The geometry and absorption of diketo-pyrrolo-pyrroles substituted with various aryls

Article abstract of DOI:10.1016/j.dyepig.2009.09.014

A series of five symmetrical and four unsymmetrical diaryl substituted diketo-pyrrolo-pyrroles was synthesized; two unsymmetrical derivatives are reported for the first time. The relationship between the theoretical excitation energies of the S₀?→?S₁ transition, computed by time dependent density functional theory and the experimental positions of 0-0 vibronic bands in the visible absorption (or fluorescence excitation) spectra was studied. Experimental data were obtained from either solution or from low temperature organic solvent glass, in which the progressions of the vibrational structure enabled correct assignment of vibronic sub-bands in some cases. Theoretical calculations predicted that a linear bathochromic and hyperchromic shift would accompany substitution of each phenyl in the parent 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione by providing a more extensively conjugated aryl centre (2-naphthyl, biphenyl, stilbenyl) for the ensuing planar derivatives. Qualitatively, the experimental bathochromic shifts of the 0-0 vibronic sub-bands were in exact agreement with theory, whilst hyperchromic shifts were affected by the very low solubility of the planar, symmetrical derivatives. Deviations from this ideal behaviour were observed for non-planar derivatives (aryl?=?stilbenyl or 1-naphthyl), for which the dihedral angles describing the aryl, out-of-plane torsions were probably underestimated by DFT.

Full text of DOI:10.1016/j.dyepig.2009.09.014

Dyes and Pigments 85 (2010) 27–36
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The geometry and absorption of diketo-pyrrolo-pyrroles
substituted with various aryls
a
a
b
,
c
,
a
*
ˇ ´
´ ˇ
ˇ
´
´
Stanislav Lunak Jr. , Lukas Havel , Jan Vynuchal
, Petra Horakova ,
d
d
a
ˇ´
ˇ
´
Jirı Kucerık , Martin Weiter , Radim Hrdina
a
´
Department of Organic Technology, Faculty of Chemical Technology, University of Pardubice, Studentska 95, CZ-532 10 Pardubice, Czech Republic
b Research Institute of Organic Syntheses, Rybitv´ı 296, CZ-533 54 Rybitv´ı, Czech Republic
c Synthesia a.s., Pardubice, Semt´ın 103, CZ-532 17 Pardubice, Czech Republic
d
ˇ
Faculty of Chemistry, Brno University of Technology, Purkynova 118, CZ-612 00 Brno, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of five symmetrical and four unsymmetrical diaryl substituted diketo-pyrrolo-pyrroles was
synthesized; two unsymmetrical derivatives are reported for the first time. The relationship between the
theoretical excitation energies of the S0 / S1 transition, computed by time dependent density functional
theory and the experimental positions of 0–0 vibronic bands in the visible absorption (or fluorescence
excitation) spectra was studied. Experimental data were obtained from either solution or from low
temperature organic solvent glass, in which the progressions of the vibrational structure enabled correct
assignment of vibronic sub-bands in some cases. Theoretical calculations predicted that a linear bath-
ochromic and hyperchromic shift would accompany substitution of each phenyl in the parent 3,6-diphenyl-
2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione by providing a more extensively conjugated aryl centre
(2-naphthyl, biphenyl, stilbenyl) for the ensuing planar derivatives. Qualitatively, the experimental
bathochromic shifts of the 0–0 vibronic sub-bands were in exact agreement with theory, whilst hyper-
chromic shifts were affected by the very low solubility of the planar, symmetrical derivatives. Deviations
from this ideal behaviour were observed for non-planar derivatives (aryl ¼ stilbenyl or 1-naphthyl), for
which the dihedral angles describing the aryl, out-of-plane torsions were probably underestimated by DFT.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 30 June 2009
Received in revised form
25 September 2009
Accepted 28 September 2009
Available online 1 October 2009
Keywords:
Diketo-pyrrolo-pyrroles (DPP)
Time dependent density functional theory
(TD DFT)
Absorption
Fluorescence
1. Introduction
five its symmetrical derivatives substituted on phenyl form
a commercially very successful family of high performance organic
The present authors have recently shown that calculations based
on time dependent density functional theory (TD DFT) could predict
the absorption maxima of diketo-pyrrolo-pyrrole (DPP) pigments
which had been substituted with various combinations of strong
electron-donating and electron-withdrawing substituents in the
para positions of both pendant phenyl rings [1]. A necessary
component of such computation was the introduction of solvent
effects by a polarized continuum model (PCM). The aim of this paper
is to challenge this methodology by interpreting the relationship
between structure and absorption of a series of symmetrical and
unsymmetrical DPPs that comprise different combinations of aryl
substituents (Table 1).
pigments [3]. One of them, BiPPBi (C.I. Pigment Red 264) was also
used in this study. These two pigments are the only ones from the
model set, for which the structure was estimated experimentally
using X-ray diffraction [4,5]. The other compounds under study were
not commercialised at the time of manuscript preparation.
Extensive literature exists on DPPs in general and, more specifically,
in relation to commercial examples; in this context, >279 references
exist for BPPB and 138 for BiPPBi were found as at 08/06/2009. In
contrast, there is a paucity of citations relating to other symmetrical
derivatives, with just 6 references to 1NPP1N, 7 for 2NPP2N and 2 for
StPPSt being found in patent literature. Four patents discuss the
asymmetrical compound BiPPB and 1 patent describes 1NPPB; as no
references relating to the 2-naphthyl compound 2NPPB and the stil-
bene analogue StPPB were found, the syntheses of these two particular
compounds was described in detail. Styryl (phenyl–ethenyl) PePPB
and PePPPe derivatives were handled only theoretically, as a synthetic
route to theses compounds is not known [6].
3,6-Diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (BPPB
in our notation) is a basic chromophore of this set; it has been
patented by CIBA [2]. BPPB itself (C.I. Pigment Red 255) and at least
* Corresponding author at: Research Institute of Organic Syntheses, Rybitvı´ 296,
CZ-533 54 Rybitvı´, Czech Republic. Tel.: þ420 466 823 351; fax: þ420 466 822971.
With exception of the authors’ previous work [1], no ab initio
calculations of DPP pigments have been published and references
ˇ
E-mail address: jan.vynuchal@vuos.com (J. Vynuchal).
0143-7208/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.09.014

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S. Lunak Jr. et al. / Dyes and Pigments 85 (2010) 27–36
28
Table 1
charged into a 0.5 l Keller flask equipped with a stirrer, reflux
Notation of the compounds studied.
condenser, thermometer and a nitrogen inlet. The reaction mixture
was heated out at 100 ꢀC for 10 h. After a cooling to room temper-
ature the product was extracted with ether (2 ꢁ 300 ml). The ether
layers were washed with water, dried with magnesium sulphate and
evaporated. The yellow product was dissolved by stirring in meth-
anol (which removed triethanolamine), filtered and dried. Yield:
19.47 g (95%). The melting point was 116–118 ꢀC (lit. [12] 117–119 ꢀC).
Ar
Ar
O
HN
O
NH
HN
O
Ar
NH
symmetrical
O
Ar
unsymmetrical
2.1.2. Syntheses of ethyl 4,5-dihydro-5-oxo-2-(2-naphhtyl)-
(1H)pyrrole-3-carboxylate (2-naphthyl-pyrrolinone ester)
BPPB
–
2.1.2.1. Preparation of ethyl 2-naphthoate. 2-Naphthoic acid
(Aldrich, 60 g, 0.348 mol), ethanol (226 ml) and sulphuric acid
(7 ml) were added to the two-necked flask equipped with ther-
mometer, stirrer and refluxing condenser. The reaction mixture was
heated to reflux for 10 h and after that the mixture was cooled to
room temperature; excess ethanol was distilled off. The ensuing
product was extracted with diethyl ether; the organic layer was
separated, washed with sodium carbonate (5%) and water and then
dried over Na2SO4. The solvent was removed under reduced pres-
sure. The amount of the oily product was 52 g (yield 75%).
BiPPBi
BiPPB
2NPP2N
1NPP1N
2NPPB
2.1.2.2. Preparation of ethyl 3-(2-naphthyl)-3-oxopropanoate. A
mixture of sodium hydride (caution: reacts violently with water,
liberating hydrogen; incompatible with water, acids, alcohols, strong
oxidizing agents; Aldrich 60%, 16.8 g, 0.42 mol), ethyl 2-naphthoate
obtained by Section 2.1.2.1 (42 g, 0.21 mol) and 1-butoxybutane
(360 ml) was added to the three neck flask equipped with ther-
mometer, stirrer, dropping funnel and refluxing condenser. The
reaction mixture was heated to 100 ꢀC and ethyl acetate (27.6 g,
0.313 mol) in 1-butoxybutane (20 g) was added to the mixture by
means of dropping funnel within about 1.5 h. The suspension was
heated to reflux for 6 h, then cooled to room temperature and
diluted with diethyl ether (100 ml). The obtained salt was filtrated
off, washed with diethyl ether (300 ml). Subsequently, the suspen-
sion was poured onto 300 g ice and acidified by hydrochloric acid to
pH ¼ 2. The product was extracted with diethyl ether, organic layer
was separated, washed with sodium carbonate (5%) and water. Then
dried over Na2SO4. The solvent was removed under reduced pres-
sure. The amount of oily product was 32.4 g (64% of theory).
1NPPB
StPPSt
StPPB
PePPB
PePPPe
to only semiempirical PPP [7] and INDO/S studies on BPPB and its
aggregates [8] were found. The latter computations were based on
X-ray geometry [7] which was later revised [4] and which under-
estimated the visible absorption maximum compared to its
experimental counterpart [9].
2. Experimental
2.1.2.3. Preparation of ethyl 4,5-dihydro-5-oxo-2-(2-naphthyl)
(1H)pyrrole-3-carboxylate. Ethyl 3-(2-naphthyl)-3-oxopropanoate
(32.4 g, 0.134 mol) obtained by Section 2.1.2.2, ethyl bromoacetate
(24.6 g, 0.147 mol), sodium carbonate (19.3 g), acetone (32 ml) and
ethyleneglycol dimethylether (32 ml) were added to the three-
necked flask equipped with thermometer, stirrer and refluxing
condenser. The reaction mixture was refluxed for 14 h. Acetone was
continuously added into the reaction mixture to keep the constant
volume due to evaporating off. Subsequently, the mixture was
cooled down at room temperature and inorganic salt was filtrated
and washed by acetone. The filtrate and acetone from the washing
process was combined and acetone was distilled off with other
volatile fractions up to 150 ꢀC under nitrogen. Acetic acid (101 ml)
and ammonium acetate (58 g) were added to the dark liquid
distilling residue. The mixture was kept under reflux at 120 ꢀC for
4 h. 16.1 g (yield 43%) of a product was obtained by filtration from
the mixture after the cooling. A sample for analysis was recrystal-
lized from toluene. The melting point was 188–193 ꢀC.
2.1. Syntheses and analysis
BPPB was synthesized from benzonitrile and diisopropyl
succinate [10]. Other symmetrical derivatives (BiPPBi, 1NPP1N,
2NPP2N and StPPSt) were synthesized in an analogous manner
from corresponding nitriles purchased from Aldrich, except for
4-cyano-stilbene, which was synthesized as described bellow. The
purity of all compounds was determined using elemental analysis,
MS and 1H NMR.
The unsymmetrical derivatives (BiPPB, 1NPPB and StPPB) were
synthesized by the base-catalysed condensation of ethyl 4,5-
dihydro-5-oxo-2-phenyl(1H)pyrrole-3-carboxylate (phenyl-pyrro-
linone ester) as described previously [11] with the above
mentioned nitriles. 2NPPB was synthesized from benzonitrile and
ethyl 4,5-dihydro-5-oxo-2-(2-naphthyl)-(1H)pyrrole-3-carboxylate
(2-naphthyl-pyrrolinone ester), which synthesis is described
bellow. The syntheses of two till unknown unsymmetrical deriva-
tives (2NPPB and StPPB) is presented in detail.
Calculated: C (72.58), H (5.37), N (4.98). Found: C (72.50),
H (5.35), N (5.11).
2.1.1. Syntheses of 4-cyano-stilbene
Triethanolamine (200 ml), 4-bromobenzonitrile(18.2 g, 0.1 mol),
styrene (10.4 g, 0.1 mol) and palladium acetate (0.23 g,1 mmol) were
MW ¼ 281 Da; positive-ion APCI-MS: m/z 282 [M þ H]þ (100%)
1H chemical shifts: 10.83 (1H, s, NH); 8.21–7.61 (7H, m, ArH);
4.05 (2H, q, CH2); 3.49 (2H, s, CH2); 1.10 (3H, t, CH3).

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S. Lunak Jr. et al. / Dyes and Pigments 85 (2010) 27–36
29
2.1.3. Syntheses of 3-(2-naphthyl)-6-phenyl-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2NPPB)
77 K) fluorescence emission and excitation spectra were measured
on Perkin Elmer LS 35 fluorescence spectrometer equipped with
commercial low temperature accessory.
tert-Amyl alcohol (33 ml) and sodium metal (caution: reacts
violently with water, liberating hydrogen; flammable solid;
incompatible with water, strong oxidizing agents; air sensitive;
0.7 g, 30 mmol) were charged into a 100 ml three-necked flask
equipped with a stirrer, reflux condenser and thermometer. The
sodium metal was dissolved under reflux in the presence of cata-
lytic amount of FeCl3 (which approximately took 1 h), whereupon
benzonitrile (1.4 g, 13.6 mmol) was added. 2-Naphthyl-pyrrolinone
ester (2.5 g, 8.9 mmol), obtained according to Section 2.1.2.3 was
continuously introduced in small amounts over 0.5 h. The ensuing
mixture was stirred under reflux for 1 h and the hot suspension was
then filtered, the filter cake suspended in 100 ml tert-amyl alcohol
and then 10 ml acetic acid was added to protolyse the salt. Pro-
tolysis was carried out at 100 ꢀC for 2 h. The resulting hot
suspension was filtered, and the filter cake was washed with hot
water to neutral washings. The filter cake was suspended in 300 ml
methanol. The suspension was heated to boiling and refluxed for
2 h. The hot suspension was filtered, washed with ethanol and hot
water Yield: 1.5 g (54%).
Thermogravimetric studies (TGA) were performed using TA
Instruments TGA Q5000 (New Castle, Delaware, USA) device in
100
m
l open platinum pans. The samꢂp1les, typically 5 mg, were
heated by thermal ramp of 10 ꢀC min from 40 ꢀC to 650 ꢀC in
dynamic nitrogen atmosphere (25 ml minꢂ1). Calorimetric analyses
(DSC) were carried out employing TA Instruments DSC Q200
calorimeter equipped with external cooler RCS90 allowing exper-
imental temperature range from ꢂ90 to 500 ꢀC. Experiments were
conducted in open TA TzeroÔ aluminum pans. Thermal history of
all samples was set up to be the same using of heatingꢂ1ramp of
3
ꢀC minꢂ1 from 40 ꢀC to ꢂ90 ꢀC and then 10 ꢀC/min to the
temperature determined by TGA as Ts (Table 2). All DSC experi-
ments were made under 50 ml minꢂ1 nitrogen purge. Before
analyses device was calibrated for temperature and enthalpy using
indium, tin and zinc standards (Perkin Elmer, Waltham, Massa-
chussets, USA). All the records were assessed by TA Universal
Analysis 2000 software version 4.4A.
The equipment for other analytical measurements and the
analytical procedures are the same as described in Ref. [1].
Calculated: C(78.09), H(4.17), N(8.28). Found C(77.19), H(4.20),
N(8.06)
2.3. Computational procedures
MW ¼ 338 Da; Negative-ion APCI-MS: m/z 337 [M ꢂ H]ꢂ (100%)
1H chemical shifts: 11.45 (2H, br s, NH); 9.03 (1H, m); 8.7 (1H,
m); 8.55(2H, m); 8.14(1H, m); 8.04 (1H, m); 8.00 (1H, m); 7.56–
7.72 (5H, m).
The geometry of all eleven compounds was optimized using
quantum chemical calculations based on DFT. Hybrid three-
parameter B3LYP functional [13] in combination with
6-311G(d,p) basis was used. No constraints were preliminary
employed, but, if the non-constrainted computations converged
to symmetrical structures, the final computations were carried
out with these symmetry constraints. No imaginary frequencies
were found after diagonalization of Hessian matrix, confirming
that the computed geometries were real minima on the ground
state hypersurfaces.
The TD DFT method was used for a computation of vertical
excitation energies on the computed geometry. The same
exchange-correlation functional (B3LYP) was used with rather
broader basis set (6-311 þ G(2d,p)) particularly efficient for TD
modeling of organic dyes [14]. Solvent effect of dimethylsulfoxide
(DMSO) was involved by non-equilibrium PCM [15].
13C chemical shifts were not determined due to a very low
solubility of the sample.
2.1.4. Syntheses of 3-(4-stilbenyl)-6-phenyl-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (StPPB)
tert-Amyl alcohol (67 ml), sodium metal (1.43 g, 97 mmol) and
a catalytic amount of FeCl3 were charged into a 250 ml three-
necked flask equipped with a stirrer, reflux condenser, thermom-
eter and nitrogen inlet. The sodium metal was dissolved under the
reflux, whereupon 4-cyano-stilbene (6.4 g, 31 mmol) obtained
according to Section 2.1.1 was added. After that phenyl-pyrrolinone
ester (7.27 g, 31 mmol) was added within 0.5 h. Finally, this mixture
was stirred and refluxed for 2 h. The reaction mixture was cooled to
a 60 ꢀC and infused into 200 ml distilled water. The mixture was
carried out at 80 ꢀC for 2 h. The resulting hot suspension was
filtered and filter cake was washed with 35 ml of hot isopropyl
alcohol and than with hot water to neutral washings. The filter cake
was suspended into methanol. The suspension was heated to
boiling and refluxed 0.5 h. The hot suspension was filtered, washed
with methanol and dried. Yield: 6.66 g (54%).
All the methods were taken from Gaussian03W program suite
[16], and the default values of computational parameters were
used. The results were analyzed using GaussViewW from Gaussian
Inc., too.
Table 2
Experimental absorption and fluorescence excitation and emission maxima and
sublimation temperatures Ts determined by TGA.
Calculated: C(79,98), H(4,65), N(7,17). Found C(79,61), H(4,62),
N(7,07)
Comp.
Absorption
Fluorescence
MTHF (77 K)
lex lem
Ts [ꢀC]
MW ¼ 390 Da; Positive-ion APCI-MS: m/z 391 [M þ H]þ (100%)
1H chemical shifts: 11.37 (2H, br s, NH); (7.54 (1H, d, J ¼ 16.7 Hz);
7.38 (1H, d, J ¼ 16.7 Hz); 8,53 (4H, m); 7.86 (2H, d), 7.63 (3H, m))
signals of Ar–CH ¼ CH–Ar; 7.70 (2H, ortho, ArH); 7.47 (2H, meta,
ArH), 7.38 (1H, para, ArH)
DMSO (20 ꢀC)
labs
3max
lem
BPPB
505
529
517
471
493
535
520
565
535
34200
30400
37200
7900
21500
17300
40100
33600
44300
517
547
534
567
558
549
535
578
554
512
–
523
498
503
–
526
–
548
515
–
527
527
511
–
531
–
550
383
463
372
278
347
477
415
?410
?428
2NPP2N
2NPPB
1NPP1N
1NPPB
BiPPBi
BiPPB
13C chemical shifts were not determined due to a very low
solubility of the sample.
2.2. Instrumental equipment
StPPSt
StPPB
The room temperature (DMSO) absorption measurements were
carried out on a Perkin–Elmer Lambda 9 absorption spectrometer.
The room temperature (DMSO, MTHF) and low temperature (MTHF,
labs, absorption maximum [nm]; lex, excitation maximum [nm]; lem, emission
maximum [nm]; 3max, molar absorptivity [l molꢂ1 cmꢂ1], values in italics come from
insufficiently soluble compounds.

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S. Lunak Jr. et al. / Dyes and Pigments 85 (2010) 27–36
30
3. Results and discussion
except 2NPPB, which was synthesized from ethyl 4,5-dihydro-5-
oxo-2-(2-naphthyl)-(1H)pyrrole-3-carboxylate (2-naphthyl-pyrro-
linone ester) and benzonitrile (Scheme 2). The reaction yields were
usually over 50%. The purity of the compounds was checked by
elemental analysis, 1H NMR and mass spectroscopy. For details of
the syntheses and analytics see Experimental.
Synthesized compounds were characterized by sublimation
temperature Ts (Table 2) determined by thermogravimetry (TGA).
In fact, the temperature Ts stands for the temperature at which
3.1. Syntheses and sublimation
The symmetrical derivatives were synthesized from diisopropyl
succinate and corresponding aromatic nitrile. The synthesis of
unsymmetrical derivatives was carried out from ethyl 4,5-dihydro-
5-oxo-2-phenyl(1H)pyrrole-3-carboxylate
ester) and corresponding aromatic nitrile (Scheme 1 for BiPPB),
(phenyl-pyrrolinone
Scheme 1.

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S. Lunak Jr. et al. / Dyes and Pigments 85 (2010) 27–36
31
Scheme 2.
Table 3
mass loss starts; kinetics of sublimation is very slow and thus
progressive increase in temperature causes later the degradation of
the sample. The char which could be seen at the end of the TGA
experiment typically reached from 10 to 40% of original sample
mass. Experiments carried out using differential scanning calo-
rimetry (DSC) revealed no phase transitions within ꢂ90 ꢀC and Ts.
TGA of StPPB and especially StPPSt show the slowest kinetics of
a sublimation process, for which Ts was determined only tentatively
(Table 2).
DFT computed relative ground state energy and PCM TD DFT computed excitation
energies (converted to lmax [nm]) and oscillator strengths (fosc) in DMSO.
Compound
Symmetry
Dihedral
anglesa
Relative
internal
energyb
S0 / S1
transition
characteristics
lmax
fosc
BPPB
C2h
0–0
–
494
0.641
2NPP2N
C2h
Cs
C2h
0–0
0–180
180–180
0.21
0.15
0.00
523
534
542
1.156
0.952
0.840
The synthesized compounds are pigments highly insoluble in
common solvents, except 1NPPB and especially 1NPP1N. The
unsymmetrical pigments (BiPPB, 2NPPB and StPPB) are slightly
better soluble than the corresponding symmetrical ones (BiPPBi,
2NPP2N and StPPSt).
2NPPB
Cs
Cs
0–0
0–180
0.09
0.00
509
519
0.886
0.718
1NPP1N
Ci
134–134
136–136
28–135
29–136
29–29
12.85
12.12
11.46
10.95
10.29
9.64
493
497
516
513
537
532
0.584
0.599
0.666
0.679
0.755
0.767
C2
C1
C1
Ci
3.2. DFT ground state geometry and energy
The restricted B3LYP calculations resulted in a strictly planar
geometry for BPPB and thus C2h symmetry [1]. The dihedral angle
between phenyl and pyrrolinone rings in unsymmetrical biphenyl
substituted compound BiPPB was also near to zero, while between
both phenyls it was about 38ꢀ. The same angle between phenyls in
biphenyl moiety was found for both rotamers (C2, Ci) of BiPPBi,
which differ by a mutual orientation of rotated terminal phenyl
rings. The ground state energy of Ci rotamer is negligibly higher
(Table 3).
C2
29–29
1NPPB
BiPPBi
C1
C1
7–136
7–29
6.19
4.87
495
515
0.611
0.703
Ci
C2
0–0
0–0
0.01
0.00
531
531
1.168
1.170
BiPPB
StPPSt
C1
0–0
–
513
0.894
C2h
Cs
C2h
0–0
0–180
180–180
0.15
0.12
0.00
600
601
601
2.012
1.959
1.923
Two (three) minima of energy were found for 2NPPB (2NPP2N),
all of them corresponding to planar geometry (Fig. 1). The most
stable rotamers are 2NPP2N (180–180) and 2NPPB (0–180), but the
energy differences are marginal (Table 3) as the steric interaction in
all two (three) rotamers is almost the same.
StPPB
Cs
Cs
0–0
0–180
0.06
0.00
550
550
1.355
1.296
The situation for1-naphthyl substituted DPPs isdifferent. Theyare
non-planar, as both 1-naphthyl and phenyl are always rotated out
from a plane of central heterocycle. Such rotation of phenyl ring of
1NPPB, induced by the rotation of 1-naphthyl bonded to the opposite
ring of a central heterocycle, is analogical to the case of N-mono-
alkylated BPPB [10]. If planar, there should exist two (three) minima
for 1NPPB (1NPP1N) by analogy with 2-naphthyl conformers,
differing in this case by a steric interaction of a naphthalene ring
PePPPe
C2h
Cs
C2h
0–0
0–180
180–180
1.89
0.89
0.00
574
588
598
1.402
1.128
0.974
PePPB
Cs
Cs
0–0
0–180
0.90
0.00
534
547
1.016
0.789
a
The first value is the dihedral angle between phenyl and pyrrolinone, the second
is between non-phenyl aryl and pyrrolinone for unsymmetrical derivatives.
b
Relative to the bolded energy of most stable isomer [kcal. molꢂ1].

ˇ´
32
S. Lunak Jr. et al. / Dyes and Pigments 85 (2010) 27–36
Fig. 1. Rotamers of 2NPP2N, 1NPP1N, StPPSt and PePPPe.
non-bonded to pyrrolinone ring with either N–H or C]O group.
Furthermore, each of them can theoretically exist in two rotameric
forms differing by a mutual sense of aryl rotations. In summary, 4 (6)
minima should exist for 1NPPB (1NPP1N).
Only two of the theoretical four minima were found for 1NPPB,
showing the naphthyl-pyrrolinone dihedral angle 29ꢀ (136ꢀ) and
phenyl-pyrrolinone angle 7ꢀ in both cases. Even if the starting
geometry favoured the rotamer with opposite sense of phenyl
rotation (173–136, 173–29), the geometry converged to the above
mentioned ones (7–136, 7–29). The energy difference is not
negligible in this case: the 1NPPB (7–136) is 1.32 kcal/mol higher in
energy than 1NPPB (7–29), so the steric interaction in an
arrangement, where the naphthalene ring non-bonded to pyrroli-
none is closer to carbonyl group, is more pronounced.
All six possible rotamers of 1NPP1N were found as the local
minima at this level of calculation. Three rotamers with the same
sense of aryl rotations corresponding to Ci symmetry for (29–29)
and (134–134) dihedral angle sets are shown in Fig. 1, too. The

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S. Lunak Jr. et al. / Dyes and Pigments 85 (2010) 27–36
33
energy difference between the pair of rotamers with opposite sense
of aryl rotations is low (less than 1 kcal molꢂ1), preferring C2
symmetry. The energy difference among three rotamers differing
by a mutual orientation of naphthalene with respect to carbonyl
group are more pronounced: (29–136) and C2 (136–136) rotamers
are higher in energy with respect to the most stable one C2 (29–29)
for 1.31 kcal molꢂ1 and 2.48 kcal molꢂ1, respectively.
Generally, any rotamer of 2-naphthyl derivatives is always more
stable than the most stable 1-naphthyl isomer. The differences
between the most stable ones are 4.87 kcal molꢂ1 between 1NPPB
(7–29) and 2NPPB (0–180), and 9.64 kcal molꢂ1 between 1NPP1N
(C2 (29–29)) and 2NPP2N (180–180).
Stilbenyl derivatives (only trans arrangement was taken into
account) are completely planar by theory, so as the styryl ones
(Fig. 1). The former ones evoke the 2-naphthyl DPPs with
a marginal differences of the ground state energy between all
rotamers, while the latter ones are closer to 1-naphthyl DPPs with
indispensable differences among the rotamers’ energies (Table 3).
The experimental X-ray structures are available only for BPPB
[4] and BiPPBi [5]. The computed geometry of both compounds
well predicts aromatic character of phenyl rings and bond lengths
alternation in diketo-pyrrolo-pyrrole moiety. The main difference
between computed geometry and that obtained from X-ray
diffraction on crystals is in planarity. According to X-ray diffraction
the pendant phenyl groups on both compounds are twisted with
respect to the heterocycle plane. The dihedral angle is reported to
be equal to 7ꢀ for BPPB [4] and the molecule is of Ci symmetry in
crystal. The dihedral angle phenyl-pyrrolinone is 18–19ꢀ for BiPPBi,
the phenyl–phenyl one is 33ꢀ and the molecule is of C1 symmetry in
crystal, because the Ci symmetry is slightly distored through a non-
planarity (4ꢀ) on central dipyrrolinone heterocycle.
corresponding arrangement of 2-naphthyl substituent with respect
to pyrrolinone are taken into account: e.g. the increment in lmax is
15 nm for an arrangement with dihedral angles equal to 0ꢀ and the
values of fosc are almost the same as for biphenylyl set.
The rotamers in stilbenyl set show marginal differences of lmax
values and moderate ones for fosc. On the other hand the styryl
derivatives resemble the 2-naphthyl set; the dependence of these
spectral features on a rotameric arrangement is even more
pronounced, including the coupled batho and hypochromic trends
among the rotamers. Generally, any increase of a conjugated
system of pendant aryls leads to bathochromic shift with respect to
parent BPPB for planar DPPs. The most efficient is stilbene, fol-
lowed closely by styryl. The position of biphenyl and 2-naphthyl in
this sequence depends on a rotameric form of 2-naphthyl DPPs. The
S0 / S1 transition energies of unsymmetrical derivatives lie
approximately in the middle of the interval between the corre-
sponding values of symmetrical analogues and BPPB, oscillator
strengths are close to their arithmetic mean.
The excitation energies predicted for 1-naphthyl derivatives are
strongly dependent on a degree of non-planarity. The position of
lmax covers a wide range from the values close to the parent BPPB
for the most distored rotamers to the values similar as for their
planar 2-naphthyl isomers. Rather surprisingly, fosc depends on
a geometry only moderately, probably because the hyperchromic
trend, caused by an increase of conjugated system, is compensated
by a hypochromic trend, resulting from a planarity perturbation.
3.4. Experimental absorption and fluorescence excitation spectra
All compounds, except 1NPP1N and 1NPPB, show very limited
solubility even in DMSO at room temperature. The worst soluble
compounds are BiPPBi, 2NPP2N and StPPSt, all showing the
spectral artefacts. The spectrum of 2NPP2N (and BiPPBi, too)
shows a long wavelength tailing caused by a light scattering of the
unsoluted particles of a pigment. The spectrum of StPPSt has
furthermore a long wavelength spectral band at 612 nm, that can be
probably attributed to dimer or higher aggregate (Fig. 2). Both types
of spectral artefacts are not detected in fluorescence excitation
spectra of these compounds. Other derivatives are better soluble
and a complete agreement between absorption and fluorescence
excitation is observed.
The absorption spectra of all compounds except 1NPP1N and
1NPPB (Fig. 3) show the spectral shape very similar to parent BPPB.
Their vibronic structure, well resolved even in DMSO at room
temperature, enables the identification of 0–0 vibronic sub-band,
that can be compared with the theoretical value. This sub-band
We have discussed the non-planarity of further DPP pigments,
whose X-ray structures are available, in our previous study [1] and
ascribed the non-zero dihedral angles between phenyl and pyrro-
linone planes to packing effects in crystal phase. The phenyl–
phenyl dihedral angle in biphenyl moiety is less important for
a good prediction of spectral behaviour, so the difference about 5ꢀ
between computed and experimental values is acceptable.
3.3. PCM TD DFT vertical excitation energies in DMSO
The vertical excitation energies of the lowest electronic transition
recomputed to wavelengths (lmax) and oscillator strengths (fosc) of all
the compounds computed using PCM TD DFT (B3LYP/6-
311 þ G(2d,p)) on B3LYP/6-311G(d,p) geometry in dimethylsulfoxide
(DMSO) are summarized in Table 3. Table S1 in Supplementary
information contains the computed spectral characteristics of four
lowest S0 / Sn transitions. The lowest excited state is always of
S1(pp*) character, and the transition S0 / S1 is symmetry allowed of
HOMO / LUMO type, significantly (always more than 90 nm)
separated from the nearest forbidden state (either on carbonyl
localized np* or HOMO / LUMO þ 1 delocalized pp*).
0.8
2NPP2N
0.7
StPPSt
0.6
The difference in lmax and fosc between C2 and Ci rotamers of
BiPPBi is negligible for the first three transitions. Each phenyl
substitution brings the same bathochromic shift of S0 / S1 tran-
sition (19 nm), when going from BPPB through BiPPB to BiPPBi,
accompanied by almost linear hyperchromic shift (Table 3). On the
other hand the energies of S0 / S1 transition of 2-naphthyl
derivatives depends on a rotamer type strongly. The increment in
predicted lmax values is about 10 nm per one naphthyl rotation, the
most bathochromically shifted rotamers are 2NPP2N (180–180)
and 2NPPB (0–180). Rather surprisingly, this bathochromic trend
among the rotamers is accompanied by the hypochromic one.
Bathochromic and hyperchromic shift is also linear when going
from BPPB through 2NPPB to 2NPP2N and the rotamers with
0.5
0.4
0.3
0.2
0.1
0.0
300 350 400 450 500 550 600 650 700 750 800
(nm)
Fig. 2. The room temperature absorption spectra of DMSO solutions of StPPSt and
2NPP2N.

ˇ´
34
S. Lunak Jr. et al. / Dyes and Pigments 85 (2010) 27–36
35000.0
30000.0
25000.0
20000.0
15000.0
10000.0
5000.0
0.0
0.8
BPPB
Abs.300K
Fl.300K
Ex.77K
1NPP1N
1NPPB
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Fl. 77K
300
350
400
450
500
550
600
(nm)
350
400
450
500
550
600
650
700
750
(nm)
Fig. 3. The room temperature absorption spectra of DMSO solutions of BPPB, 1NPPB
and 1NPP1N.
Fig. 5. The room temperature absorption and fluorescence emission and low
temperature fluorescence emission and excitation spectra of 1NPPB in MTHF.
forms also an absolute spectral maximum. All compounds fluoresce
in DMSO, showing approximately a mirror symmetry relation
between excitation and emission. The wavelengths of absorption
and emission 0–0 vibronic sub-bands are summarized in Table 2.
Stokes shift lies between 10 and 20 nm. The vibronic structure is
further sharpened in low temperature (77 K) 2-methyl-tetrahy-
drofuran (MTHF) glass, at which the fluorescence emission and
excitation spectra were measured. The typical behaviour is shown
in Fig. 4 for StPPB. The excitation maxima are shifted bath-
ochromically, as MTHF glass is even more polar environment than
DMSO solution [17], and the emission maxima hypsochromically, as
the excited state relaxation processes are limited in rigid glass.
Finally, Stokes shift is decreased to 2–5 nm (Table 2). BiPPBi,
2NPP2N and StPPSt were not sufficiently soluble in MTHF even at
concentrations lower than 10ꢂ6 M to measure their low tempera-
ture fluorescence emission and excitation spectra.
1-Naphthyl derivatives show partially (1NPPB) or fully
(1NPP1N) blurred vibronic structure (Fig. 3) in DMSO, as a conse-
quence of non-planarity, so an identification of 0–0 vibronic tran-
sition is difficult, especially for 1NPP1N. Their low temperature
fluorescence excitation spectra show quite well resolved vibronic
structure for 1NPPB (Fig. 5) and rather worse resolution for
1NPP1N (Fig. 6). Evolution of the spectra when going from room to
low temperature may be considered as an evidence, that the
absolute spectral maximum corresponds to 0–0 vibronic transition
for 1NPPB and probably to 0–1 transition for 1NPP1N.
corresponding symmetrical derivative as predicted by theory. When
going from BPPB to all three unsymmetrical derivatives, a hyper-
chromic shift is observed, also in a qualitative agreement with
computational results. But molar absorptivities of all three
symmetrical derivatives are lower than those ones of corresponding
unsymmetrical ones, on the contrary to the theoretical results, pre-
dicting approximately linear increase. The main reason for this
discrepancy is an extremal insolubility of symmetrical pigments,
making impossible a correct estimation of 3max. The difference in
3max values between BPPB and BiPPB is 5900 l molꢂ1 cmꢂ1, so the
similar value should be expected according to theoretical predictions
between BiPPB and BiPPBi. Thus 3max value of BiPPBi, extrapolated
from better soluble BPPB and BiPPB, is 46 000 l molꢂ1 cmꢂ1, while
the experimental one is 17 300 l molꢂ1 cmꢂ1. The value of 2NPP2N of
3max is also lower (30 400 l molꢂ1 cmꢂ1) than the one derived by
extrapolation from BPPB and 2NPPB (40 200 l molꢂ1 cmꢂ1),
although the difference is not so dramatic as for biphenylyl set. The
same type of inconsistency between the value (54 400 l molꢂ1 cmꢂ1
)
extrapolated from BPPB and StPPB and experimental (33
600 l molꢂ1 cmꢂ1) one for StPPSt is observed, too, from the same
reasons.
There is quite a good agreement between computed and
experimental maxima for BPPB, BiPPB and BiPPBi, for which
either there is only one possible geometry or both rotamers give the
same theoretical lmax. The experimental maxima are always at
rather longer wavelengths, but the differences are acceptable
(11 nm, 7 nm and 4 nm). The situation seems to be similar to that
one described in Ref. [1]: the para substituted derivatives of BPPB
are computed rather better than the parent chromophore itself.
If we do not consider both non-planar 1-naphthyl DPPs for this
moment, we can see several clear trends. Larger conjugated system
of an aryl substituent causes a bathochromic shift; labs of unsym-
metrical derivative is exactly an arithmetic mean of parent BPPB and
0.8
0.8
Abs. 300K
Abs.300K
0.7
0.7
Fl.300K
Fl.300K
Ex.77K
Fl. 77K
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Ex.77K
Fl.77K
0.6
0.5
0.4
0.3
0.2
0.1
0.0
350
400
450
500
550
600
650
700
750
350
400
450
500
550
600
650
700
750
(nm)
(nm)
Fig. 4. The room temperature absorption and fluorescence (DMSO) and low temper-
ature (MTHF) fluorescence emission and excitation spectra of StPPB.
Fig. 6. The room temperature absorption and fluorescence emission and low
temperature fluorescence emission and excitation spectra of 1NPP1N in MTHF.

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S. Lunak Jr. et al. / Dyes and Pigments 85 (2010) 27–36
35
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
the value extrapolated from BPPB and 1NPPB (8800 l molꢂ1 cmꢂ1
)
is close to the experimental one (7900 l molꢂ1 cmꢂ1) for 1NPP1N,
that is only qualitatively in accordance with theory and only for the
most distored rotamer set, e.g. BPPB / 1NPPB (7–136) /
1NPP1N (Ci, 134–134). Even if we take into account, that the
absorption maximum of 1NPP1N corresponds to 0–1 vibronic
transition, experimental lmax of both 1-naphthyl derivatives show
a hypsochromic shift with respect to parent BPPB on the contrary
to the theoretical predictions for any rotamer. Simply said, the
computations underestimate the excitation energies and over-
estimate the probability of spectral transitions. We do think, that
the explanation of this discrepancy comes from the underestima-
tion of dihedral angles by DFT calculations of the ground state
geometry. The synthesis of further non-planar symmetrical and
unsymmetrical till unknown DPPs, with N-heteroaryles coming
from 4-cyano-quinoline and 1-cyano-isoquinoline, is now in
progress. Thus, we return to the detailed discussion of 1-naphthyl
DPPs with similar steric hindrance after collecting more experi-
mental data from their N-heteroanalogues.
Ex.300K
Fl.300K
EX.77K
Fl.77K
350
400
450
500
550
600
650
700
750
(nm)
Fig. 7. The room and low temperature fluorescence emission and excitation spectra of
2NPPB in MTHF.
No dependence of the fluorescence emission spectrum on the
excitation wavelength (and opposite) was observed for any sample,
either at room or low temperature. As the difference in absorption
maxima of rotamers of 2-naphthyl derivatives should be about
10 nm, a similar value as experimentally obtained for rotamers of 2-
naphthyl-ethylenes, we can conclude, that only one rotamer of
2NPPB and 2NPP2N is present in solution [18], although their
ground state energy is almost the same. Furthermore, a mixture of
spectrally shifted rotamers could hardly produce the spectra with
such sharp vibronic structure as shown in Fig. 7. Thus the question
arises, whether the rotamer present in solution can be estimated by
a comparison of the experimental and theoretical lmax. The
experimental values are closer for 2NPPB (0–0) rotamer and lie
between 2NPP2N (0–0) and 2NPP2N (0–180) rotamers. Thus, if we
suppose, that the same orientation of naphthalene ring is preferred
in both compounds, the rotamers characterized by dihedral angle
equal to 0 (Fig. 1) can be concluded as more probable. This
assignment is also consistent with a comparison of 2NPPB (0–0)
with BiPPB: the difference in lmax is 4 (3) nm by theory (experi-
ment) and 3max are very close as predicted.
4. Conclusions
The effect of a size of a conjugation system of aryl substituents
on the absorption spectra of diketo-pyrrolo-pyrroles was studied.
Two new unsymmetrical derivatives were synthesized in order to
complete the investigated set. PCM TD DFT calculations predict
a linear increase of both lmax and 3max with a substitution of each
phenyl in parent BPPB by larger aryl for planar derivatives. Quali-
tatively, the experimental bathochromic shifts of 0–0 vibronic sub-
bands are in exact agreement with theory, while the hyperchromic
shifts are affected by extremally low solubility of planar symmet-
rical derivatives. Quantitatively, the theory only slightly over-
estimates the excitation energies in this case. The deviations from
this ideal behaviour were observed for non-planar derivatives
(aryl ¼ stilbenyl or 1-naphthyl), for which the dihedral angles
describing the aryl out-of-plane torsions are probably under-
estimated by DFT.
On the contrary to 2-naphthyl and biphenylyl derivatives the
experimental absorption maxima of stilbenyl ones are considerably
blue shifted with respect to the theoretical values. Furthermore,
molar absorptivity of StPPB is only a bit higher than for BiPPB,
Acknowledgments
Authors acknowledge the support of the grant project No.
2A-1TP1/041 sponsored by the Ministry of Trade and Industry of
the Czech Republic.
although the theoretical prediction supposes a higher value of 3max
.
We consider two sources of this discrepancy. First, trans-stilbene,
although planar in crystal [19] and by high level quantum chemical
calculations [20], is often considered as non-planar in solution with
both phenyls partially rotated out-of-plane [21]. Such rotation
could cause a considerable hypsochromic and hypochromic shift, if
present in StPPB and StPPSt. Second, both stilbenyl derivatives
show the longest conjugated chain in one direction, that can be
a problem for TD DFT method, because of its known incorrect
asymptotic behaviour with respect to the long distance between
charges [22]. Both these effect can be less significant in styryl
derivatives PePPB and PePPPe, so it is possible, that they could be
experimentally bathochromically shifted with respect to stilbenyl
analogues, although the theory predicts the opposite relation. Thus
it is desirable to find a synthetic alternative for them [6].
Appendix. Supplementary information
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.dyepig.2009.09.014.
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