Introduction 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@ chem.kyushu-univ.jp (H.S.); shinkoh.nanbu@sophia.ac.jp (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 Downloaded by AUSTRIA CONSORTIA on July 6, 2009 Published on July 1, 2009 on http://pubs.acs.org | 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 BIMb λ/nm electronic state E/cm-1 oscillator 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 λ/nm electronic state E/cm-1 oscillator 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. Downloaded by AUSTRIA CONSORTIA on July 6, 2009 Published on July 1, 2009 on http://pubs.acs.org | 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
