Donor-substituted distyrylpyrazines: Influence of steric congestion on UV-Vis absorption and fluorescence

Article abstract of DOI:10.1002/poc.2978

Di(p-aminostyryl)pyrazines with voluminous substituents on the nitrogen and in the adjacent positions were prepared via twofold aldol condensation. Absorption and emission spectra are influenced by increasing steric hindrance because the orbital overlap between nitrogen and π-system is modulated by voluminous groups. Strong solvatochromism of the fluorescence and huge Stokes shifts result from amplified donor-acceptor interaction in the excited state. Protonation occurs at the terminal amino groups first, followed by protonation of the central pyrazine. With increasing strength of acid, absorption and emission spectra are first shifted to the blue followed by a redshift. Copyright

Full text of DOI:10.1002/poc.2978

Research Article
Received: 30 November 2011,
Revised: 6 March 2012,
Accepted: 7 May 2012,
Published online in Wiley Online Library: 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.2978
Donor-substituted distyrylpyrazines:
influence of steric congestion on UV–Vis
absorption and fluorescence
Christoph Winka and Heiner Deterta*
Di(p-aminostyryl)pyrazines with voluminous substituents on the nitrogen and in the adjacent positions were
prepared via twofold aldol condensation. Absorption and emission spectra are influenced by increasing steric
hindrance because the orbital overlap between nitrogen and p-system is modulated by voluminous groups. Strong
solvatochromism of the fluorescence and huge Stokes shifts result from amplified donor–acceptor interaction in the
excited state. Protonation occurs at the terminal amino groups first, followed by protonation of the central pyrazine.
With increasing strength of acid, absorption and emission spectra are first shifted to the blue followed by a redshift.
Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: acidochromism; conjugated oligomers; fluorescence; halochromism; solvatochromism
changes of the electronic spectra. These external stimuli can be
used to characterize local environments in artificial and bio-
INTRODUCTION
logical materials, making compounds of this type interesting
Distyrylbenzenes and related fluorophores find widespread
for sensing purposes.[31–33] In the recent past, the interest in
application in optical display devices,[1–6] as nonlinear optical
local properties increased and initiated the discovery of a range
materials,[7–9] two-photon absorbers,[10–12] and for sensing
of efficient and selective fluorescent sensing materials.[34–37]
purposes.[13,14] The combination of electron-pair donating and
Among others, donor-substituted distyrylpyrazines are appro-
electron-pair accepting (EPA) substituents on a p-system can
priate fluorophores for sensing purposes. Some had been
lead to an efficient internal charge transfer, resulting in batho-
prepared[11,38–40] and used in optoelectronic devices, as lasing
chromic shifts of the electronic spectra, dual fluorescence,[15–18]
materials,[41,42] and also as two-photon absorbing dyes.[11]
and efficient two-photon absorption. These properties depend
As part of our interest in fluorophores with switchable optical
on the length and nature of the p-system and the strength and
properties and efficient two-photon absorption,[10,43–45] we
position of donor and acceptor groups.[19] In this context, amino
report here the synthesis of linear 2,5-distyrylpyrazines (DSP)
with lateral amino groups but steric congestion around these
donors. With the central electron acceptor pyrazine, a donor–
acceptor–donor electronic structure results. The donor strength
groups as efficient donors are often combined with strong
acceptors like nitriles.[12,20] In addition to electron withdrawing
groups on the p-system, replacing benzene rings with electron
deficient heterocycles like pyridine[21–24] is a successful route to
of the aminostyryl units is modulated by steric repulsion of
increase the electron affinity. The interaction of these com-
substituents on the nitrogen and in the neighbouring ortho-
pounds with the environment can result in significant spec-
positions. The influences of steric hindrance on the electronic
troscopic changes, useful for sensing purposes.[25,26] Dipolar
spectra and the impact of the environment on absorption and
solvents stabilize species with an internal charge transfer, in
emission are reported because solvent polarity and protonation
the ground state and in the excited state. This is visible as
have a distinct effect on the absorption or emission behaviour.
solvatochromism of absorption or fluorescence spectra. Because
the extent of charge transfer between electron-pair donating
Synthesis
and EPA substituents and the p-bridge is strongly depending
on the twist angle between these groups,[27,28] steric hindrance
Condensation of benzaldehydes with dimethylpyrazine is a fast
synthetic access to donor-substituted distyrylpyrazines.[38–40,46]
Generally, the required benzaldehydes are easily prepared by
can reduce the p–p or p–p overlap. If the steric energy and the
stabilization of the charge-transfer (CT) state by solvent dipoles
are of comparable values, an interplay between these interac-
tions and therefore a modulation of the solvatochromism
results.[29,30] Furthermore, reactions of amine donors with pro-
tons or Lewis acids reduce their donor strength, whereas
these interactions with azines increase their electron accepting
power. Both, solvent polarity and acids provoke environment-
depending donor and acceptor capabilities and therefore
modulate the internal charge transfer, resulting in significant
* Correspondence to: H. Detert, Institut für Organische Chemie, Duesbergweg
10-14, D-55099 Mainz, Germany.
E-mail: detert@mail.uni-mainz.de
Dedicated to Wolfgang Rettig on the occasion of his 65th birthday
a C. Wink, H. Detert
Institut für Organische Chemie, Duesbergweg 10-14, D–55099, Mainz, Germany
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

C. WINK AND H. DETERT
R2
Vilsmeier formylation of the corresponding anilines, but this
procedure relies on the electron-donating effect of the amino
group. Whereas N,N-diisopropylaminobenzaldehyde 7a was
prepared in moderate yield (49%), no formylation of anilines 1
with higher sterical hindrance could be achieved. A sequence
of bromination of anilines 1 to bromoanilines 2 or 5 (via reduc-
tion of acetanilides 4 to ethylanilines 5), alkylation of 2 and 5 fol-
lowed by lithiation of 3, 6, and reaction with dimethylformamide
(DMF) proved to be successful for the preparation of highly
alkylated aminobenzaldehydes 7b–7f.[47] (Scheme 1)
R3
N
N
CHO
+
R4
N
R1
8
7a - 7f
KOtBu
DMF
R2
R3
N
R2
N
R4
R3
R1
The methyl groups in 2,5-dimethylpyrazine 8 are sufficiently
activated that a deprotonation with potassium-t-butylate in
DMF is possible, allowing a direct aldol condensation on both
sides of 8 with benzaldehydes 7a– 7f to form distyrylpyrazines
9–14. The yields of this condensation vary from 5% to 56%; the
highest yields (56%, 45%) have been obtained in the reactions
with highly alkylated aminobenzaldehydes 7e, 7f.[47] Attempts
to improve the yields in these condensation have been made:
combinations of the base KOtBu and the solvent DMF with
other bases (KOH, NaOtBu) or solvents (tetrahydrofuran, dioxane,
dimethyl sulfoxide) resulted in even lower yields or failed com-
pletely. The Siegrist reaction of Schiff bases of aminobenzalde-
hydes and dimethylpyrazine gave comparable yields of DSPs.
(Scheme 2; Table 1)
N
N
R4
R1
9 - 14
Scheme 2. Synthesis of DSPs (substitution pattern and yields see Table 1)
solvents. Furthermore, the extinction coefficient of 9 is signif-
icantly higher than of the related DSPs with higher steric
hindrance. Upon irradiation into the long-wavelength maximum,
all compounds are strongly fluorescent in cyclohexane solution.
The emission of 9 peaks at l = 489 nm, alkyl groups in the
2-position (10–13: lFmax = 464–469 nm) and, more pronounced,
in the 2,6-positions (14: lFmax = 454 nm) shift the fluorescence
maximum to the blue. Increasing solvent polarity shifts the
fluorescence of all compounds to the yellow region,
lFmax =557–580 nm. The solvatochromic shifts of the emission of
9 is Δn = 2998 cm–1, of the sterically more congested dyes
10–14 Δn ꢀ 4000 cm–1. Typical for quadrupolar dyes of the pheny-
lenevinylene type,[49,50] the fluorescence efficiency decreases with
the bathochromic shift: in ethanol, the fluorescence of 9 has an
intensity of 1/10 compared with the fluorescence from solution in
cyclohexane; for the sterically most hindered 14, this factor drops
to 1/68 (Fig 1).
A more detailed study of the solvatochromism was performed
with compound 13 as a representative example. The spec-
troscopic results for solutions on 13 in 15 solvents and 11
toluene/acetonitrile binary mixtures are collected in Table 3.
Because these di(aminostyryl)pyrazines are Brønstedt bases
with the basic centers participating in the p-system, a strong
impact of protonation on the electronic spectra was expected
(Table 4, Fig 2). In dichloromethane solution, a concentration of
Electronic spectra
The solid DSPs 9–14 are yellow to red crystalline compounds
with a good solubility in solvents like toluene or dichloro-
methane, giving yellow to orange colored fluorescent solutions.
(Table 2)
The UV–Vis absorption spectra of these DSPs are dominated
by a low energy band with a maximum around lmax = 400 nm
for the DSPs 10–14, only compound 9 without ortho-substitution
around the amino group absorbs with a red shifted maximum at
lmax = 442 nm in cyclohexane. Similar to other quadrupolar
distyrylpyrazines,[45] increasing solvent polarity (cyclohexane to
ethanol) has only minor effect on the absorption spectra of
10–14 (Δl ≤ 10 nm), only the absorption maximum of 9 is shifted
about Δl = 21 nm (Δn = 1026 cm–1) to the red. The maximal
solvatochromic shift is reached in dichloromethane and inverts
for all compounds to a negative solvatochromism in more polar
R3
R4
NH2
NH2
N
R2
R1
R2
R1
R2
R1
1. R3-X
2. Et-I
ii
Br2
i
3
1
Br
2
Br
1. BuLi
2. DMF
vi
iii Ac2O, Br2
R3
R4
O
NH
R1
NH
Br
N
N
R1
R2
R1
R1
LiAlH4
iv
Et-I
1. BuLi
2. DMF
v
vi
Br
CHO
Br
7
4
5
6
Scheme 1. Synthesis of 3,N,N-tri- and 3,5,N,N-tetraalkylaminobenzaldehydes
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J. Phys. Org. Chem. (2012)

STERICALLY CONGESTED DI(AMINOSTYRYL)PYRAZINES
Table 1. Substitution pattern of intermediates 2–7 and DSPs 9–14
R = ethyl
R1
isopropyl
Compounds, yields, colors and m. p.
2c: 44%
2e: 22%
2f: 37%
7a: 49%
7b: 60%
7c: 60%
7d: 58%
7e: 36%
7f: 35%
3c: 67%
3e: 46%
3f: 48%
9: 7%
4b: 63%
4d: 33%
5b: 50%
5d: 62%
6b: 97%
6d: 79%
R1
R1, R2
R3, R4
red
166 ꢁC
124 ꢁC
dec.
R1, R3, R4
R1, R4
R3, R4
R4
10: 15%
11: 5%
12: 18%
13: 56%
14: 45%
orange
yellow
orange
yellow
yellow
R3
R1
122 ꢁC
169 ꢁC
dec.
R1, R3
R1, R2
R3, R4
Table 2. Spectroscopic data of 9–14 in different solvents, Irel = Isolvent/Iethanol corrected to identical optical density at lexc
;
Δn(solv) = solvatochromic shift from cyclohexane to ethanol, value in brackets: maximal solvatochromic shift; Δn(Stokes): Stokes
shift in cyclohexane and Stokes shift in ethanol
DSP
Cyclo-hexane Toluene Dichloro-methane Aceto-nitrile Ethanol Δn(solv) Δn(Stokes) / cm–1
ENT [48]
0.006
0.099
0.309
0.460
0.654
9
lmax /nm (log e)
lmax /nm (log e)
lmax /nm (log e)
lmax /nm (log e)
lmax /nm (log e)
lmax /nm (log e)
442 (4.8)
402 (4.5)
405 (4.5)
398 (4.5)
402 (4.7)
395 (3.8)
489 (10)
468 (9)
469 (19)
464 (34)
464 (37)
454 (68)
456 (4.8)
409 (4.6)
411 (4.5)
404 (4.5)
407 (4.7)
398 (3.8)
513 (8)
500 (16)
497 (31)
496 (27)
491 (29)
475 (42)
465 (4.8)
412 (4.5)
414 (4.5)
408 (4.5)
412 (4.7)
400 (3.8)
541 (4)
459 (4.9)
407 (4.6)
410 (4.5)
405 (4.5)
408 (4.7)
395 (3.8)
544 (2)
571 (4)
563 (7)
570 (5)
568 (4)
463 (4.8)
409 (4.6)
412 (4.5)
403 (4.6)
407 (4.7)
397 (3.8)
573 (1)
575 (1)
580 (1)
569 (1)
568 (1)
1026 (1119)
425 (604)
419 (536)
312 (615)
306 (377)
10
11
12
13
14
9
10
11
12
13
14
127 (317)
lFmax/ nm (Irel
lFmax/ nm (Irel
lFmax/ nm (Irel
lFmax/ nm (Irel
lFmax/ nm (Irel
lFmax/ nm (Irel
)
)
)
)
)
)
2998 2175 / 4146
3976 3508 / 7059
4080 3369 / 7030
3977 3574 / 7239
3946 3323 / 6904
4073 3290 / 7236
533 (6)
530 (10)
529 (11)
527 (13)
520 (16)
559 (2)
557 (1)
cyclohexane
cyclohexane
toluene
dichloromethane
acetonitrile
ethanol
toluene
500
dichloromethane
acetonitrile
ethanol
1.5
1.0
0.5
400
300
200
100
0
0.0
300
350
400
450
500
550
450
500
550
600
650
700
wavelength/nm
wavelength/nm
Figure 1. Solvatochromism of 9
10–4 M trifluoroacetic acid (TFA) is sufficient to protonate 9–14,
the absorption maxima are hypsochromically shifted to lmax
379–386 nm. A second protonation step is visible in TFA of
0.1–1 M strength: new maximum in the range lmax
403–422 nm appears. Because TFA causes hypsochromic and
bathochromic shifts in the absorption spectra (379 nm ≤ lmax
≤ 422 nm), the fluorescence of 9–14 in dichloromethane/TFA
was excited in the UV (l = 350 nm). The excited dyes 9–14 are
protonated in TFA as low as 10–5 M resulting in hypsochromic
shifts to lFmax = 417–429 nm. With high TFA concentrations a
second protonation is visible as a bathochromic shift to
l = 490–515 nm. While the fluorescence spectra of 9–13 show a
single emission band over the whole concentration range,
irradiation of 14 resulted in the ‘normal’ emission (lFmax ꢀ 425
nm) and a second band at longer wavelengths (lF = 500 nm).
The fluorescence intensity of 9–14 decreases with the strength
=
a
=
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C. WINK AND H. DETERT
400
200
0
dichloromethanea
10-5 M TFA
10-4 M TFA
10-3 M TFA
10-2 M TFA
10-1 M TFA
dichloromethane
1.0
0.8
0.6
0.4
0.2
0.0
10-5 M TFA
10-4 M TFA
10-3 M TFA
10-2 M TFA
10-1 M TFA
1
M TFA
1
M TFA
300
350
400
450
500
400
450
500
550
600
650
wavelength/nm
wavelength/nm
Figure 2. Acidochromism of 12
Table 3. Solvatochromism of 13 in different solvents and binary solvent mixtures. DCM, dichloromethane; DMA, N,N-dimethyla-
cetamide; Tol/AN, binary mixture of toluene and acetonitrile (v/v); ENT* , averaged ETN values of the mixtures of toluene (ETN = 0.099)
and acetonitrile (ENT = 0.460) weighted with their volume fractions[30]
Solvent
ETN
lmax /nm lFmax /nm Δn /cm–1 Tol/ AN
ENT*
lmax /nm lFmax /nm Δn /cm–1
Cyclohexane
Toluene
0.006
0.099
0.207
0.228
0.309
0.333
0.355
0.377
0.386
0.389
0.444
0.460
0.546
0.654
0.762
402
407
408
408
412
412
411
410
412
408
416
408
409
407
409
464
496
521
519
527
525
533
534
558
528
575
568
538
568
567
3324
4409
5316
5241
5296
5224
5569
5629
6351
5570
6647
6904
5862
6964
6813
100/0
99/1
98/2
0.099
0.102
0.106
0.117
0.135
0.171
0.275
0.370
0.406
0.424
0.460
407
408
408
409
409
409
409
408
408
408
408
496
501
502
510
517
524
534
539
549
562
568
4409
4550
4589
4744
5108
5366
5723
5897
6295
6716
6904
Tetrahydrofuran
Ethyl acetate
Dichloromethane
Benzonitrile
Acetone
N,N-Dimethylacetamide
DMF
t-Butanol
Dimethyl sulfoxide
Acetonitrile
i-Propanol
Ethanol
Methanol
95/5
90/10
80/20
50/50
20/80
15/85
10/90
0/100
Table 4. Absorption and emission maxima of 9–14 in dichloromethane containing different concentrations of TFA
DSP
CH2Cl2
10–5 M TFA
10–4 M TFA
10–3 M TFA
10–2 M TFA
10–1 M TFA
1 M TFA
lmax /nm
lmax/nm
lmax/nm
lmax/nm
lmax/nm
lmax/nm
l
max/nm
9
465
412
416
408
412
400
541
533
530
529
527
520
460
405
411
405
408
400
426
425
427
425
429
417
525
379
381
381
383
383
386
425
425
426
425
425
426
380
397
380
379
379
377
424
425
425
424
425
426
382
382
380
381
383
381
509
477
487
486
489
423
493
415
414
413
406
380
403
504
494
499
494
498
393
500
422
422
418
417
419
411
515
------
------
503
504
414
490
10
11
12
13
14
9
10
11
12
13
14
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STERICALLY CONGESTED DI(AMINOSTYRYL)PYRAZINES
of the acid, in TFA ≥ 0.1 M fluorescence efficiency drops to values
less than 1/10 compared with the neutral solution or results in a
complete quenching (10, 11).
For a correlation of solvatochromic shifts and solvent para-
meters we choose Reichardts ENT scale.[48] This scale has found
major application in solvatochromism and has recently been
refined.[51,52] Furthermore, most of the solvent polarity scales
agree with each other qualitatively.[53] Although ETN as a single-
parameter approach is fairly inadequate in correlating a number
of solvent-sensitive processes, limitations are also found in the
multiparameter approaches,[54] for example of Kamlet et al.[55]
The solvatochromism of dipolar dyes has been intensively stud-
ied and good to excellent correlations of Δn and ENT have been
reported.[56,57] This is completely different for centrosymmetric
solutes. Solvatochromism of centrosymmetric molecules has
not yet become a standard tool and only a few cases have been
reported.[49,58]
Because 9–14 are centrosymmetric molecules, the solvato-
chromism of the absorption and the emission should be small.
Indeed, the absorption of DSPs 9–14 is only weakly influenced
by solvent polarity and shows a reversed solvatochromism.
Within the nonhydrogen bonding solvents cyclohexane, toluene,
dichloromethane, and acetonitrile, the largest red-shift occurs
in dichloromethane. A reversed solvatochromism has been
observed for several dipolar merocyanine dyes.[53] This was
explained as a change of the dipolar characteristics of the
chromophores with increasing solvent polarity. Some of these
merocyanines, although moderately dipolar in relatively non-
polar solvents, can be regarded as highly dipolar in strongly po-
lar solvents. Whereas the polarity increases from acetonitrile
(ENT = 0.460) to ethanol (ENT = 0.654), a bathochromic shift of the
absorption of 9–14 occurs. Because ethanol is a solvent with
the capability to act as a hydrogen bond donor and 9–14 are
dyes with formally four basic centers, the solvatochromic shifts
in ethanol should be separated from the shifts in nonhydrogen
bond donor (non-HBD) solvents.
The solvatochromic shift of the fluorescence of 9 amounts to
ΔnF = 2998 cm–1; however, the sterically more hindered dyes
10–14 show solvatochromic shifts of about ΔnF = 4000 cm–1.
Accordingly, the Stokes shifts of 10–14 increase from ΔnSt
ꢀ 3400 cm–1 in cyclohexane to ΔnSt ꢀ 7000 cm–1 in ethanol.
An internal rotation of the dialkylamino group around the
aryl-N bond in the excited state can lead to a charge transfer
from the amine to the pyrazine. This and the stabilization of
the CT state by interaction with solvent dipoles may account
for these huge shifts.[59] As has been shown for the highly substi-
tuted benzaldehyde 7f,[60] steric hindrance around the amino
group can result in large dihedral angles between the planes
of the dialkylamino group and an acceptor-substituted benzene
ring (7f: Y = 68ꢁ), thus breaking the conjugation of donor and
acceptor. This steric energy, inhibiting internal charge transfer in
the ground state, can be overcome by dipole–dipole interactions
of dipolar solvent molecules with a polar excited state of the dye.
Centrosymmetric solutes, in function of their zero dipole
moments, apparently are incapable of dipole–dipole interac-
tions; therefore, they are expected to be relatively free of
solvatochromic shifts. Nevertheless, several accounts of solvato-
chromism of centrosymmetric molecules can be found. It has
been suggested that these dislocations are the result of either
solvent dipole–solute quadrupole interactions,[61–64] or of locali-
zation of the excitation on only part of the molecule. The latter
can include breaking the symmetry[61,65] thus being the origin
for solvent impact on the electronic spectra. Johnson and
Ogilby reported solvatochromic studies on quadrupolar donor–
acceptor–donor substituted distyrylbenzenes and could not
DISCUSSION
The substitution of 2,5-distyrylpyrazines with amino groups results
in chromophores with a quadrupolar donor–acceptor–donor
(D–A–D) electronic structure. The fundamental chromophore,
unsubstituted distyrylpyrazine 15 absorbs with lmax = 382nm
and its emission peaks at lFmax = 424 nm in cyclohexane.[45]
Absorption and emission are only weakly positive solvatochro-
mic, with TFA ≥ 10–2 M protonation of the pyrazine starts and
shifts absorption and emission to the red side (1 M TFA: lmax
=
450 nm, lFmax = 535 nm). Substitution of 15 with donors of
moderate strength (methoxy, 9-carbazolyl) shifts the electronic
spectra about Δl ≤ 30 nm to the red side (in cyclohexane)
and provokes a pronounced solvatochromism of the emission
(Δn= 2030 cm–1; 2900 cm–1).[45]
In terms of electron donating capability, alkylamino groups are
generally much more efficient than alkoxy or diarylamino
groups. This is valid on condition that a sufficient overlap of
the nitrogen p-orbital with the p-system is possible. A N,
N-diisopropylamino substitution allows an efficient interaction
of the donor group with the electron deficient p-system: the
absorption of 9 peaks at lmax = 442 nm (cyclohexane). However,
a N,N,o-triethyl substitution is sufficient to disturb this interaction
by twisting the planes of the phenyl ring and the adjacent amino
group, thus reducing the donor effect of the amino groups in 10
and in the higher alkylated congeners 11–14 (Fig. 3). As judged
by lmax = 395–405 nm, the donor effects of the amino groups in
10–14 are comparable to a methoxy- or 9-carbazolyl substitu-
tion.[45] Contrary to 10–14, and to its molecular symmetry, com-
pound 9 shows a significant positive solvatochromism of the
absorption (Δn = 1026 cm–1 from cyclohexane to ethanol). This
can result from balanced interplay between steric congestion
and solvent-dipole supported charge transfer. The emission
maxima of 9 are separated from the absorption maxima by large
Stokes shifts, for example, ΔnSt = 2175 cm–1 in cyclohexane and
ΔnSt = 4126 cm–1 in ethanol, indicating serious electronic and
geometrical relaxation in the excited state.
465
460
580
455
450
445
440
435
430
425
420
415
410
405
400
395
560
540
520
500
480
460
440
420
9
10
11
12
13
14
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
solvent polarity
Figure 3. Solvatochromic displacements of the absorption and
emission maxima of 9–14. Solvent polarity: ENT ; full symbols: absorption;
open symbols: emission
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C. WINK AND H. DETERT
find a correlation between the position of the absorption band
maximum and any solvent polarity-related parameter.[50]
In the ground state, the conjugation of both donor groups in
9, and more pronounced in 10–14, is hampered by steric
hindrance. After excitation, the Franck–Condon state relaxes to
a solvent-equilibrated excited state, this can include geometric
relaxation around the donor groups resulting in an enhanced
charge transfer from the donor to the acceptor; the formation
of these CT states is favoured in more polar solvents. Strengthen-
ing of the D–A interaction can occur by rotation of both amino
groups of 9–14, either simultaneously or stepwise. Figure 4
shows the Stokes shifts of 13 depending on the ENT value of 11
nonhydrogen bond donating solvents and of 11 toluene/
acetonitrile binary mixtures. Both series clearly show three
regimes of solvent impact on electronic spectra: A significant
increase of Δn in solvents of low polarity, a weak bathochromism
in moderately polar solvents followed by pronounced impact in
highly polar solvents. Furthermore, the slopes of Δn(ENT ) in the
regimes of low and of high polarity are similar. This suggests
an improvement of the charge transfer in two successive steps.
Comparing the data for 13 in non-HBD solvents and in four
alcohols, it becomes obvious that in addition to the generalized
polarity ENT , hydrogen bonding has a specific impact on the
electronic spectra. The shifts obtained from solutions of 13 in
four alcohols do not fit into the Δn(ENT ) relation: a strong
increase in Δn was observed only at significantly higher ETN
values (ENT > 0.55).
Fluorescence quenching is another major effect of solvent
polarity on the fluorescence of 9–14. An increase in the extent
of CT character generally provides mechanisms for deactivation
of the excited state. These CT-mediated processes can be quite
pronounced in polar solvents because solvent-equilibrated
emitting states of quadrupolar D–A–D chromophores are quite
susceptible to the effects of the solvent and are stabilized in
more polar/polarizable solvents.[50] With a decreasing energy
gap between excited and ground state, internal conversion
becomes favorable.[66] The decay of the fluorescence of 9–14
in highly polar solvents implies that their emitting states have
a certain amount of charge transfer character.
all DSPs are protonated in 10–4 M TFA in dichloromethane. This
protonation results in a new absorption maximum on the blue
side of the spectrum, lmax ꢀ 382 nm, similar to the absorption
of unsubstituted DSP 15 in dichloromethane. The origin of this
hypsochromic shift is the change of the moderate p-electron
donating to a weak s-electron accepting capability of both
terminal nitrogen atoms. Unsubstituted 15 is protonated in
10–2 M TFA; the enhanced EPA character of the protonated
pyrazine shifts absorption and emission to significantly lower en-
ergies.[45] Protonation of the remaining basic site –pyrazine – in
9–14 requires TFA ≥ 10–1 M. As the electron affinity of the
pyrazine increases upon protonation, bathochromic shifts to
lmax = 411–422 nm result.
Excitation of chromophores like 9–14 results in species with
basicities that can be completely different from the ground state
molecules.[67] Typically, bases like aniline are about several
orders of magnitude more basic in their ground state than in
their excited states. This rule fails for EPA-substituted anilines[43]
and DSPs 9–14. TFA in concentrations as low as 10–5 M proto-
nates excited 9–14, as indicated by the hypsochromic shifts to
lFmax = 417–425 nm. Like in the ground state, the third proton is
transferred to the pyrazine; the new fluorescence bands appear
at lFmax ≥ 475 nm. Even 10–2 M TFA is sufficient for this step
(Fig. 5). This corresponds to the expectation that the basicity of
an azine increases upon excitation. Although 9–13 are stepwise
protonated, the behavior of excited 14 is different. The fluores-
cence spectra of 14 in 10–5 M TFA and in 10–2–1 M TFA are
explicable as superpositions of the spectra of neutral and proton-
ated 14 (10–5 M TFA) and twofold/threefold protonated 14. The
high sterical congestion around the amino groups in 14 may
prevent the amines from protonation. X-ray diffraction of 7f
showed that the two isopropyl groups force the diethylamino
group into a planar conformation with an sp2 hybridized nitro-
gen; this geometry can also be assumed for 14. A protonation-
induced rehybridization to sp3 would increase the steric
energy, thus allowing the coexistence between differently
protonated species 14. The observation of two protonated
species in absorption and in emission of 9–14 is in contrast to
studies on the analogous, but sterically not constrained DSP
2,5-di(p-diethylaminostyryl)pyrazine.[38] Harper reported that
extinction and fluorescence decreased with an increasing con-
centration of toluenesulfonic acid, only at high concentration,
a weak red shift of the emission was noted.
In addition to their geometrically restraint electron donating
ability, the amino groups in 9–14 can act as bases. This can be
the origin of the different solvatochromism of 9–14 in non-
HBD and in HBD solvents of similar ENT . In the ground state,
480
7000
6500
6000
5500
5000
540
520
500
460
440
480
420
460
4500
400
380
440
420
400
different solvents
toluene/acetonitrile
alcohols
4000
3500
-6
-5
-4
-3
-2
-1
0
3000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
log [TFA]
solvent polarity
Figure 5. Halochromism of the absorption and emission of 9. Squares:
absorption maximum; stars: emission maximum
Figure 4. Solvent impact on the Stokes shifts of 13, solvent polarity: ETN
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J. Phys. Org. Chem. (2012)

STERICALLY CONGESTED DI(AMINOSTYRYL)PYRAZINES
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CONCLUSION
Steric congestion around the amino groups of di(aminostyryl)
pyrazines modulates the internal charge transfer in these fluoro-
phores. Large solvatochromic shifts of the fluorescence and
Stokes shifts up to 7000 cm–1 result from enforced electronic
interaction conquering steric hindrance. A correlation of Δn(ENT )
for 13 shows three regimes of polarity-induced shifts. A strong
sensitivity in low-polar solvents is followed by weak solvatochro-
mism in the medium polarity range. In highly polar solvents, the
sensitivity increases strongly. The latter appears also in HBD
solvents – but at significantly higher ENT values. With increasing
solvent polarity, a stepwise planarization of the amino-aryl unit
may account for these changes. Protonation occurs first on the
terminal amino groups; their basicity increases upon excitation.
Only in strongly acidic media, the central pyrazine is protonated.
These protonations alter the charge distribution and result in
initial hypsochromic shifts followed by bathochromic shifts of
excitation and emission. These new distyrylpyrazines are very
promising fluorescent probes for the study of micropolarity of
different heterogeneous objects, including biological systems.
Because of strong solvatochromism and adjustable intramolecu-
lar charge transfer these fluorophores must have significant
nonlinear optical properties.
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SUPPORTING INFORMATION
Supporting Information
[43] H. Detert, V. Schmitt, S. Glang, Mater. Res. Soc. Symp. Proc. 2006,
937E, 2006. 0937-M07-54.
Acknowledgements
[44] H. Detert, V. Schmitt, J. Phys. Org. Chem. 2006, 19, 603.
[45] V. Schmitt, J. Fischer, H. Detert, ISRN Org. Chem. 2011, Article ID
589012. DOI: 10.5402/2011/589012
The authors gratefully acknowledge valuable advice from the
referees and generous financial support from the Deutsche
Forschungsgemeinschaft.
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[47] Experimental and analytical details are given in the supplementary
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