Photochromic spirotetrahydroazafluorenes: Part V. Why photochromic molecules with rigid region B exhibiting extremely fast bleaching process?

Article abstract of DOI:10.1080/15421400590947513

Spirotetrahydroindolizines (THIs) 1a-g and Spirotetrahydroazafluorenes (THF) 2a-g were previously synthesized. The high reaction rate of the thermal 1,5-electro-cyclization results in the half-lives of the colored betaines 2′a-g range from 3.9 to 5.4 ns, thus showing the existence of the shortest lived colored form observed so far for pyrazoline based photochromes. However, laser flash spectroscopy was successfully used for the determination of both the half-life and absorption maxima of the colored betaines 2′a-g. Time resolved transient spectroscopy shows the geometry of the betaines 2′a-g to be related to one of the two detectable intermediates 1′a-g or 1″a-g.

Full text of DOI:10.1080/15421400590947513

This article was downloaded by: [UOV University of Oviedo]
On: 28 October 2014, At: 04:37
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
Photochromic
Spirotetrahydroazafluorenes:
Part V. Why Photochromic
Molecules with Rigid Region
B Exhibiting Extremely Fast
Bleaching Process?
Saleh A. Ahmed a & Heinz Dürr b
a Chemistry Department , Faculty of Science, Assiut
University , Assiut/Egypt
b Fachbereich 11.2, Organische Chemie , Universität
des Saarlandes , Saarbrücken, Germany
Published online: 31 Aug 2006.
To cite this article: Saleh A. Ahmed & Heinz Dürr (2005) Photochromic
Spirotetrahydroazafluorenes: Part V. Why Photochromic Molecules with Rigid Region B
Exhibiting Extremely Fast Bleaching Process?, Molecular Crystals and Liquid Crystals,
431:1, 575-580
To link to this article: http://dx.doi.org/10.1080/15421400590947513
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and

are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 431, pp. 275=[575]–280=[580], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590947513
Photochromic Spirotetrahydroazafluorenes: Part V. Why
Photochromic Molecules with Rigid Region B Exhibiting
Extremely Fast Bleaching Process?
Saleh A. Ahmed
Chemistry Department, Faculty of Science, Assiut University,
Assiut=Egypt
Heinz Du¨ rr
¨
Fachbereich 11.2, Organische Chemie, Universitat des Saarlandes,
Saarbru¨cken, Germany
Spirotetrahydroindolizines (THIs) 1a-g and Spirotetrahydroazafluorenes (THF)
2a-g were previously synthesized. The high reaction rate of the thermal 1,5-electro-
cyclization results in the half-lives of the colored betaines 20a-g range from 3.9 to
5.4 ns, thus showing the existence of the shortest lived colored form observed so far
for pyrazoline based photochromes. However, laser flash spectroscopy was success-
fully used for the determination of both the half-life and absorption maxima of the
colored betaines 20a-g. Time resolved transient spectroscopy shows the geometry of
the betaines 20a-g to be related to one of the two detectable intermediates 10a-g or
100a-g.
Keywords: flash photolysis; short-lived photochromes; spirotetrahydroazafluorenes
INTRODUCTION
Photochromic dihydroindolizines DHIs and tetrahydroindolizines
THIs undergo a photo-induced change of color in solutions and in
polymer matrices when exposed to UV radiation or direct sunlight
exposure.
Many thanks are due to Prof. Dr. Aboel-Magd A. Abdel-Wahab for his help-
ful discussions and Dr. Thomas Hartmann for his assistance with the laser
measurements.
Address Correspondence to Saleh A. Ahmed, Chemistry Department, Faculty
of Science, Assiut University, 71516 Assiut=Egypt. E-mail: saleh 63@hotmail.com
275=[575]

276=[576]
S. A. Ahmed and H. Du¨rr
The thermal back reaction- the 1,5-electrocyclization- from the ring-
open betaine to DHI shows rates extending from milliseconds to sev-
eral weeks [1,2–5] depending on the substituents and structure of
the involved molecule. This manuscript will shed light on the different
intermediates controlling the photochromic behavior of the ultra-fast
bleaching process of the cis-fixed betaine forms of THFs 2.
RESULT AND DISCUSSIONS
Photochromic THIs 1a-g and THF 2a-g were previously prepared [6].
The kinetic of the 1,5-electrocyclization of betaine forms of THIs 1a-f
was measured using UV=VIS in CH2Cl2 at ambient temperature
(Table 1). The colored betaine forms of THIs 1a-f showed half-lives
(t1=2) between 10–96s depending on the subsitutents in the 100b-
phenyl ring. Millisecond (ms) flash photolysis (Table 1) was success-
fully used to determine, in addition to kmax of betaine form of THI
1g, the kinetics of the thermal back reaction. In the case of the
betaines 20a-g, the extremely fast bleaching process hindered the
measurement of both half-lives and absorption maxima of the ultrafast
electrocyclization process of the betaine 20a-g to the 2a-g.
The kinetic of the thermal 1,5-electrocylization of 20a-g were mea-
sured with a laser flash absorption apparatus described earlier [7–9].
The absorption spectra were recorded by time resolved spectroscopy.
The time domaines from 10ꢀ3 to 10ꢀ9 were monitored. Thus only
half-lives for 20a-g in the range of a few nanoseconds (t1=2 ¼ 3.1–
5.4 ns were found (Table 1, Figure 1). So ring fusion in region B of
2a-g results in an acceleration of the thermal 1,5-electrocyclization
of 20a-g to 2a-g. A stabilization of 20a-g by protonation in position 10
with mineral acids that has been shown previously [1,10,11] was not
possible in this case.

277=[577]

278=[578]
S. A. Ahmed and H. Du¨rr
FIGURE 1 Time-resolved fluorescence emission of 2c measured with a Nd-
Yag-laser at 500, 550, 600, 650 and 700 nm in CH2Cl2 and by the single photon
counting method (t4c < 0.30 ns).
According to Fromm et al. [12], these results are explained as fol-
lows: The electron withdrawing 30-acetyl is stabilized the ring-open
form 1000a-g by reducing the negative charge at C10; which has sp2
character. However, the ring closure is only initiated in the case of
a sp3-hybridization of C10. Dorweiler [13] showed that this is borne
out by MINDO3 calculations. So the electrocyclization is slowed down
by a low electron density at C10. The transoid betaine conformer
1000a-g is formed from the cisoid 10a-g by rotations around the axes
C20-C30 and C30-N40. The energy barriers between the conformers
10a-g, 100a-g and 1000a-g are small. Collet [14] could determine a rota-
tional barrier C10-C20 (with double bond character) to be 36 KJ=mol.
The geometry of 1000a-g was proved by x-ray of a stable betaine by
Mu¨nzamy [15]. According to this result, the most stable relaxed
transoid betaine 1000a-g is not totally planar but slightly twisted
between C10 and N40. The dihederal angle found is of the order of
116.5 between the axis R42-C2 and C3-R430 (R4: methyl ester). The
geometry of 100a-g is not exactly known whereas 10a-g must be close
to the one of THIs 1a-g.
In contrast to these results in betaine 20a-g the rotation between
C20 and C30 is impossible because of the annulated heterocyclic ring.

Photochromic Spirotetrahydroazafluorenes
279=[579]
The molecule 20a-g is cis-fixed. There are only rotations possible
around the C10-C20 and C3-N4 axis and the ring systems are inter-
acting during the rotation. The sp3 character of C10 is maintained to
a large extent because of decreasing withdrawal of charge by the
aromatic isoquinoline ring (C20 and C30). Thus the geometry of the
betaine 20a-g is similar to the starting material, the THF 2a-g.
Therefore, this structure is favorable for fast thermal disrotatory
[9] 1,5-electrocyclization of 20 (obtained by irradiation of 2a-g). It
is demonstrated that in the carbocyclic analogue the ring-opening
mechanism of the cyclopentenyl anion, the transitions state comes
close to the possible transition state of the reaction 20!1. The rate
constant k (ln 2=k ¼ t1=2) is much smaller than k and k Now
ꢀ3
ꢀ2
ꢀ1
the different steps and intermediates involved in the ring opening
of 2 and cyclization of 20a-g can explained with the following reac-
tion sequence (for the structure of 10, 100, 1000, 20, 200, 2000 see
reference 6):
In conclusion, novel type of photochromic THF 2a-g showed a fast
1,5-electrocyclization process, which cannot be detected by UV=Vis
or millisecond flash photoylsis technique. Ring annulation in the cis-
fixed 20a-g molecules suppresses rotation and thus produces colored
transients in the nanosecond domain. The new system is an inte-
resting candidate for pattern recognition system and it will find its
application.
REFERENCES
[1] Du¨rr, H. & Bouas-Laurent. H. (1990, 2003). Photochromism: Molecules and
Systems. Vol l,2., Elsevier: Amsterdam.
[2] Ahmed, S. A., Hartmann, T., Huch, V., Du¨rr, H., & Abdel-Wahab, A. A. (2000).
J. Phys. Org. Chem., 13, 539.
[3] Tan, Y. S., Ahmed, S. A., Du¨rr, H., Huch, V., & Abdel-Wahab, A. A. (2001). Chem.
Commun., 14, 1246.
[4] Fromm, R., Ahmed, S. A., Hartmann, T., Huch, V., Abdel-Wahab, A. A., & Du¨rr, H.
(2001). Eur. J. Org. Chem., 21, 4077.
[5] Ahmed, S. A., Abdel-Wahab, A. A., & Du¨rr, H. (2003). J. Photochem. Photobiol. A.,
154, 131.
[6] Ahmed, S. A. (2005). Mol. Cry. & Liq. Cry., 430, 295.

280=[580]
S. A. Ahmed and H. Du¨rr
[7] Ahmed, S. A. (2002). J. Phys. Org. Chem., 15, 392.
[8] Tan, Y. S., Hartmann, Th., Huch, V., Du¨rr, H., & Kossanyi, K. (2001). J. Org.
Chem., 66, 1130.
[9] Fromm, R., Born, R., Du¨rr, H., Kannengiesser, J., Breuer, H.-D., Valat, P., &
Kossanyi, J. (2000). J. Photochem. & Photobiol., 135, 85.
[10] Weber, C., Rustemeyer, F., & Du¨rr, H. (1998). Adv. Mater., 10, 1348.
[11] Du¨rr, H. (1989). Angew. Chem., 101, 427. Angew. Chem. Int. Ed. Engl. 28, 413.
[12] Dorweiler, C., Mu¨nzmay, T., Spang, P., Holderbaum, M., Du¨rr, H., Raabe, E., &
Ku¨rger, K. (1988). Chem Ber., 121, 843.
¨
[13] Collet, C. (1994). Master Thesis, Universitat des Saarlands, Saarbru¨cken.
[14] Mu¨nzamy, T. (1986). Ph. D Thesis, Universita¨t des Saarlands, Saarbru¨cken.