Photophysical properties of trans-3-(4-monomethylamino-phenyl)-acrylonitrile: Evidence of twisted intramolecular charge transfer (TICT) process

Chemical Physics 324 (2006) 733–741 www.elsevier.com/locate/chemphys

Photophysical properties of trans-3-(4-monomethylamino-phenyl)-acrylonitrile: Evidence of twisted intramolecular charge transfer (TICT) process Amrita Chakraborty, Samiran Kar 1, Nikhil Guchhait

*

Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India Received 27 August 2005; accepted 9 December 2005 Available online 21 February 2006

Abstract A donor acceptor substituted aromatic system trans-3-(4-monomethylamino-phenyl)-acrylonitrile (MMAPA) has been synthesized and its photophysical behavior has been investigated in the solvent of different polarity by steady state absorption and emission, time-resolved emission and quantum chemical calculations. The observed dual fluorescence of MMAPA in polar aprotic solvents has been assigned to emission from the locally excited and twisted intramolecular charge transfer states. The low-energy emission in protic solvent is attributed to the hydrogen-bonded complex. Potential energy surfaces for the ground and excited states along the donor (–NHMe group) and acceptor (acrylonitrile group) twist coordinates have been calculated by time-dependent density functional theory (TDDFT) and time-dependent density functional theory-polarized continuum model (TDDFT-PCM) in the gas phase and in acetonitrile solvent, respectively. Calculations predict that the stabilized excited state along the twist coordinate is responsible for the solvent dependent red shifted charge transfer emission. It is found that the twisting along the donor site is energetically favorable compared to that of the acceptor site. The canonical crossing of the excited states for the twisting of the donor group and localized nitrogen lone pair orbital of –NHMe group at the perpendicular configuration with respect to p-orbitals of benzene ring support TICT model for photo-induced charge transfer reaction in MMAPA molecule.  2006 Elsevier B.V. All rights reserved. Keywords: Trans-3-(4-monomethylamino-phenyl)-acrylonitrile; Dual fluorescence; TICT; TDDFT; TDDFT-PCM

1. Introduction The photophysics and photochemistry of donor acceptor substituted aromatic molecules have attracted considerable attention since the first observation of dual fluorescence from 4-(N,N-dimethylamino)benzonitrile (DMABN) by Lippert et al. [1]. In polar solvent, DMABN shows a short wavelength emission from the locally excited (LE) state (Lb state) along with a long wavelength emission from the charge transfer (CT) state (La state). The solvent *

Corresponding author. Tel.: +91 33 2350 8386; fax: +91 33 2351 9755. E-mail address: [email protected] (N. Guchhait). 1 Present address: CHEMGEN Pharma International, Dr. Siemens Street, Bolck GP, Section V, Salt Lake City, Kolkata 700 091, India. 0301-0104/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2005.12.032

dependency of the maximum of red shifted emission band is the characteristics of the excited state charge transfer process. This type of excited state behavior is explained as intramolecular charge transfer (ICT) accompanied by a structural reorganization. But the actual mechanism of CT reaction is still under controversial discussion. Many spectroscopic, thermodynamics and quantum chemical studies have been performed on photoinduced charge transfer reactions and various models are proposed to get an in depth understanding of the CT state [2–7]. The most accepted model namely twisted intramolecular charge transfer (TICT) was proposed by Grabowski et al. [8]. According to this model the intramolecular CT state of DMABN has a molecular geometry with the lone pair electron orbital of nitrogen of dimethylamino group is

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orthogonal to the p-orbitals of the benzene ring. The red shifted emissive CT species is stabilized in polar solvents relative to the LE state. Zachariasse et al. described this dual emission by considering solvent induced pseudo Jahn–Teller effect [9] which is, however, contradicted by many groups [2–4]. Sobolewski et al. proposed that the in-plane bending mode of nitrile group is responsible for the intramolecular CT emission in DMABN [10,11]. Till date it is found that TICT model is the widely accepted one to explain photoinduced intramolecular charge transfer reaction in many donor acceptor systems [12–16]. Most of the molecular systems so far investigated for photo-induced ICT reaction mainly have tertiary amino group as a charge donor with different acceptor moieties such as nitrile, acid, aldehyde and ester [2,3]. Even though primary and secondary amino groups have charge donor properties, the systems with these types of donor moiety for the ICT reaction are rare. So far our knowledge goes to very limited number of examples where secondary amino group only with sterically hindered ortho substituted methyl groups is used as charge donor for the ICT reaction [17,18]. In this paper, we present the photophysical properties of trans-3-(4-monomethylamino-phenyl)acrylonitrile (MMAPA) by absorption and emission spectroscopy. This could be the first example of ICT reaction where secondary amine without sterically hindered ortho-substituted group is used as charge donor. In addition, we also perform quantum chemical calculations for the ground and excited states to predict theoretically photoinduced ICT reaction in MMAPA using DFT level of theory. Excited state potential energy surfaces (PES) along the twist coordinate for both the donor and acceptor sites have been computed with the inclusion of solvent effect to support qualitatively the observed ICT process in terms of TICT model [19–24]. 2. Experimental 2.1. Materials p-Nitro benzaldehyde and triphenylphosphoranylideneacetonitrile in dry dicholoromethane were stirred at room temperature for 48 h. Generated nitro product was reduced by zinc/aqueous ammonium chloride to produce 3-(4amino-phenyl)-acrylonitrile. Then one hydrogen of amino group was blocked by di-tert-butyl pyrocarbonate {(tBOC)2O}/triethylamine/tetrahydrofuran and was methylated by methyl iodide/sodium hydride. In the next step, –BOC group was removed by trifluoroacetic acid/dichloromethane to produce the cis/trans mixture of the desired compound 3-(4-monomethylamino-phenyl)-acrylonitrile. The trans compound was separated from the mixture by column chromatography and finally recrystalized to get pure trans product. 60% trans:40% cis mixture: 1H NMR (CDCl3, 300 MHz) d 2.87 (s, -CH3, 6H), 5.07 (d, Jcis = 12.1 Hz, 1H), 5.54 (d, Jtrans = 16.6 Hz, 1H), 6.53 (m, 4H), 6.91 (d, Jcis = 12.1 Hz, 1H), 7.23–7.35 (m, 3H),

7.68 (d, J = 8.7 Hz, 2H,); Pure trans product: 1H NMR (CDCl3, 300 MHz) d 2.88 (s, 3H), 5.54 (d, J = 16.5 Hz, 1H), 6.54 (d, J = 8.7 Hz, 2H), 7.27 (m, 3H). 9

H3C

2 7

3

1

4

N

H 17

8 11

6

5

10

12

N

Structure of MMAPA

2.2. Spectroscopic measurements The absorption and emission spectra of MMAPA were recorded on a Hitachi UV/Vis U-3501 spectrophotometer and Perkin–Elmer LS50B fluorimeter, respectively. All the spectral measurements were done at 106 M concentration of solute in order to avoid aggregation and self-quenching. Solvents used for all measurements were spectral grade. The quantum yields were measured by relative method using b-naphthol in cyclohexane (/f = 0.23) as secondary standard [25]. Fluorescence lifetime measurements were carried out using Time Master Fluorimeter from Photon Technology International (PTI) [26]. The sample was excited using a thyratron gated Nitrogen flash lamp (width 1.5 ns) capable of measuring fluorescence lifetime resolved acquisitions as a flash rate of 25 kHz. The system measures fluorescence lifetime using PTI’s patented strobe technique and gated detection method. Lamp profiles were measured at the excitation wavelength using Ludox as the scatterer using slits with a band pass of 3 nm. Intensity decay P curves were fitted as a sum of exponential terms: F(t) = iai exp (t/si) where ai is a pre-exponential factor representing the fractional contribution to the time resolved decay of the component with a life time si. The software FeliX32 is used for data acquisition and analysis. The decay parameters were recovered using a nonlinear least squares iterative fitting procedure based on Marguardt algorithms [27]. The quality of fit has been assessed over the entire decay, including the rising edge, and tested with a plot of weighted residuals and the other statistical parameters, e.g., the reduced v2 and the Durbin–Watson parameters (1.6 < DW > 2.0) [28]. 2.3. Computational details All computations were performed using Gaussian-03 software [29]. Full geometry optimization was done by density functional theory (DFT) with hybrid functional B3LYP and 6-31++G(d,p) basis set. The analysis of the excited state properties was done by time dependent density functional theory (TDDFT) using the same functional and basis set [24]. Since analytical gradients are not

A. Chakraborty et al. / Chemical Physics 324 (2006) 733–741

0.5

a

2

Absorbance

3

1

4

0.3 0.2 0.1 0.0

300

350

400

Wavelength (nm)

80

b

70

1

4

3

5

60 50

2

40 30 20 10

3. Results and discussion

0

3.1. Absorption spectra The absorption spectra of trans-3-(4-monomethylamino-phenyl)-acrylonitrile (MMAPA) (1 · 106 M solution) in polar and non-polar solvents show two absorption bands at 318 nm and at 350 nm (Fig. 1a). These two bands are assigned to transition from S0 to S2 and S1 states, respectively. It is seen from the absorption spectra that the higher energy absorption peak (318 nm) almost unaltered with the nature of the medium, but the position of lower energy peak changes and it depends on the dielectric constant and hydrogen bonding ability of the solvents. In aprotic solvents the lower energy absorption band shifts towards the red side with the increase of the dielectric constant of the medium. On the other hand, in hydrogen bonding solvents, this band shifts towards the blue region depending on the hydrogen bonding ability of the solvents.

5

0.4

Intensity (a.u.)

available for the TDDFT method, all excited state calculations are limited to single point calculations. Relaxed calculations, i.e., optimization at each stage have been done for several donor–acceptor systems to get a better understanding of potential energy surfaces [4,15,30] However, a large number of computations have been performed successfully by non-relaxed calculations for donor acceptor ICT reaction [31–33]. We have defined twisting of monomethyl amino and acrylonitrile group separately as the reaction co-ordinate and freeze all other degrees of freedom (nonrelaxed calculation). It is found that twisting along the central double bond is not energetically favorable and, hence, is not considered as reactions coordinate [34]. We also extended our calculations in solvated system using non-equilibrium TDDFT-PCM model mainly considering the fact that in case of photon absorption or emission with consequent electronic state transition the solute electronic density changes very quickly. This means that only solvent electrons are capable to rearrange in order to stay in equilibrium with the solute, while the solvent molecular motion are kept frozen during the process [23]. Since TDDFTPCM calculation can provide a very good agreement with observed excitation energies, we have included non-equilibrium effect for theoretical evaluation of solvent shift.

735

400

450

500

550

Wavelength (nm) Fig. 1. Room temperature (a) absorption and (b) emission spectra of MMAPA in: (1) cyclohexane, (2) tetrahydrofuran, (3) acetonitrile, (4) ethanol, (5) water.

Table 1 Spectroscopic parameters obtained from the absorption and emission spectra of MMAPA molecule Solvent

kabs (nm)

kflu (nm)

mabs (cm1)

mflu (cm1)

Dm (cm1)

3.2. Emission spectra

Cyclohexane Methylcyclohexane Dioxane Chloroform Tetrahydrofuran Acetonitrile Isopropanol Ethanol Methanol Water

340,318 337,318 350,318 346,318 355,318 350,318 361,318 359,318 356,318 343,318

368 391 399 404 419 421 408 414 423 445

29,411 29,673 28,571 28,901 28,169 28,571 27,700 27,855 28,089 29,154

27,173 25,575 25,062 24,758 23,866 23,741 24,533 24,154 23,651 22,471

2238 4098 3509 4143 4303 4830 3167 3701 4438 6683

The fluorescence emission spectra of MMAPA in different polar and non-polar solvents are shown in Fig. 1b and the spectral data are reported in Table 1. In non-polar cyclohexane type of solvent, on 330 nm excitation, the molecule exhibits local emission at 368 nm. But the same excitation shows a dual fluorescence in polar aprotic solvent. The long wavelength emission band is observed at 400–460 nm and the short wavelength emission band at 377 nm. The position of the short wavelength emission

band is found to be almost independent on the polarity of the medium. However, the position of the long wavelength emission band depends on the polarity of the solvent. More polar nature of the solvent more is the red shift of the low energy emission band. This polarity dependent emission is assigned to the charge transfer character of the emissive state as was seen in DMABN or related molecules [2]. The effect of the polarity of the solvent on the

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emission spectra is also rationalized from the change in dipole moment of the molecule from ground to the excited state calculated by solvatochromic measurements. The Stokes shifts (Dm cm1), calculated from the maxima of the absorption and fluorescence spectra of MMAPA, are presented in Table 1. Fig. 2a shows the plot of Stokes shift (Dm) vs. solvent parameter (Df) (Lippert plot). The solvent parameter Df can be expressed by the following equation [7]: Df ¼ ½ðe  1Þ=ð2e þ 1Þ  ½ðn2  1Þ=ð2n2 þ 1Þ Here, Df describes polarity and polarisibility of the solvent. e and n are the dielectric constant and refractive index of the solvent, respectively. The dipole moment of the FC (lFC) and CT (lCT) states of MMAPA have been calculated by plotting mabs vs. Df and mflu vs. Df, respectively, using the following equations [7]: mabs ¼ ½ð1=4pe0 Þð2=hca3 Þlg ðlFC  lg ÞDf þ constant

ð1Þ

3

mflu ¼ ½ð1=4pe0 Þð2=hca ÞlCT ðlCT  lg ÞDf þ constant ð2Þ The Onsager cavity radius (a) and the ground state dipole moment (lg) were calculated using Gaussian 03 software. The calculated (B3LYP/6-31++G(d,p)) value of ‘a’ and 5000 4800

a

4

4600

˚ and 9.0 D, respectively, for the ground state lg are 4.5 A minimum structure. The lCT and lFC are the dipole moment of the charge transfer and Franck–Condon (FC) state, respectively. It is found that the calculated dipole moment obtained from solvatochromic measurements for the CT state (12.6 D) is larger than Franck–Condon (9.5 D) and ground state. This indicates that charge transfer occurs sequentially via relaxation from FC state to the CT state. The different extent of stabilization of the highly dipolar CT state in different polar solvents is responsible for the observed solvent dependent red shifted emission band. The decrease in intensity of the high-energy emission band and the appearance of the intense low energy band in polar solvent may indicate the generation of ICT state from the LE state. It is seen that the Stokes shift of the low energy emission band in polar protic solvents does not correlate as expected on the basis of the solvent polarity. As shown in Fig. 2b, the fluorescence maxima of low energy band in protic solvent shows a linear relationship with hydrogen bonding parameter (a) [35]. This observation infers that hydrogen-bonding interaction must be responsible for this Stokes shift of the low energy emission band in polar protic solvent. Decrease of fluorescence quantum yields (Table 2) with increase of the hydrogen bonding tendency of the protic solvents also support the fact that hydrogen bonding acts as nonradiative channel in the ICT path [36]. 3.3. Effect of acid and base

-1

(cm )

4400

3

2

4200 4000 3800 3600 3400

1 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

f

24500

4

b 3

-1

(cm )

Table 2 Fluorescence quantum yields of MMAPA in different polar, nonpolar solvents at room temperature Solvent

24000 2

23500 23000

1

22500 0.6

The studies of absorption and emission spectra of MMAPA at different pH range further confirm the CT nature of the emitting state [37]. The absorption and fluorescence spectra of MMAPA undergo a drastic change in the presence of acid. As seen in Fig. 3a, addition of acid generates a new band at 297 nm with the disappearance of 343 nm band. This new absorption band could be arising from the p–p* transition of benzene type of moiety hav-

0.7

0.8

0.9

1.0

1.1

Fig. 2. (a) Lippert plot of MMAPA: (1) dioxane, (2) chloroform, (3) tetrahydrofuran, (4) acetonitrile. (b) Plot of hydrogen bonding parameter (a) vs. fluorescence band position of MMAPA: (1) water, (2) methanol, (3) ethanol, (4) isopropanol.

a

/LE

/CT 3

Cyclohexane Acetonitrile

– –

7.7 · 10 1.0 · 103

– 2.09 · 101

Ethanol Methanol Water

0.86 0.98 1.10

1.7 · 103 1.6 · 103 1.0 · 103

1.878 · 101 1.728 · 101 9.7 · 102

v2

s (ns) a

0.6 ± 0.15 0.6 ± 0.15b 0.3 ± 0.15c – – 0.6 ± 0.15 (81%)d 3.37 ± 0.15 (19%)d

0.88 1.03 1.50 – – 0.96

a is the index of solvent hydrogen donor ability. Fluorescence life time (s) and v2 value in different solvents. a kex = 360 nm and kem = 370 nm. b kex = 360 nm and kem = 390 nm. c kex = 337 nm and kem = 415 nm. d kex = 337 nm and kem = 390 nm.

A. Chakraborty et al. / Chemical Physics 324 (2006) 733–741 0.10

a 2

Absorbance

1

0.05

0.00

300

350

400

Wavelength (nm) 30

b

2

Intensity (a. u.)

1 20

10

0

350

400

450

500

550

Wavelength (nm )

ing a conjugated double bond generated by protonation of MMAPA molecule. Fig. 3b depicts the fluorescence spectra of MMAPA in water and in water/H+. The emission spectrum in presence of acid also exhibits a new band at 375 nm with the disappearance of red shifted emission band. This new emission band could be generated from the protonated species of MMAPA. The ground state protonation reaction can be described by Scheme 1. Proton tries to bind to the lone pair electron of nitrogen of the donor and hence, electrons of the donor are not available for the charge transfer process in the excited state. As a result the low energy CT emission disappears. It is seen that base has no effect on the spectra of MMAPA molecule. 3.4. Fluorescence quantum yields and lifetime The room temperature fluorescence quantum yields of MMAPA in polar and non-polar solvents are shown in Table 2. As seen in some donor acceptor molecules [7], H NC

N

H+

Me

NC

H Scheme 1. Protonation reaction.

the quantum yields for the CT emission of MMAPA is higher than the LE emission band. Usually, CT nature is dominant in the polar aprotic solvents such as tetrahydrofuran and acetonitrile. However, protic solvents effectively quench long wavelength fluorescence. It is also found that the fluorescence quantum yields is markedly decreased with increase of the hydrogen-bonding parameter (a) of the protic solvent [36]. This result indicates the existence of a nonradiative channel of increasing importance with growing hydrogen bonding ability of the protic solvents, resulting of very low quantum yields in water (a = 1.1). The fluorescence lifetime of MMAPA in cyclohexane, acetonitrile and water are shown in Fig. 4 and the data are presented in Table 2. The LE emission (390 nm) shows single exponential decay with fluorescence lifetime of 0.627 and 0.634 ns in cyclohexane and acetonitrile solvent, respectively. The fluorescence decay of CT band (415 nm) band in acetonitrile solvent is 0.375 ns. The charge transfer excited state decays faster than the LE state [38,39]. The fluorescence decay in polar solvent like water is of different nature. As shown in Fig. 4a, the 390 nm emission band in water shows biexponential decay curve with lifetimes of 0.600 and 3.370 ns. The large difference in lifetime between the two components indicates that they are of different origin. We assume that the fast lifetime value corresponds to LE emission and the slow lifetime to the hydrogen-bonded species. 3.5. Temperature effect on emission spectra

Fig. 3. Absorption (a) and emission (b) spectra of MMAPA in (1) water and (2) in presence of 2 N acid solution.

Me

737

N +

H

The fluorescence spectra of MMAPA in the glass matrix are shown in Fig. 5a. The most notable observation is the emission of MMAPA at 77 K in methylcyclohexane and in ethanol glass matrix. In methylcylcohexane glass the observed spectrum is assigned to LE state emission. As seen in Fig. 5a, the emission spectrum of MMAPA in ethanol glass is different from its room temperature spectrum. It is found that the nature of emission in ethanol glass is similar to that of methylcyclohexane glass. A blue shift of the low energy emission band is observed at low temperature ethanol glass. This blue shift reflects the change in solvent properties such as polarity, polarizability and viscosity upon decreasing the temperature. High viscosity at low temperature glass may also hindered twisting motion for the relaxation path from FC to CT state. This observation may support the fact that the twisting motion is hindered and hence, photoinduced ICT band is absent in rigid glass matrix. The variation of CT emission intensity with increase of temperature from 283 to 313 K in acetonitrile solvent is shown in Fig. 5b. It is found that the intensity of the CT emission band decreases and LE band increases slightly with the increase of temperature. The decrease in the intensity of the CT band with the increase of temperature may be due to opening up of some temperature activated nonradiative channels. But we cannot say about the increase in intensity of the LE state with increase of temperature.

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A. Chakraborty et al. / Chemical Physics 324 (2006) 733–741

800

1800

a

700

1400

Intensity (a.u.)

600 500 400 300

1200 1000 800 600

200

400

100

200 0

60

70

80

90

100

110

120

350

400

Cyclohexane

1 70

80

90

100

110

Res.

4

120

Water

2 0 -2 -4

70

80

500

12

b

20

Intensity (a.u.)

Res.

Time (ns) 4 2 0 -2 -4

450

Wavelength (nm)

90

100

110

15

1

Intensity (a.u.)

Intensity

a

1600

10

8 375

10

380

385

390

395

400

Wavelength (nm)

4 4

5 700

b 0

600

400

450

Intensity

500

550

Wavelength (nm)

500 400

140

1

c

300

120

100 60

70

80

90

10 0

110

Res.

Time (ns) 4 2 0 -2 -4

ACN (LE)

Res.

100 80 60

4 40 20

70

80

90

4

100

110

ACN (CT)

0 -2 70

80

90

0

360

380

400

420

440

460

480

500

520

Wavelength (nm )

2

-4

Intensity (a.u.)

200

100

Fig. 4. Fluorescence decay curve of MMAPA: (a) lamp profile (–h–), in cyclohexane (kem = 390 nm) (–m–), in water (kem = 390 nm) (–d–); (b) lamp profile (–h–), in acetonitrile (kem = 390 nm) (solid line), in acetonitrile (kem = 415 nm) (–m–). Residuals are given at the bottom of each graph.

We are not sure whether it is simply due to the change of solvent polarity with temperature. The temperature dependent emission spectra of MMAPA in water are shown in Fig. 5c. In this case the fluorescence intensity of both the LE and CT bands decreases with increase of temperature.

Fig. 5. (a) Emission spectra of MMAPA at 77 K (–m–) and 298 K (––) in ethanol and in methylcyclohexane glass at 77 K (– –); Temperature dependent fluorescence emission spectra (kext = 330 nm) of MMAPA in (b) acetonitrile and (c) in water at (1) 283 K, (2) 293 K, (3) 303 K, (4) 313 K.

Here, also radiationless processes increase with increase of temperature. In protic solvent weak hydrogen bond can rapture with increase of temperature. 3.6. Quantum chemical calculations Ground state optimization for the global minimum structure of MMAPA has been performed using Becke’s

A. Chakraborty et al. / Chemical Physics 324 (2006) 733–741

three parameter hybrid functional (B3LYP) and 631++G(d,p) basis set. The optimized geometrical parameters of the ground state of MMAPA in vacuo are shown in Table 3. In the global minimum structure, the monomethylamino group is 8.4 out of benzene plane and the N-centre is pyramidal. According to the TICT model structurally out of plane donor group favours excited state charge Table 3 Optimized geometry for the ground state of MMAPA in vacuo at DFT (B3LYP/6-31++G(d,p)) level Bond

Calculated ˚ values in A

Angle/dihedral angle

Calculated values in 

RC1–C2 RC2–C3 RC3–C4 RC1–N7 RC4–C8 RC8–C10 RC10–C11 RC11–N12 RN7–H17 RN7–C9

1.410 1.390 1.407 1.376 1.450 1.354 1.423 1.167 1.000 1.450

\N12–C11–C10 \C11–C10–C8 \C8–C4–C3 \C2–C1–N9 \C9–N7–H17 \C3–C4–C8–C10 \C9–N7–C1–C6 \H17–N7–C1–C2

180.0 122.2 119.2 122.2 117.1 180.0 172.6 154.3

-496.40

a

transfer process from donor to the acceptor site. Therefore, from the structural point of view, TICT process is expected in MMAPA molecule. The PE curves along the twist coordinate for the ground and different low-lying singlet excited states both in vacuo and in acetonitrile solvent obtained by TDDFT calculations are shown in Fig. 6. As seen in Fig. 6a, it is clear that the torsion of the donor (monomethylamino) group leads to canonical crossing of S2 and S1 states. Whereas twisting of the acceptor (acrylonitrile) group leads to a stabilization of S1 state (CT state) and destabilization of S2 and S0 states (Fig. 6b). Out of two possible twisting paths it is seen that twisting along the donor path is energetically favorable over the twisting of acceptor path. Along the TICT path there are no other states except S2 close enough in energy, which could be involved in ICT reaction. Theoretically, S0 ! S1 excitation leads to a crossing of S2 and S1 states along the twist coordinate of the donor group and gives rise to low energy emission from the stabilized S2 state (CT state). As shown in Fig. 7, very similar energy stabilization of the excited state along the twist coordinate is observed in the solvated (acetonitrile) system. Here also

S2

-496.42

-496.44

LE State

S1

CT State

-496.48

3.16 eV S0

3.77 eV

-496.58

-496.60

0

20

40

60

80

100

Energy (Hartree)

Energy (Hartree)

a

S2

-496.46

-496.44 -496.46

739

-496.48

LE State

2.9 eV

S0 -496.60

3.47eV

-496.62

120

0

20

Twist angle (Degree) -496.42

-496.44

S2

b

S1

LE State

CT State

3.35 eV 3.77eV S0

-180

-160

-140

-120

-100

-80

Twist Angle (Degree) Fig. 6. Potential energy surface of the ground and excited states along the twisting coordinate at the (a) donor and (b) the acceptor part using TDDFT method with B3LYP functional and 6-31++G(d,p) basis set.

Energy (Hartree )

Energy (Hartree)

-496.60

60

80

100

120

b

S2

-496.46

-496.48

-496.58

40

Twist angle (Degree)

-496.44 -496.46

S1

CT State

-496.50

-496.48 -496.50

S1

LE State

CT State

3.01eV -496.60

-496.62

3.47 eV

-180

-160

S0

-140

-120

-100

-80

Twist Angle (Degree) Fig. 7. Potential energy surface of the ground and excited states in acetonitrile along the twisting coordinate at the (a) donor and (b) the acceptor part using TDDFT-PCM model.

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A. Chakraborty et al. / Chemical Physics 324 (2006) 733–741

twisting along the donor path is energetically favorable over the acceptor path. Interestingly, it is found that the TDDFT-PCM model yields a good agreement between the observed absorption bands with the calculated vertical transition energy for the 1st and 2nd excited states. Calculated emission energies at the perpendicular geometry in vacuo and in acetonitrile solvent is presented in Table 4. In vacuo, the calculated emission energies due to twisting at donor and acceptor sites are 392 nm (3.16 eV) and 370 nm (3.35 eV), respectively. On the other hand, in acetonitrile solvent, the calculated emission energies due to twisting at donor and acceptor sites are 427 nm (2.90 eV) and 411 nm (3.01 eV), respectively. The calculated emission energy due to twisting at the donor site shows better agreement with the experimental values than twisting at the acceptor site. Therefore, comparatively more stabilization of S2 state (CT state) in polar solvent predicts calculated red shifted emissions, which correlates well with the observed low energy emission band. The HOMO and LUMO molecular orbital picture for the ground and twisted forms are shown in Fig. 8. It is seen from the orbital picture that the normal ground to excited state transition in MMAPA molecule (transition in the gloTable 4 Comparison between the experimental and theoretical values of vertical transition energies (eV) of MMAPA in vacuo and in acetonitrile solvent Absorption

Emission

State

Eth

Eex

State

Etha

Ethb

Eex

Vacuo

S1 S2

3.77 4.27

– –

S1 S2

3.16 4.25

3.35 4.38

– –

Acetonitrile

S1 S2

3.47 4.26

3.54 3.89

S1 S2

2.9 4.08

3.01 4.3

2.94 –

Eth is the calculated energy value (Eexcited  Eground) at DFT level (B3LYP/6-31++G(d,p)). Eex is the experimental value. a Emission energy due to twisting of –NHMe group. b Emission energy due to twisting of acrylonitrile group.

bal minimum structure) is simply p–p* (HOMO ! LUMO) type (LE state), where the nitrogen lone pair is delocalized over the benzene plane (Fig. 8a). At the twisted configuration, the nitrogen lone pair is mostly localized over the monomethylamino group and the transition is simply n–p* (HOMO ! LUMO) type (CT state) (Fig. 8b). Moreover, the change in the nature of the CT state is reflected by the values of calculated oscillator strength, initially the value is 0.78, and latter decreases to 0.024 at the twisted state indicating a transformation from allowed to forbidden transition. Therefore, the localized nitrogen lone pair in the twist configuration (Fig. 8b) favors charge transfer in the excited state as described by TICT model [2]. 4. Conclusion In this paper, we have investigated ICT reaction by absorption and emission spectroscopy of MMAPA molecule where secondary amine without sterically hindered methyl substituted group is used as charge donor. It is found that MMAPA shows dual fluorescence in polar aprotic solvents. Solvent dependency of the low energy emission band indicates charge transfer character of the emissive species. Solvatochromic measurement suggests high dipole moment of the CT state compared to the ground and FC states. Temperature dependent emission spectra and the measured quantum yields establish the formation of hydrogen bonded complex in protic solvents. Ground state structural calculation of MMAPA at DFT level predicts that the donor site (monomethylamino group) is slightly out of plane of the benzene ring. Potential energy surfaces along the twisting coordinate of the donor and acceptor site in vacuo and in acetonitrile solvent revel the fact that both the twisting motions favour excited state stabilization process. However, twisting along the donor group is energetically more favorable than the acceptor site. It is also found that the delocalized lone pair electrons of nitrogen of the donor in the global minimum structure is localized in the perpendicular configuration and is responsible for photo-induced charge transfer reaction supported by TICT mechanism. Overall, the experimental results and theoretical calculations predict photo-induced twisted intramolecular charge transfer (TICT) reaction in MMAPA molecule. Acknowledgements

Fig. 8. Molecular orbital of MMAPA (a) in the ground state (1) HOMO and (2) LUMO; and (b) in the twisted conformer (1) HOMO and (2) LUMO.

The authors are grateful to DST, India for financial support (Project No. SP/S1/PC-1/2003). The authors also thank to Professor Tapan Ganguly, Department of Spectroscopy, IACS, and to Professor Sanjib Ghosh and Mr. Subhadeep Samanta, Department of Chemistry, Presidency College, Calcutta for allowing them low temperature and fluorescence lifetime measurements available for this work. N.G. thank Dr. S.N. Bhattacharyya, Dr. S.S.Z. Adnan, Dr. S. Cakraborty and Dr. S. Chakrabarti for their constant encouragement through out this work.

A. Chakraborty et al. / Chemical Physics 324 (2006) 733–741

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