Ultrafast investigation of photoinduced charge transfer in aminoanthraquinone pharmaceutical product

We investigated the mechanism of intramolecular charge transfer and the following radiationless dynamics of the excited states of 1-aminoanthraquinone using steady state and time-resolved absorption spectroscopy combined with quantum chemical calculations. Following photoexcitation with 460 nm, conformational relaxation via twisting of the amino group, charge transfer and the intersystem crossing (ISC) processes have been established to be the major relaxation pathways responsible for the ultrafast nonradiative of the excited S1 state. Intramolecular proton transfer, which could be induced by intramolecular hydrogen bonding is inspected and excluded. Time-dependent density functional theory (TDDFT) calculations reveal the change of the dipole moments of the S0 and S1 states along the twisted coordinate of the amino group, indicating the mechanism of twisted intra-molecular charge transfer (TICT). The timescale of TICT is measured to be 5 ps due to the conformational relaxation and a barrier on the S1 potential surface. The ISC from the S1 state to the triplet manifold is a main deactivation pathway with the decay time of 28 ps. Our results observed here have yield a physically intuitive and complete picture of the photoinduced charge transfer and radiationless dynamics in anthraquinone pharmaceutial products.

model systems. Of special interest are the derivatives with strong intramolecular hydrogen bonds to the quinone oxygen. A dominant hydrogen-bonding interaction between the carbonyl oxygen with solvent might have a distinctive influence on the fluorescence behaviour. Fluorescence quantum yields were measured to be 0.006~0.0715 in different solvents by steady absorption and time-resolved spectra 25,26 . In addition, the nonradiative relaxation mechanism is attributed to internal conversion to the S 0 state. However, the lowest excited states of aminoanthraquinones have intramolecular charge transfer characters. It cannot be negligble that Wasielewski pointed out that AQ derivatives undergo rapid intersystem crossing to yield relatively long-lived triplet states that are capable of oxidizing purine nucleobases 27 .
In this paper, we report on a joint experimental and calculational study of the excited-state dynamics of 1-aminoanthraquinone in solution after excitation to the charge transfer electronic state. We employ femtosecond transient absorption spectroscopy to monitor the temporal evolution of the photoexcited 1-aminoanthraquinone in solution and elucidate unimolecular deactivation pathway of, especially twisted intramolecular charge transfer and following ISC processes. The characteristic spectra bands were measured and analyzed in detail combined with quantum chemical calculations. The kinetic traces of transient absorption disclose a mechanism of geometry relaxation and contribution of the triplet states. Quantum chemical calculations are also performed to help to understand the suggested mechanism.

Results and Discussion
Steady and transient absorption spectra. The absorption and fluorescence spectra of 1-NH 2 -AQ in ethanol are shown in Fig. 1. As can be seen, the absorption spectrum at wavelength of λ > 250 nm reveals several distinguishable absorption bands. These bands correspond to the transitions from the ground state to the different excited states. A relative much broader absorption band is peaked at 480 nm and associated with a transition from the S 0 state to the first optically bright S 1 state, which has a large oscillator strength. Following an excitation at 460 nm to the S 1 state, a redshifted and broad emission is observed. An enormously large Stokes shifted band with respect to the S 0 -S 1 absorption band is located at 620 nm in the fluorescence spectra. The emission with such a large Stokes shift indicates that the molecule has undergone a significant rearrangement.
All optimized geometries are determined to be planar structures with C s symmetry on B3LYP method with 6-311G basis set and confirmed to be stationary points by vibrational frequencies analysis. The chromophoric groups reside on the same plane in the structure of the ground state. Jürgen Troe et al. also confirmed that the equilibrium conformation is almost complete planarity on the HF/6-31G(d,p) 28 . All calculations under the planar structure reveal that the lowest excited S 1 state is a bright state and originates a 59 ← 58 transition, whereas the second excited S 2 state mainly originates a 59 ← 57 transition and is a dark state for its zero transition dipole moment. The molecular orbits of HOMO and LUMO at optimized S 0 and S 1 structures are shown in Fig. 2, respectively. The 57 and 58 orbits are both π orbits and correspond to the HOMO-1 and HOMO, respectively, whereas the 59 orbit is the LUMO and belongs to a π * orbit. The calculations show that the excitation to the S 1 state is strongly allowed with the main contribution of the transition from the HOMO to the LUMO. For the optimized geometry of the S 0 state, the HOMO is largely localized on the amino group, whereas the LUMO is largely localized at the whole anthraquinone group. The calculated vertical excitation energies of the excited states at a planar form using several methods with different basis sets are listed in Table 1. The original transition to the first populated excited state of 1-NH 2 -AQ was determined to be 2.6325 eV using fluorescence excitation spectra 28 . The calculated vertical excitation energies of the S 1 state are good match with the original transition to the S 1 state. The value at B3LYP/6-311G level is the most consistent with the experimental value.
Since the B3LYP/6-311G level gives the minimum energy of the ground state and the most consistent energy value of the S 1 state, the other calculations in this work are performed at the same method level. The pump wavelength is 460 nm and coincides with the peak of the π π * absorption band of the molecules. The transient absorption spectra of 1-NH 2 -AQ in ethanol are measured from 500 to 680 nm, as shown in Fig. 3. It obviously divides into two main broad bands. The region at wavelength < 590 nm is a positive signal and elucidated by the excited state absorption (ESA). And the other region from 590 to 680 nm is a negative signal and originating from the stimulated emission (SE). The two main bands become weaker as the pump-probe delay time increases. Both of them almost disappear until 900 ps. To properly describe the dynamics observed in the visible range, we performed a singular value decomposition (SVD) analysis on the 2D data matrix. The resulting kinetic amplitude vectors were globally fitted. Three time-constants of 5 ps, 28 ps, and 550 ps determined by global fit results are listed in Table 2. The decay associated difference spectra (DADS) shown in Fig. 4. do overlap, indicating that the temporal development of the spectral features cannot be described with a single exponential      Roughly, the amplitudes of three components in ESA band are opposite to those in SE band respectively, and the weights of the component τ 3 in both bands are the main contribution since their amplitudes are obviously larger than others.

Intramolecular hydrogen bonding. Intramolecular hydrogen bonding and dynamics of intramolecu-
lar proton transfer (PT) have been investigated in three intramolecular hydrogen-bonded molecules, 1, 8dihydroxyanthraquinone (1,8-DHAQ), 1-NH 2 -AQ, and 9-hydroxyphenalenone (9-HPA), respectively 28 28 . They checked the presence of a HT barrier along the reaction path calculated with CIS method with electron correlation by TDDFT. According to the TDDFT calculations, the HT barrier in 1-NH 2 -AQ is 3300 cm −1 . The excess energy in the S 1 state following the excitation with 460 nm is obviously lower than the barrier. It is disadvantage for ESIPT due to the difficulty to pass through the higher barrier.

Intramolecular charge transfer.
Optimized structures of the ground and first excited states of 1-NH 2 -AQ are shown in Fig. 2, respectively. It is interesting that the optimized geometry of the S 1 state is no longer a planar, but a twisted. The amino group twists and is almost perpendicular to the anthraquinone plane. The dihedral angle between the NH 2 group and anthraquinone at the optimized structure of the S 1 state becomes about 90°. Furthermore, the excited state optimized geometrical parameters reveal an interesting feature that the C=O bond nearby the amino group and the C-N bond increase from 1.270 and 1.363 Å in the S 0 state to 1.321 and 1.442 Å in the S 1 state, respectively. It originates from the strong donating nature of the amino group, which induces a large charge localization at the amino group. The dipole moments of the S 0 and S 1 states are determined to be 2.3818 D and 7.8076 D based on B3LYP/6-311G, respectively. The dipole moment of the excited state, in which the -NH 2 group is twisted, is obviously larger than that having the coplanar conformation. A change of the transition dipole moment of ~5.4258 D manifests a charge transfer process in molecule upon an excitation. The electron cloud distributions of both conformations also show the transfer of the charge. It is also apparent from the HOMO-LUMO electron distribution in the ground and the excited state geometries. As shown in Fig. 2, in the optimized geometry of the S 1 state, the LUMO electron density is more largely localized at the anthraquinone group as compared to that in the S 0 state. It is obvious that the LUMO electron has been pulled from electron-donating -NH 2 group to electron-withdrawing anthraquinone group. The electron density distributions suggest that the S 1 state is a strong ICT character.
As mentioned above, the conformation of 1-NH 2 -AQ maintains a coplanar structure in Franck-Condon region after excitation and relaxes to a twisted structure on the potential surface of the S 1 state. The parameters including the energies and dipole moments of the S 0 and S 1 states as a function of the twisted angle between the amino and anthraquinone groups are calculated using B3LYP method with 6-311G basis set and listed in Table 3. It is noticed that the oscillator strengths are decreasing as the twisted angle increasing. The oscillator strength becomes zero at 90° of the angle. It is obviously suggested that the fluorescence just emissions from an initially locally excited state not via a relaxed ICT state. No dual fluorescence was also observed in other ICT molecular system [32][33][34] . The dipole moments of the S 0 state is decreasing as the twisted angle increasing, whereas the dipole moments of the S 1 state has the opposite trend. The dipole moment of the planar conformation in the S 0 state is 2.3818 D. Following excitation to the Franck-Condon region, the dipole moment suddenly increases to 6.7567 D. Furthermore, the dipole moment of the S 1 state increases to 7.8076 D with increasing of the twisted angle between the amino group and the anthraquinone moiety. It is elucidated that the twisted intramolecular charge transfer is associated with the conformational relaxation on the potential surface of the S 1 state. It is also proved by the decrease of the charge localized at N atom from 0.906 e − in the S 0 state to 0.650 e − in the S 1 state. However, the timescale of twisted intra-molecular charge transfer (TICT) is usually measured to be a few hundred femtoseconds in some barrierless systems [32][33][34] . It is considered the relative energies of the S 1 state to the ground state at different -NH 2 twisted angles to properly depict the potential. For 1-NH 2 -AQ, the energy at the 0° twisted angle in the S 1 state is just 2.6756 eV, whereas it becomes 2.9351 eV at the 60° twisted angle. It indicates that there exists a small barrier about 0.2595 eV on the potential surface of the S 1 state along the twisted coordinate of the amino group. Müller et al. also suggested that a single-minimum-type potential exists in the S 1 states of 1-NH 2 -AQ 28 . The TICT process in 1-NH 2 -AQ is estimate to be longer since the small barrier on the amino twisting potential surface exists. The fast decay component of 5 ps is definitely attributed to twisted intramolecular charge transfer.
As mentioned in Table 3, it is obvious that the energies of the S 0 and T 2 states are increasing as the twisted angle increasing, whereas the energies of the S 1 and T 1 states are decreasing. It is estimated that the potential energy surfaces of the S 1 and T 2 states become isoenergetic nearby the twisted angle of ~40°. These calculations also govern the presence of a prominent conical intersection between the S 1 and T 2 states. It is possible that the deactivation of the S 1 state is directed to the intersystem crossing (ISC) channel. In the case of 1-NH 2 -AQ, the intersystem crossing to the triplet state is a major deactivation channel from the S 1 state and in this derivative a close-lying T 2 state seems to be responsible for the high k isc rate. The second time component of 28 ps was assigned to the ISC from the S 1 state to the triplet manifold. The potential surface of the ground, triplet and singlet excited states. of 1-NH 2 -AQ along as the change of twisted angle using B3LYP/6-311G are plotted in Fig. 6. Moreover, the energy gap between the S 1 and S 0 states in Franck-Condon region is 2.6756 eV. However, the energies of the S 0 and S 1 states at the optimized geometry of the S 1 state were performed at the same method level and determined to be 1.6699 eV and 2.2748 eV, respectively. The energy gap of both states is only 0.6049 eV at the optimized geometry of the S 1 state. It can be deduced that the energy gap decreases from 2.6756 eV to 0.6049 eV along the conformation relaxation coordinate. The obviously decrease trend of the energy gap is benefited to an ultrafast internal conversion (IC) from the S 1 state to the S 0 state. Venkataraman et al. pointed out that the S 1 state is mainly deactivated through IC to the ground state. The rate constant of IC is determined to be 5.3 × 10 8 and 2.3 × 10 9 s −1 in toluene and methanol, respectively 25 . It is obvious that the rate constant of IC is much larger than that of ISC. Yoshihara et al. pointed out that the absorption and fluorescence spectra is dependence on the strong solvent polarity and the lifetime of S 1 state is about 400 ± 100 ps in ethanol by picosecond fluorescence studies 26 .

Angle (°)
S 0 (eV) T 1 (eV) T 2 (eV) S 1 (eV) f (S 1 ) μ(S 0 ) (D) μ(S 1 ) (D)   It agreed well with the value of τ 3 = 550 ps obtained in our measurements. A longer decay component is observed in ethanol and assigned to be the lifetime of the S 1 state.

Conclusions
In this paper, we inspected the mechanism of intramolecular charge transfer and following radiationless dynamics of the excited states of 1-NH 2 -AQ using time-resolved absorption spectroscopy combined with quantum chemical calculations. Two main absorption bands were illustrated by the excitation at 460 nm to the S 1 state. The involvement of -NH 2 group rotation become as the main coordinate in the excited state relaxation dynamics. The optimized structure of the ground state is confirmed to be a planar conformation with C s symmetry, whereas the structure of the S 1 state is a twisted conformation with the amino group perpendicular to the anthraquinone plane. The difference of the dipole moments of the S 0 and S 1 states is ~5.4258 D and manifests a charge transfer process in molecule upon an excitation. The increase of dipole moment in the S 1 state with the change of the twisted angle is elucidated that the TICT is associated with the conformational relaxation on the potential surface of the S 1 state. The fast decay component of 5 ps is definitely attributed to twisted intramolecular charge transfer. Afterwards, the ISC from the S 1 state to the triplet manifold is a main deactivation pathway with the decay time of 28 ps. The long-lived triplet state plays the role of oxidizing purine nucleobases. According to the ES band and the larger rate constant of IC, the population of the S 1 state is decayed by way of fluorescence. A general photoinduced mechanism is drawn in Fig. 6 according to the experiments and quantum chemical calculations.

Experimental Method
1-aminoanthraquinone (1-NH 2 -AQ, 99% purity) was purchased from Sigma, and used without further purification. Ethanol (99% purity) purchasing from Aladdin was used as a solvent. The concentration of 1-NH 2 -AQ in ethanol was 1 mM at room temperature and a fresh sample was prepared for each measurement. The absorption and emission spectra were recorded on the UV-VIS spectrometer (INESA, L6) and the spectrometer (Princeton, SpectraPro 2500i) in a 1 mm quartz cell, respectively. Ultrafast broadband absorption measurements were performed based on a Ti:sapphire femtosecond laser system. Details of the femtosecond laser system have been described elsewhere 35,36 . Briefly, the seed beam is generated by a commercial Ti:sapphire oscillator pumped by a CW second harmonic of an Nd:YVO 4 laser, and then amplified by an Nd:YLF pumped regenerative amplifier to generate a 1 kHz pulse train centered at 800 nm of approximately 35 fs pulse width and with maximum energy of 1 mJ/pulse. A fraction of the laser is frequency doubled in a 1 mm thick BBO crystal, yielding pulses at 400 nm with an energy of 100 μ J, which are used to pump the NOPA. The excitation pulse energy at 460 nm used here is about 2 μ J by an attenuation. The NOPA pulse needs to be temporally compressed in order to obtain the minimum pulse width compatible with their bandwidth. A white light continuum generated by focusing the fundamental light at 800 nm on a 1 mm sapphire plate is reflected from the front and back surfaces of a quartz plate to obtain the probe and reference beams. The pump and probe pulses intersect in the sample at an angle of ~ 4°, and the reference beam is transmitted through the sample at a different spot. The relative polarization of the pump and probe pulses is set to the magic angle for all the measurements. A linear translation stage is used to delay the probe beam to monitor the pump-probe dynamics. The resulting spectra are detected by a CCD camera (PI-MAX, 1024 × 256 pixel array) equipped with a spectrometer (Princeton, SpectraPro 2500i). The instrumental response function of the system, determined by cross correlation between the excitation and probe pulses using the optical Kerr-gate method, is typically better than 150 fs.
All quantum chemical calculations are performed using the Gaussian09W suit of program 37 . The geometries of the ground and excited states of 1-NH 2 -AQ are optimized using MP2 and B3LYP with 6-311G basis set in gas phase and ethanol solution, respectively. The stationary points are also confirmed by the vibrational frequencies analysis. The energies of excited states are performed using the B3LYP function based on optimized geometries of the ground and excited states, respectively. The B3LYP function provides accurate excited-state ordering, excited-state transition energies, oscillator strengths, transition dipole moments and singlet-triplet energy gaps, particularly when solvent effects are taken into account 38,39 , which has been performed in other molecular systems 32,34,40,41 . Solvent effects are expected to lead to large ground-and excited-state energy changes in heteroaromatic compounds. Thus, the effect of the bulk solvent dielectric on the ground-state geometries and on the excited-state vertical energies was modeled by performing self-consistent reaction field (SCRF) calculations using the polarizable continuum model (PCM) with the integral quation formalism 42,43 .