Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 67–72
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Intermolecular hydrogen bond and twisted intramolecular charge transfer in excited state of fast violet B (FVB) in methanol solution Shuo Chai a,b,⇑, Shu-Lin Cong a a b
School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The twisted intramolecular charge
transfer of FVB is investigated by TDDFT method. The hydrogen bonding strengthening in the excited state is demonstrated. The mechanism of TICT is clarified.
a r t i c l e
i n f o
Article history: Received 22 November 2013 Received in revised form 13 January 2014 Accepted 19 January 2014 Available online 31 January 2014 Keywords: Twisted intramolecular charge transfer Intermolecular hydrogen bond Excited state dynamics Time-dependent density functional theory
a b s t r a c t The excited state hydrogen bonding dynamics and corresponding photophysical processes of fast violet B (FVB) in hydrogen-donating methanol (MeOH) solution are investigated by using time-dependent density functional theory (TDDFT) method. In the FVB molecule, there are AC@O, ANAH groups which could act as hydrogen acceptor and donor. It is demonstrated that both the intramolecular hydrogen bond O HAN in FVB and intermolecular hydrogen bond C@O HAO between FVB and MeOH are formed in the ground state S0 and strengthened in the excited state S1. The absorption spectra are obviously red shifted for the hydrogen-bonded complex in comparison with FVB monomer in the low energy range. The theoretical investigation demonstrates that the twisted intramolecular charge transfer takes place in the excited states for both isolated FVB and hydrogen-bonded complex, and the dominant twisting is along N2AC3 bond. The potential energy curve is investigated to understand the photophysics process of FVB and hydrogen-bonded complex. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Hydrogen bond plays a significant role in modern photochemistry, biochemistry and life science [1–8]. The intramolecular ⇑ Corresponding author at: School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China. Tel.: +86 411 84706122; fax: +86 411 84709304. E-mail address: [email protected] (S. Chai). http://dx.doi.org/10.1016/j.saa.2014.01.092 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
hydrogen bonds integrate the compound in a stable conformation and the intermolecular hydrogen bonds connect the hydrogen donor and acceptor in a special location. Both the intra- and intermolecular hydrogen bonds can affect a series of photophysical processes of chemical and biological compounds, such as internal conversion (IC), photoinduced proton transfer (PPT), photoinduced electron transfer (PET) and twisted intramolecular charge transfer (TICT) [9–12]. In the past few decades, numerous theoretical and experimental efforts have been devoted to the understanding and
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application of hydrogen bonding dynamics and corresponding photophysical processes [13–19]. The ground state hydrogen bonds in organic chromophores and supramolecular systems have been explored extensively by ultrafast spectroscopy measurement, molecular mechanics study and quantum chemistry calculation [20–23]. However, few studies on excited state hydrogen bonds have been reported, because of the complexity of the excited state dynamics. The electronically excited state intramolecular and intermolecular hydrogen bonds are of fundamental importance in many photoelectronic applications, such as fluorescence probe, optical sensor and light emitting devices [24–28]. When the chromophore with hydrogen donor/acceptor group is dissolved into a protic solvent, the intermolecular hydrogen bond is formed between solute and solvent, which is one of the site-specific interactions in solution. Since 2007, Zhao and Han et al. have investigated the excited state hydrogen bonds in solution by using the time-dependent density functional theory (TDDFT) calculations as well as ultrafast spectroscopy experiments to demonstrate the strengthening or weakening of hydrogen bond in excited states and show the importance of excited state hydrogen bonding dynamics in photophysics and photochemistry [29–39]. Numerous chromophores have been reported in terms of excited state hydrogen bond, Coumarin 102, Aminofluorenones, Perylene materials, etc. [30,35,38]. The hydrogen bond strengthening/weakening happens in the electronically excited states, which leads to the electronic spectra red/blue shift and affects the photophysical processes significantly. In our previous work, we have demonstrated the facilitating effect of excited state hydrogen bond strengthening on proton transfer and photoexcitation dynamics [40,41]. Twisted intramolecular charge transfer is one of conformation relaxations in chemistry and biology systems and the dual fluorescence of organic chromophores is usually attributed to the twisted intramolecular charge transfer experimentally [42–46]. The dual fluorescence of dimethylaminofluorenone (DMAF) and dimethylaminobenzonitrile (DMABN) attracted the interest of researchers and the twisted intramolecular charge transfer was employed to explain this significant photochemical phenomenon in the past investigations [47–49]. The twisted intramolecular charge transfer processes of aminofluorenones are explored by using TDDFT calculations and the potential energy curves are scanned along the twisted dihedral angle by Zhao et al. [30]. Fast violet B (FVB) is one of organic dye molecules. Because of the coexistence of hydrogen acceptor and donor, FVB is very sensitive to both the intramolecular and intermolecular interactions, such as hydrogen bond, solute–solvent interaction, steric interaction, and so forth. Therefore FVB is an excellent model compound to understand the intramolecular and intermolecular interactions both in the ground and excited states. Prabhu et al. investigated the steady-state and timeresolved spectra of fast blue RR and fast violet B to detect the effects of solvents and cyclodextrin complexation [50]. However, the conformation relaxation of FVB happening in electronically excited states and corresponding photophysical properties remain unclear. In the present work, the density functional theory (DFT) and TDDFT methods are employed to investigate the excited state dynamics and corresponding photophysical processes of FVB in methanol solution. The TDDFT method has been demonstrated to be a reliable tool to investigate the excited state dynamics [51–54]. The geometries of isolated FVB and hydrogen-bonded FVB-MeOH complex both in the ground and excited states are optimized with B3LYP functional. The electronic excitation energy is calculated using TDDFT method and the absorption spectra are analyzed for the investigated systems. The frontier molecular orbitals are described to understand the charge transfer in the excited states. The polar hydrogen-donating solvent effect on the photophysical properties is discussed. It is demonstrated that both the
intramolecular and intermolecular hydrogen bonds are strengthened and the twisted intramolecular charge transfer takes place in the electronically excited state S1. The conformation twisting mechanism is clarified in this work.
Theoretical method The DFT and TDDFT methods were employed to study the structural, electronic properties and the related photophysical processes of isolated FVB and hydrogen-bonded complex FVB-MeOH. The optimized geometries of the ground state were obtained by using DFT method. Both the vertical excited energies calculation and geometric optimization in the excited state were performed by using TDDFT method. Becke’s three parameter hybrid exchange functional with Lee–Yang–Parr gradient-corrected correlation (B3LYP functional) and 6-311++G (d, p) basis set were employed in all the DFT and TDDFT calculations. The local minima were confirmed by the absence of an imaginary mode in vibrational analysis calculations. In this work we mainly paid attention to the intermolecular hydrogen bonding interactions between organic chromophore and protic solvent methanol, thus the outer solvent environment was not considered. In the present work, all the electronic structure calculations were performed using Gaussian 09 program suite [55].
Discussion and results Fig. 1 shows the optimized geometries of isolated FVB and hydrogen-bonded FVB-Methanol complex in the ground state with DFT method, where some relevant atoms are labeled. For convenience, in our discussion the molecule of FVB is divided into two parts, moiety-A and moiety-B, they are connected with AN2AC3A, as shown in Fig. 1. In our calculations only the strongest intermolecular hydrogen bond in the inner solvation shell is considered. The outer solvent environment will lead to only a small shift of the spectra and have a little influence on the photophysical properties. The intermolecular hydrogen bond C3@O2 H3AO3 (with a binding energy of 29.54 kJ/mol) between benzamido group and methanol is discussed, which is much stronger than the N2AH2 O3 (with a binding energy of 17.32 kJ/mol) between amino group and methanol. Due to the steric effect the molecule cannot keep planar conformation. In the moiety-A, because of the interaction of intramolecular hydrogen bond O1 H1AN1, the stable five-membered ring is formed. In the polar protic solution the hydrogen acceptor C3@O2 is active, which is responsible for the formation of intermolecular hydrogen bond. The FVB and methanol molecules are connected by intermolecular hydrogen bond O3AH3 O2, and the bond length is 1.88 Å. The existence of the intermolecular hydrogen bond leads to the significant twisting of the molecular structure and the molecular conformation is largely influenced by the intermolecular hydrogen bond. The dihedral angle AC1AC2AN2AH2 of the monomer and hydrogen-bonded complex are 1.27° and 24.66°, respectively, and the obvious difference denotes the big twisting along C2AN2 bond. With the introduction of hydrogen-donating methanol molecule, the dihedral angle AH2AN2AC3AO2 is changed from 172.88° to 170.38° and AO2AC3AC4AC5 is changed from 26.46° to 26.38 °, which mean the twisting along N2AC3 and C3AC4 are small. Because of the existence of intermolecular hydrogen bond the bond length LC3AO2 is slightly increased from 1.22 to 1.23 Å and also the intramolecular hydrogen bond length is decreased by 0.02 Å from 2.27 to 2.25 Å. From the geometries of our investigated systems in the ground state one can note the remarkable conformation change after the hydrogen bond formation.
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Fig. 1. The optimized geometries of FVB and FVB-MeOH in the ground states, the atom numbers are labeled.
Table 1 Calculated electronic excitation energies (EE, in eV) and corresponding oscillator strengths (OS) of FVB and FVB-MeOH at the TDDFT/B3LYP/6-311++G (d, p) level. FVB
S1 S2 S3 S4 S5 S6
FVB-MeOH
EE
OS
EE
OS
3.314 3.904 4.094 4.235 4.330 4.495
0.218 0.011 0.002 0.012 0.010 0.083
3.198 3.857 4.138 4.153 4.335 4.510
0.192 0.002 0.015 0.003 0.054 0.100
The calculated electronic excitation energies and corresponding oscillator strengths of low-lying excited states for FVB and FVB-MeOH are based on the optimized geometries in the ground state, as listed in Table 1. All the calculations are performed by using TDDFT method. The calculated excitation with maximum oscillator strength corresponds to the S1 state for both FVB and FVB-MeOH, indicating that the two molecules are initially photoexcited to the S1 state. The excitation energies of S1 state are 3.314 and 3.198 eV for FVB and FVB-MeOH, respectively. We can see that the excitation energy decreases significantly for the hydrogenbonded FVB-MeOH complex because of the solute–solvent intermolecular hydrogen bonding interaction. According to the demonstration of Zhao about the relation of excitation energy and the strength of hydrogen bond [5,33,38], we predict that the hydrogen bond is strengthened in the electronically excited states.
Fig. 3. Simulated absorption spectra of FVB and FVB-MeOH.
From the excitation energy calculation we know that the S1 state corresponds to the molecular orbital transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) for both FVB and FVB-MeOH. The frontier molecular orbitals are analyzed and the HOMOs and LUMOs of isolated FVB and hydrogen-bonded complex are shown in Fig. 2. The charge densities are localized at the FVB molecule for
Fig. 2. Frontier molecular orbitals of isolated FVB and FVB-MeOH.
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Fig. 4. The optimized geometries of FVB and FVB-MeOH in the electronically excited state S1.
Table 2 Calculated geometric parameters (bond lengths in Å and dihedral angles in °) and intramolecular and intermolecular hydrogen bond lengths of FVB and FVB-MeOH in the ground and excited states. LC2AN2
LN2AH2
LN2AC3
LC3@O2
LC3AC4
LO3AH3
LN1AH1 O1
LO3AH3 O2
FVB
S0 S1
1.42 1.33
1.01 1.01
1.37 1.52
1.22 1.25
1.51 1.41
– –
2.27 2.25
– –
FVB-MeOH
S0 S1
1.42 1.34
1.01 1.01
1.37 1.50
1.23 1.26
1.50 1.41
0.97 0.98
2.25 2.24
1.88 1.78
both monomer and hydrogen-bonded complex, but at the different moieties of FVB for HOMO and LUMO. The existing or not of intermolecular hydrogen bond does not influence the orbital distribution. The obvious intramolecular charge transfer (ICT) is observed for both FVB and FVB-MeOH. The charges in the HOMOs are mainly localized in moiety-A, while most charges are transferred to moiety-B in the LUMOs. We can conclude that the orbital transition from the HOMO to the LUMO involves the intramolecular charge redistribution. We also perform the Mulliken population analysis of isolated FVB and FVB-MeOH complex to investigate the charge distribution in the ground and excited states. The charge have a redistribution between the ground and excited states and there is more electron density distribution located around the C@O group of FVB-MeOH complex after the photoexcitation. The absorption spectra of FVB and FVB-MeOH are calculated, shown in Fig. 3. The absorption of these two systems are mostly distributed in the range of 250–450 nm. The spectra of hydrogenbonded complex FVB-MeOH are red shifted in comparison with FVB monomer in the low energy range, the absorption peaks are 374 and 388 nm, respectively. In the high energy range the calculated absorption maximum are 276 and 274 nm based on the TDDFT method, and the spectrum shift is not obvious. We noticed the experimental absorption spectra measured with spectrophotometer by Prabhu et al. are 306 nm in nonpolar cyclohexane solution and 294 nm in protic methanol solution [50]. Thus our theoretical results coincide with the experimental data and the theoretical calculations are reasonable. Since we neglect the contribution of outer solvation environment, a small shift of calculated spectra is found in comparison with experimental one, which is negligible to determine the photophysical properties. The red shift of the absorption spectra of FVB-MeOH in the low energy range is remarkable, resulting from the intermolecular hydrogen bond strengthening in the low excited state. The geometries of FVB and FVB-MeOH in the electronically excited state S1 are optimized using TDDFT method, as shown in Fig. 4. The apparent differences of the structures in the ground and excited states are observed for FVB and FVB-MeOH. The FVB
AC1AC2AN2AH2
AH2AN2AC3AO2
AO2AC3AC4AC5
1.27 6.54
172.88 83.69
26.46 1.00
24.66 10.65
170.38 82.60
26.38 0.89
molecule is largely twisted for both monomer and hydrogenbonded complex and it is deduced that the TICT process takes place in the excited states. The charge donor and acceptor of FVB are connected by ACANA, and the internal rotations along different bonds take place, leading to the apparent conformation differences in the excited states. Table 2 lists some geometric parameters of isolated FVB and hydrogen-bonded FVB-MeOH in the electronically ground and excited states. From Table 2 we recognize the twisting motions of FVB in the excited states. From the calculated dihedral angles one can note that the molecules of FVB and FVB-MeOH twist along AC2AN2A, AN2AC3A and AC3AC4A. For the FVB monomer the dihedral angle AO2AC3AC4AC5 changes from 26.46° in the ground state to 1.00° in the excited state, and AC1AC2AN2AH2 changes from 1.27° to 6.54°. Both the bond lengths of AC3AC4A and AC2AN2A decrease about 0.1 Å. The dihedral angle AH2AN2AC3AO2 changes from 172.88° in the ground state to 83.69° in the excited state. The molecular twisting in the excited state is mainly along AN2AC3A bond and the bond length increases from 1.37 to 1.52 Å. The geometry of FVB in hydrogen-bonded complex is quite similar to the isolated one both in the ground and excited states. The bond length of intermolecular hydrogen bond LO3AH3 O2 decreases 0.1 Å in the excited state. With the optimized excited state geometries we conclude that the intermolecular hydrogen bond is strengthened in the excited state. We also find that the double bond C3@O2 is lengthened by 0.3 Å in the excited states which assists in the hydrogen bond strengthening. The intramolecular hydrogen bond N1AH1 O1 is slightly shortened from 2.27 to 2.25 for FVB and from 2.25 to 2.24 Å for hydrogen-bonded complex. In short, in the electronically excited states S1 both the intramolecular and intermolecular hydrogen bonds are strengthened. Fig. 5 shows the potential energy curves of TICT process of FVB and FVB-MeOH drived by the photoexcitation, and the corresponding energies of respective states are labeled. The energy of optimized ground state geometry is taken as the zero point. It is noted that FVB and FVB-MeOH are photoexcited to the excited state S1, and the corresponding energies are 76.42 and 73.74 kcal/mol, respectively. In the excited state S1, the twisted
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with that in the ground state. Our theoretical absorption is accordant with the experimental results. Because of the hydrogen bond strengthening the absorption spectra of hydrogen-bonded complex is apparently red shifted in the low energy range. The twisted intramolecular charge transfer takes place and the FVB molecule rotates along three bonds C2AN2, N2AC3 and C3AC4 in the excited state S1. The dihedral angle AH2AN2AC3AO2 changes significantly from 172.88° to 73.69°, denoting that the dominant twisting is along N2AC3 bond. The twisted mechanism in the excited state has been clarified by theoretical calculations. Furthermore, the photophysical processes of isolated FVB and hydrogenbonded FVB-MeOH in the ground and excited states are described with the help of potential energy curves. Acknowledgements The quantum chemical calculations in this work were performed on the Linux cluster of group 1101, Dalian Institute of Chemical Physics. This work was supported by the National Natural Science Foundation of China (Grant No. 11304029) and the Fundamental Research Funds for Central Universities of China (Grant No. DUT12RC(3)55). References
Fig. 5. Potential energy (in kcal/mol) curves of TICT process of FVB and FVB-MeOH, the energy of optimized S0 structure is taken as zero-point energy.
intramolecular charge transfer and charge redistribution take place and get to the local minimized point of S1, the energies are 45.45 and 40.23 kcal/mol for FVB and FVB-MeOH, respectively. The dihedral angles of the geometries in the ground and excited states are labeled in the figure. During this charge transfer process, the structures of FVB are largely twisted and the intramolecular and intermolecular hydrogen bonds are strengthened.
Conclusion The intramolecular and intermolecular hydrogen bonds of FVB in methanol solution have been theoretically investigated by using DFT and TDDFT methods and the excited state dynamics has been discussed in details. The geometries of isolated FVB and hydrogenbonded FVB-MeOH are fully optimized in both ground and excited states. Due to the steric effect, the geometry of FVB cannot keep the coplanar conformation. There are an intramolecular hydrogen bond N1AH1 O1 between the amino and ether moieties in the conformation of FVB and an intermolecular hydrogen bond O3AH3 O2 between hydroxyl moiety of methanol and carbonyl moiety of FVB in the methanol solution. Moreover, from the frontier molecular orbital analysis of isolated and hydrogenbonded systems we depict the obvious charge transfer of FVB between HOMO and LUMO. It has been demonstrated that both the intramolecular and intermolecular hydrogen bonds are strengthened in the electronically excited state S1 in comparison
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Intermolecular hydrogen bond and twisted intramolecular charge transfer in excited state of fast violet B (FVB) in methanol solution Shuo Chai a,b,⇑, Shu-Lin Cong a a b
School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The twisted intramolecular charge
transfer of FVB is investigated by TDDFT method. The hydrogen bonding strengthening in the excited state is demonstrated. The mechanism of TICT is clarified.
a r t i c l e
i n f o
Article history: Received 22 November 2013 Received in revised form 13 January 2014 Accepted 19 January 2014 Available online 31 January 2014 Keywords: Twisted intramolecular charge transfer Intermolecular hydrogen bond Excited state dynamics Time-dependent density functional theory
a b s t r a c t The excited state hydrogen bonding dynamics and corresponding photophysical processes of fast violet B (FVB) in hydrogen-donating methanol (MeOH) solution are investigated by using time-dependent density functional theory (TDDFT) method. In the FVB molecule, there are AC@O, ANAH groups which could act as hydrogen acceptor and donor. It is demonstrated that both the intramolecular hydrogen bond O HAN in FVB and intermolecular hydrogen bond C@O HAO between FVB and MeOH are formed in the ground state S0 and strengthened in the excited state S1. The absorption spectra are obviously red shifted for the hydrogen-bonded complex in comparison with FVB monomer in the low energy range. The theoretical investigation demonstrates that the twisted intramolecular charge transfer takes place in the excited states for both isolated FVB and hydrogen-bonded complex, and the dominant twisting is along N2AC3 bond. The potential energy curve is investigated to understand the photophysics process of FVB and hydrogen-bonded complex. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Hydrogen bond plays a significant role in modern photochemistry, biochemistry and life science [1–8]. The intramolecular ⇑ Corresponding author at: School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China. Tel.: +86 411 84706122; fax: +86 411 84709304. E-mail address: [email protected] (S. Chai). http://dx.doi.org/10.1016/j.saa.2014.01.092 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
hydrogen bonds integrate the compound in a stable conformation and the intermolecular hydrogen bonds connect the hydrogen donor and acceptor in a special location. Both the intra- and intermolecular hydrogen bonds can affect a series of photophysical processes of chemical and biological compounds, such as internal conversion (IC), photoinduced proton transfer (PPT), photoinduced electron transfer (PET) and twisted intramolecular charge transfer (TICT) [9–12]. In the past few decades, numerous theoretical and experimental efforts have been devoted to the understanding and
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application of hydrogen bonding dynamics and corresponding photophysical processes [13–19]. The ground state hydrogen bonds in organic chromophores and supramolecular systems have been explored extensively by ultrafast spectroscopy measurement, molecular mechanics study and quantum chemistry calculation [20–23]. However, few studies on excited state hydrogen bonds have been reported, because of the complexity of the excited state dynamics. The electronically excited state intramolecular and intermolecular hydrogen bonds are of fundamental importance in many photoelectronic applications, such as fluorescence probe, optical sensor and light emitting devices [24–28]. When the chromophore with hydrogen donor/acceptor group is dissolved into a protic solvent, the intermolecular hydrogen bond is formed between solute and solvent, which is one of the site-specific interactions in solution. Since 2007, Zhao and Han et al. have investigated the excited state hydrogen bonds in solution by using the time-dependent density functional theory (TDDFT) calculations as well as ultrafast spectroscopy experiments to demonstrate the strengthening or weakening of hydrogen bond in excited states and show the importance of excited state hydrogen bonding dynamics in photophysics and photochemistry [29–39]. Numerous chromophores have been reported in terms of excited state hydrogen bond, Coumarin 102, Aminofluorenones, Perylene materials, etc. [30,35,38]. The hydrogen bond strengthening/weakening happens in the electronically excited states, which leads to the electronic spectra red/blue shift and affects the photophysical processes significantly. In our previous work, we have demonstrated the facilitating effect of excited state hydrogen bond strengthening on proton transfer and photoexcitation dynamics [40,41]. Twisted intramolecular charge transfer is one of conformation relaxations in chemistry and biology systems and the dual fluorescence of organic chromophores is usually attributed to the twisted intramolecular charge transfer experimentally [42–46]. The dual fluorescence of dimethylaminofluorenone (DMAF) and dimethylaminobenzonitrile (DMABN) attracted the interest of researchers and the twisted intramolecular charge transfer was employed to explain this significant photochemical phenomenon in the past investigations [47–49]. The twisted intramolecular charge transfer processes of aminofluorenones are explored by using TDDFT calculations and the potential energy curves are scanned along the twisted dihedral angle by Zhao et al. [30]. Fast violet B (FVB) is one of organic dye molecules. Because of the coexistence of hydrogen acceptor and donor, FVB is very sensitive to both the intramolecular and intermolecular interactions, such as hydrogen bond, solute–solvent interaction, steric interaction, and so forth. Therefore FVB is an excellent model compound to understand the intramolecular and intermolecular interactions both in the ground and excited states. Prabhu et al. investigated the steady-state and timeresolved spectra of fast blue RR and fast violet B to detect the effects of solvents and cyclodextrin complexation [50]. However, the conformation relaxation of FVB happening in electronically excited states and corresponding photophysical properties remain unclear. In the present work, the density functional theory (DFT) and TDDFT methods are employed to investigate the excited state dynamics and corresponding photophysical processes of FVB in methanol solution. The TDDFT method has been demonstrated to be a reliable tool to investigate the excited state dynamics [51–54]. The geometries of isolated FVB and hydrogen-bonded FVB-MeOH complex both in the ground and excited states are optimized with B3LYP functional. The electronic excitation energy is calculated using TDDFT method and the absorption spectra are analyzed for the investigated systems. The frontier molecular orbitals are described to understand the charge transfer in the excited states. The polar hydrogen-donating solvent effect on the photophysical properties is discussed. It is demonstrated that both the
intramolecular and intermolecular hydrogen bonds are strengthened and the twisted intramolecular charge transfer takes place in the electronically excited state S1. The conformation twisting mechanism is clarified in this work.
Theoretical method The DFT and TDDFT methods were employed to study the structural, electronic properties and the related photophysical processes of isolated FVB and hydrogen-bonded complex FVB-MeOH. The optimized geometries of the ground state were obtained by using DFT method. Both the vertical excited energies calculation and geometric optimization in the excited state were performed by using TDDFT method. Becke’s three parameter hybrid exchange functional with Lee–Yang–Parr gradient-corrected correlation (B3LYP functional) and 6-311++G (d, p) basis set were employed in all the DFT and TDDFT calculations. The local minima were confirmed by the absence of an imaginary mode in vibrational analysis calculations. In this work we mainly paid attention to the intermolecular hydrogen bonding interactions between organic chromophore and protic solvent methanol, thus the outer solvent environment was not considered. In the present work, all the electronic structure calculations were performed using Gaussian 09 program suite [55].
Discussion and results Fig. 1 shows the optimized geometries of isolated FVB and hydrogen-bonded FVB-Methanol complex in the ground state with DFT method, where some relevant atoms are labeled. For convenience, in our discussion the molecule of FVB is divided into two parts, moiety-A and moiety-B, they are connected with AN2AC3A, as shown in Fig. 1. In our calculations only the strongest intermolecular hydrogen bond in the inner solvation shell is considered. The outer solvent environment will lead to only a small shift of the spectra and have a little influence on the photophysical properties. The intermolecular hydrogen bond C3@O2 H3AO3 (with a binding energy of 29.54 kJ/mol) between benzamido group and methanol is discussed, which is much stronger than the N2AH2 O3 (with a binding energy of 17.32 kJ/mol) between amino group and methanol. Due to the steric effect the molecule cannot keep planar conformation. In the moiety-A, because of the interaction of intramolecular hydrogen bond O1 H1AN1, the stable five-membered ring is formed. In the polar protic solution the hydrogen acceptor C3@O2 is active, which is responsible for the formation of intermolecular hydrogen bond. The FVB and methanol molecules are connected by intermolecular hydrogen bond O3AH3 O2, and the bond length is 1.88 Å. The existence of the intermolecular hydrogen bond leads to the significant twisting of the molecular structure and the molecular conformation is largely influenced by the intermolecular hydrogen bond. The dihedral angle AC1AC2AN2AH2 of the monomer and hydrogen-bonded complex are 1.27° and 24.66°, respectively, and the obvious difference denotes the big twisting along C2AN2 bond. With the introduction of hydrogen-donating methanol molecule, the dihedral angle AH2AN2AC3AO2 is changed from 172.88° to 170.38° and AO2AC3AC4AC5 is changed from 26.46° to 26.38 °, which mean the twisting along N2AC3 and C3AC4 are small. Because of the existence of intermolecular hydrogen bond the bond length LC3AO2 is slightly increased from 1.22 to 1.23 Å and also the intramolecular hydrogen bond length is decreased by 0.02 Å from 2.27 to 2.25 Å. From the geometries of our investigated systems in the ground state one can note the remarkable conformation change after the hydrogen bond formation.
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Fig. 1. The optimized geometries of FVB and FVB-MeOH in the ground states, the atom numbers are labeled.
Table 1 Calculated electronic excitation energies (EE, in eV) and corresponding oscillator strengths (OS) of FVB and FVB-MeOH at the TDDFT/B3LYP/6-311++G (d, p) level. FVB
S1 S2 S3 S4 S5 S6
FVB-MeOH
EE
OS
EE
OS
3.314 3.904 4.094 4.235 4.330 4.495
0.218 0.011 0.002 0.012 0.010 0.083
3.198 3.857 4.138 4.153 4.335 4.510
0.192 0.002 0.015 0.003 0.054 0.100
The calculated electronic excitation energies and corresponding oscillator strengths of low-lying excited states for FVB and FVB-MeOH are based on the optimized geometries in the ground state, as listed in Table 1. All the calculations are performed by using TDDFT method. The calculated excitation with maximum oscillator strength corresponds to the S1 state for both FVB and FVB-MeOH, indicating that the two molecules are initially photoexcited to the S1 state. The excitation energies of S1 state are 3.314 and 3.198 eV for FVB and FVB-MeOH, respectively. We can see that the excitation energy decreases significantly for the hydrogenbonded FVB-MeOH complex because of the solute–solvent intermolecular hydrogen bonding interaction. According to the demonstration of Zhao about the relation of excitation energy and the strength of hydrogen bond [5,33,38], we predict that the hydrogen bond is strengthened in the electronically excited states.
Fig. 3. Simulated absorption spectra of FVB and FVB-MeOH.
From the excitation energy calculation we know that the S1 state corresponds to the molecular orbital transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) for both FVB and FVB-MeOH. The frontier molecular orbitals are analyzed and the HOMOs and LUMOs of isolated FVB and hydrogen-bonded complex are shown in Fig. 2. The charge densities are localized at the FVB molecule for
Fig. 2. Frontier molecular orbitals of isolated FVB and FVB-MeOH.
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Fig. 4. The optimized geometries of FVB and FVB-MeOH in the electronically excited state S1.
Table 2 Calculated geometric parameters (bond lengths in Å and dihedral angles in °) and intramolecular and intermolecular hydrogen bond lengths of FVB and FVB-MeOH in the ground and excited states. LC2AN2
LN2AH2
LN2AC3
LC3@O2
LC3AC4
LO3AH3
LN1AH1 O1
LO3AH3 O2
FVB
S0 S1
1.42 1.33
1.01 1.01
1.37 1.52
1.22 1.25
1.51 1.41
– –
2.27 2.25
– –
FVB-MeOH
S0 S1
1.42 1.34
1.01 1.01
1.37 1.50
1.23 1.26
1.50 1.41
0.97 0.98
2.25 2.24
1.88 1.78
both monomer and hydrogen-bonded complex, but at the different moieties of FVB for HOMO and LUMO. The existing or not of intermolecular hydrogen bond does not influence the orbital distribution. The obvious intramolecular charge transfer (ICT) is observed for both FVB and FVB-MeOH. The charges in the HOMOs are mainly localized in moiety-A, while most charges are transferred to moiety-B in the LUMOs. We can conclude that the orbital transition from the HOMO to the LUMO involves the intramolecular charge redistribution. We also perform the Mulliken population analysis of isolated FVB and FVB-MeOH complex to investigate the charge distribution in the ground and excited states. The charge have a redistribution between the ground and excited states and there is more electron density distribution located around the C@O group of FVB-MeOH complex after the photoexcitation. The absorption spectra of FVB and FVB-MeOH are calculated, shown in Fig. 3. The absorption of these two systems are mostly distributed in the range of 250–450 nm. The spectra of hydrogenbonded complex FVB-MeOH are red shifted in comparison with FVB monomer in the low energy range, the absorption peaks are 374 and 388 nm, respectively. In the high energy range the calculated absorption maximum are 276 and 274 nm based on the TDDFT method, and the spectrum shift is not obvious. We noticed the experimental absorption spectra measured with spectrophotometer by Prabhu et al. are 306 nm in nonpolar cyclohexane solution and 294 nm in protic methanol solution [50]. Thus our theoretical results coincide with the experimental data and the theoretical calculations are reasonable. Since we neglect the contribution of outer solvation environment, a small shift of calculated spectra is found in comparison with experimental one, which is negligible to determine the photophysical properties. The red shift of the absorption spectra of FVB-MeOH in the low energy range is remarkable, resulting from the intermolecular hydrogen bond strengthening in the low excited state. The geometries of FVB and FVB-MeOH in the electronically excited state S1 are optimized using TDDFT method, as shown in Fig. 4. The apparent differences of the structures in the ground and excited states are observed for FVB and FVB-MeOH. The FVB
AC1AC2AN2AH2
AH2AN2AC3AO2
AO2AC3AC4AC5
1.27 6.54
172.88 83.69
26.46 1.00
24.66 10.65
170.38 82.60
26.38 0.89
molecule is largely twisted for both monomer and hydrogenbonded complex and it is deduced that the TICT process takes place in the excited states. The charge donor and acceptor of FVB are connected by ACANA, and the internal rotations along different bonds take place, leading to the apparent conformation differences in the excited states. Table 2 lists some geometric parameters of isolated FVB and hydrogen-bonded FVB-MeOH in the electronically ground and excited states. From Table 2 we recognize the twisting motions of FVB in the excited states. From the calculated dihedral angles one can note that the molecules of FVB and FVB-MeOH twist along AC2AN2A, AN2AC3A and AC3AC4A. For the FVB monomer the dihedral angle AO2AC3AC4AC5 changes from 26.46° in the ground state to 1.00° in the excited state, and AC1AC2AN2AH2 changes from 1.27° to 6.54°. Both the bond lengths of AC3AC4A and AC2AN2A decrease about 0.1 Å. The dihedral angle AH2AN2AC3AO2 changes from 172.88° in the ground state to 83.69° in the excited state. The molecular twisting in the excited state is mainly along AN2AC3A bond and the bond length increases from 1.37 to 1.52 Å. The geometry of FVB in hydrogen-bonded complex is quite similar to the isolated one both in the ground and excited states. The bond length of intermolecular hydrogen bond LO3AH3 O2 decreases 0.1 Å in the excited state. With the optimized excited state geometries we conclude that the intermolecular hydrogen bond is strengthened in the excited state. We also find that the double bond C3@O2 is lengthened by 0.3 Å in the excited states which assists in the hydrogen bond strengthening. The intramolecular hydrogen bond N1AH1 O1 is slightly shortened from 2.27 to 2.25 for FVB and from 2.25 to 2.24 Å for hydrogen-bonded complex. In short, in the electronically excited states S1 both the intramolecular and intermolecular hydrogen bonds are strengthened. Fig. 5 shows the potential energy curves of TICT process of FVB and FVB-MeOH drived by the photoexcitation, and the corresponding energies of respective states are labeled. The energy of optimized ground state geometry is taken as the zero point. It is noted that FVB and FVB-MeOH are photoexcited to the excited state S1, and the corresponding energies are 76.42 and 73.74 kcal/mol, respectively. In the excited state S1, the twisted
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with that in the ground state. Our theoretical absorption is accordant with the experimental results. Because of the hydrogen bond strengthening the absorption spectra of hydrogen-bonded complex is apparently red shifted in the low energy range. The twisted intramolecular charge transfer takes place and the FVB molecule rotates along three bonds C2AN2, N2AC3 and C3AC4 in the excited state S1. The dihedral angle AH2AN2AC3AO2 changes significantly from 172.88° to 73.69°, denoting that the dominant twisting is along N2AC3 bond. The twisted mechanism in the excited state has been clarified by theoretical calculations. Furthermore, the photophysical processes of isolated FVB and hydrogenbonded FVB-MeOH in the ground and excited states are described with the help of potential energy curves. Acknowledgements The quantum chemical calculations in this work were performed on the Linux cluster of group 1101, Dalian Institute of Chemical Physics. This work was supported by the National Natural Science Foundation of China (Grant No. 11304029) and the Fundamental Research Funds for Central Universities of China (Grant No. DUT12RC(3)55). References
Fig. 5. Potential energy (in kcal/mol) curves of TICT process of FVB and FVB-MeOH, the energy of optimized S0 structure is taken as zero-point energy.
intramolecular charge transfer and charge redistribution take place and get to the local minimized point of S1, the energies are 45.45 and 40.23 kcal/mol for FVB and FVB-MeOH, respectively. The dihedral angles of the geometries in the ground and excited states are labeled in the figure. During this charge transfer process, the structures of FVB are largely twisted and the intramolecular and intermolecular hydrogen bonds are strengthened.
Conclusion The intramolecular and intermolecular hydrogen bonds of FVB in methanol solution have been theoretically investigated by using DFT and TDDFT methods and the excited state dynamics has been discussed in details. The geometries of isolated FVB and hydrogenbonded FVB-MeOH are fully optimized in both ground and excited states. Due to the steric effect, the geometry of FVB cannot keep the coplanar conformation. There are an intramolecular hydrogen bond N1AH1 O1 between the amino and ether moieties in the conformation of FVB and an intermolecular hydrogen bond O3AH3 O2 between hydroxyl moiety of methanol and carbonyl moiety of FVB in the methanol solution. Moreover, from the frontier molecular orbital analysis of isolated and hydrogenbonded systems we depict the obvious charge transfer of FVB between HOMO and LUMO. It has been demonstrated that both the intramolecular and intermolecular hydrogen bonds are strengthened in the electronically excited state S1 in comparison
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