Intramolecular hydrogen bonding and photoinduced intramolecular proton and electron transfer in 2-(2′-hydroxyphenyl)benzothiazole

Journal of Molecular Structure: THEOCHEM 806 (2007) 105–112 www.elsevier.com/locate/theochem

Intramolecular hydrogen bonding and photoinduced intramolecular proton and electron transfer in 2-(2 0 -hydroxy phenyl)benzothiazole Dongjie Sun a, Jinghai Fang b, Guanghua Yu b, Fengcai Ma b

a,*

a Department of Physics, Liaoning University, Shenyang 110036, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100080, PR China

Received 23 October 2006; accepted 12 November 2006 Available online 21 November 2006

Abstract Intramolecular hydrogen bonding (IHB) and photoinduced intramolecular proton and electron transfer in 2-(2 0 -hydroxyphenyl)benzothiazole (HBT) were investigated theoretically. The IHB causes planarization of the molecule, which results in the change of the energy level of LUMO, and the change of density of state (DOS) of HOMO. The IHB and intramolecular proton transfer (IPT) was studied by IR spectra, which shows the influence of IHB to the IR spectra and the dynamics of IPT. The excited state properties of them are investigated with 2D and 3D real space representations, which revealed that the intramolecular charge transfer (ICT) also occurs when IPT happens. From the 3D transition density, the orientation of transition dipole moment for enol reverses, compared to that of normal. The orientation of ICT between enol and keto is opposite, which results from the opposite orientation of transition dipole moment between them.  2006 Elsevier B.V. All rights reserved. Keywords: 2-(2 0 -Hydroxyphenyl)benzothiazole; Intramolecular charge transfer; Density functional theory

1. Introduction Hydrogen bonding, the formation of a weak bond between a hydrogen donor, e.g., an O–H group, and a hydrogen acceptor group, represents a fundamental local interaction in nature, determining microscopic molecular structure and conformation on the static electronic spectroscopy of aromatic molecules [1–5]. Hydrogen bonding can also have a profound effect on the dynamic of aromatic molecules. In dramatic cases, hydrogen bonds can be a pathway for excited-state intra- and intermolecular proton-transfer reactions, which are of fundamental importance for acid-base chemistry [6–14]. In the case of the adiabatic, barrierless excited state intramolecular proton transfer processes a proton (hydroxyl or amino) which is *

Corresponding author. E-mail address: [email protected] (F. Ma).

0166-1280/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.11.015

attached covalently to an atom A in the ground state of the molecule, in the excited electronic state, migrates to a neighboring hydrogen-bonded atom B at a distance of ˚ , to produce a ‘‘phototautomer’’ [6–16]. In the dynam<2 A ics of the intramolecular and intermolecular hydrogen transfer, the electron also transfers simultaneously, since the proton and electron transfer are closely coupled in the photoinduced dynamics processes [9]. Intramolecular and intermolecular charge and energy transfer are also fundamental chemical processes in a variety of phenomena in physics, chemistry, materials and biology [17–21]. Intramolecular bonding, excited state intramolcular proton transfer (IPT) in 2-(2 0 -hydroxyphenyl)benzothiazole (HBT, see Fig. 1) have been extensively investigated experimentally [22–27] with a variety of spectroscopy (transient absorption, time-resolved fluorescence, time-resolved infrared (IR) and Raman spectroscopy) and theoretical methods [28,29], which was also reviewed [30]. Elsaesser

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Fig. 1. The molecular structure of HBT at the normal, enol and keto forms.

and Kaiser reported a pioneering picosecond IR experiment on HBT [22], where a first direct characterization of local changes of molecular geometries due to hydrogen transfer from the enol to keto state was demonstrated by inspection of IR absorbance changes in the O–H stretching band centered at 3000 cm1 and in the fingerprint region between 1400 and 2000 cm1. Femtosecond IR spectroscopy has also been reported on the intramolecular proton transfer enol–keto reaction of HBT [23]. Here, the C@O stretching marker mode at 1530 cm1, as well as other fingerprint vibrations, has been monitored after excitation with the UV pump tuned between 310 and 350 nm. Detailed steady-state resonance Raman of enol-HBT [24] and femtosecond UV/vis pump probe studies [25] show the substantial displacement of a subset of low-frequency Raman-active vibrations in HBT.

Potential energy (eV)

5

Absorption

ETS

S0

4

Absorption

S1 Fluorescence

3 2 1 keto

GTS

normal

0

enol

Fig. 2. The optimized ground state and the transition energy in absorption and in fluorescence for the normal, enol and keto forms. ETS and GTS stand for the transition states at the ground and first excited states.

Table 1 The calculated transition energies (TE) and oscillator strengths (in the parentheses) for the normal, enol and keto Normal

S1

Enol

Keto

TE

CIC

TE

CIC

TE

CIC

304(0.43)

0.13(H-2 fi L)

334(0.34)

0.13(H-1 fi L) 0.64(H fi L)

446(0.29)

0.61(H fi L)

0.13(H-1 fi L) 0.63(H fi L) CI expansion coefficients (CIC) for the orbital transition on the excitation are also shown.

a

b

0.4

HO18 16 2 1 7N 15 81117 3 6 9 12 14 4 5 10S 13

1.7

Bond Length (A)

0.2

Mulliken charges

1.8

0.0

-0.2

H O 1 15 14 N 2 6 13 8 10 3 5 S 11 12 4

Normal (S0) Enol (S0)

-0.4

Keto (S1) -0.6 0

2

4

6

8

10

Label of Atoms

12

14

O18 16 2 1 7H N 8 17 15 11 6 3 9 12 14 4 5 10S 13

1.6

1.5

1.4

Normal (S0) Enol (S0)

1.3

Keto (S1) 16

0

2

4

6

8

10

12

14

16

18

Bond

Fig. 3. The Mulliken charge distribution (a) on the atoms (Atomic charges with hydrogens have been summed into heavy atoms), and the bond length (b) for the normal (at ground state), enol (at ground state) and keto (at excited state) forms.

D. Sun et al. / Journal of Molecular Structure: THEOCHEM 806 (2007) 105–112

a

1 2 N O Total

1.8 1.6 1.4

DOS

1.2

H O

proton are typically accompanied by rearrangements of the electronic charge densities, and it is more appropriate to describe the observed features as being caused by the net transfer of a hydrogen atom [9,30]. In this paper, the influence of intramolecular hydrogen boding to molecular structure and the optical physical properties of HBT, and photoinduced dynamics of intramolecular proton and electron transfer on the excited state of HBT, are studied theoretically with quantum chemical method as well as the two dimensional (2D) site [3,17,35–38] and 3D cube [3,39–43] representations.

2

N

1 S

1.0 0.8

Absorption S0->S1

0.6 0.4

HOMOs LUMOs

0.2 0.0

107

2. Methods

-0.2 -7

-6

-5

-4

-3

-2

-1

0

Energy (eV)

b

1.8

H

1.6

O

2

1 2 N O Total

N

1

1.4

S

1.2

DOS

1.0 0.8

Absorption S0->S1

0.6 0.4

HOMOs LUMOs

0.2 0.0 -0.2 -7

-6

-5

-4

-3

-2

-1

0

Energy (eV)

DOS

c

2

1 2 N O Total

1

H

O

N S

2

1

Fluorescence S0<- S1 0

LUMOS HOMOs -7

-6

-5

-4

-3

-2

-1

0

Energy (eV)

Fig. 4. The single electronic spectra and DOS for the normal, enol and keto.

Many experimental and theoretical efforts have been made to study the intramolecular and intermolecular hydrogen bonding and excited state hydrogen transfer [1– 16,22–32], but few studies [8,9,33,34] have been done about investigation of the intramolecular proton and electron transfer simultaneously. In reality the motions of the

All the quantum chemical calculations were carried out with the GAUSSIAN 03 suite [44]. The geometry optimizations of HBT (see Fig. 1) for the ground states have been done with density functional theory (DFT) [45], B3LYP functional [46] and 6-31G(D) basis set. The geometry optimization for the lowest excited state of keto has been performed using CIS method [47] with 6-31G(D) basis set. Normal coordinate calculations were carried out at the minimum geometries in the ground and excited states, and the calculated frequencies were scaled by 0.965 [23]. With the optimized geometries, electronic transition energies were calculated using time-dependent density functional theory (TD-DFT) [48], B3LYP functional and 6-31G(D) basis set. At the excited-state optimal geometry, the transition frequency and oscillator strength corresponds to the vertical fluorescence process [43]. Absorption and fluorescence points were treated at the TD-B3LYP/6-31G(D)// DFT-B3LYP/6-31G(D) and TD-B3LYP/6-31G(D)//CIS/ 6-31G(D) levels, respectively, in conventional quantumchemical notation ‘‘single point//optimization level’’ [43]. The single electronic spectra and density of state (DOS) were calculated with GaussSum 1.0 [49]. The two dimensional (2D) site and 3D cube representations have been detailed described in Refs [3,17,35–43]. Briefly, 3D transition density shows the orientation and strength of the transition dipole moment, and the charge difference density reveals the orientation and results of intramolecular charge transfer (ICT) [3,39–43]. The 2D contour plot of transition density matrix reveals the electron–hole coherence and sizes of delocalization (along the diagonal) and exciton (along the off diagonal) [3,17,35– 38]. The single electronic spectra and density of state (DOS) reveal the energy levels of HOMOs and LUMOs, and the DOS at the HOMOs and LUMOs [50]. 3. Results and discussion The calculated transition energies and oscillator strengths of HBT at the normal, enol and keto forms were listed in Table 1, which also can be seen from Fig. 2. From Fig. 2, it can be found that the enol is of the potential energy minimum at ground state, which results from the intramolecular hydrogen bonding (IDB). From the optimized

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geometries at ground states, the IDB causes the dihedral angle between the two subunits decreases from 31.5 to 0.0, so the IDB causes planarization of the molecule [3]. The influence of the IDB to the electronic structure of enol can also be seen from Fig. 3(a). The IDB causes the increase of negatively charge on N. The influence of the

a

intramolcular proton transfer (IPT) to the electronic structure of Keto can also be seen from Fig. 3(a). The IPT results in the decrease of the negatively charge on N; while which results in the increase of negatively charge on O. So, when the IPT occurs, ICT also appears (the electron transfers from N to O). From Fig. 3(b), the IPT causes the bond

b

100

normal

400

enol

S0

80

S0

60

Intensity

Intensity

300

200

40

100

20

0

0 0

500

1000

1500

2000

2500

3000

0

3500

500

1000

2000

2500

3000

3500

2500

3000

3500

Frequency (cm )

Frequency (cm )

c

1500

-1

-1

d 800

240

keto S1

700

keto S0

600

160

Intensity

Intensity

500 400 300

80 200 100 0

0 0

1000

2000

0

3000

500

1000

-1

1500

2000 -1

Frequency (cm )

Freqency (cm )

f

e

600 H

400

S0

Normal Enol

500

S1

S H

S0 300

O

N

O

N

400

Intensity

Intensity

S

200

H

300

O

N S

200

100 100

0

0

0

500

1000

1500

2000

2500 -1

Frequency (cm )

3000

3500

0

500

1000

1500

2000

2500

3000

3500

-1

Frequency (cm )

Fig. 5. The calculated IR spectra, where the calculated frequencies were scaled by 0.965. The full width at half-maximum of the Gaussian curves is set 10 cm1.

D. Sun et al. / Journal of Molecular Structure: THEOCHEM 806 (2007) 105–112 9

keto S1 enol S0

7

ground state, the IR spectra of enol at the ground state, the IR spectra of keto at the excited state and ground state, respectively, which are the initial, reactant, interme-

a

normal

16 14

6

#2

Force Constant (au)

8

normal S0

109

5

keto S0

12

4 2600

2800

3000

3200

3400

3600

3800

Absorption

10 8 6

-1

#1

Frequency (cm )

4

Fig. 6. The force constant via frequency.

2

S1

b

2

4

#1

6

8

10

12

14

16

14

16

#2 Enol

16

#2

14

Absorption

12 10 8

#1

6 4 2 2

S1

c

4

#1

6

8

10

12

#2

keto

16

#2

14 12

1.0

Fluorescene

0.0030 0.0027

10

0.8

0.0024 0.0021

0.6

0.0018

8

0.0015

0.4

0.0012 9.000E-

6

0.2

6.000E-

#1

#8 increase, since it becomes the single bond from the double bond; and the C–O bond become the C@O bond, so the Bond #16 decreases. The bond #11 connected to the two subunits also become small, since it is double bond in Keto. It is interesting to compare the single electronic spectra and density of state (DOS) among the three forms. Comparing Figs. 4(a) and (b), one will find that the IDB does not change the energy level of HOMO; while the energy level of LUMO becomes lower, so the energy differences (eHL) between HOMO and HUMO is decreased, and the spectra in absorption is red shifted, which is consistent with the calculated transition energy in absorption (see Table 1 and Fig. 2). Comparing the DOS on LUMO and HOMO for the enol, the DOS on the #2 is larger than that on the #1 for HOMO; while the DOS on the #2 is smaller than that on the #1 for HOMO. So there is an ICT from #2 to #1 in optical absorption (we will later detailed discuss it with charge difference density). From Fig. 4(c), two conclusions can be obtained for keto: (1) the orientation of ICT is from #1 to #2 in fluorescence; which reverses the orientation of ICT for enol; (2) the energy of LUMO is higher and the energy level of HOMO is lower, so the transition energy will be red shifted. It should be noted that eHL at the ab initio level does not closely relate to excitation energy (i.e., band gap eExciton) due to the absence of exciton binding energy (eBinding) of the electron–hole pairs and the lattice relaxation [51]. The relationship between the gap of the HOMO–LUMO and the band-gap energy were detailed studied [52,53]. Vibrational spectroscopy can grasp local excitations of hydrogen-bonded groups and thus gives insight into local interactions and microscopic dynamics. So, the IR spectra in the process of IPT were investigated [see Figs. 5(a)–(f)], which elucidates molecular structure evolution during ultrafast chemical reactions. To confirm the accuracy of the calculation, the calculated IR spectrum was compared to the experimental data (see Fig. A.1 in the appendix). Figs. 5(a)–(d) are the IR spectrum of normal at the

3.000E-

0.0

4

0

2

S1

2

4

#1

6

8

10

12

14

16

#2

Fig. 7. The contour plots of transition density matrix for the normal, enol and keto. The color bar is at the right of the last figure. The #1 and #2 of the molecules can be seen in Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

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D. Sun et al. / Journal of Molecular Structure: THEOCHEM 806 (2007) 105–112

Transition density

Charge difference density

Absorption

Absorption

µ

e-

Fluorescence

µ Fig. 8. Charge difference density for the normal, enol and keto. The green and red colors stand for the hole and electron, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)

diate, and product states. Site-specific information, such as the N–H, O–H and O@C vibrational modes are assigned in Figs. 5(a)–(d), which [Figs. 5(a)–(d)] stand for the four statuses of dynamics of IR spectra. To study the influence of the IHB to the IR spectrum, Figs. 5(a) and (b) were compared [see Fig. 5(e)]. One can find the O–H vibrational mode for enol is red shifted, compared to that for the normal mode; and the IR intensity of the O–H vibrational mode for enol is significantly stronger than that for normal. The red-shift of the O–H vibrational mode for enol at the ground state reflects a decrease of the effective force constant of the hydrogen stretching oscillator (see Fig. 6), resulting from IHB. To directly study the influence of the IPT to the IR spectra in the dynamics, the IR spectra (Figs. 5(b)–(d)) are compared (see Fig. 5(f)). Compared to the O–H vibrational mode for enol, the N–H vibrational mode is blue shifted at the excited state, so the IPT makes the effective force constant of N–H at the excited state be larger than that of O–H for normal (see Fig. 6). Comparing the N–H vibrational mode at the ground sate and that at excited state, the N–H vibrational mode at the ground state is red shifted, which reflects a decrease of the effective force constant of N–H stretching mode (see Fig. 6). So, the changes of the frequencies of O–H and N–H modes, resulting from IHB and IPT, reflect the changes of the effective force constants in the processes of IHB and IPT. Since the motions of the proton are typically accompanied by rearrangements of the electronic charge densities [9,30], electron–hole coherence and ICT were studied with 2D contour plot of transition density matrix, 3D transition density and charge difference density, respectively. From Fig. 7(a), the electron–hole pairs are delocalized along the whole molecule, so it is a delocalized excited state. Form Figs. 7(b) and (c), one will find the electron–hole pairs are mainly localized on the #2, furthermore, there

is electron–hole coherence between #1 and #2, which means there is an ICT between #1 and #2 for them on the excitation. To reveal the orientation of the ICT, the charge difference density (CDD) is employed (see Fig. 8). From CDDs, one can find that the orientation of ICT for enol is from #2 to #1; while the orientation of ICT for keto is from #1 to #2. The orientation of ICT is controlled by the orientation of the transition dipole moment. The orientation of the transition dipole moment for enol is from #2 to #1 (the orientation of the transition dipole moment is from the electron to the hole), so the ICT is from #2 to #1 (see transition dipole moment in Fig. 8). The orientation of the transition dipole moment for keto is opposite to that for enol, so the orientation of ICT is also opposite (it is from #1 to #2). Comparing the orientation and strength of transition dipole moments between normal and keto, it can be found that the orientation is the same, but the strength of transition dipole moment for keto is much stronger than that of normal, so the ICT of keot is much stronger than that of normal. It should be noted that there are two sub-transition dipole moments with the same orientation for normal. So the ICT for normal is within subunits with the same orientation (see charge difference density for normal). Comparing the orientation of transition dipole moment of normal and enol, one can find the orientations between them are opposite, so the IHB causes the orientation of the transition dipole moment reverse. 4. Conclusion IHB and photoinduced intramolecular proton and electron transfer in HBT were investigated theoretically. The calculated results reveal that several important conclusions: (1) the IHB causes planarization of the molecule; (2) the IHB and IPT significantly change the energy levels of HOMOs and LUMOs, and the DOS at the HOMOs and

D. Sun et al. / Journal of Molecular Structure: THEOCHEM 806 (2007) 105–112

111

ε [l / mol cm]

500

250

0 1700

1600

1500

1400

1300

1200

1100

1000

1700

1600

1500

1400

1300

1200

1100

1000

1600

1500

1400

1300

1200

1100

1000

140 120

Intensity

100 80 60 40 20 0 1700

-1

Frequency (cm ) Fig. A.1. The experimental IR spectrum (top, from Ref. [23]) and the calculated IR spectrum (below), and the calculated frequencies were scaled by 0.965. The full width at half-maximum of the Gaussian curves is set 10 cm1.

LUMOs; (3) the IBH and the IPT makes the IR spectra of them significant different; and (4) the ICT also occurs when IPT happens, and the orientations of ICT between enol and keto are opposite. To reveal the orientation of ICT, the transition dipole moments of them were studied, which shows the orientation and strength of transition dipole moment. Acknowledgement This work was supported by NNSFC (10374040). Appendix A To confirm the accuracy of the calculation, the calculated IR spectrum is compared to the experimental data. The calculated spectrum is consistent with the experimental spectra. Some of vibrational modes of them were assigned. References [1] A.P. Monkman, L.O. Pa˚lsson, R.W.T. Higgins, C. Wang, M.R. Bryce, J.A.K. Howard, J. Am. Chem. Soc. 124 (2002) 6049. [2] E.T.J. Nibbering, T. Elsaesser, Chem. Rev. 104 (2004) 1887. [3] M.T. Sun, J. Chem. Phys. 124 (2006) 054903.

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