Theory Study On The Excited State Of Amide Group And Amino Anthraquinone Derivatives

We have performed a series of calculations using densityfunctional theory (DFT) and time-dependent density functionaltheory (TD-DFT) for1-methylamideanthraquinone (MAAQ) and threedifferent amino anthraquinone derivatives (AQ, MAQ and DMAQ).The main results are outlined as follows:1. In the S0state of MAAQ, amide group is coplanar withanthraquinone, and an intramolecular hydrogen bond C=O…H-N isformed. The S0â†'S1transition has a distinct Ï€â†'Ï€*feature. Twostable structures (planar nMAAQ and twisted tMAAQ) have beenobtained in the S1state of MAAQ, and nMAAQ is lower by0.105eVthan tMAAQ in energy, so nMAAQ is the dominant conformation inthe S1state of MAAQ and the emission spectra of tMAAQ cannot beobserved in the solution of MAAQ. Furthermore, tMAAQ has adistinct intramolecular charge transfer character, and twistedintramolecular charge transfer (TICT) process occurs in the S1stateof MAAQ. Excited state intramolecular proton transfer (ESIPT)between C=O and N-H was not observed in the S1state of MAAQ.Upon addition of fluoride anion, only twisted conformations wereobtained in both S0and S1states of MAAQ-F-. An intermolecularhydrogen bond F…H-N is formed in the S0state, and ESIPT happens in the S1state for MAAQ-F-. Thus, MAAQ-F-shows obviously differentabsorption and fluorescence spectra with and without fluorideanion.2. We theoretically studied the excited states of intramolecularhydrogen bonds of three different amino anthraquinone derivatives(AQ, MAQ and DMAQ) using the TD-DFT method. We fully optimizedthe isolated AQ, MAQ and DMAQ, and their hydrogen bondcomplexes AQ-EtOH, MAQ-EtOH DMAQ-EtOH in the ground state, theexcited singlet state and the excited triplet state. Theintramolecular hydrogen bond C=O…H-N can be formed between thecarbonyl group and amino group for the isolated AQ and MAQ.Furthermore, the amino groups are coplanar with the anthraquinonemolecules. However, no intramolecular hydrogen bond for DMAF canbe formed, and in isolated DMAQ, the most stable conformation ofDMAF is out-of-plane structure. On the other hand, in all of thehydrogen bond complexes, intermolecular hydrogen bonds C=O…H-O is formed formed between aminoanthraquinones and ethanolmolecules. We have proved in theory, as compared with the groundstate, the intermolecular hydrogen bonds can be significantlyenhanced in the singlet excited state. Therefore, hydrogen bondstrengthening can lower the excitation energy of a related excitedstate. The internal conversion process from S1to S0state of theseaminoanthraquinones is facilitated by the intermolecular hydrogenbond strengthening. In addition, the intermolecular hydrogen bondsC=O…H-O in the singlet excited state is weaker than ground state,in the triplet excited state stronger than ground state. At the sametime, the change of intermolecular hydrogen bond in AQ-EtOH is stronger than in MAQ-EtOH between the singlet excited state andthe triplet excited state.We calculated potential energy curves of different electronicstates along the twisted dihedral angles for isolated AQ, MAQ,DMAQ and hydrogen-bonded complexes AQ-EtOH, MAQ-EtOH,DMAQ-EtOH. We note that due to the intramolecular hydrogen bondthere is a high energy barrier, so the twisted intramolecular chargetransfer process cannot occurs in the ground and triplet excitedstates of isolated AQ and the ground of isolated MAQ. However, thestable structure of isolated DMAQ is when the dihedral anglesformed between dimethylamino groups and anthraquinone ring iscalculated to be83.1°in the ground state,96.87°in the singleexcited state and96.84°in the triplet excited state, respectively.Furthermore, we can note that all the potential energy curves of thehydrogen-bonded complexes are correspondingly lowered incomparison with those of the isolated AQ, MAQ, and DMAQ. So theintermolecular hydrogen bonding can lower the energy levels of allthe potential energy curves. However, their main characters aresimilar to those of the isolated AQ, MAQ, and DMAQ. This shows thatthe hydrogen bonds can not significantly affect the twistedconformation of the excited state of aminoanthraquinones.