Spectroscopic Correlations between Supermolecules and Molecules. Anatomy of the Ion-Modulated Electronic Properties of the Nitrogen Donor in Monoazacrown-Derived Intrinsic Fluoroionophores

Article abstract of DOI:10.1021/jo035462k

The synthesis, absorption and emission spectra, fluorescence quantum yields, and fluorescence lifetimes of three compound series of trans-4,4′-disubstituted aminostilbenes (1-3) are reported. The chromo-/fluoroionophoric behavior of the monoaza-15-crown-5

Full text of DOI:10.1021/jo035462k

Sp ectr oscop ic Cor r ela tion s betw een Su p er m olecu les a n d
Molecu les. An a tom y of th e Ion -Mod u la ted Electr on ic P r op er ties of
th e Nitr ogen Don or in Mon oa za cr ow n -Der ived In tr in sic
F lu or oion op h or es†
J ye-Shane Yang,* Chung-Yu Hwang, Chia-Chun Hsieh, and Shih-Yi Chiou
Department of Chemistry, National Central University, Chung-Li, Taiwan 32054
jsyang@cc.ncu.edu.tw
Received October 6, 2003
The synthesis, absorption and emission spectra, fluorescence quantum yields, and fluorescence
lifetimes of three compound series of trans-4,4′-disubstituted aminostilbenes (1-3) are reported.
The chromo-/fluoroionophoric behavior of the monoaza-15-crown-5- (A15C5) and monoaza-18-crown-6
(A18C6)-derived species (1A5-3A5 and 1A6-3A6) in acetonitrile and dichloromethane are also
investigated. Great similarities in electronic spectroscopic properties (chemical shifts, wavelength,
intensity, and lifetime) between the metal ion-complexed supermolecules and the corresponding
chloro-substituted molecules have been observed: namely, 1A5/Ca2+-3A5/Ca2+ ∼ 1A6/Ba2+-3A6/
Ba2+ ∼ 1C-3C in acetonitrile and 1A5/Na+-2A5/Na+ ∼ 1A6/K+-2A6/K+ ∼ 1C-2C in dichlo-
romethane. Such spectroscopic correlations allow us to define the metal ion-modulated electronic
character of the azacrown nitrogen atom in the ground and excited states and, in turn, to gain
insights into the observed fluoroionophoric behavior of these probes in terms of the size and direction
of fluorescence shifts and intensity variations.
In tr od u ction
ring, charge number of cation, counterion electrostatic
interactions, and solvent donicity.1-3 For a specific type
of coronands such as N-aryl-substituted monoaza-15-
crown-5 (A15C5), more coronate stability-correlated pa-
rameters have been derived, including the sum of bond
angles about the amino nitrogen,7 the NMR chemical
shifts of the N-phenyl and crown protons,8 and the
calculated charge density on the dimethylamino group.9
Understanding of the difference in electronic nature
of the donor atoms in coronands vs coronates is particu-
larly important in understanding how the cation-coro-
nand interactions govern the spectral changes of coronand-
based intrinsic chromo-/fluoroionophores. For example,
in many monoazacrown-derived systems (e.g., compound
1A5) the azacrown nitrogen donor is directly integrated
with the aromatic chromo-/fluorophore. As a result, the
spectroscopic behavior is expected to be affected more by
the nitrogen than the oxygen donors of the azacrown
moiety. The event of cation binding modulates the
electronic nature of the nitrogen donor and concomitantly
the electronic structure of the chromo-/fluorophore. The
intrinsic difference in absorption/emission properties
between the ion-free (a molecule) and the ion-bound (a
supermolecule) species thus dictates the chromo-/fluor-
oionophoric behavior, namely the size and direction of
The cation-coronand interaction is well documented1-3
and constitutes one of the features for many important
areas of chemical investigation such as ion recognition
and separation,3,4 ion sensing and detection,4,5 and con-
struction of molecular devices.4,6 The stability of a cation-
coronand complex (a coronate)3 is determined by the
collective and cooperative interactions between the cation
and each donor atom of the coronand, which competes
with the cation-solvent interaction (solvation). Param-
eters that affect the value of the coronate stability
constant (K) include the relative cation and coronand
cavity sizes, number and stereochemical arrangement of
coronand donor atoms, substitution on the macrocycle
† This paper is dedicated to Professor Kwang-Ting Liu on the
occasion of his 65th birthday.
(1) Izatt, R. M.; Bradshaw, J . S.; Nielsen, S. A.; Lamb, J . D.;
Christensen, J . J . Chem. Rev. 1985, 85, 271-339 and references
therein.
(2) Vo¨gtle, F.; Weber, E. Crown Ethers and Analogues; Patai, S.,
Rappoport, Z., Eds.; J ohn Wiley & Sons: Avon, UK, 1989; Chapter 4.
(3) Gokel, G. W. Crown Ethers and Cryptands; Royal Society of
Chemistry: Letchworth, UK, 1991.
(4) Comprehensive Supramolecular Chemistry; Atwood, J . L., Davies,
J . E. D., MacNicol, D. D., Vogtle, F., Lehn, J .-M., Eds.; Pergamon:
Oxford, UK, 1996; Vols. 1 and 10.
(5) (a) De Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.;
Huxley, A. J . M.; McCoy, C. P.; Rademacher, J . T.; Rice, T. E. Chem.
Rev. 1997, 97, 1515-1566. (b) Valeur, B.; Leray, I. Coord. Chem. Rev.
2000, 205, 3-40. (c) Bricks, J . L.; Slominskii, J . L.; Kudinova, M. A.;
Tolmachev, A. I.; Rurack, K.; Resch-Genger, U.; Rettig, W. J . Photo-
chem. Photobiol. A: Chem. 2000, 132, 193-208.
(7) J onker, S. A.; Van Dijk, S. I.; Goubitz, K.; Reiss, C. A.;
Schuddeboom, W.; Verhoeven, J . W. Mol. Cryst. Liq. Cryst. 1990, 183,
273-282.
(8) Fery-Forgues, S.; Bourson, J .; Dallery, L.; Valeur, B. New J .
Chem. 1990, 14, 617-623.
(6) Balzani, V.; Go´mez-Lo´pez, M.; Stoddart, J . F. Acc. Chem. Res.
1998, 31, 405-414.
(9) Rurack, K.; Sczepan, M.; Spieles, M.; Resch-Genger, U.; Rettig,
W. Chem. Phys. Lett. 2000, 320, 87-94.
10.1021/jo035462k CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/07/2004
J . Org. Chem. 2004, 69, 719-726
719

Yang et al.
SCHEME 1
both wavelength shifts and intensity variations. Although
a correlation between the K values or related parameters
and the size of cation-induced spectral shifts has been
successful and meaningful for a particular probe that
interacts with different cations,7,8,10 such a correlation
could not apply to the situation of a group of different
probes. For one reason, the K value is a measure of the
collective cation-coronand interactions and does not
necessarily reflect the relative extent of electronic per-
turbation on the particular donor atom(s) that is also a
part of the chromo-/fluorophore (e.g, the azacrown N atom
in 1A5). In addition, the cation-coronand interactions
could be affected upon electronic excitation of the fluo-
rophore with an extent depending on the nature of the
fluorophore. For instance, a weakening of the cation-
nitrogen interactions has been noticed for donor (aza-
crown)- or donor-acceptor-type systems (e.g., 4A5) in the
excited state on the basis of a much less binding-induced
shift in emission vs absorption bands.11-13 In contrast,
the fluorescence response is significant in donor (azac-
rown)-donor-type systems (e.g., 1A5), presumably due
to the difference in the photoinduced internal charge
transfer (ICT) process, where the ion-bound azacrown
nitrogen no longer behaves as an electron donor.14 In this
context, it is desired to have a direct definition of the
electronic character of the coronate donor atoms in both
the ground and the excited states in order to gain insight
into the observed chromo-/fluoroionophoric behavior among
different systems.
As demonstrated by the donor-donor stilbenes
1A5-3A5 and 1A6-3A6, we report herein an approach
toward an anatomy of the cation-modulated electronic
properties of the nitrogen donor in A15C5- and mono-
aza-18-crown-6 (A18C6)-derived intrinsic probes by elec-
tronic spectroscopic correlations of the ion-bound super-
molecules with structurally related but “uncrowned”
molecules. We have observed that both the absorption
and fluorescence properties of supermolecules 1A5/Ca2+
-
3A5/Ca2+ and 1A6/Ba2+-3A6/Ba2+ closely resemble those
of the corresponding Cl-substituted molecules (1C-3C)
in acetonitrile despite the distinct fluoroionophoric be-
havior displayed by each probe. The similarity in elec-
tronic properties for 1A5/Ca2+-3A5/Ca2+ and 1C-3C in
acetonitrile-d6 is further supported by the similar 1H
NMR chemical shifts of the stilbene protons. Spectro-
scopic correlations between 1A5/Na+-2A5/Na+ and 1A6/
K+-2A6/K+ and 1C-2C in dichloromethane are also
observed. Implications of such correlations in metal ion-
azacrown interactions and the fluoroionophoric behavior
of A15C5- and A18C6-derived intrinsic probes will be
discussed.
(10) Rurack, K.; Bricks, J . L.; Reck, G.; Radeglia, R.; Resch-Genger,
U. J . Phys. Chem. A 2000, 104, 3087-3109.
Resu lts a n d Discu ssion :
(11) (a) Le´tard, J .-F.; Lapouyade, R.; Rettig, W. Pure Appl. Chem.
1993, 65, 1705-1712. (b) Dumon, P.; J onusauskas, G.; Dupuy, F.; Pe´e,
P.; Rullie`re, C.; Le´tard, J .-F.; Lapouyade, R. J . Phys. Chem. 1994, 98,
10391-10396. (c) Mathevet, R.; J onusauskas, G.; Rullie`re, C.; Le´tard,
J .-F.; Lapouyade, R. J . Phys. Chem. 1995, 99, 15709-15713.
(12) (a) Crochet, P.; Malval, J .-P.; Lapouyade, R. Chem. Commun.
2000, 289-290. (b) Rurack, K.; Rettig, W.; Resch-Genger, U. Chem.
Commun. 2000, 407-408.
(13) (a) Rurack, K.; Koval’chuck, A.; Bricks, J . L.; Slominskii, J . L.
J . Am. Chem. Soc. 2001, 123, 6205-6206. (b) Malval, J .-P.; Gosse, I.;
Morand, J .-P.; Lapouyade, R. J . Am. Chem. Soc. 2002, 124, 904-905.
(14) Delmond, S.; Le´tard, J .-F.; Lapouyade, R.; Mathevet, R.;
J onusauskas, G.; Rullie`re, C. New J . Chem. 1996, 20, 861-869.
Syn th esis. As shown in Scheme 1, the synthesis of
compound series 1-3 started with the conventional
Wittig15 or Horner-Wadsworth-Emmons16 reactions for
the construction of the 4,4′-disubstituted trans-stilbene
backbone bearing a bromo or nitro group along with the
desired R2 substituents A-I (5 and 6). The starting
(15) Maercker, A. Org. React. 1965, 14, 270-490.
(16) Wadsworth, W. S., J r. Org. React. 1977, 25, 73-253.
720 J . Org. Chem., Vol. 69, No. 3, 2004

Correlations between Supermolecules and Molecules
TABLE 1. Ma xim a of UV-Vis Absor p tion (λAbs) a n d
F lu or escen ce (λf), Stok es Sh ifts (∆νst), F lu or escen ce
Deca y Tim es (τf), a n d F lu or escen ce Qu a n tu m Yield s (Φf)
for 1-4 a n d Som e of Th eir Meta l Ion Com p lexes in
Aceton itr ile a t Room Tem p er a tu r e (ca . 298 K)a
b
compd
1A5
λabs (nm)
λf (nm)
∆νst (cm-1
)
Φf (%) τf (ns)
367c
416, 434c
452c
420
3438
5654
3438
5955
3626
6033
3728
5789
5145
4395
4215
5799
5669
5356
4735
4042
5852
6579
6730
5771
6567
5646
4922
4583
5655
4656
5680
4902
5514
5207
8883
3896
59d
1.01d
0.28
1.25
0.35
1.07d
0.25
0.75
2.78
1A5/Ca2+ 360c
8c
1A6
367
52
8
1A6/Ba2+ 358
455
1B
1C
2A5
366e
357
378
422e
455
57e
5
440
466
457
449
444
463
465
463
453
442
469
521
516
456
494
455
439
447
424
447
426
449
423
25
42
2A5/Ca2+ 367
2A5/Ba2+ 370
2A5/Mg2+ 375
materials of A15C5- and A18C6-substituted benzalde-
hyde were prepared according to the method of Dix and
Vo¨gtle.17 A reduction of the nitro group in 6 by tin(II)
chloride produced compound 7,18 which was followed by
N-methylation19 with formaldehyde and sodium cyano-
borohydride and by N-arylation20 with palladium catalyst
and phosphine ligands to form compounds 1 and 3,
respectively. The corresponding Pd-catalyzed amination
reaction between 5 and diphenylamine afforded com-
pound 2. Typical synthetic procedures and related dis-
cussion have been previously reported for the preparation
of 2H21 and 3H.22
2A6
374
26
50
0.73
2.61
2A6/Ca2+ 365
2A6/Ba2+ 366
2A6/Mg2+ 371
2A6/K+
2B
2C
2D
2E
2F
2G
2H
2I
373
375
368
388
383
361
373
362
361
371
21
43
17
18
53
30
51
62
12
42
11
40
10
45
1f
0.68
2.86
2.78
2.83
2.81
2.88
2.55
2.44
0.47
1.24
0.58
1.32
0.44
1.26
3A5
P h otop h ysica l P r op er ties of Am in ostilben es. The
spectroscopy and excited-state behavior of diaminostil-
benes 1A5 and 1B in acetonitrile have been previously
investigated.14,23,24 Despite the structural symmetry, 1B
exhibits a fairly large excited-state dipole moment (6-9
D),23 indicating the presence of strong ICT upon electronic
excitation. Both the absorption and emission spectra of
1A5 are similar to those of 1B, which suggests that the
A15C5 oxygen donors in 1A5 have little electronic effect
on the diaminostilbene fluorophore. Likewise, the elec-
tronic spectra of 2A5 and 3A5 are similar to those of 2B
and 3B, respectively (Table 1). The fluorescence quantum
yields (Φf) and lifetimes (τf) of compounds 1A5-3A5 are
also similar to those of 1B-3B, although the correspond-
ing values are slightly larger in the former compounds.
A larger value of Φf or τf for the A15C5- vs the dimethy-
lamino (DMA)-substituted compounds has been observed
in other systems and attributed to a reduced mobility of
the bulkier A15C5 vs DMA donor group.10,25
3A5/Ca2+ 342
3A6
370
3A6/Ba2+ 343
3B
3C
4A5
368
343
356f
437f
424f
339,353
4A5/Ca2+ 308f
1f
4C
298,311
a
Data were collected from sample solutions prepared under
b
atmosphere without degassing or inert gas bubbling. ∆νst ) νabs
- νfl of spectral maxima or weighted maxima. c Data from ref 14.
Data from this work. e Data from ref 23a. f Data from ref 11b.
d
between the ortho hydrogens on the stilbene (Hf) and the
methylene groups in the azacrown.26 An increase in the
twist angle is expected to cause a reduction in the
electron-donating ability of the azacrown nitrogen and
in the π-conjugation length of the fluorophore. This is
consistent with the observation of a slightly blue-shifted
absorption but red-shifted fluorescence for 1A6-3A6 vs
1A5-3A5.
As shown in Table 1, a replacement of one of the two
DMA groups in 1B by a diphenylamino (2B) or a
dipyridylamino (3B) group results in a dramatic change
in spectroscopic behavior. In comparison to those of 1B,
both the absorption and emission spectra of 2B and 3B
are red-shifted due to the increase in π-conjugation length
of the chromophores.21 On the basis of the size of the
Stokes shift, the relative extent of photoinduced ICT
character is 3B > 2B > 1B. The reverse trend is observed
for their fluorescence quantum yield and lifetimes (i.e.,
1B > 2B > 3B). Since intersystem crossing has been
shown to be efficient for 1B24 and 2H21 in both polar and
nonpolar solvents, the reduced fluorescence efficiency of
2B and 3B vs 1B could be in part attributed to an
increased rate in intersystem crossing as a result of a
decreased singlet-triplet energy gap.
An increase of the ring size on going from A15C5 to
A18C6 (i.e., 1A5-3A5 vs 1A6-3A6) only slightly affects
the photophysical properties of diaminostilbenes (Table
1). This could be attributed to a small increase in the
C-N twist angle between the phenyl and the azacrown
CNC planes as a result of the increased steric hindrance
(17) Dix, J . P.; Vo¨gtle, F. Chem. Ber. 1980, 113, 457-470.
(18) Bellamy, F. D.; Ou, K. Tetrahedron Lett. 1984, 25, 839-842.
(19) Borch, R. F.; Hassid A. I. J . Org. Chem. 1972, 37, 1673-1674.
(20) (a) Wolfe, J . P.; Wagaw, S.; Marcoux, J .-F.; Buchwald, S. L. Acc.
Chem. Res. 1998, 31, 805-818. (b) Hartwig, J . F. Angew. Chem., Int.
Ed. 1998, 37, 2046-2067.
(21) Yang, J .-S.; Chiou, S.-Y.; Liau, K.-L. J . Am. Chem. Soc. 2002,
124, 2518-2527.
(22) Yang, J .-S.; Lin, Y.-H.; Yang, C.-S. Org. Lett. 2002, 4, 777-
780.
(23) (a) Le´tard, J .-F.; Lapouyade, R.; Rettig, W. J . Am. Chem. Soc.
1993, 115, 2441-2447. (b) Le´tard, J .-F.; Lapouyade, R.; Rettig, W.
Chem. Phys. Lett. 1994, 222, 209-216.
(24) Lewis, F. D.; Weigel, W.; Zuo, X. J . Phys. Chem. A 2001, 105,
4691-4696.
(25) Lednev, I. K.; Ye, T.-Q.; Hester, R. E.; Moore, J . N. J . Phys.
Chem. A 1997, 101, 4966-4972.
(26) Le´tard, J .-F.; Delmond, S.; Lapouyade, R.; Braun, D.; Rettig,
W.; Kreissler, M. Recl. Trav. Chim. Pays-Bas 1995, 114, 517-527.
J . Org. Chem, Vol. 69, No. 3, 2004 721

Yang et al.
TABLE 2. Ma xim a of UV-Vis Absor p tion (λa bs) a n d
F lu or escen ce (λf), Stok es Sh ifts (∆νst), F lu or escen ce
Deca y Tim es (τf), a n d F lu or escen ce Qu a n tu m Yield s (Φf)
for 1-4 a n d Som e of Th eir Meta l Ion Com p lexes in
Dich lor om eth a n e a t Room Tem p er a tu r e (ca . 298 K)a
It has been shown that the photophysical behavior of
trans-4,4′-disubstituted stilbenes containing a DMA group
such as 1C, 1D, 1E, and 1I is strongly dependent on the
other substituent.27 A nice correlation between the
fluorescence quantum yield and the Hammett σ-con-
stants has been observed: the stronger the “push-pull”
character of the stilbene is, as defined by the electronic
difference between the two substituents, the smaller the
fluorescence quantum yield becomes.26 Such a feature is
essentially retained for the diphenylamino-substituted
analogues, i.e., compounds 2B-2I. It should be noted that
the one having the largest fluorescence quantum yield
in the compounds 1 series is 1B but it is 2I in the
compounds 2 series (Table 1). In addition, the fluores-
cence quantum yield is generally larger for 2C-2I than
1C-1I, which could be attributed to the “amino conjuga-
tion effect”.21 A more detailed discussion of such an effect
for 1H and 2H has been recently reported.21 Since this
effect is associated with the double bond torsion in the
singlet state, the singlet-state mechanism for trans f cis
photoisomerization appears to be important in these
aminostilbenes, consistent with the previous conclusion
based on 1C and 1I.27 It also should be noted that the
fluorescence lifetimes are much less sensitive to substitu-
tion than the fluorescence quantum yields in both
systems 1 and 2.
The spectroscopic data for compounds 1 and 2 deter-
mined in less polar dichloromethane solvent are shown
in Table 2. When compared with the corresponding data
in acetonitrile, the position of the absorption is slightly
red shifted by 3-6 nm (negative solvatochromism), but
the shift for the fluorescence is relatively larger and
hypsochromic (positive solvatofluorochromism). A similar
solvent effect has been observed for 1A5 and trans-4,4′-
diaminostilbene.14,28 The solvent effect on the fluorescence
position is consistent with the significantly larger ICT
character of these aminostilbenes in the excited vs ground
state. As for the fluorescence quantum yields and life-
times, the values are decreased for 1A5, 1A6, and 1B but
increased for 2A5-2I on going from acetonitrile to dichlo-
romethane. A strongly fluorescent twisted ICT state
(TICT) has been proposed to participate in the excited
states of 1A5 and 1B.14,23 The difference in solvent effect
between compounds 1 and 2 might result from a different
role of such a TICT state. More investigation of this issue
is required to reach a pertinent conclusion.
b
compd
λabs (nm) λf (nm) ∆νst (cm-1
)
Φf (%) τf (ns)
1A5
370
363
362
369
361
363
369
362
382
374
375
380
374
381
381
373
374
375
373
380
373
395
388
366
379
366
367
414
445
433
417
441
435
415
436
428
464
464
465
449
432
429
457
458
443
450
428
452
503
501
445
477
442
434
2872
5076
4530
3119
5025
4560
3004
4689
2814
5186
5115
4810
4466
3099
2937
4928
4904
4094
4587
2951
4686
5436
5813
4850
5421
4698
4206
28
8
6
29
10
9
29
6
0.85
0.29
0.34
0.90
0.33
0.66
0.91
0.23
1.02
2.47
2.38
1A5/Ca2+
1A5/Na+
1A6
1A6/Ba2+
1A6/K+
1B
1C
2A5
46
55
73
2A5/Ca2+
2A5/Ba2+
2A5/Mg2+
2A5/Na+
2A5/K+
2A6
54
2.30
52
70
74
0.88
1.85
2.63
2A6/Ca2+
2A6/Ba2+
2A6/Na+
2A6/K+
2B
2C
2D
2E
2F
62
40
58
31
30
62
42
58
63
2.13
0.91
2.46
2.40
2.08
2.60
2.46
2.53
2.37
2G
2H
2I
a
Data were collected from sample solutions prepared under
b
atmosphere without degassing or inert gas bubbling. ∆νst ) νabs
- νfl of spectral maxima.
the previous report,11 the absorption spectrum of 1A5 is
blue shifted in the presence of Ca2+, due to the azacrown
nitrogen-Ca2+ interactions that reduce the chromophoric
π-system, and the corresponding fluorescence spectrum
is red shifted, due to the increased ICT character in the
excited state. While the absorption spectra of 2A5 and
3A5 are also blue-shifted upon the addition of Ca2+, the
fluorescence shifts to the red for 2A5 but to the blue for
3A5. Furthermore, unlike the Ca2+-induced fluorescence
intensity weakening and fluorescence lifetime shortening
in 1A5, the Ca2+-binding of 2A5 and 3A5 leads to fluores-
cence enhancement by a factor of 1.7-3.5 and lifetime
increase by a factor of 2.6-3.7. It is interesting to note
that the chromo-/fluoroionophoric behavior of 1A6-3A6
to Ba2+ closely resembles that of 1A5-3A5 to Ca2+ (Table
1). In other words, the electronic effect exerted by A15C5/
Ca2+ is similar to that exerted by A18C6/Ba2+ for the
fluorophores 1-3.
Com p lexa tion Beh a vior of 1A5-3A5 a n d 1A6-3A6.
The effect of the alkali and alkaline earth metal ions Li+,
Na+, K+, Mg2+, Ca2+, and Ba2+ in perchlorate salts on
the electronic absorption and emission spectra of 1A5-
3A5 and 1A6-3A6 in acetonitrile has been investigated.
The spectra are slightly or not at all affected by the
monovalent alkali metal ions, and the largest spectral
shifts for 1A5-3A5 and 1A6-3A6 are induced by Ca2+ and
Ba2+, respectively, in accord with the selectivity previ-
ously observed for other azacrown-derived systems.17,26,29,30
Figure 1 shows the absorption and fluorescence spectra
of 1A5-3A5 in acetonitrile in the absence and the
presence of excess Ca2+ (∼100 equiv). In agreement with
The complex stability constants, expressed as logK,
determined by absorption titration spectra31 for 1A5-3A5
and 1A6-3A6 with Na+, K+, Ca2+, and Ba2+ in acetonitrile
are shown in Table 3. Despite the distinct fluorescence
responses (position, intensity, and lifetime) of 1A5-3A5
to Ca2+, the logK values of 1A5/Ca2+-3A5/Ca2+ are only
(30) (a) Lo¨hr, H.-G.; Vo¨gtle, F. Acc. Chem. Res. 1985, 18, 65-72. (b)
Fery-Forgues, S.; Le Bris, M.-T.; Guette´, J .-P.; Valeur, B. J . Phys.
Chem. 1988, 92, 6233-6237. (c) Cazaux, L.; Faher, M.; Lopez, A.;
Picard, C.; Tisnes, P. J . Photochem. Photobiol. A: Chem. 1994, 77, 217-
225.
(31) Connors, K. A. Binding ConstantssThe Measurement of Mo-
lecular Complex Stability; J ohn Wiley & Sons: New York, 1987;
Chapter 4.
(27) Papper, V.; Pines, D.; Likhtenshtein, G.; Pines, E. J . Photochem.
Photobiol. A: Chem. 1997, 111, 87-96.
(28) Zhao, Y.; Khodorkovsky, V.; Cohen, J .; Priel, Z. J . Photochem.
Photobiol. A: Chem. 1996, 99, 23-28.
(29) Rurack, K. Spectrochim. Acta A 2001, 57, 2161-2195.
722 J . Org. Chem., Vol. 69, No. 3, 2004

Correlations between Supermolecules and Molecules
TABLE 4. Ch em ica l Sh iftsa of Stilben e P r oton s (Ha -Hf)
fr om th e 1H NMR Sp ectr a (500 MHz) of 1-3 (2 m M)b in
Aceton itr ile-d 3
compd
1A5
δHa
δHb
δHc
δHd
δHe
δH
f
6.72
6.72
6.74
6.73
6.98
6.98
6.99
6.98
7.00
7.00
7.00
6.98
7.35
7.35
7.41
7.40
7.41
7.40
7.45
7.45
7.49
7.49
7.53
7.55
6.85
6.86
6.93
6.95
6.99
6.99
7.06
7.07
7.06
7.07
7.13
7.15
6.85
6.86
7.10
7.08
6.90
6.92
7.15
7.14
6.94
6.97
7.20
7.18
7.32
7.35
7.48
7.54
7.36
7.39
7.51
7.57
7.38
7.41
7.54
7.59
6.66
6.72
7.33
7.27
6.68
6.73
7.36
7.29
6.70
6.74
7.37
7.30
1B
1C
1A5/Ca2+
2A5
2B
2C
2A5/Ca2+
3A5
3B
3C
3A5/Ca2+
a
Data were recorded with respect to the solvent peak at δ 1.93
ppm. The mole ratio of 1A5-3A5:Ca2+ is 1:100.
b
F IGURE 1. Absorption and fluorescence (λex ) 365 nm)
spectra of (A) 1A5, 1A5/Ca2+, and 1C, (B) 2A5, 2A5/Ca2+, and
2C, and (C) 3A5, 3A5/Ca2+, and 3C in acetonitrile.
TABLE 3. Com p lex Sta bility Con sta n ts (log K) of
1A5-3A5 a n d 1A6-3A6 in Aceton itr ilea
compd
Na+
K+
Ca2+
Ba2+
1A5
2A5
3A5
1A6
2A6
3A6
3.4 (5.2)
3.6 (5.1)
3.9
n.d.
n.d.
n.d.
n.d
n.d
4.5 (5.0)
4.2 (4.8)
4.5
4.9b (5.7)
4.8 (5.4)
4.7
5.4
5.1
4.5
4.4
4.7
5.4
5.1
5.0
F IGURE 2. Normalized absorption and fluorescence spectra
of (A) 1A5, 1A5/Na+, and 1C and (B) 2A5, 2A5/Na+, and 2C in
dichloromethane.
n.d.
4.9
which could be attributed to the larger number of donor
atoms in A18C6 vs A15C5. It is also noted that 1A5-
3A5 show a larger selectivity than 1A6-3A6 toward Ca2+
a
The values given in parentheses were determined in dichlo-
b
romethane. Data from ref 14.
and Ba2+
.
slightly different from one another. The similarity in logK
value for 1A5/Ca2+-3A5/Ca2+ is also consistent with the
similar Ca2+-induced shifts in their 1H NMR spectra.
According to Fery-Forgues et al. on the studies of
4-substituted Ph-A15C5 derivatives,8 the difference be-
tween the Ca2+-induced change in the chemical shift of
the ortho and that of meta protons, i.e., [∆δ(Hortho) - ∆δ-
(Hmeta)], is proportional to their logK values. As shown
in Table 4, the values of [∆δ(Hf) - ∆δ(He)] are nearly
the same (0.39-0.40 ppm) in all three cases. Apparently,
the N-substituents in the R1 group merely cause a small
influence on the A15C5-Ca2+ interactions. Therefore, the
observed difference in fluoroionophoric behavior among
1A5-3A5 should simply result from the intrinsic differ-
ence in the fluorophores (vide infra). In comparison,
complexes 1A6/Ba2+-3A6/Ba2+ possess larger logK values,
The chromo-/fluoroionophoric behavior of 1A5, 2A5, 1A6,
and 2A6 in less polar dichloromethane solvent has also
been investigated (Figure 2). The corresponding studies
on 3A5 and 3A6 were not performed, because in dichlo-
romethane the interaction between the bis(2-pyridyl)-
amino (R1) group and metal ions seems not negligible and
might complicate the spectral analysis. As indicated by
the larger logK values for the same complexes (Table 3),
the cation-azacrown interactions are stronger in dichlo-
romethane vs acetonitrile, due to the poorer cation
solvation interactions. While this is accompanied by a
larger spectral shift in the cases of the monovalent Na+
and K+, the spectral shifts induced by the divalent ions
Ca2+ and Ba2+ are decreased (Table 2). As a result, the
cation selectivity by these fluoroionophores is decreased.
For example, the spectra of both 2A5 and 2A6 could not
J . Org. Chem, Vol. 69, No. 3, 2004 723

Yang et al.
differentiate Ca2+ from Ba2+, and the absorption shifts
induced by Na+, K+, Ca2+, and Ba2+ are essentially the
same for 2A6 in dichloromethane. Like the cases between
1A5/Ca2+-3A5/Ca2+ and 1A6/Ba2+-3A6/Ba2+ in acetoni-
trile, the spectroscopic properties of 1A5/Na+ and 2A5/
Na+ closely resemble those of 1A6/K+ and 2A6/K+, re-
spectively, in dichloromethane.
Su p er m olecu le-Molecu le Cor r ela tion a n d Its Im -
p lica tion s. In addition to the above-mentioned correla-
tions between different complexes, spectroscopic corre-
lations between complexes (supermolecules) and molecules
are also observed. As demonstrated by complexes 1A5/
Ca2+-3A5/Ca2+ in acetonitrile, their spectroscopic data
are in great similarity, with a difference within the
experimental uncertainties (i.e., (2 nm in λabs and λfl,
(10% in Φf, and (0.10 ns in τf) to those for the
corresponding Cl-derivatives 1C-3C (Table 1 and Figure
1). Such results suggest that the interactions between
Ca2+ and the A15C5 nitrogen atom in acetonitrile are all
similar in 1A5/Ca2+-3A5/Ca2+, consistent with their
similar logK values (Table 3), and that the Ca2+
-
modulated nitrogen atom in A15C5/Ca2+ has an elec-
tronic nature corresponding to a chlorine atom. In
particular, such a correlation exists in the excited state
as well as in the ground state, indicating that electronic
excitation of the fluorophores has a negligible effect on
the interactions between A15C5 and Ca2+, and the
complexes are essentially stable during their excited-
state lifetimes (0.28-2.78 ns). On the basis of the
Hammett substituent constants,32 the electronic charac-
ter of a p-Cl substituent (σ ) 0.23) is electron withdraw-
ing with a strength between that of a p-F (σ ) 0.06) and
a p-carbonyl ester group (σ ) 0.50) and is very different
from that of an electron-donating p-amino group (σ )
-0.66). The dependence of the spectroscopic properties
of 4,4′-disubstituted aminostilbenes on the R2 substituent
could be illustrated by the comparison of compounds 2A-
2I (Table 1). Thus, it is such a difference between 1A5-
3A5 and the corresponding 1C-3C that determines the
observed chromo-/fluoroionophoric behavior for 1A5-3A5
upon interacting with Ca2+ in acetonitrile. Likewise, a
spectroscopic correlation of 1A6/Ba2+-3A6/Ba2+ ∼ 1C-
3C in acetonitrile also exists. In dichloromethane, the
azacrown nitrogen in A15C5/Ca2+ and A18C6/Ca2+ has
an induction strength larger than that of Cl but lower
than that of CF3 based on the relative fluorescence
position of the complexes 2A5/Ca2+ and 2A6/Ba2+ and
compounds 2A-2I (Table 2). However, the substituent
effect of A15C5/Na+ and A18C6/K+ reaches a level similar
to that of Cl in dichloromethane, i.e., 1A5/Na+-2A5/Na+
∼ 1A6/K+-2A6/K+ ∼ 1C-2C (Figure 2).
F IGURE 3. 1H NMR spectra (500 MHz) of 3A5, 3B, 3C, and
3A5/Ca2+ in acetonitrile-d3. The substrate concentration is ca.
2 mM. The solid circles denote peaks due to the N-pyridyl
protons, and the dashed lines are to correlate the stilbene
protons.
the pyridyl protons are either unchanged or slightly
shifted, which supports our previous conclusion based on
3H that interactions between the dipyridylamino group
and Ca2+ in acetonitrile are negligible.22 In accord with
the electronic spectra, the 1H NMR spectrum of 3C
resembles that of 3A5/Ca2+ with a small deviation (0.02-
0.07 ppm) comparable to that (0-0.04 ppm) between the
spectra of 3A5 and 3B. It should be noted that a chemical
shift can be affected not only by the electronic inductive
and resonance properties but also by the size and
1
anisotropy of substituents. Thus, the observed H NMR
correlation between 1C-3C and 1A5/Ca2+-3A5/Ca2+ is
remarkable in view of their dramatic differences in size
and anisotropy (sphere vs bulky nonsphere shape).
In principle, the interactions of cations with azacrown
ether will reduce the electron-donating strength of the
nitrogen atom, and the maximum effect will occur when
the nitrogen atom becomes a fully charged ammonium
ion. A continued spectrum of electronic character ranging
from the unperturbed amino group to the fully charged
ammonium ion should exist for the cation-modulated
nitrogen atom, depending on the extent of interactions,
and, in turn, on those parameters that affect the complex
stability constant. Thus, the spectroscopic properties of
a complex could not always correlate with those of a
molecule, because the number of molecular substituents
with well-defined electronic effects is limited. Even worse,
the substituent effect on the photophysical behavior of
an aromatic system depends not only on the electron
“push-pull” character but also on the size, bonding, and
geometry of the substituent. For example, some molec-
ular substituents such as Br, I, carbonyl, and nitro groups
are know to facilitate intersystem crossing of the excited
fluorophore (e.g., stilbene), leading to lower fluorescence
The ground-state electronic correlation between 1A5/
Ca2+-3A5/Ca2+ and 1C-3C in acetonitrile-d3 is further
supported by 1H NMR spectroscopy. As is depicted by the
case of 3A5 (Figure 3), the introduction of Ca2+ leads to
a significant downfield shifting (∆δ ) 0.60 ppm) of the
stilbene protons (Hf) localized in the ortho position to the
A15C5. The extent of downfield shifting decreases for
those (Hb-He) at farther distances than Hf to the A15C5
and becomes negligible for the protons (Ha) near the
dipyridylamino group (Table 4). The chemical shifts of
(32) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in
Organic Chemistry; Happer & Row: New York, 1987; p 144.
724 J . Org. Chem., Vol. 69, No. 3, 2004

Correlations between Supermolecules and Molecules
quantum yields.33 For substituents that possess π-con-
jugated bonding such as CN and carbonyl groups, the
π-conjugation effect on the spectra often surpasses the
induction effect (e.g., the fluorescence maximum 2D (521
nm) > 2A5/H+ (492 nm) in acetonitrile). Moreover, some
cations, particularly the paramagnetic cations (e.g., Ni2+
and Cu2+), could interact with the excited fluorophore not
only by electrostatic coordination but also by energy or
electron transfer, leading to fluorescence quenching.34
With these in mind, the nice supermolecule-molecule
correlation in both positions of spectra and the yields and
lifetimes of fluorescence for systems 1-3 appear to be
novel and unique, and it is anticipated that changes in
any one of the parameters (e.g., solvent and counterion)
will remove the observed correlations: namely, A15C5/
Ca2+(ClO4-)2 ∼ A18C6/Ba2+(ClO4-)2 ∼ Cl in acetonitrile
and A15C5/Na+(ClO4-) ∼ A18C6/K+(ClO4-) ∼ Cl in
dichloromethane.
Nonetheless, implications of these correlations deserve
our attention. An electronic correlation of a nitrogen atom
with a chlorine atom is reminiscent of their similarity in
electronegativity (3.04 vs 3.16).35 The main factor that
differentiates the electronic effect of an amino group from
that of a chloro substituent in aromatic compounds is the
resonance effect of the lone pair electrons. A good overlap
between the carbon and nitrogen 2p orbitals makes the
amino group an excellent electron donor, whereas the
poor overlap between the carbon 2p and chlorine 3p (or
sp3) orbitals keeps the chlorine atom as an electron-
withdrawing substituent. In this context, a reduction in
C-N orbital overlap will reduce the amino group reso-
nance effect and meanwhile the difference in electronic
effect between the amino nitrogen and the chlorine atom.
This could be achieved by pyramidalizing (sp3 hybridiza-
tion) the nitrogen atom and/or by twisting the C-N bond.
Indeed, an increase of the degree of pyramidalization of
the azacrown nitrogen upon metal ion complexation has
been observed in many other systems,7-10,36 and the
degree of pyramidalization in some cases correlates well
with the chromoionophoric shift.7 Azacrown-derived probes
that undergo complexation-induced C-N twisting have
also been reported.37 Since the alkali and alkaline-earth
metal ions that provide the strongest interactions for
A15C5 and A18C6 are Ca2+ and Ba2+, respectively, it
appears that the maximum electrostatic perturbation of
metal ions on the azacrown nitrogen in acetonitrile is to
localize the nitrogen lone-pair electrons. Our results also
suggest that a decrease in cation solvation on going from
acetonitrile to dichloromethane strengthens the electro-
static interaction between the azacrown and the metal
ions in such a way that Ca2+ and Ba2+ can further
polarize the N-M2+ coordination bond and results in a
partially positive-charged nitrogen, and that Na+ and K+
become capable of localizing the nitrogen lone pair
electrons. Regarding the distinct size of azacrown vs Cl,
the nice spectroscopic correlations observed herein indi-
cate that the excited-state behavior of the cation-bound
fluorophores 1-3 is insensitive to the motion of the
cation-bound azacrown moiety.
For comparison, the spectroscopic properties of donor-
substituted stilbene 4A5, its complex 4A5/Ca2+, and 4C
in acetonitrile were also examined (Table 1). While the
absorption maximum of 4A5/Ca2+ is approximately the
same as that of 4C, their fluorescence positions are rather
different (Table 1).38 The slightly lower value of logK
(4.4)11 for 4A5/Ca2+ in comparison to that for 1A5/Ca2+
-
3A5/Ca2+ might indicate a slightly weaker interaction
between Ca2+ and the azacrown nitrogen, but this ap-
pears not sufficient to account for the large discrepancies
in fluorescence maximum (4744 cm-1) between 4A5/Ca2+
and 4C. Instead, a weakening of the Ca2+-N bonding,
resulting in a weaker electronic influence of Ca2+ on the
azacrown nitrogen, on going from the ground state to the
excited state is implicated. This is in accord with the
previous studies on donor-donor- vs donor-(acceptor)-
type ICT systems, where the latter systems, including
4A5/Ca2+, are destined to undergo the excited-state Ca2+
decoordination reaction.11-13
Con clu d in g Rem a r k s
Novel spectroscopic correlations between metal ion-
complexed supermolecules and structurally related mol-
ecules have been observed, including 1A5/Ca2+-3A5/Ca2+
∼ 1A6/Ba2+-3A6/Ba2+ ∼ 1C-3C in acetonitrile and 1A5/
Na+-2A5/Na+ ∼ 1A6/K+-2A6/K+ ∼ 1C-2C in dichlo-
romethane. While comparisons between complexes and
molecules in the ground state by 1H NMR or UV-vis
absorption spectroscopy have been previously performed
for some azacrown-derived intrinsic fluoroionophores,8,29
the correlations reported herein have for the first time
allowed a more accurate description of the metal ion-
modulated electronic character of the azacrown nitrogen
in the excited state as well as in the ground state. A
persistent correlation in both the absorption (ground
state) and emission (excited state) spectra suggests that
the cation-azacrown interactions in these donor-donor
stilbenes are little affected by electronic excitation and
the subsequent relaxation of the fluorophores. The simi-
lar complex stability constants but distinct fluoroiono-
phoric behavior observed for systems 1-3 show that the
N-substituents can strongly affect the intrinsic properties
of the aminostilbene fluorophore but not the azacrown-
metal ion interactions. With such correlations, the elec-
tronic nature of the metal ion-perturbed fluorophores
could be further investigated based on the electronically
correlated molecules, since both experimental and theo-
retical studies on a molecule vs supermolecule are more
feasible. A modulation of the azacrown nitrogen with a
character from strongly electron donating (free probe) to
moderately electron withdrawing resembling a chlorine
atom (complexed probe) appears to be sufficient for
(33) (a) Saltiel, J .; Sun, Y.-P. Photochromism, Molecules and Sys-
tems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, The
Netherlands, 1990; pp 64-164. (b) Saltiel, J .; Charlton, J . L. Rear-
rangements in Ground and Excited States; de Mayo, P., Ed.; Academic
Press: New York, 1980; Vol. 3, pp 25-89.
(34) Varnes, A. W.; Dodson, R. B.; Wehry, E. L. J . Am. Chem. Soc.
1972, 94, 946-950.
(35) Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC
Press: Boca Raton, FL, 1997.
(36) Alfimov, M. V.; Churakov, A. V.; Fedorov, Y. V.; Fedorova, O.
A.; Gromov, S. P.; Hester, R. E.; Howard, J . A. K.; Kuz’mina, L. G.;
Lednev, I. K.; Moore, J . N. J . Chem. Soc., Perkin Trans. 2 1997, 2249-
2256.
(37) Witulski, B.; Weber, M.; Bergstra¨sser, U.; Desvergne, J .-P.;
Bassani, D. M.; Bouas-Laurent, H. Org. Lett. 2001, 3, 1467-1470.
(38) Compound 4C shows vibronic structures in the electronic
spectra. For the purpose of comparison with 4A5/Ca2+, the weighted
spectral maxima for the absorption and emission of 4C were estimated
to be ca. 308 and 350 nm, respectively.
J . Org. Chem, Vol. 69, No. 3, 2004 725

Yang et al.
were recorded with 1 × 10-5 M acetonitrile or dichloromethane
solutions for all cases. The fluorescence four-wall cuvette was
charged with 3 mL of fluoroionophores and a magnetic stir
bar. The acetonitrile solution of perchlorate salts used for
complexation studies is at a concentration of 0.005 M. Aliquots
of salt solution were added at the prescribed increments.
Solutions were allowed to equilibrate for 15 min before the
measurementwas taken. Experimentation with longer equili-
bration times did not produce noticeable differences. The
spectroscopic properties of complexes were determined in the
presence of excess metal ions (∼100 equiv), which is beyond
the ending of titration (<50 equiv), where no further shifts
were observed in both the absorption and fluorescence spectra.
Complex stability constants were determined based on the
absorption titration curves.31
Ma ter ia ls. Solvents for organic synthesis were reagent
grade or HPLC grade but all were HPLC grade for spectra
and quantum yield measurements. Compounds 1A5,8 1B,40
1C,27 2D,41 2H,22 2I,41 5B,27 5C,42 5D,43 5E,44 5F ,45 5H,22 5I,46
6B,27 6C,27 7B,27 and 7C27 have been previously reported, and
our melting points and/or 1H NMR spectra conform to the
literature values. The characterization data for the other
compounds are provided in the Supporting Information.
creating a sensitive ICT-based probe in acetonitrile.
However, the stability constant and thus the extent of
electronic perturbation of metal ions on the azacrown
nitrogen is expected to be substantially lower in aqueous
solutions. A cooperative binding of the azacrown with
other ligands in the probe molecule might be a solution
for retaining or improving the metal ion-azacrown
interactions in aqueous conditions. Azacrown ethers are
also employed in forming fluoroionophores that signal the
complexation event by the mechanism of photoinduced
electron transfer (PET).5 Since the energetics of PET from
the azacrown nitrogen (donor) to the excited fluorophore
(acceptor) depend on the electronic character of the
nitrogen donor, our results also should be informative
for the PET- and azacrown-based probes. Finally, it is
interesting to point out that the knowledge of cation-
coronand interactions in terms of selectivity and stability
has offered a basis for the development of chromo-/
fluoroionophores, and now our results show the feedback
of these probes, providing a better understanding of the
cation-coronand interactions in terms of the binding
character between the cation and the donor atoms.
Ack n ow led gm en t. Financial support for this re-
search was provided by the National Science Council
(NSC 91-2113-M-008-011 and NSC 92-2113-M-008-006).
Exp er im en ta l Section
Su p p or tin g In for m a tion Ava ila ble: Detailed character-
1
ization data and H and 13C NMR spectra of new compounds.
Meth od s. The degassed solution of anthracene in hexane
(Φf ) 0.27)39 was used as standard for the fluorescence
quantum yield determinations of systems 1-3 in normal
atmosphere with solvent refractive index correction. All fluo-
rescence spectra are uncorrected and an error of (10% is
estimated for the fluorescence quantum yields. The lifetime
data were determined with excitation and emission wave-
lengths at the absorption and fluorescence maxima of the
corresponding substrates. All the fluorescence decay curves
can be fit with a single exponential. The goodness of nonlinear
least-squares fit was judged by the reduced ø2 value (<1.3 in
all cases), the randomness of the residuals, and the autocor-
relation function. Fluorescence and absorption titration spectra
This material is available free of charge via the Internet at
http://pubs.acs.org.
J O035462K
(40) Stewart, F. H. C. J . Org. Chem. 1961, 26, 3604-3605.
(41) Yang, J .-S.; Wang, C.-M.; Hwang, C.-Y.; Liau, K.-L.; Chiou, S.-
Y. Photochem. Photobiol. Sci. 2003, 2, 1225-1231.
(42) Tewari, R. S.; Kumari, N.; Kendurkar, P. S. J . Chem. Eng. Data
1976, 21, 125-131.
(43) Bance, S.; Barber, H. J .; Woolman, A. M. J . Chem. Soc. 1943,
1-4.
(44) Fuson, R. C.; Cooke, H. G., J r. J . Am. Chem. Soc. 1940, 62,
1180-1183.
(45) Gascoyne, J . M.; Mitchell, P. J .; Phillips, L. J . Chem. Soc.,
Perkin Trans. 2 1977, 1051-1057.
(39) Dawson, W. R.; Windsor, M. W. J . Phys. Chem. 1968, 72, 3251-
3260.
(46) Diana, G. D.; Carabateas, P. M.; J ohnson, R. E.; Williams, G.
L.; Pancic, F.; Collins, J . C. J . Med. Chem. 1978, 21, 889-894.
726 J . Org. Chem., Vol. 69, No. 3, 2004