Absorption and fluorescence of soluble polar diketo-pyrrolo-pyrroles

Article abstract of DOI:10.1016/j.dyepig.2011.05.004

Six soluble derivatives of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole- 1,4-dione N-alkylated on pyrrolinone ring with polar substituents in para positions of pendant phenyl rings were synthesized; five of them are reported for the first time. Absorpti

Full text of DOI:10.1016/j.dyepig.2011.05.004

Dyes and Pigments 91 (2011) 269e278
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Absorption and fluorescence of soluble polar diketo-pyrrolo-pyrroles
a
b
,
a,c,
Stanislav Lunák Jr. , Martin Vala , Jan Vynuchal d, Imad Ouzzane b, Petra Horáková a,
*
ꢀ
ꢀ
b
b
ꢀ ꢀ
ꢀ
ꢀa
Petra Mozísková , Zdenek Eliás , Martin Weiter
a Faculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-530 09 Pardubice, Czech Republic
b
ꢀ
Faculty of Chemistry, Centre for Materials Research, Brno University of Technology, Purkynova 464/118, CZ-612 00 Brno, Czech Republic
c Research Institute of Organic Syntheses, Rybitví 296, CZ-533 54 Rybitví, Czech Republic
d Synthesia a.s., Pardubice, Semtín 103, CZ-532 17 Pardubice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Six soluble derivatives of 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione N-alkylated on
pyrrolinone ring with polar substituents in para positions of pendant phenyl rings were synthesized; five
of them are reported for the first time. Absorption and fluorescence spectra were studied in solvents of
different polarity. The compounds show small solvatochromism of absorption and a moderate positive
solvatochromism of fluorescence, especially when substituted by strong electron-donating piperidino
substituent. A significant decrease of fluorescence quantum yields and its biexponential decay for dipolar
derivatives in polar solvents was tentatively ascribed to the formation of twisted intramolecular charge
transfer (TICT) excited state. All six compounds show fluorescence in polycrystalline solid-state with the
maxima covering a range over 200 nm in visible and near infrared region.
Received 26 October 2010
Received in revised form
29 April 2011
Accepted 4 May 2011
Available online 19 May 2011
Keywords:
Diketo-pyrrolo-pyrrole
Fluorescence
Ó 2011 Elsevier Ltd. All rights reserved.
Solvatochromism
Solid-state fluorescence
Twisted intramolecular charge transfer
Organic electronics
1. Introduction
We have recently published the syntheses and spectral prop-
erties of parent DPP chromophore 3,6-diphenyl-2,5-dihydro-pyr-
Although originally developed as high performance organic
pigments [1,2,3], various structural modifications made diketo-
pyrrolo-pyrroles (DPPs) interesting as advanced materials for
modern optical and electronic technologies. The devices based on
DPPs copolymerized with e.g. carbazoles [4] and especially with
oligothiophenes [5,6] reached the promising efficiencies as organic
field-effect transistors (OFET) [5] and bulk heterojunction (BHJ)
photovoltaic solar cells (OPV) [4,6]. Aside from DPP copolymers,
which form rather specific area with dramatically increasing
number of references, DPP monomers have been recently found
also to be perspective in photovoltaics, either in the BHJ OPV [7], or
in dye-sensitized solar cells (DSSC) [8] type. The common structural
features of DPP derivatives designed for these purposes are:
The substituted (usually alkylated, in some cases acylated [9])
pyrrolinone nitrogens, changing the insoluble pigments to
molecules enabling wet solution based processing, and the
presence of electron-donating groups as a counterpart to diketo-
pyrrolo-pyrrole core with an electron-accepting character.
rolo[3,4-c]pyrrole-1,4-dione (I) and its electron-donor (piperidino)
and electron-acceptor (cyano) symmetrically and unsymmetrically
substituted derivatives (IIeVI, Fig. 1) [10]. The derivatives have
shown bathochromic and hyperchromic shift of absorption and
bathofluoric shift of fluorescence with respect to parent compound
I invoked above all by piperidino electron-donating substituent.
Dimethyl sulfoxide (DMSO) was found to be the only common
solvent able to dissolve all these pigments. In order to make these
compounds better treatable we have decided to substitute them
on pyrrolinone nitrogens and so eliminate the intermolecular
hydrogen bonding [1]. On the contrary to more usual N-alkylation
by alkylhalogen, used in our previous studies [11,12], we applied
ethyl bromoacetate in this case (Fig. 1). Such substitution was
reported only once resulting in compound VII with highly distorted
structure in crystal according to X-ray diffraction [13] giving thus
a good chance to be highly soluble because of the absence of
pep
stacking, another insolubility implicating phenomenon aside from
COeNH hydrogen bonding [14]. Compounds VIIIeXII were never
reported, thus they are fully characterized in Experimental. This is
the first case, to the best of our knowledge, when simple push-pull
substituted well soluble DPP derivative (XII) is studied.
* Corresponding author. Tel.: þ420 541 149 411; fax: þ420 541 149 398.
E-mail address: vala@fch.vutbr.cz (M. Vala).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.004

ꢀ
270
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278
R1
R1
O
O
CH3CH2OOCCH2
N
BrCH2COOCH2CH3
K2CO3,NMP
HN
O
N
NH
CH2COOCH2CH3
O
R2
R2
Symbol
VII
R1
H
R2
H
Symbol
R1
R2
H
I
H
II
VIII
IX
CN
CN
H
CN
CN
H
III
CN
CN
N
N
N
N
N
N
X
XI
H
IV
V
H
N
N
XII
CN
VI
CN
Fig. 1. General synthetic route and notation of the compounds under study.
DPP derivatives are known for a long time to be strongly fluo-
a similar way as described earlier and confirmed by melting point
210e212 ꢀC (lit. [13]. 207e208 ꢀC). N-methyl-2-pyrrolidone (NMP),
ethyl bromoacetate and potassium carbonate were purchased from
Sigma-Aldrich, so as the three solvents used in the spectral studies.
rescent in solution [15]. The only reported exceptions to the best
of our knowledge are the compounds, in which the pyrrolinone
carbonyl group underwent a nucleophilic substitution by hetero-
arylacetonitrile [16] or bis(trimethylsilyl)carbodiimide [17] forming
the products loosing the true DPP character. The aim of this study
was thus first to investigate in detail the dependence of absorption
and fluorescence spectra and fluorescence efficiencies in solution
on the character of pendant phenyl substitution in VIIIeXII
with respect to on-phenyl unsubstituted VII and N-unsubstituted
precursors IeVI. Contrary to hardly soluble pigments IeVI, it was
possible also to measure a solvatochromism on VIIeXII, which was
never studied in detail before.
2.1.1. Preparation of diethyl-3-(phenyl)-6-(4-cyanophenyl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione-diacetate (VIII)
Compound II (8 g, 0.0256 mol), potassium carbonate (35.4 g,
0.256 mol) and NMP (480 ml) were charged to a three-necked flask.
Reaction mixture was heated to 120 ꢀC. Ethyl bromoacetate (42.8 g,
0.256 mol) in NMP (185 ml) was added to the reactor dropwise
during 40 min. Then the reaction was stirred and heated to 120 ꢀC
for 2 h. After cooling, the reaction was slowly poured onto ice-cold
water (1400 ml). The obtained precipitate was filtered off and
washed with water until neutral washings. The crude product was
recrystallized from a mixture of methanol and water (2:1). Yield:
2.02 g (16.29%) of compound VIII. m.p. 139e141 ꢀC.
As we observed the luminescence of some of these N,N0-
dialkylated DPPs in solid-state just by naked eye during the
samples handling (opposite to totally non-luminescent precursors
IeVI), we studied also this not particularly common behaviour but
being of growing interest [18,19,20]. Since the pioneering work
of Langhals [21], the solid-state fluorescence of several DPP deriv-
atives was mentioned in literature [9,22,23], but the full under-
standing of this phenomenon requires more systematic studies on
the representative series of derivatives.
Calculated: C (66.90), H (4.78), N (8.66), Found: C (66.84), H
(4.73), N (8.46)
MW ¼ 485 Da; Positive-ion APCI-MS: m/z 486 [M þ H]þ, (100%)
1H chemical shifts: 8.12 (2H, m, ArH); 7.99 (2H, m, ArH); 7.84 (2H,
m, ArH); 7.66 (3H, m, ArH); 4.64 (2H, s, eNCH2,); 4.63 (2H, s, eNCH2,);
4.115 (2H, q, J ¼ 7.1 Hz, eOCH2); 4.110 (2H, q, J ¼ 7.1 Hz, eOCH2); 1.142
(3H, t, J ¼ 7.1 Hz, eCH2CH3); 1.140 (3H, t, J ¼ 7.1 Hz, eCH2CH3)
13C chemical shifts: 168.37 (1C, >C]O); 168.29 (1C, >C]O);
161.30 (1C, >C]O); 161.03 (1C, >C]O); 149.52 (1C, ArC); 145.37
(1C, ArC); 133.02 (2C, ArC); 132.09 (1C, ArC); 131.33 (1C, ArC);
129.26 (2C, ArC); 129.21 (2C, ArC); 128.68 (2C, ArC); 126.88 (1C,
ArC); 118.26 (1C, eC^N); 113.52 (1C, ArC); 109.90 (1C, ArC); 108.47
2. Experimental
2.1. Syntheses and analyses
The synthesis of the starting derivatives IeVI was described in
Ref. [10]. Compound VII was synthesized from compound I in

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S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278
271
(1C, ArC); 61.41 (1C, NeCH2); 61.38 (1C, NeCH2); 43.38 (1C, eOCH2);
43.26 (1C, eOCH2); 13.92 (2C, eCH2CH3)
was recrystallized from methanol. Yield: 3.57 g (54.3%) compound
XI, m.p. 213e217 ꢀC.
Calculated: C (68.99), H (6.75), N (8.94), Found: C (68.80), H
(6.85), N (8.78)
2.1.2. Preparation of diethyl-3,6-di-(4-cyanophenyl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione-diacetate (IX)
MW ¼ 626 Da; Positive-ion APCI-MS: m/z 627 [M þ H]þ (100%)
1H chemical shifts: 7.75 (4H, m, ArH); 7.01 (4H, m, ArH); 4.64
(4H, s, eNCH2); 4.16 (4H, q, J ¼ 7.2 Hz, eOCH2); 3.41 (8H, m,
eCH2CH2CH2N); 1.64 (12H, m, eCH2CH2CH2N and eCH2CH2CH2N);
1.18 (6H, t, J ¼ 7.2 Hz, eCH2CH3)
Compound III (5 g, 0.0148 mol), potassium carbonate (20.42 g,
0.148 mol) and NMP (272 ml) were charged into a three-necked
flask. Reaction mixture was heated to 120 ꢀC and ethyl bromoace-
tate (24.7 g, 0.148 mol) in NMP (116 ml) was added dropwise to the
reactor during 40 min. Then the reaction was stirred and heated to
120 ꢀC for 2 h. After cooling, the reaction was slowly poured onto
ice-cold water (760 ml). The obtained precipitate was filtered off
and washed with water until neutral washings. The crude product
was recrystallized from methanol. Yield: 0.81 g (10.7%) of orange
compound IX, m.p. 177e179 ꢀC.
13C chemical shifts: 130.27 (4C, ArC); 113.67 (4C, ArC); 61.19 (2C,
NeCH2); 47.83 (4C, eCH2CH2CH2N); 43.80 (2C, eOCH2); 24.97 (4C,
eCH2CH2CH2N); 14.04 (2C, eCH2CH3); Remaining signals were not
detected due to low solubility of the sample.
2.1.5. Preparation of diethyl-3-(4-cyanophenyl)-6-(4-
Calculated: C (68.49), H (6.12), N (7.73), Found: C (68.49), H
(6.04), N (7.63)
piperidinophenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-
1,4-dione-diacetate (XII)
1H chemical shifts: 8.14 (4H, m, ArH); 8.01 (4H, m, ArH); 4.64
(4H, s, eNCH2); 4.10 (4H, q, J ¼ 7.1 Hz, eOCH2); 1.13 (6H, t, J ¼ 7.1 Hz,
eCH2CH3)
Compound VI (3 g, 0.0076 mol), potassium carbonate (10.5 g,
0.076 mol) and NMP (140 ml) were charged into three-necked flask
(500 ml). Reaction mixture was heated to 120 ꢀC and ethyl bro-
moacetate (14 g, 0.082 mol) in NMP (60 ml) was added dropwise to
the reactor during 40 min. Then the reaction was stirred and heated
to 120 ꢀC for 2 h. After cooling, the reaction was slowly poured onto
ice-cold water (400 ml). The obtained precipitate was filtered off
and washed with water until neutral washings. The crude product
was recrystallized from the mixture of n-hexan and methanol (7:3).
Yield: 2.26 g (70.4%) of compound XII, m.p. 192e195 ꢀC.
Calculated: C (67.59), H (5.67), N (9.85), Found: C (67.40), H
(5.72), N (9.65)
13C chemical shifts: 168.23 (2C, >C]O); 160.97 (2C, >C]O);
146.92 (2C, ArC); 133.05 (4C, ArC); 131.06 (2C, ArC); 129.33 (4C,
ArC); 118.36 (2C, eC^N); 113.82 (2C, ArC); 109.82 (2C, ArC); 61.45
(2C, NeCH2); 43.40 (2C, eOCH2); 13.90 (2C, eCH2CH3)
2.1.3. Preparation of diethyl-3-(phenyl)-6-(4-piperidinophenyl)-
2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione-diacetate (X)
Dry and pure NMP (150 ml), compound IV (3.7 g, 0.01 mol) and
potassium carbonate (15.2 g, 0.11 mol) were charged to a three-
necked flask. Reaction mixture was heated to 120 ꢀC. Ethyl bro-
moacetate (18.4 g, 0.11 mol) in NMP (80 ml) was added dropwise to
the reactor during 40 min. Then the reaction mixture was stirred
and heated to 120 ꢀC for 2 h. After cooling, the reaction was slowly
poured onto ice-cold water (500 ml). The obtained precipitate
was filtered off and washed with water until neutral washings. The
crude product was recrystallized from methanol. Yield: 3.2 g (59%)
of compound X, m.p. 207e214 ꢀC.
MW ¼ 568 Da; Positive-ion APCI-MS: m/z 569 [M þ H]þ (100%)
1H chemical shifts: 8.08 (2H, m, ArH); 8.01 (2H, m, ArH); 7.87
(2H, m, ArH); 7.10 (2H, m, ArH); 4.69 (2H, s, eNCH2); 4.65 (2H, s,
eNCH2); 4.17 (2H, q, J ¼ 7.0 Hz, eOCH2); 4.11 (2H, q, J ¼ 7.1 Hz,
eOCH2); 3.50 (4H, s, eCH2CH2CH2N); 1.65 (6H, s, eCH2CH2CH2N
and eCH2CH2CH2N); 1.19 (3H, t, J ¼ 7.1 Hz, eCH2CH3); 1.15 (3H, t,
J ¼ 7.0 Hz, eCH2CH3)
13C chemical shifts: 168.62 (1C, >C]O); 168.57 (1C, >C]O);
162.07 (1C, >C]O); 160.80 (1C, >C]O); 152.95 (1C, ArC); 150.68
(1C, ArC); 141.33 (1C, ArC); 132.92 (2C, ArC); 131.85 (1C, ArC);
131.07 (2C, ArC); 129.04 (2C, ArC); 118.44 (1C, eC^N); 114.04 (1C,
ArC); 113.36 (2C, ArC); 112.73 (1C, ArC); 110.35 (1C, ArC); 105.81
(1C, ArC); 61.39 (1C, NeCH2); 61.32 (1C, NeCH2); 47.63 (2C,
eCH2CH2CH2N); 44.05 (1C, eOCH2); 43.37 (1C, eOCH2); 25.05 (2C,
eCH2CH2CH2N); 24.10 (1C, eCH2CH2CH2N); 14.04 (1C, eCH2CH3);
13.97 (1C, eCH2CH3)
Calculated: C (65.88), H (4.34), N (10.97), Found: C (65.47), H
(4.44), N (10.82)
MW ¼ 510 Da; Positive-ion APCI-MS: m/z 511 [M þ H]þ (100%)
1H chemical shifts: 7.83 (2H, m, ArH); 7.78 (2H, m, ArH); 7.61
(3H, m, ArH); 7.1 (2H, m, ArH); 4.66 (2H, s, eNCH2); 4.61 (2H, s,
eNCH2); 4.17 (2H, q, J ¼ 7.1 Hz, eOCH2); 4.12 (2H, q, J ¼ 7.1 Hz,
eOCH2); 3.48 (4H, m, eCH2CH2CH2N); 1.64 (6H, m, eCH2CH2CH2N
and eCH2CH2CH2N); 1.19 (6H, t, J ¼ 7.1 Hz, eCH2CH3); 1.15 (6H, t,
J ¼ 7.1 Hz, eCH2CH3)
13C chemical shifts: 168.65 (1C, >C]O); 168.61 (1C, >C]O);
161.99 (1C, >C]O); 161.06 (1C, >C]O); 152.74 (1C, ArC); 149.20 (1C,
ArC); 144.33 (1C, ArC); 131.05 (1C, ArC); 130.70 (2C, ArC); 129.03 (2C,
ArC); 128.45 (2C, ArC); 127.59 (1C, ArC); 114.51 (1C, ArC); 113.47 (2C,
ArC); 108.75 (1C, ArC); 105.91 (1C, ArC); 61.29 (1C, NeCH2); 61.21
(1C, NeCH2); 47.69 (2C, eCH2CH2CH2N); 43.88 (1C, eOCH2); 43.34
(1C, eOCH2); 25.00 (2C, eCH2CH2CH2N); 24.07 (1C, eCH2CH2CH2N);
14.02 (1C, eCH2CH3); 13.97 (1C, eCH2CH3)
2.2. Structure and purity characterization
2.2.1. Mass spectrometry
The ion trap mass spectrometer MSD TRAP XCT Plus system
(Agilent Technologies, USA) equipped with APCI probe was used.
Positive-ion and negative-ion APCI mass spectra were measured in
the mass range of 50e1000 Da in all the experiments. The ion trap
analyzer was tuned to obtain the optimum response in the range of
the expected m/z values (the target mass was set from m/z 289
to m/z 454). The other APCI ion source parameters: drying gas
flow rate 7 dm3 minꢁ1, nebulizer gas pressure 60 psi, drying gas
temperature 350 ꢀC, nebulizer gas temperature 350 ꢀC. The samples
were dissolved in a mixture of DMSO/acetonitrile and methanol in
various ratios. All the samples were analyzed by means of direct
infusion into LC/MS.
2.1.4. Preparation of diethyl-3,6-di-(4-piperidinophenyl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione-diacetate (XI)
Compound V (5 g, 0.011 mol), potassium carbonate (15.2 g,
0.11 mol) and NMP (205 ml) were charged into three-necked flask.
Reaction mixture was heated to 120 ꢀC and ethyl bromoacetate
(18.38 g, 0.11 mol) in NMP (80 ml) was added dropwise to the
reactor during 40 min. Then the reaction was stirred and heated to
120 ꢀC for 2 h. After cooling, the reaction was slowly poured onto
ice-cold water (600 ml). The obtained precipitate was filtered off
and washed with water until neutral washings. The crude product
2.2.2. Elemental analysis
Perkin-Elmer PE 2400 SERIES II CHNS/O and EA 1108 FISONS
instruments were used for elemental analyses.

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S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278
272
2.2.3. Nuclear magnetic resonance
3. Results and discussion
Bruker AVANCE 500 NMR spectrometer operating at
500.13 MHz for 1H was used for measurements of the 1H NMR
spectra. The compounds were dissolved in hexadeuteriodimethyl
sulfoxide used as deutered standard. The 1H chemical shifts were
3.1. Syntheses
Although looking quite simple (Fig. 1), N,N-dialkylation of DPPs
is a two-step process complicated by extremely low solubility
of starting pigments, which sometimes leads to the presence of
mono-alkylated intermediate in reaction mixture, even if an excess
of both alkylating agent and HBr neutralizing potassium carbonate
is used [11]. In-depth study of this reaction was recently carried out
[29]. The reports on the syntheses targeted directly on N-mono-
alkylated DPPs are relatively rare, but there was recently shown,
that these compounds can be highly sensitive and selective
fluorescent sensors for fluoride anions [30]. All six DPP derivatives
VIIeXII in the presented study were prepared with moderate to
high yields and the special purification procedures like chroma-
tography or fractional crystallization were not necessary in order
to obtain the product of the desired quality. We ascribe this fact to
higher reactivity of bromine on ethyl bromoacetate as compared
to e.g. n-butyl bromide. The compounds show good solubility over
a wide range of solvents, so we have carried out the spectral
measurements in highly polar DMSO, in order to have direct
comparison with our previous results obtained for N-non-alkylated
pigment precursors IeVI [10] or N-butylated analogues of VII and
XI [11,12], in acetonitrile, as a representative of less polar solvents,
and in almost non-polar but easily polarizable toluene.
referred to the central signal of the solvent (
d
¼ 2.55).
2.3. Optical characterization
2.3.1. UVeVIS absorption and fluorescence spectroscopy
The referred UVeVIS absorption spectra in solution were
recorded using Varian Carry 50 UVeVIS spectrometer. The fluo-
rescence spectra in solution were measured in 90ꢀ configuration at
Aminco Bowmann S2 fluorimeter. The solid-state luminescence
spectra were recorded on Perkin-Elmer LS 55 equipped with an
accessory for solid-state measurements from the same producer.
Polycrystalline samples were placed under quartz plate and the
emission spectra were recorded using front face geometry.
2.3.2. Fluorescence quantum yields
The fluorescence quantum yields (4F) in solution were calcu-
lated according to the comparative method [24], where for each
test sample gradient of integrated fluorescence intensity versus
absorbance F ¼ f(A) is used to calculate the 4F using two known
standards. The standards were previously cross-calibrated to verify
the method. This calibration revealed accuracy about 5%. As the
reference we used Rhodamine B and Rhodamine 6G with 4Fl 0.49
[25] and 0.950 ꢂ 0.005 [26], respectively. The excitation wave-
length was chosen to be the same as for the laser excited experi-
ments i.e. 532 nm. Since the VII has very low absorption coefficient
at this exciting wavelength, we used Fluorescein (0.91 ꢂ 0.02) [20]
and Coumarin 6 (0.78) [27] because of the better spectral overlap.
The excitation wavelength in this case was 460 nm.
3.2. DFT computed structure
As expected, DFT optimized geometries of VIIeXII predict
non-zero dihedral angles, describing phenyl-pyrrolinone rotation
(Table 1), on the contrary to strictly planar precursors IeVI [10]. The
computed values of dihedral angles are the result of a compromise
between sterical effect of methylene and ortho phenyl hydrogens
(more or less the same for all six derivatives), invoking nonplanarity,
and conjugation effect (dependent primarily on para phenyl
substituent of each derivative), maximal for planar arrangement.
The effect of the substituent on opposite pendant phenyl is marginal
(less than 1ꢀ). Average values are thus 26ꢀ, 36ꢀ and 38ꢀ for piper-
idino, cyano and unsubstituted phenyls, respectively.
2.3.3. Fluorescence lifetimes
The fluorescence lifetime sF was measured using Andor Sham-
rock SR-303i spectrograph and Andor iStar ICCD camera. The
samples were excited by third harmonic of EKSPLA PG400
Nd:YAG laser (355 nm) with light pulse time duration of w30 ps.
The temporal resolution of the system is approximately 25 ps. In
order to avoid chromatic aberrations, the emitted light from the
sample was collected by two off-axis mirrors.
As these dihedral angles are crucial for the interpretation of
absorption spectra, they should be verified by comparison with
the experiment. The only known X-ray diffraction structure of
compound VII [13] is a bit problematic from this point of view.
2.4. Computational procedures
The molecule is highly unsymmetrical in crystal with
a
¼ 36.5ꢀ, i.e.
quite close to the computed value 38.2ꢀ, and
b
¼ 68.8ꢀ, i.e. totally
All theoretical calculations for compounds VIIeXII were carried
out on exactly same level as for previously reported precursors IeVI
[10], in order to be directly comparable. The ground state (S0)
geometry was optimized using quantum chemical calculations
based on DFT. Hybrid three-parameter B3LYP functional in combi-
nation with 6-311G(d,p) basis was used. No constraints were
preliminary employed, but, as the nonconstrainted computations
converged to symmetrical structures for compounds VII, IX and
XI, the final computations were carried out with Ci symmetry
constraint. No imaginary frequencies were found by vibrational
analysis, confirming that the computed geometries were real
minima on the ground state hypersurfaces.
TD DFT computations of the vertical excitation energies were
carried out on the computed S0 geometries. The same exchange-
correlation functional (B3LYP) was used in TD DFT calculations with
rather broader basis set (6-311þG(2d,p)). Solvent effect of DMSO
was involved by non-equilibrium PCM.
out of a reasonable agreement. These results show a dramatic role
of packing forces in DPP crystal. As discussed earlier [31], there are
always some perturbation of planarity even for theoretically planar
DPP pigments, bearing evidence of relatively flat minima of ground
state geometry with respect to phenyl-pyrrolinone rotation. One
can expect, that the equilibrium between sterical and conjugation
Table 1
DFT computed phenylepyrrolinone dihedral angles and PCM (DMSO) TD DFT
computed excitation energies converted to wavelengths.
Compound
a
[ꢀ]
b
[ꢀ]
l00 [nm]
fosc
VII
VIII
IX
X
XI
38.2
34.7
36.3
38.0
26.6
35.2
38.2
38.2
36.3
26.1
26.6
25.5
465
495
509
518
546
561
0.512
0.563
0.615
0.835
1.138
0.923
XII
All methods were taken from Gaussian09W program suite [28],
and the default values of computational parameters were used. The
results were analyzed using GaussViewW from Gaussian Inc, too.
a
[ꢀ] ¼ R1-phenyl-pyrrolinone dihedral angle;
b
[ꢀ] ¼ R2-phenyl-pyrrolinone dihe-
dral angle; l00 [nm] ¼ theoretical excitation energy; fosc ¼ oscillator strength.

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S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278
273
Table 2
effects may be even more fragile for N-alkylated derivatives and
thus the effect of packing forces may be more dramatic. That is
probably the case of the X-ray structure of VII. Unfortunately, such
distorted molecular structure can give only limited evidence on the
accuracy of the computed structures. We have tested the relevance
of DFT method to predict the dihedral angles of N,N-disubstituted
compound I on another two known X-ray structures, in which
Ci molecular symmetry in crystal is retained. The results were quite
encouraging. The computed value for N,N-dimethyl I (30.3ꢀ)
matches well the experimental one (30.4ꢀ [32]), and an agreement
for N,N-diallyl I (theor. 31.2ꢀ, exp. 35.8ꢀ [13]) is also acceptable.
There are no X-ray data for N,N-di-n-butyl I available, but a lot of
them exist, for its derivatives (see bellow), so we computed also the
dihedral angle for this compound. Its value (26.4ꢀ) is considerably
lower, than for compound VII (38.2ꢀ).
The spectroscopic properties of VIIeXII in a) DMSO, b) acetonitrile and c) toluene.
Compound lA (nm)
3
(
lA
)
lF(nm)
4F
DlStokes
(cmꢁ1
sF (ns)
(l molꢁ1 cmꢁ1
)
)
a) DMSO
VII
VIII
IX
460
471
480
516
15,800
16,000
13,300
32,200
519
547
557
601
0.56 2470
0.51 2950
0.72 2880
0.12 2740
7.34 ꢂ 0.08
7.07 ꢂ 0.03
6.50 ꢂ 0.04
1.17 ꢂ 0.02
6.7 ꢂ 1.2
X
XI
XII
540
541
45,100
42,300
603
654
0.45 1940
<0.01 3190
3.45 ꢂ 0.02
0.217 ꢂ 0.002
3.72 ꢂ 0.10
b) Acetonitrile
VII
VIII
IX
454
19,200
16,900
19,400
35,400
512
538
549
587
0.31 2490
0.75 2960
0.32 2930
0.01 2960
6.70 ꢂ 0.04
5.78 ꢂ 0.03
6.67 ꢂ 0.04
1.12 ꢂ 0.01
2.70 ꢂ 0.20
3.70 ꢂ 0.03
<0.10
464
473
500
We searched the Cambridge structural database (CSD) using
ConQuest procedure [33] in order to find the structures similar to the
derivatives VIIeXII. There were found four files with X-ray struc-
tures of DPP derivatives with both pyrrolinone nitrogen substituted
by n-butyl and at least one pendant phenyl either unsubstituted,
or substituted with amino or cyano groups in para position: KAKMAL
X
XI
XII
523
524
50,500
34,800
590
638
0.20 2170
<0.01 3410
3.57 ꢂ 0.02
c) Toluene
(R1 ¼ CN, R2 ¼ H,
a
¼ 45.2ꢀ,
b
¼ 32.6ꢀ) [34], XATKIN (R1 ¼ R2 ¼ CN,
VII
VIII
IX
X
XI
462
472
482
499
520
527
18,500
22,570
17,700
35,000
44,400
36,500
520
544
558
563
575
603
0.36 2410
0.43 2800
0.29 2830
0.34 2280
0.38 1840
0.16 2390
5.93 ꢂ 0.04
4.24 ꢂ 0.02
6.48 ꢂ 0.04
4.00 ꢂ 0.02
3.20 ꢂ 0.02
4.41 ꢂ 0.05
a
b
b
¼
b
¼ 43.1ꢀ), NAWREJ (R1 ¼ H, R2 ¼ diphenylamino,
a
a
¼ 32.8ꢀ,
¼ 41.0ꢀ,
¼ 30.5ꢀ) and XATKEJ (R1 ¼ H, R2 ¼ 4-MeO-phenyl,
¼ 42.3ꢀ) [35]. Another relevant data come from recently published
N,N-dibenzylated DPPs. Such group recall probably similar steric
XII
hindrance with respect to phenyl, as dihedral angles of chlorinated
derivative (R1 ¼ R2 ¼ Cl,
a
¼ 41.6ꢀ,
b
¼ 43.9ꢀ) [23] are very similar to
lA e the position of the absorption maximum;
3 e molar absorption coefficient at
the lA lF e the position of the fluorescence maximum; 4F e the fluorescence
;
N,N-dibutylated dibromo derivative found in CSD in XATJAE file
quantum yield; DlStokes e the Stokes shift (lFelA); sF e the observed fluorescence
lifetime.
(R1 ¼ R2 ¼ Br,
a
¼
b
¼ 45.0ꢀ) [35]. So the results for N,N-dibenzylated
dimorfolino derivative (R1 ¼ R2 ¼ morfolino,
a
¼
b
¼ 28.1ꢀ) [23] are
close to those obtained for diphenylamino substituent (file NAWREJ)
in accordance with an interpretation based of a mixing of two
resonance structures for dimorfolino (and generally diamino) DPP
derivatives [23].
central DPP core composed of two coupled merocyanines
(H-chromophore). We consider that the second and third contri-
butions are almost constant over the whole series and the differ-
ences in hypsochromic shifts caused by N-alkylation go almost
exclusively on account of the first one, i.e. the substituent depen-
dent changes in the ground state planarity.
The highest hypsochromic shift with respect to non-alkylated
precursor was found for di-cyano substituted IX (59 nm), mono-
cyano substituted VIII (53 nm) and rather lower for unsubstituted
VII (45 nm). On the other hand the lowest hypsochromic shift was
observed for di- and mono-piperidino substituted XI (20 nm) and X
(24 nm), while the shift of unsymmetrical compound XII (35 nm)
with both types of substituents lies between the symmetrically
Finally, DFT predicted decrease of the phenyl-pyrrolinone
dihedral angle accompanying p-piperidino substitution is in
accordance with the relevant experimental data, at least qualita-
tively. On the other hand, small lowering of this dihedral angle
connected with p-cyano substituent is inconsistent not only with
the data coming from both above mentioned p-cyano substituted
derivatives, but even with a general trend represented by other
electron-accepting substituents, i.e. halogens.
In order to test the sensitivity of dihedral angles of CN
substituted derivatives on a quantum chemical method, we carried
out the calculations on HartreeeFock (HF) level with the same
6-311G(d,p) basis set. We obtained the dihedral angles considerably
5x104
higher (
a
a
¼ 52.4ꢀ,
b
¼ 52.1ꢀ for VIII,
a
¼
b
¼ 52.4ꢀ for IX and,
¼ 52.5ꢀ,
b
¼ 47.9ꢀ for XII) compared to those ones coming from
VII
DFT calculations (Table 1). But these computations also do not
considerably distinguish 4eCNephenyl-pyrrolinone and phenyl-
pyrrolinone dihedral angles.
4x104
VIII
IX
X
3x104
XI
XII
3.3. Absorption and fluorescence spectra in DMSO
2x104
The absorption maxima of VIIeXII in DMSO show hypsochromic
and hypochromic shifts (Table 2a, Fig. 2) with respect to corre-
sponding precursors IeVI [10]. Hypsochromic shift is in fact a net
effect of three contributions: 1) An increase of excitation energy
due the less efficient conjugation because of the loss of molecular
planartity, 2) the redistribution of the intensities of vibronic
sub-bands from 0e0 maximum of IeVI in favour of 0e1 in VIIeXII,
as shown by a successive N-alkylation of I and V [11,12] and 3) the
opposite effect e the decrease of excitation energy due to the
increase of electron-donating strength of a pyrrolinone nitrogens in
1x104
0
400
500
600
Wavelength (nm)
Fig. 2. Molar absorption coefficients of the studied DPP derivatives in DMSO.

ꢀ
274
S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278
a
b
VII
1.0
VII
VIII
IX
1.0
0.8
0.6
0.4
0.2
0.0
VIII
IX
X
XI
XII
0.8
0.6
0.4
0.2
0.0
X
XI
XII
500
600
700
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 3. Normalized absorption (a) and fluorescence (b) spectra of VIIeXII in DMSO.
substituted IX and XI. Although soluteesolvent interaction may
play some role, we relate this behaviour mainly to the dependence
of the dihedral angle describing the phenyl-pyrrolinone rotation on
a nature of para substituent of this pendant phenyl. The conclusion
is thus clear: piperidino substitution dramatically decreases
the phenyl-pyrrolinone dihedral angles in accordance with DFT
predictions, while cyano substitution increases these angles
considerably, contrary to theory.
PCM TD DFT computed data relate to the experimental maxima
only qualitatively. More precisely, the series can be divided into two
groups of derivatives. The first one (VII, X and XI) show the devia-
tion between computed l00 (Table 1) and experimental lA (Table 2a)
2e6 nm, while the difference for cyano substituted derivatives
is significantly bigger (20e22 nm for mono-cyano substituted
VIII and XII and even 29 nm for di-cyano IX). As there were no such
discrepancies between theory and experiment for planar derivatives
IeVI [10], we ascribe this inconsistency also to underestimated
dihedral angles in DFT calculated geometries.
Finally, the suspicion revealed in part 3.2. filled up and the
absorption spectra clearly show, that the dihedral angle between
p-cyano-phenyl and pyrrolinone rings should be relatively higher
than for unsubsubstituted phenyl-pyrrolinone case, contrary
for the DFT prediction. PCM (DMSO) TD DFT 6-311 þ G(2d,p)
calculations carried out on the above mentioned more distorted HF
geometry resulted in a considerable blue shift (l00 ¼ 426.nm for
VIII, 435 nm for IX and 477 nm for XII), compared the values
obtained on DFT geometry (Table 1), i.e. the values of l00 are in this
case much lower compared to the experimental lA (Table 2a)
showing an evidence on nonrealistic distortion coming from HF
geometries.
pairs I/VII and III/IX). N,N-dialkylated electron-donor substituted
derivatives
(þ14 nm) and XI (þ15 nm) show moderate
X
bathofluoric shifts as compared to IV and V, respectively. Push-pull
derivative XII shows almost identical maximum as VI (the differ-
ence is þ1 nm), i.e. its value lies between III/IX and V/XI pairs as in
absorption. As a consequence the Stokes shift of compound XII is
absolutely the highest one (3190 cmꢁ1), i.e. significantly higher
than for non-alkylated compound VI (2060 cmꢁ1). An increment
of a Stokes shift increase connected with N,N-disubstitution is
relatively similar (1090e1270 cmꢁ1) for all three pairs of piperidino
substituted compounds and rather higher (1930e2070 cmꢁ1) for
unsubstituted or only cyano substituted pairs.
The main reason for the general increase of the Stokes shift
due to N-alkylation is caused by the fact, that it is considered as
a difference between absorption and fluorescence maxima. The
maxima correspond to 0e0 vibronic transition in fluorescence
spectra for all twelve compounds, to 0e0 vibronic transition in
absorption for IeVI [10] and to 0e1 transition for VIIeXII (Fig. 3). An
increase of Stokes shift caused by a redistribution of vibronic bands
intensities in absorption may not be strictly constant, but probably
quite similar for all six pairs. The rest of the changes in Stokes shift
goes on account of the differences in internal (geometrical) and
external (solvent) relaxation, when going from vertical Franck-
Condon (FC) state to relaxed excited state. Internal relaxation is
probably mainly connected with the changes of above discussed
dihedral angles. In order to have a better view on external contri-
bution, it was necessary to carry out the spectral measurements in
other solvents with different (lower) polarity.
3.4. Solvatochromism
The hypsochromic shift of VII with respect to N,N-dibutylated
I (7 nm) [12] and opposite bathochromic shift of XI with respect
It was impossible to measure the absorption and fluorescence
solvatochromism of IeVI because of their insolubility in other than
highly polar solvents able to form H-complexes with solute. The
spectral data for N-alkylated derivatives in toluene and acetonitrile
are summarized in Table 2b, c and the spectra are shown on Figs. 4
and 5. The shape of the absorption spectra is the same in all three
solvents, i.e. the vibronic structure is completely unresolved even in
toluene, while the vibronic structure of fluorescence spectra is
best resolved for all compounds in toluene, in which clearly 0e0
vibronic transition is the absolute maximum, and its resolution
decreases in acetonitrile and is almost lost in DMSO.
to N,N-dibutylated
V
(ꢁ4 nm) [12] do not support, that
eCH2COOCH2CH3 grouping is sterically significantly more efficient
than eCH2CH2CH2CH3 as it would relate to the computed differ-
ence of phenyl rotation in VII (38.2ꢀ) and N,N-dibutylated I (26.4ꢀ).
The absorption spectra do not corfirm that unsymmetrical highly
distorted X-ray structure of VII [13] is retained in solution.
The relation between fluorescence maxima of corresponding
members of N-alkylated and non-alkylated sets is different from
absorption and much less clear on the first view (Table 2a, Fig. 3).
The maxima of unsubstituted I with respect to VII are almost the
same (þ2 nm on behalf of VII), while IX shows hypsofluoric
shift with respect to III (ꢁ8 nm). The fluorescence maximum of
compound VIII shows also a hypsofluoric shift with respect to II
(ꢁ3 nm, i.e. exactly between þ2 nm and ꢁ8 nm for symmetrical
Compound VII shows small negative solvatochromism, when
going from toluene to acetonitrile (ꢁ8 nm) and almost the same
positive shift from acetonitrile to DMSO (þ6 nm). The shifts of its
fluorescence maxima are almost identical, thus Stokes shift is the

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S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278
275
a
b
VII
VIII
IX
1.0
0.8
0.6
0.4
0.2
0.0
1.0
VII
VIII
IX
X
0.8
0.6
0.4
0.2
0.0
X
XI
XII
XI
XII
300
400
500
600
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 4. Normalized absorption (a) and fluorescence (b) spectra in acetonitrile.
same in all three solvents. The solvent induced shifts of IX are
nearly the same within 1 nm. The spectral shifts when going from
toluene to acetonitrile markedly evoke the situation found for
BODIPY dyes [36] and the interpretation is the same. While the
absorption maximum of their hybrid VIII lies exactly between VII
and IX in all solvents, the fluorescence maximum is generally closer
to IX and a small growth of Stokes shift with solvent polarity from
toluene (2800 cmꢁ1) to acetonitrile (2960 cmꢁ1) is observed. Thus
VIII is only a bit more polar in relaxed than in FC excited state.
The introduction of piperidino group brings two general trends
with respect to excited state relaxation in compounds XeXII: The
contribution of internal relaxation is decreased, which well relates
with lower rotation of p-piperidino-phenyl in FC state, while the
external contribution is increased, as the electron-donating
substitution changes the intramolecular charge distribution. The
former statement can be documented by lower Stokes shift of XI
(1840 cmꢁ1) with respect to VII (2410 cmꢁ1) in non-polar toluene.
The latter sentence is generally proved by a dependence of Stokes
shift on a solvent polarity, i.e. its increase is less than 160 cmꢁ1 for
VIIeIX and notably higher for XI (330 cmꢁ1), X (680 cmꢁ1) and
especially XII (1020 cmꢁ1) when going from toluene to acetonitrile.
Thus, the largest observed Stokes shift of XII (3190 cmꢁ1 in DMSO)
is in fact mainly done by donor-acceptor substitution (2060 cmꢁ1 in
DMSO for VI [10]) and the additional effect of N-alkylation goes
mainly on account of a redistribution of vibronic intensities in
absorption spectrum of XII.
The solvent induced shifts in absorption of XeXII are less than
4 nm when going from toluene to acetonitrile, reflecting very
similar polarity of the ground and excited FC states. Positive
solvatochromism of fluorescence is moderate: 15 nm for symmet-
rical XI, and 24 and 35 nm for asymmetrical IX and XII, respectively,
that can be ascribed to relaxed excited state solvent stabilization.
The positive solvatochromism in absorption of XeXII when going
from acetonitrile to DMSO is almost constant (16e17 nm), very
similar to the corresponding shift in fluorescence (13e16 nm) and
a bit higher than for VIIeIX (6e7 nm in absorption and 7e9 nm in
fluorescence). It means, that further increase of solvent polarity
does not lead to additional excited state relaxation and the
energy of the excited FC and relaxed states is lower only because
the excited state charge distribution interacts more favourably than
the ground state itself with the reaction field of more polar solvent
(induced by ground state distribution) [37].
The highest solvent induced rise of Stokes shift, i.e. for compound
XII between toluene and acetonitrile (þ1020 cmꢁ1), is significantly
lower than for 4-(dimethylamino)-40-cyano-1,4-diphenylbutadiene
(DCB, þ4640 cmꢁ1 between n-hexane and acetonitrile [38]), in
which only central 1,4-diphenyl-butadiene backbone of XII is push-
pull substituted. This difference goes exclusively on account of much
lower value of positive fluorescence solvatochromism of compound
XII (þ35 nm) with respect to DCB (þ129 nm), while the sol-
vatochromism of absorption is almost the same (ꢁ3 nm for XII
and þ2 nm for DCB), i.e. negligible between above mentioned
a
b
1.0
VII
1.0
0.8
0.6
0.4
0.2
0.0
VII
VIII
IX
VIII
IX
0.8
X
X
XI
XI
XII
XII
0.6
0.4
0.2
0.0
300
400
500
600
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 5. Normalized absorption (a) and fluorescence (b) spectra in toluene.

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S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278
276
3.5. Photophysical behaviour
Fluorescence quantum yields (4F) and lifetimes (
VII
VIII
IX
1.0
0.8
0.6
0.4
0.2
0.0
s
F) for VIIeXII
in all three solvents are summarized in Table 2. Corresponding
photophysical data for non-alkylated precursors I (4F ¼ 0.74,
sF ¼ 6.21 ns) and V (4F ¼ 0.58, sF ¼ 3.66 ns) in DMSO were reported
previously [12] and we now supply them by 4F and sF for further
two piperidino substituted precursors IV (4F ¼ 0.48, sF ¼ 3.49 ns)
and VI (4F ¼ 0.14, sF ¼ 3.90 ns) also in DMSO and not published in
Ref. [10]. Both these compounds show monoexponential fluores-
cence decay.
X
XI
XII
All six compounds VIIeXII also show a relatively high 4F in
toluene and the fluorescence decay is strictly monoexponential
with lifetimes similar to corresponding non-alkylated precursors
in DMSO. However, there is a dramatic change for compounds
X and XII when going to polar solvents. The quantum yields
of fluorescence are significantly decreased, especially for XII
(Table 2b, c), and the decay is biexponential. It implies, that some
specific process connected with the excited state intramolecular
charge transfer (ICT) must be present. Such behaviour may be
connected with a conformational change in excited state known
as twisted intramolecular charge transfer (TICT). 4-piperidino-
benzonitrile is known to undergo such process even more will-
ingly than 4-dimethylamino-benzonitrile, a prototype molecule
with respect to TICT [41]. Nevertheless, it is generally very diffi-
cult to prove this idea, if not supported by two emission bands in
steady-state fluorescence. According to our opinion, the observ-
able fluorescence of XII in polar solvents comes from the minor
portion of excited molecules, for which the charge separating
twist did not pass. Nevertheless this explanation is only specu-
lative and the final solution of this problem would require further
sophisticated photophysical experiments that are out of the scope
of this article.
500
600
700
800
900
Wavelength (nm)
Fig. 6. Solid-state fluorescence spectra of DPP derivatives.
non-polar and polar solvents. Thus, although compounds X and XII
behave qualitatively like push-pull like chromophores, quantita-
tively their fluorescence solvatochromism is quite limited.
Compound XI is formally a quadrupolar molecule with D-p-A-p-
D character. Such compounds may or may not undergo an excited
state symmetry-breaking in highly polar solvents [39]. The shifts of
absorption (3 nm) and fluorescence (15 nm) maxima of XI, when
going from toluene to acetonitrile, is relatively small (although not as
small as for e.g. squaraines [39]), indicating very small (if any) excited
state perturbation. Compound XI is thus an intermediate quad-
rupolar chromophor with expected high two-photon absorption
cross-section. Their relatively high values estimated for N-octylated
p-diphenylamino substituted DPPs confirm this idea [40].
Fig. 7. Polycrystalline samples of IeXII under daylight (top) and fluorescence of the samples of VIIeXII under UV irradiation (365 nm) with the same settings of Panasonic DMC-FZ7
camera (bottom).

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S. Lunák Jr. et al. / Dyes and Pigments 91 (2011) 269e278
277
3.6. Solid-state fluorescence
biexponential decay for dipolar derivatives in polar solvents was
tentatively ascribed to the formation of non-fluorescent TICT
excited state. All compounds show fluorescence in polycrystalline
solid-state with the maxima covering a range over 200 nm in visible
and near infrared region, where the solid-state fluorescence is quite
rare [18].
All six studied DPP derivatives show pronounced solid-state
fluorescence, on the contrary to any of their non-alkylated
precursors IeVI. The spectra are shown on Fig. 6. The fluores-
cence of symmetrical VII, IX and XI is strong, easily observable by
naked eye under UV irradiation (Fig. 7). The fluorescence of
unsymmetrical VIII and X is less intense, but observable. Solid-state
fluorescence of push-pull derivative XII is almost not observable
(and totally undetectable by a camera e Fig. 7) partly because of its
significantly lower intensity and also as it falls almost fully into the
infrared region. The values of emission maxima in polycrystalline
phase are 568 nm (VII), 639 nm (VIII), 648 nm (IX), 708 nm (X),
670 nm (XI) and 784 nm (XII), and hence, their sequence corre-
sponds to that one in polar solvent (Table 2a) except the changeover
of X and XI pair. The corresponding changeover of dipolar VIII
and formally quadrupolar IX did nor occur, but the fluorescence
maximum of hybrid VIII in solid state is significantly closer to
parent IX than to VII. The spectral maxima are shifted bath-
ochromically with respect to even the most polar solvent (DMSO).
The smallest shift was found for compound VII (49 nm) without
any polar substituent, the moderate one for centrosymmetric
compounds XI (67 nm) and IX (91 nm), and the highest one for
polar VIII (92 nm) and X (107 nm) and, especially, for push-pull
substituted XII (130 nm).
Although the intermolecular interactions in highly organized
crystal phase cannot be in principle described by simple sol-
uteesolvent terminology, the red shifts probably predicate about the
polarity inside the crystal environment, i.e. 1) it is effectively more
polar than DMSO in all cases and 2) the highest effect is of course
in crystals composed from the molecules with non-zero dipole
moment. The changeover of solid-state fluorescence maxima of X
and XI is thus a logical continuation of the trend observed in solvents
with increasing polarity (Table 2aec). Although the crystal of XII has
evidently more polar environment than in DMSO solution, the
fluorescence of XII does not definitely diminishes in it. We consider
this phenomenon as further support of TICT role in the deactivation
cascade of XII in DMSO, while the excited state twisting is disabled
in rigid crystal environment. Almost identical behaviour and inter-
pretation was reported for push-pull substituted 1,6-diphenyl-1,3,5-
hexatriene [42].
Acknowledgement
The research was supported by the Academy of Sciences of the
Czech Republic via project KAN401770651, by the Ministry of
Industry and Trade of the Czech Republic via 2A-1TP1/041 project,
by the Czech Science Foundation via P205/10/2280 project and by
the project “Centre for Materials Research at FCH BUT” No. CZ.1.05/
2.1.00/01.0012 from ERDF.
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Generally, the solid-state fluorescence of organic pigments is
considered as a property of individual molecule conserved in crystal
phase [43]. In other words, the quenching process connected
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