Novel fluorescent stilbene analogs involving a carbazole moiety

Article abstract of DOI:10.1016/S0040-4039(00)01489-1

The synthesis of a series of novel efficient TPA chromophores involving a carbazole moiety from N-(p-formyl)phenylcarbazole precursors and their linear absorption and fluorescent properties are described. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01489-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8573–8576
Novel fluorescent stilbene analogs involving a carbazole
moiety
Mark Sigalov, Amos Ben-Asuly, Lev Shapiro, Arkady Ellern and Vladimir Khodorkovsky*
Department of Chemistry, Ben-Gurion University of the Negev, 84105 Beer Sheva, Israel
Received 29 August 2000; accepted 6 September 2000
Abstract
The synthesis of a series of novel efficient TPA chromophores involving a carbazole moiety from
N-(p-formyl)phenylcarbazole precursors and their linear absorption and fluorescent properties are
described. © 2000 Published by Elsevier Science Ltd.
Recently, chromophores with a large two-photon absorption cross section have received much
research interest due to their applications in multiphoton fluorescent imaging,1 3D optical data
storage2 and activation of photodynamic localized treatment.3 Conjugated symmetrical analogs
of p-diaminostilbene are known to possess pronounced two-photon absorption (TPA) proper-
ties.4 Recently, we found that replacing the dialkylamino groups with a carbazole moiety gives
rise not only to a further increase in the TPA coefficient l, but also enhances the photostability
of the resulting chromophores.5 Here we report on the synthesis and fluorescence of these
chromophores and their analogs.
The parent compound, 4,4%-bis(9-carbazolyl)stilbene 2a has been previously synthesized in a
low yield by the Ullmann reaction from 4,4%-dibromostilbene and carbazole.6
Our synthetic strategy is outlined in Scheme 1. It employs 9-(4%-formylphenyl)carbazole 1a as
the key precursor. Recently, this compound was also synthesized by Pd/P(t-Bu3) catalyzed
reaction of p-chlorobenzaldehyde with carbazole,7 but no details were reported. Our approach
involving Ullmann arylation of carbazole with p-bromo benzaldehyde8 gave practically the same
yields of 1a as reported in Ref. 7, and is effective on a preparative scale also for the synthesis
of other derivatives, such as 1b.9 The 3,6-dibromo derivative 1c was obtained by direct
bromination of 1a.10 Coupling of 1a and 1c by TiCl4/Zn11 in dry dioxane afforded stilbene
derivatives 2a,c in good yields.10 Derivative 2b was obtained by formylation of 2a using the
known procedure.12
* Corresponding author. E-mail: hodor@bgumail.bgu.ac.il
0040-4039/00/$ - see front matter © 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)01489-1

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Scheme 1.
Distyrylbenzene derivatives 3a–c and diphenylhexatriene derivative 4 were synthesized by
reacting 1 with the corresponding phosphonates in DMSO in the presence of t-BuOK.10

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The crystal structure of 2a13 shows that the dihedral angle between the carbazole and stilbene
moieties is 51.4° and, according to molecular modeling results, the fully planar conformation is
sterically hindered.
For comparison, derivative 5,10 in which the carbazolyl moiety can be co-planar to the distyryl
moiety, was synthesized from 3-formyl-N-phenyl carbazole by an analogous procedure.
Data on linear absorption and fluorescent properties of derivatives 2–5 are collated in Table
1. Interestingly, all chromophores that exhibit high fluorescence efficiencies (3, 4 and 5), also
possess the strongest TPA properties.5 The photostability of 5, in which the carbazolyl moiety
can adopt the planar conformation, is noticeably lower than that of 3 and 4. The presence of
electron-accepting substituents in the carbazole moiety diminishes the fluorescence yield drasti-
cally. The fluorescence is also quenched in the presence of electron acceptors that can form
charge transfer complexes with the carbazole moiety. Recently, we employed this effect for
designing a system capable of reversible photo-induced emission switching.14
Table 1
Absorption, fluorescence and quantum yield of derivatives 2–5
Compound
Solvent
uabs (nm)
uem (nm)
Quantum yield
2a
2b
2c
3a
3b
3c
4
Toluene
Toluene
Dichloromethane
Toluene
Toluene
Toluene
Toluene
Toluene
348
397, 418
395, 415
402, ꢀ420
426, 448
458, 485
433, 439
434, 459
431, 457
0.67
0.20
0.14
0.70
0.77
0.87
0.68
0.92
332, 343
344, 358
378
406
387
388
392
5
Acknowledgements
This work was supported in part by the grants from the Israeli Ministry of Science and Soreq
NRC.
References
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submitted for publication.

8576
9. 1b: A mixture of 1.15 g (4.12 mmol) of 3,6-di-tert-butylcarbazole,15 0.76 g (4.13 mmol) of 4-bromobenzaldehyde,
1.0 g of potassium carbonate, 0.05 g of Cuꢀbronze and 10 ml of nitrobenzene was boiled under reflux for 1 h and
then the solvent was removed by steam distillation. The residue was extracted with hot petroleum ether (60–80°C)
and the solid was crystallized from petroleum ether. Yield 0.7 g (44%), mp 111–112°C. 1H NMR (200 MHz,
CDCl3): 10.10 (s, 1H, CHO), 8.18 (dd, 2H, J=0.8 Hz, 1.6 Hz, H-4,5), 8.11 (d, 2H, J=8.6 Hz, ArH), 7.79 (d, 2H,
J=8.6 Hz, ArH), 7.44–7.55 (m, 4H, H-1,2,7,8), 1.40(s, 18H, CH3); HRMS (CI): m/z (M+1) cac. fr C27H30NO
1
384.2336, found 384.2327. 1c: Yield 94%, mp 240–242°C; H NMR (200 MHz, CDCl3): 10.13 (s, 1H, CHO), 8.21
(d, 2H, J=1.6 Hz, H-4,5), 8.14 (d, 2H, J=8.4 Hz, ArH), 7.73 (d, 2H, J=8.4 Hz, ArH), 7.53 (dd, 2H, J=1.6,
8.7 Hz, H-2,7), 7.35 (d, 2H, J=8.7 Hz, H-1,8); HRMS (CI): m/z (M+1) cac. fr C19H12Br2NO 427.9285, found
427.9313.
10. 2a: Yield 78%, mp 304–306°C. 2b: Yield 46%, mp>300°C (decomp.). 2c: Yield 65%, mp 419–420°C (decomp.). 3a:
Yield 40%, mp 338°C. 3b: Yield 43%, mp 356°C. 3c: Yield 7.4%, mp 318–320°C. 4: Yield 6%, mp 248–250°C. 5:
1
Yield 13%, mp 268°C. The structure of these compounds was confirmed by H NMR, 13C NMR and HRMS
spectra.
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13. X-ray analysis of 2a: A colorless prism (0.5×0.3×0.3 mm) of C38H26N2 was used for X-ray intensity data
,
measurements on a standard Bruker SMART CCD 1000™ diffractometer (u(Mo Ka)=0.71073 A, graphite
monochromator, a scan width of 0.3° in ꢀ and an exposure time of 10 s/frame, detector-crystal distance 5.05 cm).
A total of 1080 frames were collected and integrated with Bruker SAINT software package using a wide-frame
integration algorithm. The crystal is monoclinic, space group C2/c, at 298 K, a=24.77(1), b=9.8395(5),
3
,
c=12.3545(7), i=117.873(3)°, Z=4, V=2662.5(2) A , F(000)=1072. 17 844 reflections were collected (2q<
41.62°), of which 1507 reflections were independent (Rint=14.00%) and 1396 with Fo>4|(Fo). Data were corrected
for absorption using the SADABS program.
The structure was solved by direct methods and refined by least-squares in the full-matrix approximation: 182
parameters, R1=5.26%, wR2=15.21%, GOF=1.073. The Bruker SHELX software package was used for all
calculations.
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.