Theoretical Study of Photophysical Properties of Bisindolylmaleimide Derivatives


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Theoretical Study of Photophysical Properties of Bisindolylmaleimide Derivatives
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Bisindolylmaleimide derivatives such as arcyriarubin and
arcyriaflavin, which are isolated from the fruiting bodies of the
slime mold (Arcyria denudata),1 contain both two indole
subunits and a maleimide subunit. These compounds correspond
to a core structure of selective inhibitors of protein kinase C
(PKC) or DNA topoisomerase, as staurosporine,2 rebeccamycin,3
and ICP-1,4 which have a bisindolylmaleimide or indolocarbazole
skeleton with a C-N linkage to a sugar moiety. The ability
of selective inhibition or regulation of the metabolism of cells
makes them therapeutically important anticancer agents.5 The
PKC isoenzyme family members phosphorylate a wide variety
of protein targets, and they are involved in diverse cellular
signaling or signal transduction.6 Consequently the PKC inhibitors
such as these bisindolylmaleimide derivatives are promising
therapy agents for autoimmune diseases and tumors.4,7
On another matter, fluorescent, chemiluminescent, and bioluminescent
compounds have been developed for luminescence
assays,8-10 especially for specific detection of certain biomolecules.
11-13 Various indole derivatives were also synthesized,
and their luminescent characteristics were investigated.14,15 A
number of bisindolylmaleimide derivatives exhibit strong
fluorescence accompanying a large Stokes shift with respect to
the absorption wavelength, which leads to suitable luminescence
assays available for a specific detection.16,17 Most bisindolylmaleimides
are vivid red crystals1 and some of them exhibit
red luminescence in their solid phase; amorphous films of
N-methylated derivatives have been applied for fabrication of
red light-emitting diodes (LEDs).18
The origin of the large Stokes shift of the emission of
bisindolylmaleimides in solution has not been fully understood.
In general, it is caused by a significant difference between the
equilibrium geometry of the lowest electronic excited state and
that of the electronic ground state,19-22 but a large Stokes-shift
may also occur due to the energy relaxation from higher excited
states to the lowest excited state.23 It is considered that the
intramolecular charge transfer (ICT) is responsible for the
phenomenon. ICT appears in the electronic ground and/or
electronically excited states of the indolylmaleimide derivatives,
because these molecules have the well-known electron donoracceptor
feature provided by the indole and maleimide groups.
Kaletas et al. investigated the solvatochromic behavior to verify
whether the ICT character dominates the spectroscopic properties
of arcyriarubin A (N-H bisindolylmaleimide, BIM) and
arcyriaflavin A (cyclized N-H bisindolylmaleimide, C-BIM).17
The molecular structures of BIM and C-BIM are displayed in
Figure 1. According to the Kamlet-Taft treatment,24 the
observed emission spectra show slightly solvatochromic trends.
* To whom correspondence should be addressed. E-mail: sekiya@ (H.S.); (S.N.).
† Graduate School of Molecular Chemistry.
‡ Research Institute for Information Technology.
§ Graduate School of Pharmaceutical Sciences.
| Present address: Department of Materials and Life Sciences, Faculty
of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku,
Tokyo 102-8554, Japan.
Figure 1. Molecular structures of BIM and C-BIM.
J. Phys. Chem. A XXXX, xxx, 000 A
10.1021/jp9043489 CCC: $40.75  XXXX American Chemical Society
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Published on July 1, 2009 on | doi: 10.1021/jp9043489
However their quantum chemical calculations with density
functional theory (DFT) indicate that all of the orbitals, which
are involved in the electronic transitions exhibit a delocalization
of the electron density over the whole molecule, therefore, we
cannot expect ICT in which the electron density transferred from
one indole unit to the maleimide part. They finally concluded
that no ICT occurs in BIM and C-BIM. On the other hand,
immediately after the report of Kaletas et al., Kosower and de
Souza pointed out that the slopes of the plots of emission
energies against ET(30) for these bisindolylmaleimide derivatives,
BIM (0.37) and C-BIM (0.54), would establish that the
emissions arise from the charge transfer.25 Thus, it is still a
subject of controversy whether ICT does occur in BIM and
C-BIM or not. Furthermore, the DFT calculation is suitable to
explore the features of the electronic ground state around the
equilibrium geometry, whereas it could not be enough to
describe the electronic structures related to the electronically
excited states, especially in the donor-acceptor type of the
electronic state. At least two configuration state functions (CSFs)
are required for the ab initio calculation of the present molecular
system. In order to look into the possibility of the donor-acceptor
type transition, the multiconfiguration self-consistent field
(MCSCF) calculations with the complete active space (CAS)
is employed to determine the molecular orbitals (MOs), and
then the multireference perturbation calculation is performed
with CSFs obtained by the MOs.
In this paper we investigate the photophysical properties of
BIM and C-BIM on the grounds of our results of quantum
chemistry calculations. The molecular structures at the potential
minima and the potential energies for the electronic ground (S0)
state, the lowest electronic excited (S1) state, and the second
excited (S2) state of these conformers (isomers) were obtained
using the multireference perturbation theory along with use of
CASSCF calculations. Details of these calculations are described
in the method section. In the results and discussion section, we
discuss the properties with our findings from BIM and those
of C-BIM separately. In order to assign the species that are
responsible for the absorption in an aprotic polar solvent, N,Ndimethylformamide
(DMF), the vertical excitation energies and
the transition dipole moments of the isomers are compared with
Figure 2. Optimized structures of neutral BIM obtained by CASPT2/
cc-pVDZ calculations and their relative potential energies are indicated
in units of kcal mol-1.
TABLE 1: Potential Energies of the S0, S1, and S2 States
and the Oscillator Strengths and the Excitation Energies for
the Electronic Transitions of the Three Isomers of Neutral
state E/cm-1
strength our theo. expa
N1 S0 0
S1 30508 0.32 327 366
S2 37317 0.14 268
N2 S0 0 (325)
S1 30574 (30899) 0.28 327
S2 36548 (36873) 0.16 274
N3 S0 0 (1254)
S1 28949 (30245) 0.18 345
S2 37456 (38712) 0.12 267
a Figures in parentheses in the column of E(energy)/cm-1 stand
for the relative energy to the S0 state of the most stable isomer.
b Reference 16.
Figure 3. Optimized structures of deprotonated anions (M1(-), M2(-),
I1(-), and I2(-)) of BIM obtained by CASPT2/cc-pVDZ calculations
and their relative potential energies are indicated in units of kcal mol-1.
TABLE 2: Potential Energies of the S0, S1, and S2 States
and the Oscillator Strengths and the Excitation Energies for
the Electronic Transitions of the Four Isomers of
Deprotonated Anion of BIMa
state E/cm-1
strength our theo. expb
M1(-) S0 0
S1 30835 <0.01 324
S2 35146 <0.01 284
M2(-) S0 0 (1030)
S1 30375 (31405) <0.01 329
S2 34042 (35072) <0.01 294
I1(-) S0 0 (1156)
S1 22231 (23387) 0.42 450 452
S2 30590 (31746) 0.05 327 (366)
I2(-) S0 0 (3307)
S1 24592 (27899) 0.52 406
S2 31348 (34655) <0.01 319
a Two of the four are the deprotonated form of the maleimide NH
group (M1(-) and M2(-)), and the other are the deprotonated form
of the indole NH group (I1(-) and I2(-)). Figures in parentheses in
the column of E (energy) /cm-1 stand for the relative energy to the
S0 state of the most stable isomer. b Reference 16.
B J. Phys. Chem. A, Vol. xxx, No. xx, XXXX Saita et al.
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Published on July 1, 2009 on | doi: 10.1021/jp9043489
the spectroscopic data.16,17 Detailed discussions of the characters
of the low-lying electronic states and the occurrence of the ICT
are made on the basis of the results of the multireference
perturbation calculation. We also discuss in this section the
origin of the Stokes-shifted fluorescence of these bisindolylmaleimides.
Finally we summarize and compare the results of
BIM and C-BIM in the conclusion section


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