Photoisomerization of cis,cis-1,4-diphenyl-1,3-butadiene in glassy media at 77 K: The bicycle-pedal mechanism

Article abstract of DOI:10.1039/b516319f

The cis-trans photoisomerization of cis,cis-1,4-diphenyl-1,3-butadiene in a soft isopentane glass at 77 K gives significant two-bond photoisomerization in contrast to solution and hard glassy media where only one-bond photoisomerization takes place. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b516319f

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Photoisomerization of cis,cis-1,4-diphenyl-1,3-butadiene in glassy
media at 77 K: the bicycle-pedal mechanism{
a
Jack Saltiel,* Tallapragada S. R. Krishna, Andrzej M. Turek and Ronald J. Clark
a
b
a
Received (in Cambridge, UK) 17th November 2005, Accepted 15th February 2006
First published as an Advance Article on the web 28th February 2006
DOI: 10.1039/b516319f
The cis–trans photoisomerization of cis,cis-1,4-diphenyl-1,3-
butadiene in a soft isopentane glass at 77 K gives significant
two-bond photoisomerization in contrast to solution and hard
glassy media where only one-bond photoisomerization takes
place.
photoisomerization of the 1,4-diphenyl-1,3-butadienes (cc-DPB,
12
ct-DPB, tt-DPB). Photoisomerization of the DPBs in solution
1
3
14
was reported by Zechmeister and later studied by Whitten and
1
5
Yee and their coworkers. Recently, Liu and coworkers reported
the photoisomerization of the DPBs in EPA (5 : 5 : 2) glass at
1
6
7
7 K. Starting from cc-DPB, sequential one-bond isomerization
12–15
gives a ct-DPB-rich photostationary state in solution,
Volume restricted media, such as protein environments, or glasses
at low temperatures, exert control over cis–trans photoisomeriza-
tion of conjugated olefins. Two mechanisms involving concerted
and
16
pure tt-DPB in EPA glass.
DPB isomer fluorescence spectra in IP at 77 K are shown in
Fig. 1. Spectra in MCH at 77 K are similar. Irradiations of dilute
1
rotation about more than one bond in S (the first excited singlet
2
6
state) were postulated to explain the specificity and high photo-
isomerization quantum yields of the retinyl moieties of rhodopsin
and bacteriorhodopsin: Warshel’s bicycle-pedal mechanism (BP)
involves simultaneous rotations in S about the C –C and C –C
solutions (y4 6 10 M) of the DPBs in glassy isopentane (IP) or
MCH were carried out in cylindrical sample tubes immersed in
liquid N in the phosphorescence accessory of a Hitachi F-4500
2
1
1
2
3
4
fluorimeter using the 150-W Xe fluorimeter lamp as the excitation
1
bonds of a 1,3-diene moiety and Liu’s hula-twist mechanism (HT)
involves simultaneous rotation about a double bond and an
adjacent essential single bond (equivalent to a 180u translocation of
source. Reaction progress was followed by fluorescence spectro-
scopy at 77 K. In MCH glass at 77 K cc-DPB gives only ct-DPB in
agreement with the earlier reports in solution and in EPA
2
13–15
glass.
17
one CH unit). These mechanisms are expected to reduce the
However, our results in the softer IP glass at 77 K
volume requirements associated with torsional relaxation by
confining most of the motion to the vicinity of the isomerizing
double bonds while minimizing the motion of bulky substituents.
The recent claim that the tachysterol products obtained on irradia-
tion of previtamin D at 92 K in EPA glass (ether : isopentane :
reveal a major cc-DPB A tt-DPB two-bond photoisomerization
process. Fluorescence spectra as a function of irradiation time of
cc-DPB in IP at 77 K are shown in Fig. 2. The well-resolved
vibronic bands of tt-DPB fluorescence are obvious from the start,
in stark contrast to spectra from a parallel experiment in MCH
3
ethyl alcohol = 5 : 5 : 2) are those predicted by the HT motion,
4
has stimulated the revival of the HT mechanism. Theoretical
1
calculations favour 1,3-bond formation in the 2 Ag states of
that show only build-up of ct-DPB fluorescence. Principal
10,18
component analysis (PCA) treatment
of the spectra in Figs.
1 and 2 reveals a 3-component system. As can be seen in Fig. 3,
eigenvector combination coefficients for the spectra of the
irradiated glassy IP solutions (Fig. 2) fall within the normalization
triangle defined by the combination coefficients of the pure isomer
spectra in Fig. 1. Treatment of the analogous spectra in glassy
polyenes at conical intersections for ultra fast radiationless decay
5,7,8
5
–7
to the ground state, and it has been argued
that such struc-
tures can lead to HT products. Involvement of cyclopropylmethyl-
ene intermediates in the direct cis–trans photoisomerization of
1
,3-butadienes as a possible alternative to allylmethylene inter-
9
mediates had been proposed earlier. Until now, no experimental
evidence had been advanced in support of the BP mechanism.
We noted reservations concerning the HT postulate in a paper
showing that the photoisomerization of cis-1-(2-naphthyl)-2-
phenylethene in methylcyclohexane (MCH) glass at 77 K is
1
0
conformer specific, giving the one-bond twist (OBT) product as
1
1
in fluid solution. We now present a related study on the
a
Department of Chemistry and Biochemistry, Florida State University,
Tallahassee, FL 32306-4390, USA. E-mail: saltiel@chem.fsu.edu;
Fax: +1 850 644 8281; Tel: +1 850 644 5405
Jagiellonian University, Faculty of Chemistry, 30 060 Cracow, Poland.
b
E-mail: turek@chemia.uj.edu.pl; Fax: +48-12-634-05-15;
Tel: +48-12-663-2262
{
Electronic supplementary information (ESI) available: Cartesian coor-
dinates, drawings, and total energies of optimized structures (minima on
); tables of X-ray data for cc-DPB consisting of crystallographic
S
0
Fig. 1 Normalized corrected fluorescence spectra of tt-DPB (red) ct-
DPB (green) and cc-DPB (blue) in IP at 77 K, lexc = 320 nm.
parameters, positional parameters, bond distances, bond angles, and
torsional angles. See DOI: 10.1039/b516319f
1
506 | Chem. Commun., 2006, 1506–1508
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Fig. 4 X-Ray structure of cc-DPB; four molecules in the asymmetric
unit viewed roughly end on (see ESI{).
˚
a monoclinic cell of dimensions a = 7.140 A, b = 7.126 A, c =
˚
˚
2
2.660 A, a = 90u, b = 92.132u, c = 90u with a volume of
3
˚
152.49 A , (ii) a triclinic cell with the initial two distances a little
1
˚
over 10 A, doubling the volume of the cell in (i), or (iii) a
Fig. 2 Fluorescence spectra of a cc-DPB solution in IP recorded as a
function of irradiation (lrad = lexc = 320 nm) time at 77 K.
˚
monoclinic cell with the initial values above 14 A, quadrupling
1 1
the cell volume in (i). Use of space groups P2 or P2 /c with the
smallest cell yielded a composite structure consistent with the
average of two cc-DPB molecules, one with phenyls in parallel
planes and the other with phenyls in roughly orthogonal planes.
Refinement to distinct molecules (see the four molecule asym-
metric unit in Fig. 4) was achieved only by use of the Cc space
group with the largest unit cell, but with a relatively high R value
1
of 9.8%{ (see ESI for more details on problems we encountered{).
Common to all solutions was that the average phenyl–diene
dihedral angle is 40u in alternating layers of molecules with the two
phenyls in parallel planes and molecules with the two phenyls in
roughly perpendicular planes.
Calculated and X-ray structures of cc-DPB are in reasonable
2
3
agreement. Gaussian 98 B3LYP calculations with the 6-31G(d,p)
basis set predict that the lowest energy structure has each of the
two phenyls rotated in opposite directions 31.5u to the diene plane.
The structure with the phenyls in parallel planes rotated 39.6u to
Fig. 3 Combination coefficients for the spectra in Figs. 1 and 2.
MCH shows that irradiation leads to mixture spectra whose
combination coefficients lie exclusively on the cc-DPB–ct-DPB
side of the normalization triangle.
21
the diene plane is predicted to lie 1.5 kcal mol above it, Fig. 5
(ESI{). The latter structure is almost identical to the upper X-ray
conformer structure shown in Fig. 4 (see, also, Fig. 6).
Correction for the 4% tt-DPB fluorescence present in the
spectrum at time 0, gives ct-DPB/tt-DPB = 0.7 ¡ 0.1 for the
spectra in Fig. 2. ct-DPB/tt-DPB product ratios are almost
constant but vary from experiment to experiment with slow
cooling favouring the ct-DPB product. However, conversion of cc-
DPB to tt-DPB is much faster than conversion of ct-DPB to tt-
DPB under the same conditions. This direct two-bond cc-DPB A
tt-DPB pathway appears to be the first example of Warshel’s BP
photoisomerization mechanism in an amorphous medium.
Estimated fluorescence quantum yields in IP at 77 K with trans-
Least motion considerations suggest that the conformation,
with phenyls in parallel planes can more readily yield tt-DPB via
the BP mechanism. The arrows in Fig. 6 show the concerted
19
stilbene in MCH at 77 K as standard are 1, 0.7 and 0.3 (25%
o
Fig. 5 Stationary point geometries on the cc-DPB S surface; the
uncertainty) for tt-, ct- and cc-DPB, respectively. The radiative rate
8
structure on the left corresponds to the global energy minimum (see ESI{).
21
20
constant, k
its molar absorptivity at lmax (compare with 8 6 10 s , the
F
= 3 6 10 s , of cc-DPB can be based roughly on
7
21
21
experimentally determined value for cis-stilbene ). It follows, that
if all the radiationless decay is along the BP channel, the lower
9
21
limit for that rate constant at 77 K is 1 6 10 s . To compete
with cc-DPB radiative decay, the activation energy of the BP
2
1
mechanism must be smaller than 2 kcal mol
.
The crystals of cc-DPB were disordered and not of sufficiently
high quality to allow straightforward refinement of X-ray
diffraction data. Numerous space groups were attempted on six
2
2
data sets collected on four different crystals. Depending upon the
random selection of reflections, the indexing analysis produced: (i)
Fig. 6 The BP mechanism shown for the cc-DPB X-ray structure with
the phenyls in parallel planes.
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4
(a) R. S. H. Liu and G. S. Hammond, Proc. Natl. Acad. Sci. USA, 2000,
97, 11153; (b) R. S. H. Liu, Acc. Chem. Res., 2001, 34, 555; (c) R. S.
torsional motions about the two former double bonds in the diene
moiety that convert cc-DPB to tt-DPB. Volume demand is
minimized because most of the motion is confined to the diene
unit, which can be viewed in the lowest singlet excited state as a
H. Liu and G. S. Hammond, Chem.–Eur. J., 2001, 7, 4537; (d) R. S.
H. Liu, Pure Appl. Chem., 2002, 74, 1391; (e) R. S. H. Liu and
G. S. Hammond, Photochem. Photobiol. Sci., 2003, 2, 835; (f) R. S.
H. Liu and G. S. Hammond, in Handbook of Organic Photochemistry
and Photobiology, ed. W. M. Horspool, F. Lenci, CRC Press: London,
1,4-biradicaloid or zwitterionic species, with the phenyl groups
held stationary. However, our observation of exclusive cc-DPB to
tt-DPB photoisomerization in the solid state (crystals or powder)
in a crystal to crystal reaction, albeit in kinetically distinct stages,
indicates that both cc-DPB conformers in Fig. 4 serve as tt-DPB
2
nd edn, 2004, pp. 26/1–26/11; (g) G. Krishnamoorthy, S. Schieffer,
J. Pescatore, R. Ulsh, R. S. H. Liu and J. Liu, Photochem. Photobiol.
Sci., 2004, 3, 1047.
5
(a) M. Olivucci, F. Bernardi, P. Celani, I. N. Ragazos and M. A. Robb,
J. Am. Chem. Soc., 1994, 116, 1077; (b) P. Celani, M. Garavelli,
S. Ottani, F. Bernardi, M. A. Robb and M. Olivucci, J. Am. Chem.
Soc., 1995, 117, 11584; (c) M. Garavelli, P. Celani, F. Bernardi,
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M. Garavelli, B. R. Smith, M. J. Bearpark, F. Bernardi, M. Olivucci
and M. A. Robb, J. Am. Chem. Soc., 2000, 122, 5568.
6 For crossing to the 2 A state see: (a) C. Woywood, W. C. Livingood
and J. H. Frederick, J. Chem. Phys., 2000, 112, 613; (b) C. Woywood,
W. C. Livingood and J. H. Frederick, J. Chem. Phys., 2000, 112, 626; (c)
C. Woywood, W. C. Livingood and J. H. Frederick, J. Chem. Phys.,
24
precursors.
The lower energy conformation with phenyls in perpendicular
planes can, with minimal torsional motion in the diene moiety,
readily form a cis-phenallylbenzyl intermediate on the way to ct-
DPB. Formation of the conventional OBT intermediate requires
little, if any, motion of the phenyl rings since they lie in roughly
orthogonal planes at the outset and can account for one-bond
photoisomerization of cc-DPB in MCH glass. As in the case of
1
g
2001, 114, 1631; (d) C. Woywood, W. C. Livingood and J. H. Frederick,
J. Chem. Phys., 2001, 114, 1645.
1
0
c-NPE, we apply Occam’s razor to favour the OBT over the HT
7
8
For a recent review of polyene photophysics see: W. Fuß, Y. Haas and
S. Zilberg, Chem. Phys., 2000, 259, 273.
A. M. M u¨ ller, S. Lochbrunner, W. E. Schmid and W. Fuss, Angew.
Chem., Int. Ed., 1998, 37, 505.
mechanism in low T rigid media when the two pathways predict
25
the same product.
We can only speculate concerning the difference in behaviour of
the lowest excited singlet states of cc-DPB in IP and MCH glassy
media at 77 K. If thermodynamic equilibrium were maintained
during the cooling process, and if the difference in size and shape
of the cavities occupied by cc-DPB in the two media were not a
factor, then the lower melting IP (113.3 K vs. 146.6 K for MCH)
would have favoured the lower energy conformer with the phenyls
in planes roughly approaching orthogonality. However, thermo-
dynamic equilibration requires slow cooling and we observed the
highest relative yields of two-bond isomerization when the sample
9 (a) J. Saltiel, L. Metts and M. Wrighton, J. Am. Chem. Soc., 1970, 92,
227; (b) J. Saltiel, J. T. D’Agostino, E. D. Megarity, L. Metts,
K. R. Neuberger, M. Wrighton and O. C. Zafiriou, Org. Photochem.,
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10 J. Saltiel, T. S. R. Krishna and A. M. Turek, J. Am. Chem. Soc., 2005,
27, 6938.
1 (a) J. Saltiel, N. Tarkalanov and D. F. Sears, Jr., J. Am. Chem. Soc.,
995, 117, 5586; (b) J. Saltiel, G. Krishnamoorthy and D. F. Sears, Jr.,
Photochem. Photobiol. Sci., 2003, 2, 1162.
3
1
1
1
1
12 For a review, see: J. Saltiel and Y.-P. Sun, in Photochromism, Molecules
and Systems, ed. H. D u¨ rr and H. Bouas-Laurent, Elsevier, Amsterdam,
1
990, p. 64.
3 J. H. Pinckard, B. Wille and L. Zechmeister, J. Am. Chem. Soc., 1948,
70, 1938.
tube was plunged into liquid N . It is possible that the shape of the
2
1
solvent cavities in the IP host favours the conformer with phenyls
in parallel planes.
14 L. R. Eastman, Jr., B. M. Zarnegar, J. M. Butler and D. G. Whitten,
J. Am. Chem. Soc., 1974, 96, 2281.
5 W.A.Yee,S.J.HugandD.S.Kliger,J.Am.Chem.Soc.,1988,110,2164.
This work was supported by National Science Foundation
Grant No. CHE-0314784. We thank Dr Olga Dmitrenko for the
DFT calculations.
1
16 L. Yang, R. S. H. Liu, K. L. Boarman, N. L. Wendt and J. Liu, J. Am.
Chem. Soc., 2005, 127, 2404.
1
7 For organic glass viscosities see: (a) H. Greenspan and E. Fischer,
J. Phys. Chem., 1965, 69, 2466; (b) G. A. von Salis and H. Labhart,
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Notes and references
1
8 J. Saltiel, D. F. Sears, Jr., J.-O. Choi, Y.-P. Sun and D. W. Eaker,
{ Crystallographic data for cc-DPB: C16H14, M = 206.27, monoclinic,
˚
space group Cc, a = 14.2760(8) A, b = 14.2485(8) A, c = 22.6565(13) A, a =
J. Phys. Chem., 1994, 98, 35.
9 J. Saltiel, A. Marinari, D. W.-L. Chang, J. C. Mitchener and
˚
˚
23
1
3
˚
9
0u, b = 92.136(2)u, c = 90u, V = 4605.4(5) A , rcalcd = 1.190 Mg m and
E. D. Megarity, J. Am. Chem. Soc., 1979, 101, 2982.
0 N. J. Turro, Modern Molecular Photochemistry, Benjamin/Cummings
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Z = 16. With the use of 11375 unique reflections (I > s(I)) collected at
2
˚
1
00(2) K with Mo Ka radiation (l = 0.71073 A) on a Bruker SMART
APEX diffractometer at a detector distance of 5 cm. The number of frames
taken was typically 2400 using 0.3 degree omega scans at 20 s frame
collection time. The first 50 frames were repeated at the end of data
collection and no significant crystal decomposition in the course of the
measurements was detected. Integration was performed using the
programme SAINT which is part of the Bruker suite of programmes.
Absorption corrections were made using SADABS. XPREP was used to
suggest the space groups and the structure was solved by direct methods
2
1 J. Saltiel, A. S. Waller and D. F. Sears, Jr., J. Am. Chem. Soc., 1993,
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2 A Bruker SMART APEX diffractometer was used.
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1
2
2
J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E.
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and refined by SHELXTL. The refinement converged to a final R
.0980, wR = 0.2340 (I > 2s(I)) and GOF = 1.155 with the largest
difference peak and hole as 0.666 and 20.339 e A respectively. CCDC
90241. For problems with the data analysis and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b516319f
1
=
0
2
23
˚
2
1
2
A. Warshel, Nature (London), 1976, 260, 679.
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3
A. M. M u¨ ller, S. Lochbrunner, W. E. Schmid and W. Fuß, Angew.
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1
508 | Chem. Commun., 2006, 1506–1508
This journal is ß The Royal Society of Chemistry 2006