Phthalazine-trione as a blue-green light-emitting moiety: Crystal structures, photoluminescence and theoretical calculations

Article abstract of DOI:10.1039/c8nj02976h

Phthalazine (2,3-diazanaphthalene) and phthalhydrazide (2,3-dihydro-1,4-phthalazinedione) compounds are heterocycles with several biological properties. Most recently, analogous phthalazine-triones also emerged as drug candidates. Herein, we introduced a

Full text of DOI:10.1039/c8nj02976h

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Phthalazine-trione as a blue-green light-emitting
moiety: crystal structures, photoluminescence
and theoretical calculations†
Cite this: New J. Chem., 2019,
43, 1313
Felipe Terra Martins, *a Lauro June Queiroz Maia,b Gisane Gasparotto,b
Ana Karoline Silva Medanha Valdo,a Jose Antonio Nascimento Neto,a
´
ˆ
Leandro Ribeiro,a Yuri de Freitas Rego,c Cleiton Moreira da Silvac and
c
ˆ
Angelo de Fatima
´
Phthalazine (2,3-diazanaphthalene) and phthalhydrazide (2,3-dihydro-1,4-phthalazinedione) compounds
are heterocycles with several biological properties. Most recently, analogous phthalazine-triones also
emerged as drug candidates. Herein, we introduced a phthalazine-trione moiety as a promising fluorophore
in the blue and green spectral regions. This desired optical property was rationalized based on their crystal
structure and time-dependent density functional theory (TD-DFT) calculations. Under ultraviolet (UV)
excitation (ca. 360 nm), two light emission maxima at ca. 460 nm and 480 nm were observed for ten
phthalazine-triones, regardless of the substitution pattern. This conservation in the light-emitting property was
rationalized by our time-dependent DFT calculations for the three phthalazine-triones whose crystal
geometries were also determined in this study (4-chlorophenyl, 3-methoxyphenyl and 4-nitrophenyl
derivatives) and for those available in literature (4-fluorophenyl and 4-bromophenyl derivatives). Electronic
transitions with the largest oscillator strengths were near 360 nm and were in excellent agreement with the
experimental excitation energy (UV-visible and photoluminescence spectra). Lifetime values of selected
samples could also be determined, which were between 1.59 ns and 3.21 ns. In all compounds, the Frontier
molecular orbitals involved in these excitations were distributed over the phthalazine-trione moiety and were
not localized on the changeable substituent.
Received 14th June 2018,
Accepted 3rd December 2018
DOI: 10.1039/c8nj02976h
rsc.li/njc
Introduction
Phthalazine (2,3-diazanaphthalene) and phthalhydrazide
(2,3-dihydro-1,4-phthalazinedione) derivatives are hetero-
cycles containing hydrazine moieties. Compounds containing
these structures have shown significant biological and pharmaco-
logical activities such as antifungal,1 VEGFR-2 inhibitor,2
vasorelaxant,3 cardiotonic,4 and antiproliferative.5 Additionally,
these heterocycles also have interesting luminescent properties
(Fig. 1).6 Phthalazine-BF2 complex (A) displayed blue phosphor-
escence when subjected to UV radiation.7 Cyclometalated
benzo[g]phthalazine iridium(III) complex (B) was reported to
a
´
´
Instituto de Quımica, Universidade Federal de Goias, Campus Samambaia,
Fig. 1 Phthalazine derivatives presenting luminescent properties.
ˆ
CP 131, Goiania, GO, 74001-970, Brazil. E-mail: felipe@ufg.br
b
c
´
´
Instituto de Fısica, Universidade Federal de Goias, Campus Samambaia, CP 131,
ˆ
Goiania, GO, 74001-970, Brazil
´
ˆ
´
exhibit near-infrared-emitting phosphorescence.8 Polymer
light-emitting devices doped with the iridium(III) complex (C)
showed high photoluminescence quantum efficiency of 96%.9
Luminescence was also noted in highly functionalized
Grupo de Estudos em Quımica Organica e Biologica (GEQOB), Departamento de
´
ˆ
Quımica, Instituto de Ciencias Exatas, Universidade Federal de Minas Gerais,
ˆ
Av. Pres. Antonio Carlos, 6627, Belo Horizonte, MG, 31270-901, Brazil
† CCDC 1836431–1836433. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8nj02976h
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phthalazine-dione derivatives (D), which exhibited fluorescence at Hydrogens had their isotropic atomic displacement parameters set
486–492 nm.10 These luminescence properties could be harnessed to 1.2Uiso(C), except in the case of methyl groups where this value
into new and more efficient technologies such as organic light- was 1.5Uiso(C). The complete X-ray diffraction dataset for all struc-
emitting diodes (OLEDs), phosphorescent organic light-emitting tures is available under the CCDC number codes shown in Table 1,
diodes (PhOLEDs), laser dyes and luminescent materials.
Recently, 2H-indazolo[2,1-b]phthalazine-trione derivatives were
synthesized by microwave-assisted one-pot three-component con-
densation reaction using p-sulfonic acid calix[4]arene as catalyst
and green solvents.5 Even though the exploration of their
medicinal properties has begun, no other technological appli-
cation has been reported thus far. Herein, we present a detailed
study of the crystalline structure and luminescence properties
of phthalazine-triones, which could be understood based on
density functional theory (DFT) calculations. Phthalazine-
trione was found to be a new robust blue-green light-emitting
fluorophore, regardless of the substitution pattern.
wherein a summary of X-ray diffraction data is also presented.
Photoluminescence (PL) and lifetime measurements
PL excitation and emission spectra were recorded using a
double monochromator and a Hamamatsu photomultiplier
tube as the detector (Fluorolog FL3-221 from Horiba Jobin-Yvon),
under excitations from a Xe arc lamp delivering 450 W. The
bandpass was fixed at 0.5 nm and readings were recorded every
1.0 nm. The PL measurements were performed in both crystals
and the chloroform solution. PL decay curves were collected at
475 nm with a bandpass of 6 nm for selected compounds in
a chloroform solution, under an excitation at 390 nm from a
NanoLED source operating at 1 MHz with 1.2 ns pulse duration.
Experimental
Theoretical calculations
Synthesis
In order to rationalize the conformational features and the
strong photoluminescence observed for phthalazine-triones,
we performed DFT calculations17,18 with full optimization for
the crystal geometries reported here and those available in the
literature (F and Br analogs)5,19 at the B3LYP/6-31G(d,p) level of
theory.20,21 Next, Frontier molecular orbitals (FMOs) and time-
dependent DFT (TD-DFT) calculations were carried out at the
same level of theory by inputting the fully optimized geometries.
Frequency calculations confirmed the nature of the located
stationary point. For all structures, positive definite Hessian
matrices were found. All calculations were accessed with the
Gaussian 09 program package.22
Phthalazine-triones 1–11 (Scheme 1) were synthesized accord-
ing to the literature.5 Briefly, 1.0 mmol of phthalhydrazide,
1.2 mmol of dimedone, 1.5 mmol of aldehyde and 1.5 mol% of
p-sulfonic calix[4]arene were mixed in 1.0 mL of ethyl lactate.
The mixture was heated under microwave radiation at 130 1C
for 10 minutes. After cooling to room temperature, water was
added. Then, the solution was cooled to ꢀ20 1C for precipita-
tion. The solid was filtrated and purified by silica gel column
chromatography.
Single crystal X-ray diffraction
Suitably sized single crystals of 2, 5 and 11 were obtained and
exposed to an X-ray beam using a Bruker-AXS Kappa Duo
diffractometer with an APEX II charge-coupled device (CCD)
detector (Cu Ka radiation, 296 K). Bruker programs, SAINT and
SADABS,11 were used for cell refinement and data indexing,
integration and reduction. Multi-scan absorption correction
was performed for our Cu Ka dataset.12 Structure solution
and refinements were performed using SHELXL-201413 within
WinGX.14 Structure analysis and artwork preparation were
performed with mercury15 and ORTEP-3.16 Non-hydrogen and
hydrogen atoms were refined anisotropically and isotropically,
respectively. All CH hydrogens were added to their corres-
ponding carbons following a riding model with fixed bond
angles and lengths (0.93 Å, 0.96 Å, 0.97 Å and 0.98 Å in
aromatic, methyl, methylene and methine groups, respectively).
Results and discussion
Suitably shaped single crystals of 2, 5 and 11 were successfully
obtained from exhaustive crystallization assays. Even though
C15 is a chirality center, all compounds crystallized in centro-
symetric space groups (P21/n for 2, P21/c for 5 and 11), i.e., both
enantiomers were present in the racemic crystals. In compli-
ance with the structure elucidation studies already reported in
the literature for the analogs, the S enantiomer was chosen to
be the crystallographic asymmetric unit as well as atom labeling
scheme followed that of the F analog.5 Before this study, the crystal
structure was determined for the other two phthalazine-triones
that differed from 2 only by the halogen atom in the para position
of phenyl ring attached at C15. One of these two literature analogs,
namely, that bearing Br {13-(4-bromophenyl)-3,3-dimethyl-
2,3,4,13-tetrahydro-1H-indazolo[1,2-b]phthalazine-1,6,11-trione;19
herein after numbered as 12 even though it was not synthesized
by us} is isostructural to our chlorinated compound 2. Both have
the same molecular conformation, crystal packing, similar unit
cell parameters and an equal space group. In contrast, the literature
antecedent owning F in lieu of Cl/Br {13-(4-fluorophenyl)-3,3-
dimethyl-2,3,4,13-tetrahydro-1H-indazolo[1,2-b]phthalazine-1,6,11-
trione}5 differs by its intermolecular arrangement and therefore
Scheme 1 General synthesis of phthalazine-triones.
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Table 1 Summary of crystal data and refinement statistics for phthalazine-triones elucidated in this study
2
5
11
Structural formula
C23H19ClN2O3
P21/n
4/1
C24H22N2O4
P21/c
4/1
C23H19N3O5
P21/c
4/1
Space group
Z/Z0
a (Å)
b (Å)
c (Å)
6.2427(2)
13.2218(4)
24.3513(8)
93.176(2)
2006.86(11)
1.347
16.4477(4)
7.3740(2)
16.5927(4)
97.642(2)
1994.58(9)
1.340
5.7534(7)
18.2637(17)
18.7499(19)
98.750(8)
1947.3(4)
1.424
b (1)
V (Å)3
Calculated density (mg mꢀ3
)
y range for data collection (1)
Data collected
Unique reflections
3.636–66.545
9240
3421
2.710–66.573
9844
3432
3.397–66.776
8457
3242
Unique reflections with I 4 2s(I)
2232
0.0492
2593
0.0329
1920
0.0458
Symmetry factor (Rint
)
Parameters refined
262
1.029
271
1.030
281
1.071
Goodness-of-fit on F2
Final R1 factor for I 4 2s(I)
wR2 factor for all data
0.0483
0.1352
0.0441
0.1225
0.0644
0.2124
Largest Dr peaks (e Åꢀ3
)
0.238/ꢀ0.252
0.156/ꢀ0.235
0.275/ꢀ0.288
CCDC
1836431
1836433
1836432
Table 2 Selected conformational descriptors (in 1) for the phenyl ring E
twist in theoretical and crystal geometries (CgN2–C9 denotes the centroid
calculated for these atoms)
unit cell constants, even though it crystallizes in the same space
group as its related halogenated analogs. Only this halogen
replacement was enough to profoundly change the pattern of
weak intermolecular contacts, which was also related to a slight
conformation change in the halogen-substituted phenyl ring
(see below).
Only one molecule was found in the asymmetric unit of the
two known phthalazine-triones and the three phthalazine-
triones elucidated here. The non-hydrogen atom and ring
labeling scheme is shown in Fig. 2.
Angle between the planes
C16–C15–CgN2–C9 and C16 to C21 N1–C15–C16–C17 torsion
DFT
C11-endo
DFT
C11-exo
DFT
DFT
Compd Crystal
Crystal C11-endo C11-exo
2
3
5
11
17.10
4.87
21.60
30.89
17.34
28.18
25.10
17.25
30.68
28.45
17.84
18.42
16.11
30.73
17.99
43.1(3) 30.70
61.6(2) 33.83
37.7(2) 41.43
85.9(4) 28.08
43.1(4) 30.42
74.08
74.70
42.55
28.06
74.21
In the F analog, the average plane of the halogen substituted
phenyl ring E transversely crossed the five membered hetero-
cycle C. The imaginary plane formed with C16, C15 and the
12
centroid of the N2–C9 (CgN2–C9) bond formed a dihedral angle Interestingly, in the nitro compound 11, the phenyl ring was also
of 4.871 with the F-substituted phenyl ring (Table 2). The slightly twisted relative to the imaginary plane transversal to the
N1–C15–C16–C17 torsion also helped to describe this conforma- five-membered ring (the corresponding angle between these
tional feature [61.57(15)1]. This phenyl ring, which was already in planes was 30.891), but towards the opposite side of the hetero-
the Cl/Br derivatives and also the meta-methoxy derivate 5, was cyclic nitrogens [N1–C15–C16–C17 = 85.9(4)1]. This conformational
slightly bent towards the nitrogen due to the rotation on the feature was seemingly associated with the cyclohexenone pucker-
C15–C16 bond axis. The aforementioned dihedral angle measured ing. In the solid state of all phthalazine-triones, cyclohexenone
17.101/17.341 and 21.601 in such compounds, respectively, while adopted the half-chair conformation, with C11 lying away from the
the N1–C15–C16–C17 torsion was 43.1(3)1/43.1(4)1 and 37.7(2)1. other five coplanar atoms (C10–C9–C14–C13–C12 r.m.s.d. = 0.0489 Å,
0.0217/0.202 Å, 0.0495 Å and 0.0512 Å in F, Cl/Br, meta-methoxy
and para-nitro derivatives, respectively). In all these structurally
elucidated phthalazine-triones, C11 was puckered on the same side
of the phenyl ring E relative to the phthalazine-trione mean plane,
describing a C11-endo conformation. On the contrary, C11 in 11
deviated from this least-square plane, but pointed towards
the opposite side of the substituted phenyl ring in a so-called
C11-exo pucker (Fig. 2 and 3). In this last structure, C22 and
C23 methyl carbons occupied equatorial and axial positions,
respectively. In F/Cl/Br/MeO analogs, C22 and C23 were instead
in axial and equatorial positions.
Fig. 2 Asymmetric unit of the phthalazine-triones reported in this study.
Non-hydrogen atoms are drawn with their 30% probability ellipsoid, while
hydrogen atoms are arbitrary radius spheres. The labelling scheme of the
In order to probe the conformational preference for the
non-hydrogen atoms and rings common to all compounds followed those
phthalazine-triones as a function of the phenyl ring E substitu-
tion pattern, we full-optimized the crystal geometries reported
shown once. For clarity, the labelling of key atoms used in the conforma-
tional descriptions was repeated in 11.
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N2 were lower in the F/Cl/Br derivatives [0.0710/0.0574/0.0495 Å,
largest deviations of 0.1504(12)/0.103(3)/0.094(4) Å for C10 in F
analog and C13 in both Cl/Br analogs] than in 5 [0.190 Å, largest
deviation of 0.438(2) Å for C12] and 11 [0.235 Å, largest deviation of
0.376(5) Å for C4]. These last two phthalazine-triones underwent a
ruffling in their molecular plane formed with the four fused rings.
Rings A and B formed a plane slightly inclined out compared
with that formed with rings C and D (the angle between the
planes crossing through rings A/B and C/D (except C11) was
2.90(4)/2.90(3)/2.37(6)1, 12.12(3)1 and 16.05(7)1 in the F/Cl/Br analogs
and in meta-methoxy and para-nitro analogs, respectively).
The crystal packing of the Cl/Br derivatives had chains
composed of 21-screw axis symmetry-related molecules arrayed
in a zigzag fashion along the b axis (Fig. 4). Within these chains,
the main intermolecular interactions are CHꢁ ꢁ ꢁO non-classical
hydrogen bond assembling R22(10) motifs. Both carbonyl groups
from the phthalazine framework and their vicinal aromatic CH
Fig. 3 Molecular superimposition for 2 (green), 5 (red) and 11 (blue), all
determined here, and for the literature analogs, 3 (yellow) and 12 (brown).5,19
here and those available in the literature (F and Br analogs)5,19
and their corresponding conformers with the opposed cyclo-
hexenone puckering upon flipping C11 through the average
chair plane of ring D. Two minimal energy conformers bearing
C11 on opposite sides of ring D were found for all five
compounds, which had practically equal energies differing by
less than 0.13 kcal molꢀ1. For halogenated derivatives, the
conformation around C15–C16 followed the conformation
found in the solid state. Accounting for the phthalazine-
trione plane, if C11 was on the same side of ring E, the last
was slightly bent towards the heterocyclic nitrogens (see Table 2
for values of these conformational descriptors). This addition-
ally reveals that the alignment of the ring E mean plane with
the transversal plane of ring C was a consequence of the
intermolecular contacts assembled by the F-substituted phenyl
ring (see below). The conformational descriptors for the phenyl
ring twist differed between the crystal and theoretical C11-endo
geometries (Table 2). The theoretical conformational para-
meters of the F analog resembled those of the Cl/Br parents
(Table 2). If C11 was on the opposite side of ring E, the last was
twisted in an opposed direction to the heterocyclic nitrogens
(Table 2). Interestingly, this conformational relationship
between the cyclohexenone puckering and the phenyl twist
found theoretically in the gas phase of the halogenated deriva-
tives, which occurred also in the solid state, was not experi-
enced in 11 and 5 in the gas phase. For these two compounds,
the phenyl ring E was twisted towards the heterocyclic nitro-
gens in both minimal energy points, regardless of the C11
puckering mode. This shows that the rotation on the C15–C16
is due to intermolecular interactions adopted by the nitro-
phenyl ring as described in the sequence. In Table 2, the angle
between the planes crossing through C16–C15–CgN2–C9 and C16
was similar in both C11-endo and C11-exo theoretical confor-
mers, as well as their values of the N1–C15–C16–C17 torsion do
not differ significantly.
Fig. 4 Overall crystal packing of 2 and details of the main intermolecular
interactions. Circled regions denote the halogenꢁ ꢁ ꢁhalogen interactions.
Cg denotes the centroid calculated through the non-hydrogen endocyclic
ring atoms. The succeeding capital letter refers to the ring as labeled in
Fig. 2. Also, for clarity, only CH hydrogens involved in the displayed
contacts will be shown.
Another striking conformational feature is the high copla-
narity of the four fused rings in the halogen-based compounds
(except for C11). The r.m.s.d. values for the least-square plane
fitted through the sixteen atoms C1 to C10, C12 to C15, N1 and
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groups were engaged in the formation of the supramolecular
ring motif (Fig. 4). T-shaped CHꢁ ꢁ ꢁp contacts between one CH
moiety from phthalazine and the p-system from the phenyl ring
E helped to stabilize the primarily hydrogen bonded CHꢁ ꢁ ꢁO
chains. These chains were further face-to-tail stacked along the
a axis on top of each other by means of R22(12) motifs and
CHꢁ ꢁ ꢁp interactions. Again, these ring motifs were composed of
two CHꢁꢁꢁO non-classical hydrogen bonds having one methylene
group and one CH moiety from the substituted phenyl ring as
donors to the carbonyl oxygen from phthalazine and cyclo-
hexenone. This additional CHꢁ ꢁ ꢁp interaction engaged the
methine CH and the benzene moiety from phthalazine.
Another R22(12) graph set enclosed by the methyl and cyclo-
hexenone carbonyl groups was found along the a axis between
molecules that were not directly stacked on top of each other
(Fig. 4). The geometry of all discussed intermolecular contacts
are grouped in Table 3 for the phthalazine-triones determined
here and the F analog.5
R22(12) motifs were also found in the F derivative (Fig. 5).
However, these were formed with one CHꢁ ꢁ ꢁF non-classical
hydrogen bond and another CHꢁ ꢁ ꢁO non-classical hydrogen
bond. These motifs assembled chains running along the [101]
direction. These chains are now face-to-face stacked along the
b axis on top of each other through pꢁ ꢁ ꢁp interactions (Fig. 5).
The carbonyl group from cyclohexenone accepted hydrogen
bonds from methine, in which two centrosymetric interactions
gave rise to another R22(10) motif and one CH moiety belonging
Fig. 5 Overall crystal packing of 3 and details of its main intermolecular
interactions.5
to phthalazine, thus completing the three-dimensional crystal
packing through the assembly of chains along the c axis.
Therefore, just the Cl/Br substitution for F altered much of the
intermolecular ordering in the phthalazine-triones despite the
reduced impact from conformational changes. Due to its
highest electronegativity, fluorine was involved in non-classical
hydrogen bonding, while Cl/Br were interacting with themselves in
halogenꢁꢁꢁhalogen contacts (Fig. 4).
Table 3 Geometry of selected main intermolecular contacts found in
phthalazine-triones determined here and in the literature analog 35
D–H Hꢁ ꢁ ꢁA
(Å) (Å)
D–Hꢁ ꢁ ꢁA
(1)
Compd D–Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA (Å)
2
C3–H3ꢁ ꢁ ꢁO2
0.93 2.47
0.93 2.66
3.077(3)
3.569(3)
3.361(3)
3.333(3)
3.558(3)
3.774(3)
3.506(2)
3.8567(15)
123
166
137
174
146
148
144
—
C5–H5ꢁ ꢁ ꢁO1
The CHꢁꢁꢁO hydrogen bonded R22(10) motifs described before in
Cl/Br derivatives were also found in 5, but the translation sym-
metry molecules were chained along the b axis in the last (Fig. 6).
C12–H12bꢁ ꢁ ꢁO2 0.97 2.59
C17–H17ꢁ ꢁ ꢁO3
C22–H22cꢁ ꢁ ꢁO3
C6–H6ꢁ ꢁ ꢁCgE
C15–H15ꢁ ꢁ ꢁCgA
Cl1ꢁ ꢁ ꢁCl1
0.93 2.41
0.96 2.72
0.93 2.95
0.98 2.66
—
—
3
5
C5–H5ꢁ ꢁ ꢁO3
C15–H15ꢁ ꢁ ꢁO3
C17–H17ꢁ ꢁ ꢁF1
C18–H18ꢁ ꢁ ꢁO2
CgAꢁ ꢁ ꢁCgB
0.93 2.39
0.98 2.58
0.93 2.57
0.93 2.53
3.085(2)
3.533(2)
3.390(2)
3.424(2)
3.581(2)
130
160
145
157
—
—
—
C6–H6ꢁ ꢁ ꢁO1
0.93 2.57
3.234(2)
3.416(2)
3.477(2)
3.578(3)
3.762(2)
3.985(2)
3.524(2)
129
160
168
149
146
148
—
C12–H12bꢁ ꢁ ꢁO1 0.97 2.49
C15–H15ꢁ ꢁ ꢁO2
C23–H23cꢁ ꢁ ꢁO3
0.98 2.51
0.96 2.70
C12–H12aꢁ ꢁ ꢁCgE 0.97 2.92
C23–H23aꢁ ꢁ ꢁCgA 0.96 3.14
CgAꢁ ꢁ ꢁCgB
—
—
11
C5–H5ꢁ ꢁ ꢁO3
C10–H10aꢁ ꢁ ꢁO5
C6–H6ꢁ ꢁ ꢁCgE
C18–H18ꢁ ꢁ ꢁCgA
O1ꢁ ꢁ ꢁC1
0.93 2.38
0.97 2.57
0.93 3.13
0.93 3.09
3.294(6)
3.452(5)
3.915(4)
3.589(4)
3.119(5)/3.141(5)
2.873(5)
3.098(6)
167
151
144
116
—
—
—
—
—
—
—
—
—
Fig. 6 Overall crystal packing of 5 and details of its main intermolecular
interactions.
O2ꢁ ꢁ ꢁN3
O4ꢁ ꢁ ꢁC8
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In addition, the methine group substituted one aromatic CH as giving rise to a R22(13) moiety (Fig. 6). The cross-linking among
the donor in 5. Another CHꢁ ꢁ ꢁO non-classical hydrogen bond neighboring chains was from pꢁ ꢁ ꢁp contacts on a side of the
involving one methyl and carbonyl group from cyclohexenone phthalazine average plane and from CHꢁ ꢁ ꢁO and CHꢁ ꢁ ꢁp on
contributed to keep 5 molecules in contact with these chains, another side (Fig. 6).
While there was a parallel disposition of molecules into
chains for all conformationally similar phthalazine-triones, 11
molecules had their phthalazine and nitro substituted phenyl
planes oriented almost perpendicular due to dipolar inter-
actions between one phthalazine carbonyl group and one nitro
group (Fig. 7). Additionally, the two CHꢁ ꢁ ꢁO interactions
between one phthalazine CH group and the cyclohexenone
oxygen as well as between one CH2 group and the nitro group
acted for assembling R22(19) motifs between these Oꢁ ꢁ ꢁN inter-
acted molecules. All these contacts kept the molecules onto
molecular layers, extending infinitely along the (010) plane.
T-shaped CHꢁ ꢁ ꢁp contacts were also identified on these mole-
cular layers (Fig. 7). Another dipolar interaction between the
phthalazine carbonyl groups was responsible for the face-to-tail
stacking of the molecules along the a axis (Fig. 7).
In Fig. 8, the PL emission spectra of the phthalazine-triones
are presented under an excitation wavelength around 360 nm.
A broad band emission between 420 nm and 550 nm (blue and
green emissions), with two maxima at ca. 460 nm and 480 nm
and one shoulder at ca. 510 nm, was observed for some
compounds structurally studied here (F, Cl and MeO analogs)
and others previously synthesized.5 Slight shifts (up to 5 nm)
in the excitation wavelengths as well as the maxima and
shoulder emission wavelengths were found as a result of
the substituent at C15. However, the emission pattern was
conserved in all light emitting phthalazine-triones, revealing
Fig. 7 Overall crystal packing of 11, the molecular layer formed onto the
(010) plane and details of the main intermolecular interactions found there.
Fig. 8 PL emission spectra for phthalazine-triones under excitation of 360 nm.
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that such a solid-state property came from their common
molecular moiety.
Even though PL seems to be mainly related to the presence
of an intramolecular core conserved in our compounds, the
control of light emission features by crystalline materials has
been obtained with an adequate choice of intermolecular
interactions. This has been commonly employed in single-
component crystal forms, including polymorphs and multi-
component crystal forms.23 Crystal engineers use several types
of intermolecular interactions to tune emission wavelength,
lifetime and efficiency. A few of those intermolecular inter-
actions include halogenꢁ ꢁ ꢁhydrogen and lone pairꢁ ꢁ ꢁhalogen
bonds,24 classical hydrogen bonds,25 p–p interactions and
stacking fashions.25,26 Even the enhancement of excited state
intramolecular proton transfer (ESIPT), a photochemical process
much invoked to improve light-emitting properties, can be demon-
strated through non-classical hydrogen bonds and p–p inter-
actions in the solid state.27
The observation that PL arose from the phthalazine-trione
moiety was supported by our TD-DFT calculations for all
phthalazine-triones whose crystal geometries were available.
After their full geometric optimization, the electronic transi-
tions with the largest oscillator strengths were found near
360 nm, in excellent agreement with the excitation energy. In
all compounds, the FMOs involved in these excitations were
distributed over the phthalazine-trione moiety and were not
localized on the C15 substituent (Fig. 9). Therefore, we demon-
strate that the phthalazine-trione moiety is a promising fluoro-
phore in the blue-green spectral regions. At last, compound 11
was not photoluminescent, which is due to the strong self-
absorption from the NO2 group in the visible emission region.28
The transmission spectra of ten compounds in a form-
aldehyde solution (0.1 mmol Lꢀ1) are shown in Fig. 10. All
compounds presented similar absorption bands in the range
from 240 to 440 nm, having two main absorptions at B250 nm
and B370 nm (with some lateral absorption). A strong and
broad absorption band around 370 nm was in good agreement
with the calculations shown in Fig. 9, which illustrated the largest
oscillator strengths near 360 nm due to the HOMO–LUMO transi-
tion placed over the phthalazine-trione moiety.
We present in Fig. 11(a) the PL emission spectra for samples
1 to 10 in formaldehyde solution (0.1 mmol Lꢀ1), under
Fig. 9 Molecular orbitals involved in the time-dependent DFT calculated
[B3LYP/6-31G(d,p)] transition with the largest oscillator strength (f) for
phthalazine-triones whose crystal structures were determined here (2, 5
and 11) and those previously elucidated (3 and 12).5,19 The highest
occupied molecular orbital (HOMO) [or HOMOꢀ1 in 5] and the lowest
unoccupied molecular orbital (LUMO) are on the left and right columns
and mean the ground and excited states, respectively. The transition
energy and wavelength are in eV and nm, respectively.
Fig. 10 Transmittance spectra of the ten phthalazine-triones studied here
in a formaldehyde solution (0.1 mmol Lꢀ1).
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Table 4 Lifetime values of 2, 3, 5 and 8 under excitation at 390 nm using a
NanoLED source (1 MHz and peak pulse of 1.2 ns) and monitoring emission
at 475 nm with a bandpass of 6 nm
excitation at 360 nm using a continuous Xe arc lamp. Broad
band emissions were observed from 400 to 600 nm with the
Sample
t1 (ns)
t2 (ns)
A1
A2
w2
2
3
5
8
2.33 ꢂ 0.04
3.21 ꢂ 0.06
1.59 ꢂ 0.02
2.46 ꢂ 0.06
—
—
1.00
1.00
0.68
1.00
0
0
0.32
0
1.81
1.53
1.64
1.39
7.87 ꢂ 0.06
—
maximum around 475 nm, and all samples have a similar
spectral shape. Some samples presented a PL emission spec-
trum shifted slightly to the red region (6, 7 and 8), which can be
related to the substituent at C15 (n-propyl in 6, cyclohexyl in 7,
and n-butyl in 8). All PL emission spectra for compounds
dissolved in formaldehyde were similar to those in the solid
state. However, the PL emission spectra were broader, as
expected, due to interactions with solvent molecules.
PL emission decay curves for four selected samples (2, 3, 5
and 8) are illustrated in Fig. 11(b). These curves could be well-
fitted with exponential functions:
I(t) = A1eꢀt/t + A2eꢀt/t
(1)
1
2
All fittings are illustrated in Fig. 11(b) and the parameters
are listed in the Table 4. The quality of the fittings was
measured by w2, and all values were between 1.39 and 1.81,
close to unity. Samples 2, 3 and 8 possessed only one compo-
nent, with a lifetime between 2.33 and 3.21 ns. However,
sample 5 possessed two components, one at 1.59 ns and
another at 7.87 ns. All these results corroborate that the main
emissions are due to HOMO–LUMO transitions distributed
over the phthalazine-trione rings. The second component of 5
may be attributed to the 3-methoxyphenyl substituent. Indeed,
HOMOꢀ1 was also slightly distributed over this substituent in
5 (Fig. 9), which therefore contributed to the main transition
calculated by TD-DFT.
Conclusions
In this study, we determined the crystal structures of three
phthalazine-triones discovering interesting relationships between
the conformation of their rings and between these conformations
and the intermolecular interaction patterns. Furthermore, these
interplays could be better understood by comparing crystal geo-
metries to theoretical geometries calculated in the gas phase.
However, the main contribution of this study is the discovery of
a new class of blue and green light emitting compounds. These
compounds have conserved the luminescence profile since this
optical property came from their common phthalazine-trione
moiety. Our theoretical approaches using time-dependent DFT
allowed us to conclude that the main transition responsible for the
observed emissions involves FMOs distributed over this common
moiety although they are not localized on the changeable
substituent. The absorption bands, PL emission and lifetime
of compounds were evaluated, the results of which correlated
well with the TD-DFT results. Similar PL emission spectra were
Fig. 11 (a) PL emission spectra of 1 to 10 in a formaldehyde solution
(0.1 mmol Lꢀ1), under excitation at 360 nm using a continuous Xe arc lamp.
(b) PL decay curves of 2, 3, 5 and 8 under excitation at 390 nm using a
NanoLED source (1 MHz and peak pulse of 1.2 ns) and monitoring emission
at 475 nm with a bandpass of 6 nm.
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12 L. Krause, R. Herbst-Irmer, G. M. Sheldrick and D. Stalke,
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observed in solution and solid-state phases. The main lifetime
of emissions at 475 nm were between 1.59 ns and 3.21 ns due to
HOMO–LUMO transitions. Therefore, this study contributes
significantly to the material chemistry field by sourcing a new
robust fluorophore that can be further explored to conceive
blue-green light-emitting devices.
Conflicts of interest
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There are no conflicts to declare.
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Acknowledgements
The authors acknowledge CNPq, CAPES, FAPEMIG and FAPEG
for financial support and research fellowships. This study
was financed in part by the Coordenaçao deAperfeiçoamento de
˜
´
Pessoal de Nıvel Superior – Brasil (CAPES) – Finance Code 001.
22 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
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