National Science Foundation (grant no. CHE-
0314784) for partial support of this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902724.
8082
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 8082 –8085

Angewandte
Chemie
Figure 2. Stationary point geometries on the DPH S0 surface; ccc-
DPHA is lower in energy by 0.23 kcalmolꢁ1
.
Figure 1. X-ray crystal structure (top) and crystal packing (bottom) of
ccc-DPH molecules.
double bonds). ccc-DPH possesses an axis of chirality and
exists in enantiomerically pure form in the crystal, as was first
observed for 1,1’-binaphthyl.[14]
Gaussian 98[15] B3LYP calculations with the 6-31G(d,p)
basis set predict that in the gas phase ccc-DPH exists as two
conformers, ccc-DPHA and ccc-DPHB (Figure 2). Conformer
ccc-DPHB closely resembles the X-ray structure since the two
phenyl groups are each rotated in opposite directions 348 to
the triene plane; ccc-DPHB is predicted to lie 0.23 kcalmolꢁ1
higher in energy than conformer ccc-DPHA. The phenyl
groups of ccc-DPHA lie in parallel planes rotated 34.18
relative to the triene plane, and in contrast to ccc-DPHB this
conformer has no chirality axis (see the Supporting Informa-
tion for calculation details). ccc-DPHA and ccc-DPHB are
predicted to lie 8.5 and 8.7 kcalmolꢁ1, respectively, above the
global energy minimum of ttt-DPH on the ground-state
energy surface of DPH (both 9.1 kcalmolꢁ1 higher in relative
free energy);[16] this prediction is consistent with the absence
of the ccc-DPH isomer in thermally equilibrated DPH
mixtures.[13,17]
Figure 3. Powder X-ray diffraction patterns measured in the course of
irradiation of ccc-DPH (l=400 nm); the upper pattern is for ttt-DPH.
ccc-DPH crystal are similar. The evolution of product
fluorescence is analogous to that for the spectra of tt-DPB
and tt-DTB photoproducts arising from the irradiation of
solid cc-DPB and cc-DTB, respectively.[7] The 0–0 vibronic
band at 449 nm for ttt-DPH decreases in relative intensity as
the reaction progresses and is replaced by the 0–1 band at
469 nm as the most intense band in the spectrum (Figure 4).
The 0–1 band, blue-shifted by about 11 nm, is also the most
intense band in the fluorescence spectrum of pure ttt-DPH
crystals. Differences in the fluorescence spectra and powder
X-ray spectra of pure ttt-DPH crystals and the ttt-DPH
photoproduct show that the spectra are sensitive to intermo-
lecular interactions that depend on the crystal packing
arrangement. Fluorescence excitation spectra were also
measured in the course of irradiation of solid ccc-DPH. The
structureless ccc-DPH fluorescence excitation spectrum,
lmax = 400 nm, is replaced by the broader, vibronically struc-
tured ttt-DPH fluorescence excitation spectrum with the 0–0
band at 418 nm.
Irradiation of ccc-DPH in crystal or powder form at
400 nm (150 W Xe lamp of a Hitachi F4500 spectrophotom-
eter) converts it directly to the all-trans isomer (ttt-DPH), the
1
only isomer detected in H NMR spectra measured in the
course of the irradiation (up to 68% conversion, see the
Supporting Information). HPLC analysis of a sample irradi-
ated to 6.4% conversion is consistent with this conclusion (see
the Supporting Information). Powder X-ray diffraction meas-
urements show it to be a crystal-to-crystal reaction (Figure 3).
Reaction progress was also followed by fluorescence spec-
troscopy. The broad emission band of the ccc-DPH powder is
replaced by the red-shifted structured emission band of
ttt-DPH (Figure 4). Changes observed on irradiation of the
Angew. Chem. Int. Ed. 2009, 48, 8082 –8085
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8083

Communications
Simultaneous two-bond photoisomerizations leading to
ctt- and tct-DPH are minor reactions in MCH, but are
significant processes, possibly following BP pathways, in AN.
In both solvents simultaneous three-bond photoisomerization
is a primary process. In MCH, formation of ttt-DPH is the
major photoreaction, leading to the possibility that it may be
the precursor of significant fractions of the observed ctt- and
tct-DPH photoproducts by sequential absorption of a second
photon. Successive UV/Vis spectra recorded in MCH indicate
the evolution of photoisomer formation (Figure 6).
Figure 4. Fluorescence spectra obtained in the course of irradiation of
ccc-DPH powder (l=400 nm) compared with the fluorescence spec-
trum of solid ttt-DPH.
The stereospecific three-bond photoisomerization of ccc-
DPH in the solid state is all the more remarkable when one
considers that on excitation in solution ccc-DPH gives all five
DPH isomers. Preliminary photoisomerization quantum
yields obtained for low conversions on irradiation of ccc-
DPH at 365 nm in degassed methylcyclohexane (MCH) and
acetonitrile (AN) solutions at 208C are given in Table 1. The
Figure 6. Successive UV/Vis spectra recorded in the course of irradi-
ation (l=365 nm) of 1.1ꢀ10ꢁ5 m ccc-DPH in degassed MCH at 208C.
Table 1: Quantum yields for ccc-DPH photoisomerization in solution.[a]
Solvent
fcct
fctc
fctt
ftct
fttt
Simultaneous three-bond photoisomerization is consis-
tent with the expected reversal of single-/double-bond
character in the triene moiety on S0!S1 excitation. The
initial structureless fluorescence spectrum of ccc-DPH in the
solid state (Figure 4) shows that, as in the cases of cc-DPB and
cc-DTB,[7] the light-induced ccc-DPH!ttt-DPH reaction is
not adiabatic. A two-stage process is envisioned with a first
stage involving torsional motions in S1 that bring the molecule
to a conical intersection at an intermediate geometry between
1ccc-DPH* and 1ttt-DPH* and a second S0 stage in which the
formation of ground-state ttt-DPH is completed. The overall
reaction is consistent with two simultaneous BP processes
occurring in disrotatory fashion about the central bond
(Scheme 2).
MCH
AN
0.012
0.06
0.006
0.009
0.002
0.02
0.003
0.01
0.02
0.03
[a] Solutions were ccc-DPH=1.46ꢀ10ꢁ3 m and 1.02ꢀ10ꢁ3 m in MCH and
AN, respectively; analysis was by HPLC as previously described.[2]
photoisomerization of ttt-DPH in AN was used as actino-
meter.[2] As in the case of ttt-DPH, the photoisomerization
quantum yields are very sensitive to the polarity of the
solvent.[2] Isomerization of the terminal bond is favored over
central one-bond photoisomerization (OBT) in both solvents.
However, as observed for ttt-DPH,[2] OBT at the terminal
bond is dramatically enhanced in the more polar solvent, such
that formation of cct-DPH is the major photoreaction in AN
(Figure 5).
Scheme 2. The disrotatory double BP process in the S1 stage of the
ccc-DPH!ttt-DPH photoreaction.
Received: May 21, 2009
Revised: July 24, 2009
Published online: September 25, 2009
Figure 5. Time evolution of DPH isomers on irradiation (l=365 nm)of
6.5ꢀ10ꢁ4 m ccc-DPH in degassed AN at 208C (HPLC).
8084 www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 8082 –8085

Angewandte
Chemie
Laohhasurayotin, T. S. R. Krishna, Angew. Chem. 2008, 120,
Keywords: photoisomerization · reaction mechanisms · trienes
.
1257 – 1260; Angew. Chem. Int. Ed. 2008, 47, 1237 – 1240.
[9] Y. Sonoda, Y. Kawanishi, S. Tsuzuki, M. Goto, J. Org. Chem.
2005, 70, 9755 – 9763.
[1] a) B. S. Hudson, B. E. Kohler, Annu. Rev. Phys. Chem. 1974, 25,
437 – 460; b) B. S. Hudson, B. E. Kohler, K. Schulten in Excited
States, Vol. 6 (Ed.: E. C. Lim), Academic Press, New York, 1982,
pp. 1 – 95; c) J. Saltiel, Y.-P. Sun in Photochromism, Molecules
and Systems (Eds.: H. Dꢁrr, H. Bouas-Laurent), Elsevier,
Amsterdam, 1990, pp. 64 – 164.
[10] K. P. C. Vollhardt, L. S. Winn, Tetrahedron Lett. 1985, 26, 709 –
712.
[11] 1H NMR (300 MHz, CDCl3): d = 7.32–7.38 (m, 3H, Ph), 7.22–
7.28 (m, 2H, Ph), 6.74–6.86 (m, 2Hb), 6.61–6.67 (m, 2Hc), 6.55,
6.58 (d, 2Ha). UV (hexane)l (nm)(e in mꢁ1 cmꢁ1) 229 (1.05 ꢃ 104),
262 (1.04 ꢃ 104), 321 (3.94 ꢃ 104).
[2] a) J. Saltiel, D.-H. Ko, S. A. Fleming, J. Am. Chem. Soc. 1994,
116, 4099 – 4100; b) J. Saltiel, S. Wang, L. P. Watkins, D.-H. Ko, J.
[12] Crystallographic data for ccc-DPH: C18H16, M = 232.31, mono-
clinic, space group C2, a = 11.5889 ꢀ, b = 5.3763(7) ꢀ, c =
10.4492(13) ꢀ, a = 908, b = 92.3198, g = 908, V= 650.51(14) ꢀ3,
Phys. Chem.
A 2000, 104, 11443 – 11450; c) J. Saltiel, G.
Krishnamoorthy, Z. Huang, D.-H. Ko, S. Wang, J. Phys. Chem.
A 2003, 107, 3178 – 3186.
1calcd = 1.186 Mgmꢁ3 and Z = 2.
A Bruker SMART APEX
diffractometer at a detector distance of 5 cm with the sample
at 173 (2) K and MoKa radiation (l = 0.71073 ꢀ) was used to take
2400 frames with 20 s collection time. The result was 4123
reflections with 794 (I > s(I), [R(int) = 0.0272]) being independ-
ent and a data to parameter ratio of about 10:1. See the
Supporting Information for structural details.
[3] a) J. Saltiel, A. S. Waller, D. F. Sears, Jr., J. Am. Chem. Soc. 1993,
115, 2453 – 2465; b) J. Saltiel, A. S. Waller, D. F. Sears, Jr., C. Z.
Garrett, J. Phys. Chem. 1993, 97, 2516 – 2522.
[4] a) J. Saltiel, J. T. DꢂAgostino, J. Am. Chem. Soc. 1972, 94, 6445 –
6457; b) J. Saltiel, Y.-P. Sun, J. Phys. Chem. 1989, 93, 6246 – 6250.
[5] A. Warshel, Nature 1976, 260, 679 – 683.
[6] a) R. S. H. Liu, A. E. Asato, Proc. Natl. Acad. Sci. USA 1985, 82,
259 – 263; b) R. S. H. Liu, D. Mead, A. E. Asato, J. Am. Chem.
Soc. 1985, 107, 6609 – 6614; c) R. S. H. Liu, G. S. Hammond,
Proc. Natl. Acad. Sci. USA 2000, 97, 11153 – 11158; d) R. S. H.
Liu, Acc. Chem. Res. 2001, 34, 555 – 562.
[7] a) J. Saltiel, T. S. R. Krishna, R. J. Clark, J. Phys. Chem. A 2006,
110, 1694 – 1697; b) J. Saltiel, T. S. R. Krishna, .S. Laohhasur-
ayotin, K. Fort, R. J. Clark, J. Phys. Chem. A 2008, 112, 199 – 209.
[8] a) J. Saltiel, T. S. R. Krishna, A. M. Turek, R. J. Clark, Chem.
Commun. 2006, 1506 – 1508; b) J. Saltiel, M. A. Bremer, S.
[13] K. Lunde, L. Zechmeister, J. Am. Chem. Soc. 1954, 76, 2308 –
2313.
[14] K. R. Wilson, R. E. Pincock, J. Am. Chem. Soc. 1975, 97, 1474 –
1478.
[15] See the Supporting Information for full citation for Gaussian98.
[16] O. Dmitrenko, J. Saltiel, unpublished calculations.
[17] J. Saltiel, J. M. Crowder, S. Wang, J. Am. Chem. Soc. 1999, 121,
895 – 902.
Angew. Chem. Int. Ed. 2009, 48, 8082 –8085
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8085