In the title compound, C
11H
7NO
4, there is a dihedral angle of 45.80 (7)° between the planes of the benzene and maleimide rings. The presence of O—H
O hydrogen bonding and weak C—H
O interactions allows the formation of
R33(19) edge-connected rings parallel to the (010) plane. Structural, spectroscopic and theoretical studies were carried out. Density functional theory (DFT) optimized structures at the B3LYP/6–311 G(d,p) and 6–31++G(d,p) levels are compared with the experimentally determined molecular structure in the solid state. Additional IR and UV theoretical studies allowed the presence of functional groups and the transition bands of the system to be identified.
Supporting information
CCDC reference: 817045
Starting materials and reagents were purchased from Aldrich and used as
received. The title compound was prepared by taking [mixing?] equimolar
quantities of p-aminobenzoic acid (1.00 g, 7.3 mmol) and maleic
anhydride in DMF [N,N-dimethylformamide?] (20 ml) under
a nitrogen atmosphere at ambient temperature for 1 h. Cyclodehydration of the
maleamic acid, intermediate to maleimide, was carried out by treating it with
fused sodium acetate and acetic anhydride for 2 h at 343 K. A yellow–orange
precipitate was obtained by adding water to the solution. Crystals were
dissolved in methanol and left evaporating to give a yellowish prism with a
melting point of 491 (1) K. The synthesis showed a 60% yield.
All H atoms were located from difference maps, positioned geometrically and
refined using a riding model with C—H = 0.93–0.97 Å and Uiso(H)
= 1.2Ueq(C). Friedel pairs were merged in the data set used for final
structure refinement. The DFT quantum-chemical calculations were performed at
the B3LYP/6–311 G(d,p) level (Becke, 1993; Lee et al.,
1988). The
performance of 6–31++G(d,p) and 6–311 G(d,p) basis functions (Bauschlicher &
Partridge, 1995) was checked in these calculations as implemented in
GAUSSIAN03 (Frisch et al., 2004). DFT structure
optimization of
(I) was performed, starting from the X-ray geometry. The harmonic vibrational
analysis at the same level of theory confirmed the stability of the ground
state as denoted by the absence of imaginary frequencies.
Data collection: COLLECT (Nonius, 2000); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 1999).
4-(2,5-dioxo-2,5-dihydro-1
H-pyrrol-1-yl)benzoic acid
top
Crystal data top
C11H7NO4 | Dx = 1.486 Mg m−3 |
Mr = 217.18 | Melting point: 361.0(10) K |
Orthorhombic, P212121 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2ac 2ab | Cell parameters from 3141 reflections |
a = 7.3326 (5) Å | θ = 2.9–27.5° |
b = 9.8832 (5) Å | µ = 0.12 mm−1 |
c = 13.3922 (11) Å | T = 294 K |
V = 970.53 (11) Å3 | Prisms, pale-yellow |
Z = 4 | 0.18 × 0.13 × 0.10 mm |
F(000) = 448 | |
Data collection top
KappaCCD diffractometer | 895 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.063 |
Graphite monochromator | θmax = 27.5°, θmin = 3.2° |
CCD rotation images, thick slices scans | h = −8→9 |
6051 measured reflections | k = −12→12 |
1278 independent reflections | l = −17→16 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.049 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.128 | H-atom parameters constrained |
S = 1.13 | w = 1/[σ2(Fo2) + (0.0644P)2 + 0.0425P] where P = (Fo2 + 2Fc2)/3 |
1278 reflections | (Δ/σ)max < 0.001 |
145 parameters | Δρmax = 0.19 e Å−3 |
0 restraints | Δρmin = −0.21 e Å−3 |
Crystal data top
C11H7NO4 | V = 970.53 (11) Å3 |
Mr = 217.18 | Z = 4 |
Orthorhombic, P212121 | Mo Kα radiation |
a = 7.3326 (5) Å | µ = 0.12 mm−1 |
b = 9.8832 (5) Å | T = 294 K |
c = 13.3922 (11) Å | 0.18 × 0.13 × 0.10 mm |
Data collection top
KappaCCD diffractometer | 895 reflections with I > 2σ(I) |
6051 measured reflections | Rint = 0.063 |
1278 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.049 | 0 restraints |
wR(F2) = 0.128 | H-atom parameters constrained |
S = 1.13 | Δρmax = 0.19 e Å−3 |
1278 reflections | Δρmin = −0.21 e Å−3 |
145 parameters | |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
σ(F2) is used only for calculating R-factors(gt) etc.
and is not relevant to the choice of reflections for refinement.
R-factors based on F2 are statistically about twice as large
as those based on F, and R- factors based on ALL data will be
even larger. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
O1 | 0.7660 (3) | 0.2398 (3) | 0.97977 (17) | 0.0591 (6) | |
O2 | 0.5223 (3) | 0.1942 (3) | 0.88701 (19) | 0.0612 (7) | |
H2 | 0.4692 | 0.2131 | 0.9391 | 0.092* | |
O3 | 1.3783 (4) | 0.0171 (3) | 0.6183 (2) | 0.0773 (9) | |
O4 | 0.8918 (4) | 0.1485 (3) | 0.4308 (2) | 0.0784 (8) | |
N1 | 1.1066 (3) | 0.0912 (2) | 0.55032 (19) | 0.0486 (6) | |
C1 | 0.7010 (4) | 0.2050 (3) | 0.9006 (3) | 0.0482 (8) | |
C2 | 0.8062 (4) | 0.1723 (3) | 0.8095 (2) | 0.0435 (7) | |
C3 | 0.7299 (4) | 0.0990 (3) | 0.7323 (3) | 0.0515 (8) | |
H3 | 0.6100 | 0.0691 | 0.7376 | 0.062* | |
C4 | 0.8283 (4) | 0.0696 (3) | 0.6478 (3) | 0.0513 (8) | |
H4 | 0.7765 | 0.0187 | 0.5968 | 0.062* | |
C5 | 1.0061 (4) | 0.1167 (3) | 0.6394 (2) | 0.0465 (7) | |
C6 | 1.0854 (4) | 0.1896 (3) | 0.7166 (3) | 0.0505 (8) | |
H6 | 1.2047 | 0.2205 | 0.7108 | 0.061* | |
C7 | 0.9863 (4) | 0.2159 (3) | 0.8017 (3) | 0.0494 (8) | |
H7 | 1.0398 | 0.2630 | 0.8542 | 0.059* | |
C8 | 1.0436 (5) | 0.1128 (3) | 0.4524 (3) | 0.0565 (9) | |
C9 | 1.2005 (6) | 0.0837 (3) | 0.3871 (3) | 0.0685 (11) | |
H9 | 1.1991 | 0.0882 | 0.3177 | 0.082* | |
C10 | 1.3412 (6) | 0.0508 (4) | 0.4410 (4) | 0.0715 (11) | |
H10 | 1.4567 | 0.0306 | 0.4165 | 0.086* | |
C11 | 1.2885 (5) | 0.0505 (3) | 0.5466 (3) | 0.0559 (8) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.0516 (13) | 0.0840 (14) | 0.0416 (13) | 0.0015 (12) | −0.0062 (10) | −0.0078 (12) |
O2 | 0.0425 (11) | 0.0877 (16) | 0.0535 (15) | −0.0054 (12) | 0.0030 (11) | −0.0186 (13) |
O3 | 0.0540 (15) | 0.0934 (19) | 0.085 (2) | 0.0122 (14) | −0.0166 (15) | −0.0259 (17) |
O4 | 0.0872 (19) | 0.0975 (19) | 0.0507 (16) | 0.0296 (17) | −0.0090 (16) | 0.0009 (14) |
N1 | 0.0446 (14) | 0.0603 (15) | 0.0409 (15) | 0.0026 (13) | 0.0018 (12) | −0.0028 (13) |
C1 | 0.0433 (16) | 0.0540 (16) | 0.047 (2) | −0.0028 (14) | −0.0039 (14) | 0.0002 (14) |
C2 | 0.0435 (15) | 0.0519 (15) | 0.0351 (16) | −0.0003 (14) | −0.0038 (13) | −0.0014 (13) |
C3 | 0.0421 (16) | 0.0652 (18) | 0.047 (2) | −0.0076 (15) | −0.0012 (15) | −0.0065 (15) |
C4 | 0.0466 (16) | 0.0628 (18) | 0.0444 (18) | −0.0049 (15) | −0.0045 (15) | −0.0100 (15) |
C5 | 0.0444 (16) | 0.0536 (16) | 0.0415 (19) | 0.0017 (15) | −0.0016 (15) | −0.0010 (14) |
C6 | 0.0438 (15) | 0.0614 (17) | 0.046 (2) | −0.0064 (15) | −0.0022 (15) | 0.0000 (15) |
C7 | 0.0441 (16) | 0.0585 (17) | 0.0456 (19) | −0.0070 (15) | −0.0024 (14) | −0.0049 (14) |
C8 | 0.069 (2) | 0.0521 (17) | 0.048 (2) | 0.0035 (16) | −0.0021 (18) | 0.0009 (16) |
C9 | 0.093 (3) | 0.057 (2) | 0.055 (2) | −0.008 (2) | 0.023 (2) | −0.0044 (17) |
C10 | 0.065 (2) | 0.072 (2) | 0.077 (3) | −0.009 (2) | 0.024 (2) | −0.021 (2) |
C11 | 0.0484 (18) | 0.0574 (17) | 0.062 (2) | −0.0018 (15) | 0.0001 (17) | −0.0149 (17) |
Geometric parameters (Å, º) top
O1—C1 | 1.212 (4) | C3—H3 | 0.9300 |
O2—C1 | 1.327 (4) | C4—C5 | 1.389 (4) |
O2—H2 | 0.8200 | C4—H4 | 0.9300 |
O3—C11 | 1.210 (5) | C5—C6 | 1.388 (4) |
O4—C8 | 1.202 (4) | C6—C7 | 1.377 (4) |
N1—C11 | 1.394 (4) | C6—H6 | 0.9300 |
N1—C8 | 1.407 (4) | C7—H7 | 0.9300 |
N1—C5 | 1.424 (4) | C8—C9 | 1.474 (6) |
C1—C2 | 1.480 (5) | C9—C10 | 1.301 (6) |
C2—C3 | 1.381 (4) | C9—H9 | 0.9300 |
C2—C7 | 1.392 (4) | C10—C11 | 1.466 (6) |
C3—C4 | 1.373 (4) | C10—H10 | 0.9300 |
| | | |
C1—O2—H2 | 109.5 | C7—C6—C5 | 119.6 (3) |
C11—N1—C8 | 109.0 (3) | C7—C6—H6 | 120.2 |
C11—N1—C5 | 125.2 (3) | C5—C6—H6 | 120.2 |
C8—N1—C5 | 125.7 (3) | C6—C7—C2 | 120.3 (3) |
O1—C1—O2 | 122.1 (3) | C6—C7—H7 | 119.9 |
O1—C1—C2 | 125.3 (3) | C2—C7—H7 | 119.9 |
O2—C1—C2 | 112.6 (3) | O4—C8—N1 | 124.9 (3) |
C3—C2—C7 | 119.4 (3) | O4—C8—C9 | 129.6 (4) |
C3—C2—C1 | 121.4 (3) | N1—C8—C9 | 105.5 (3) |
C7—C2—C1 | 119.3 (3) | C10—C9—C8 | 109.8 (4) |
C4—C3—C2 | 121.0 (3) | C10—C9—H9 | 125.1 |
C4—C3—H3 | 119.5 | C8—C9—H9 | 125.1 |
C2—C3—H3 | 119.5 | C9—C10—C11 | 109.1 (4) |
C3—C4—C5 | 119.3 (3) | C9—C10—H10 | 125.4 |
C3—C4—H4 | 120.4 | C11—C10—H10 | 125.4 |
C5—C4—H4 | 120.4 | O3—C11—N1 | 124.8 (3) |
C6—C5—C4 | 120.5 (3) | O3—C11—C10 | 128.5 (4) |
C6—C5—N1 | 119.9 (3) | N1—C11—C10 | 106.6 (3) |
C4—C5—N1 | 119.6 (3) | | |
| | | |
O1—C1—C2—C3 | 161.9 (3) | C3—C2—C7—C6 | 1.9 (5) |
O2—C1—C2—C3 | −19.1 (4) | C1—C2—C7—C6 | −178.4 (3) |
O1—C1—C2—C7 | −17.8 (5) | C11—N1—C8—O4 | 179.9 (3) |
O2—C1—C2—C7 | 161.2 (3) | C5—N1—C8—O4 | −4.6 (5) |
C7—C2—C3—C4 | −0.6 (5) | C11—N1—C8—C9 | −0.5 (3) |
C1—C2—C3—C4 | 179.7 (3) | C5—N1—C8—C9 | 175.0 (3) |
C2—C3—C4—C5 | −1.2 (5) | O4—C8—C9—C10 | 178.6 (4) |
C3—C4—C5—C6 | 1.7 (5) | N1—C8—C9—C10 | −0.9 (4) |
C3—C4—C5—N1 | −177.5 (3) | C8—C9—C10—C11 | 1.9 (4) |
C11—N1—C5—C6 | 43.8 (4) | C8—N1—C11—O3 | −175.8 (3) |
C8—N1—C5—C6 | −131.0 (3) | C5—N1—C11—O3 | 8.7 (5) |
C11—N1—C5—C4 | −137.0 (3) | C8—N1—C11—C10 | 1.6 (3) |
C8—N1—C5—C4 | 48.2 (4) | C5—N1—C11—C10 | −174.0 (3) |
C4—C5—C6—C7 | −0.4 (5) | C9—C10—C11—O3 | 175.0 (4) |
N1—C5—C6—C7 | 178.8 (3) | C9—C10—C11—N1 | −2.2 (4) |
C5—C6—C7—C2 | −1.4 (5) | | |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2···O1i | 0.82 | 1.90 | 2.672 (3) | 156 |
C3—H3···O3ii | 0.93 | 2.39 | 3.103 (4) | 134 |
C6—H6···O4iii | 0.93 | 2.68 | 3.392 (4) | 134 |
Symmetry codes: (i) x−1/2, −y+1/2, −z+2; (ii) x−1, y, z; (iii) x+1/2, −y+1/2, −z+1. |
Experimental details
Crystal data |
Chemical formula | C11H7NO4 |
Mr | 217.18 |
Crystal system, space group | Orthorhombic, P212121 |
Temperature (K) | 294 |
a, b, c (Å) | 7.3326 (5), 9.8832 (5), 13.3922 (11) |
V (Å3) | 970.53 (11) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.12 |
Crystal size (mm) | 0.18 × 0.13 × 0.10 |
|
Data collection |
Diffractometer | KappaCCD diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 6051, 1278, 895 |
Rint | 0.063 |
(sin θ/λ)max (Å−1) | 0.649 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.049, 0.128, 1.13 |
No. of reflections | 1278 |
No. of parameters | 145 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.19, −0.21 |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2···O1i | 0.82 | 1.90 | 2.672 (3) | 156.0 |
C3—H3···O3ii | 0.93 | 2.39 | 3.103 (4) | 133.6 |
C6—H6···O4iii | 0.93 | 2.68 | 3.392 (4) | 134.4 |
Symmetry codes: (i) x−1/2, −y+1/2, −z+2; (ii) x−1, y, z; (iii) x+1/2, −y+1/2, −z+1. |
Comparison of selected geometric data for (I) (Å, °) from
calculated (DFT) data and X-ray. topBond lengths | X-ray | B3LYP/ | B3LYP/ |
| | 6-31++G(d,p) | 6-311G(d,p) |
O1-C1 | 1.209 (3) | 1.2168 | 1.2080 |
O2-C1 | 1.325 (3) | 1.3593 | 1.3565 |
C5-N1 | 1.425 (3) | 1.4251 | 1.4243 |
C8-O4 | 1.202 (4) | 1.2128 | 1.2038 |
C11-O3 | 1.212 (4) | 1.2127 | 1.2036 |
C1-C2 | 1.482 (4) | 1.4856 | 1.4854 |
| | | |
Bond angles | | | |
O4-C8-N1 | 125.0 (3) | 126.32 | 126.58 |
O3-C11-N1 | 124.9 (3) | 126.31 | 126.57 |
C11-N1-C5 | 125.1 (2) | 125.31 | 125.26 |
N1-C5-C4 | 119.6 (2) | 119.83 | 119.95 |
C2-C1-O2 | 112.6 (2) | 113.19 | 112.90 |
C2-C1-O1 | 125.3 (2) | 124.97 | 124.99 |
Comparison of the observed and calculated vibrational frequencies
in cm-1 for (I) topAssignement | Observed | Calculated |
C-H angular deformation | 767 | 717 |
out of the aromatic plane. | 831 | 849 |
| 855 | 873 |
| | |
C-H scissor deformation at | 1027 | 1076 |
C=C of maleimide plane. | | |
| | |
Vibrational axial deformation | 1146 | 1109 |
of C—O of carboxyl group. | | |
| | |
Axial deformation of C—N at | 1180 | 1125 |
the maleimidic skeleton. | | |
| | |
C—H angular deformation | 1293 | 1192 |
in the aromatic plane. | 1214 | 1218 |
| 1312 | 1226 |
| | |
Axial deformation of C—N | 1398 | 1386 |
between maleimide and | | |
benzene rings. | | |
| | |
Axial deformation of carbonyl | 1720 | 1793 |
C=O. | | |
The structure determination of 4-carboxyphenylmaleimide [systematic name: 4-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)benzoic acid], (I), is part of a series of structure determinations on phenylmaleimide derivatives (Moreno-Fuquen et al., 2003, 2006, 2008). There is considerable interest in the development of N-substituted maleimides as photoionizers for free radical polymerization, where the maleimide can produce the initiating radical species (Andersson et al., 1996; Teerenstra et al., 2000). Miller et al. (2001) synthesized a good number of N-aromatic maleimides to evaluate their utility as free radical photoinitiators. As a result of this evaluation they found that the photochemical properties of N-arylmaleimide systems depend on the values of the dihedral angle between the benzene and imidic rings (Miller et al., 2000). Even with good crystallographic information on N-phenylmaleimide derivatives reported in the literature, the search for new, related systems remains important for the analysis of polymerization processes in which they are involved. Calculations by density functional theory (DFT) on N-phenylmaleimide compounds, modelling the torsional deformation between the rings and showing the energy barrier to planarity, are also reported (Miller et al., 1999). The present work describes structural, spectroscopic and theoretical studies on 4-carboxyphenylmaleimide.
The title compound shows a dihedral angle of 45.80 (7)° between the mean planes of the benzene and maleimide rings (see Fig. 1). This structural behaviour is repeated in similar systems, e.g. p-nitrophenylmaleimide, 42.98 (5)° (Moreno-Fuquen et al., 2003), p-chlorophenylmaleimide, 47.54 (9)° (Moreno-Fuquen et al., 2008) and 2-p-toluidino-N-p-tolylmaleimide, 42.6 (1)° (Watson et al., 2004), where the interplanar angles of these systems are close to that observed in (I), and their bond distances and bond angles are very similar. O—H···O hydrogen bonds of moderate character (Emsley, 1984) and weak C—H···O intermolecular interactions are observed in (I) (see Table 1; Nardelli, 1995). Although C—H···O interactions appear to be very weak, these contacts may have a determining effect on the formation of different packing motifs (Desiraju et al., 1993), they can play significant roles in molecular conformation (Saenger & Steiner, 1998) and they are essential in molecular recognition processes (Shimon et al., 1990). With regard to the structure (I), the O2 atom acts as hydrogen-bond donor to the carboxyl atom, O1i, in the molecule at (x - 1/2, -y + 1/2, -z + 2). At the same time, the C3 atom acts as donor to the O3ii atom in the molecule at (x - 1, y, z). The molecules of (I) form an infinite chain of edge-connected R33(19) rings (Etter, 1990) running parallel to the (010) plane (see Fig. 2). Neighbouring chains interact through very weak C—H···O contacts in which the C6 atom acts as hydrogen-bond donor to the carbonyl atom O4iii in the molecule at (x + 1/2, -y + 1/2, -z + 1), forming R22(12) rings, completing the two-dimensional array.
The presence of substituents in the benzene ring forces the system to produce several conformations between the benzene and maleimide rings (Miller et al., 2000). The position of the substituent on the benzene ring, the volume of the substituent and its intra- and intermolecular interactions are essential factors when analysing the structural behaviour of these systems. The presence of the carboxyl group in the para position allows the analysis of the influence of the substituent on the inter-ring torsion angle along N1—C5. To gain a better understanding of the properties of compound (I), we further explored the stability of this compound in the gaseous state, calculating the harmonic frequencies and comparing the results with those observed in the fundamental vibrational frequencies. Additionally, theoretical studies of the UV spectra were undertaken. Previous studies on similar systems (Miller et al., 1999) showed that calculations at the DFT-B3LYP level were consistently closer to experimental values.
Calculations by density functional theory DFT-B3LYP, with two basis sets 6–31++G(d,p) and 6–311 G(d,p) of bond lengths and bond angles, were performed. These values were compared with experimental values of the title system (see Table 2). From these results we can conclude that basis set 6–311 G(d,p) is better behaved in its approach [better suited?] to the experimental data.
Calculations using basis set 6–311(d,p) modelled torsional deformations between aryl and maleimide rings, showing different conformations with different energy barriers. Calculations on isolated 4-carboxyphenylmaleimide showed a minimum rotational energy for a rotamer with an inter-ring dihedral angle of 35.11°. This result shows a significant correlation with the experimental value of 45.80 (7)°.
The vibrational analysis of the title compound shows the expected infrared bands attributed to the constituents of the complex. The spectrum shows several well defined bands: an intense and broad band in the IR spectrum at 1720 cm-1 can be assigned to the axial deformation of carbonyl C═O which is also observed in the simulated spectrum at 1793 cm-1. The C═O band of the carboxyl group is masked within the same carbonyl C═O band. These and other observed and calculated bands with their assignements are shown in Table 3. The comparison of the observed fundamental frequencies of (I) and the IR spectrum simulated by DFT calculation (B3LYP) showed a good agreement between frequencies (see Fig. 3).
Compound (I) shows an absorption band in the UV region at α = 246.5 nm in methanol. The most intense bands obtained near this region in B3LYP/6–311 G(d,p) calculations for an isolated molecule are around λ = 243 nm [oscillator strength = 0.413 (exp) and 0.330 (calc)]. These bands are attributed to an intramolecular charge transfer (ICT) from the highest occupied molecular orbital (HOMO) to an orbital close to the lowest unoccupied molecular orbital (LUMO+1). The calculations reveal that these are π orbitals, primarily localized in the plane extending from the phenyl to the maleimide ring. These orbitals are shown in Fig. 4.