Femtosecond Excited-State Dynamics of 4-Nitrophenyl

ARTICLE pubs.acs.org/JPCA

Femtosecond Excited-State Dynamics of 4-Nitrophenyl Pyrrolidinemethanol: Evidence of Twisted Intramolecular Charge Transfer and Intersystem Crossing Involving the Nitro Group Shahnawaz Rafiq, Rajeev Yadav, and Pratik Sen* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, PIN-208 016, India ABSTRACT: Ultrafast excited-state relaxation dynamics of a nonlinear optical (NLO) dye, (S)-()-1-(4-nitrophenyl)-2pyrrolidinemethanol (NPP), was carried out under the regime of femtosecond fluorescence up-conversion measurements in augmentation with quantum chemical calculations. The primary concern was to trace the relaxation pathways which guide the depletion of the first singlet excited state upon photoexcitation, in such a way that it is virtually nonfluorescent. Ground- and excited-state (singlet and triplet) potential energy surfaces were calculated as a function of the NO2 torsional coordinate, which revealed the perpendicular orientation of NO2 in the excited state relative to the planar ground-state conformation. The fluorescence transients in the femtosecond regime show biexponential decay behavior. The first time component of a few hundred femtoseconds was ascribed to the ultrafast twisted intramolecular charge transfer (TICT). The occurrence of charge transfer (CT) is substantiated by the large dipole moment change during excitation. The construction of intensity- and area-normalized timeresolved emission spectra (TRES and TRANES) of NPP in acetonitrile exhibited a two-state emission on behalf of decay of the locally excited (LE) state and rise of the CT state with a Stokes shift of 2000 cm1 over a time scale of 1 ps. The second time component of a few picoseconds is attributed to the intersystem crossing (isc). In highly polar solvents both the processes occur on a much faster time scale compared to that in nonpolar solvents, credited to the differential stability of energy states in different polarity solvents. The shape of frontier molecular orbitals in the excited state dictates the shift of electron density from the phenyl ring to the NO2 group and is attributed to the charge-transfer process taking place in the molecule. The viscosity dependence of relaxation dynamics augments the proposition of considering the NO2 group torsional motion as the main excited-state relaxation coordinate.

1. INTRODUCTION Molecules exhibiting nonlinear optical (NLO) properties have received significant attention because of their possible applications as NLO devices in the fields of spectroscopy, telecommunications, optical data storage and processing, etc.13 In general, second-order NLO-active chromophores consist of strong electrondonating (D) and electron-withdrawing (A) moieties capping a π-conjugated spacer. Such kinds of Dπ-bridgeA combinations are commonly referred to as “pushpull” systems.16 It is believed that the capability of a molecule to exhibit NLO phenomena is related to the degree of charge separation and this charge redistribution takes place on an ultrafast time scale.79 During the past few decades, the excited-state intramolecular charge-transfer (ICT) dynamics of such type of “pushpull” systems have been the subject of many experimental and computational studies.79 The important thing about twisted intramolecular charge-transfer (TICT) processes is that the π-orbitals of donor and acceptor moieties are oriented at some angle with respect to each other.79 Photoexcitation causes the formation of a locally excited (LE) singlet state, which relaxes to the more polar TICT singlet state, and emission is thus an outcome of both the states.711 The widely studied compound in this regime is r 2011 American Chemical Society

dimethylaminobenzonitrile, which shows a very prominent TICT emission.79 There are many other molecules which have been studied until date to investigate the charge-transfer dynamics, but very little concern has been paid toward studying the overall excited-state dynamics of compounds containing a nitro group as an acceptor moiety separated from the donor group through a π-conjugated spacer. The occurrence of nonbonding electrons in many substituted aromatic hydrocarbon systems govern the presence of upper excited states, which in some cases significantly increase the spinorbit coupling and hence drastically change the photophysics and photochemistry of the molecule.1217 This feature essentially hastens the intersystem crossing (isc) channels rendering such aromatic molecules highly nonfluorescent with fluorescence quantum yields of ca. 103. Thus the excited-state process will mainly occur from the triplet state instead of the singlet.17,18 In 1980, Hamanoue and co-workers first suggested the involvement of upper nπ* triplet states as a receiver of electrons from singlet state S1 during isc in 9-nitroanthracene.12,13 Received: January 18, 2011 Revised: May 9, 2011 Published: July 13, 2011 8335

dx.doi.org/10.1021/jp2005524 | J. Phys. Chem. A 2011, 115, 8335–8343

The Journal of Physical Chemistry A The first identification of upper triplet states with adequate electronic configuration to couple to the S1 state in nitroaromatics was shown by Zugazagoitia et al.19 The spinorbit coupling due to the presence of the NO2 group was assigned to the observed lack of fluorescence and high triplet yield of this molecule.12,13 Also, the electron-withdrawing nature of the NO2 group permits the extension of ring aromaticity and hence helps in stabilizing the electronically excited states, and, as well, imparting a charge-transfer character.17,1921 Transient absorption measurements on several nitroaromatic systems have inferred a few picoseconds buildup time for the triplet population.22,23 Ernsting and coworkers probed the deactivation kinetics of p-nitroaniline through transient absorption technique and semiempirical calculations. They revealed that the internal conversion process was controlled by the twisting motion of the NO2 group to the perpendicular conformation toward the local minimum of the potential energy surface along the torsional coordinate.24,25 In 2007, Morales-Cueto et al. measured the transient fluorescence from several nitrated polycyclic aromatic hydrocarbons and proposed isc as the main decay channel of the S1 state with the time duration of a few tens of femtoseconds.26 Recently Crespo-Hernandez et al. in 2008, on the basis of transient absorption measurements and quantum chemical calculations, reported an isc time constant of ∼7 ps corresponding to the decay from the ππ* S1 state to the nπ* T3 state for 1-nitropyrene.21 The orientation of the nitro group has been suggested to be responsible for controlling the rate of isc from singlet to triplet states. The torsional angle of the nitro group upon excitation changes its orientation relative to the ground state, which increases the efficiency of spinorbit coupling between the singlet and triplet states, and hence, instead of showing fluorescence, the molecule decays through an isc channel.19 Apart from experimental findings, many computational studies have been, as well, devoted to understand the photophysics of nitrated aromatic systems. Time-dependent density functional theory (TD-DFT) was employed to estimate the relative energies of singlet and triplet manifolds which assisted in correlating the rate of isc with the energy alignment of singlet and triplet states.16,17,21 From these calculations, Crespo-Hernandez et al. observed that, in 1-nitropyrene, there is a local minimum at the nitro group torsional angle of 27.46° in the fully optimized ground state; however, in the first singlet excited state the nitro group is almost coplanar with the aromatic system.21 In this contribution, we present detailed femtosecond fluorescence up-conversion measurements and quantum chemical calculations on (S)-()-1-(4-nitrophenyl)-2-pyrrolidinemethanol (NPP). NPP is a nitrated pushpull molecule exhibiting NLO properties. The emphasis is being given to understand its excited-state relaxation mechanism.

2. EXPERIMENTAL SECTION 2.1. Materials. (S)-()-1-(4-Nitrophenyl)-2-pyrrolidinemethanol (NPP, Figure 1) was purchased from Sigma-Aldrich (St. Louis, U.S.A.) and used as received. The solvents used in this study were of HPLC grade and obtained from Spectrochem (India) and used without further purification. 2.2. Steady-State Measurements. The UVvis absorption spectra of sample solutions were recorded on a commercial spectrophotometer (UV-2401, Schimadzu), whereas fluorescence spectra were obtained on a commercial fluorimeter (Fluorolog, Jobin-Yvon). The quantum yield measurements were performed using coumarin 153 as reference. 2.3. Femtosecond Fluorescence Up-Conversion Measurements. The fluorescence transients have been measured by a

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Figure 1. Structure of (S)-()-1-(4-nitrophenyl)-2-pyrrolidinemethanol (NPP).

femtosecond fluorescence up-conversion setup (FOG-100, CDP Corp.). The details of the setup are discussed elsewhere.27 Briefly, the sample has been excited at 405 nm using the second harmonic of a mode-locked Ti:sapphire laser (Tsunami, Spectra Physics), pumped by a 5 W Millennia (Spectra Physics). To generate the second harmonic we used a nonlinear crystal (1 mm BBO, θ = 25°, φ = 90°). The fluorescence emitted from the sample was up-converted in another nonlinear crystal (0.5 mm BBO, θ = 38°, φ = 90°) by using the fundamental beam as a gate pulse. The up-converted light is dispersed in a monochromator and detected by photon-counting electronics. The femtosecond fluorescence decays were deconvoluted using a Gaussian shape for the instrument response function having a full width at half-maximum (fwhm) of 250 fs (obtained through water Raman scattering) using commercial software (IGOR Pro, WaveMetrics). The molecule was excited at 405 nm, and all measurements were done at 20 °C. 2.4. Computational Methods. Ground- and excited-state quantum chemical calculations were performed using the Gaussian 03 program.28 The ground-state optimization of NPP was carried out at the DFT level using the B3LYP hybrid functional and 6-311++g(d,p) basis set. Vertical excited-state calculations of low-lying states in vacuum were pursued with TDDFT using the B3LYP functional and the same 6-311++g(d,p) basis set. The ground- and excited-state single-point energies were calculated along the twist coordinate of the NO2 group. The atoms specifying the twist angle are O1N2C3C4 as shown in Figure 1. The nonequilibrium vertical excitation energies of the fully optimized NPP structure in different solvent media were performed employing TD-PBE0/NE-IEFPCM/6-311++G(d,p)|| B3LYP/IEFPCM/6-311++G(d,p) level of theory for ground- and excited-state calculations.29,30

3. RESULTS 3.1. Steady-State Absorption and Emission Spectra. NPP exhibits a single broad structureless absorption band in the solution phase. Figure 2a shows the steady-state absorption spectra of NPP in four solvents of different polarities. In nonpolar tetrahydrofuran, the absorption maximum was centered at 392 nm. On increase in polarity the absorption maximum monotonically red-shifted, and in the case of water it was found at 422 nm, reflecting a strong solvatochromism. The emission spectra (shown in Figure 2b) also reflect strong solvatochromism as is dictated by the appreciable red shift of emission maxima with increasing solvent polarity. In tetrahydrofuran (THF), the emission maximum is centered at ca. 453 nm, whereas in water it is ca. 520 nm. In acetonitrile, NPP shows the molar extinction coefficient of 22 800 M1 cm1 and the oscillator strength was calculated to be 0.33. The fluorescence quantum yield of NPP was found to be ∼5 ( 1 104 in acetonitrile using coumarin 153 as reference, which indicates its nonfluorescent nature. The observed strong positive solvatochromism is an indicative of larger dipole moment change during photoexcitation. 8336

dx.doi.org/10.1021/jp2005524 |J. Phys. Chem. A 2011, 115, 8335–8343

The Journal of Physical Chemistry A

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Figure 2. Steady-state absorption spectra (a) and fluorescence spectra (b) of NPP in four different solvents: tetrahydrofuran (black, - - ), ethanol (blue, ), acetonitrile (green, 3 3 3 ) and water (red, —). The respective absorption maxima are 391, 392, 399, and 422 nm, and the respective emission maxima are 453, 458, 486, and 523 nm. The broken region in each of the emission spectra is the solvent Raman peak which was manually deleted in order to make the spectra look clear.

Figure 3. Plots of (a) Stokes shift (ν̅ a ν̅ f) vs solvent polarity parameter f(ε, n); (b) sum of absorption and emission maxima (ν̅ a + ν̅ f) vs solvent polarity parameter f(ε, n) + 2g(n). The labels shown in the graph are solvents (1) dimethyl sulfoxide, (2) acetonitrile, (3) methanol, (4) ethanol, (5) isopropyl alcohol, (6) butanol, (7) dimethylformamide, (8) octanol, and (9) water used in this study. The respective slopes are m1 = 5360 and m2 = 15 850.

The ground- and excited-state dipole moments were calculated using the solvatochromic theory established by Kawski and co-workers,31ad whose applicability has been authenticated on a large number of molecules.32ae The theory differs from the LippertMattaga33 propositions, which consider the polarizability of both the ground and excited state to be zero (R = 0). Kawski and co-workers did not neglected the polarizability; rather, the isotropic polarizability (R) of the solute was introduced as 2R/a3 = 1 (where a is the Onsager cavity radius), and the following solvent reaction field factors were deduced: ! 2n2 + 1 ε 1 n2 1 f ðε, nÞ ¼ 2 ð1Þ n + 2 ε + 2 n2 + 1 gðnÞ ¼

3 n 1 2 ðn2 + 2Þ2 4

ð2Þ

These factors depend upon dielectric constant, ε, and refractive index, n, of the solvent. The steady-state absorption and emission characteristics of NPP were determined as a function of these solvent polarity parameters f(ε, n) and f(ε, n) + 2g(n) as shown in Figure 3, parts a and b, respectively. The respective slopes are m1 = 5360 and m2 = 15 850. On the basis of above solvent polarity parameters, the ground-state dipole moment is given by m2 m1 hca3 μg ¼ 2 2m1

!1=2 ð3Þ

Assuming the symmetry of the investigated solute molecule remains unchanged upon electronic transition, and also the ground- and excited-state dipole moments are parallel (also predicted by TD-DFT calculations), which renders the 8337



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polarizability of both states similar, the excited-state dipole moment can be calculated as m2 + m1 μ ðm2 > m1 Þ ð4Þ μe ¼ m2 m1 g It was also observed by Kawski31d that the shape of the molecule does not have a significant effect on the determined values of dipole moments; however, the dipole moments do depend upon the size of the Onsager cavity. In the present case, the Onsager cavity radius was taken as 50% of the long axis of the optimized ground-state structure of NPP, which yielded a = 4.76 Å. By substituting the value of all the parameters in eqs 3 and 4, the dipole moment values obtained are μg = 7.4 D and μe = 14.9 D with a change in transition dipole moment of 7.5 D. This clearly manifests a chargetransfer process in the molecule upon photoexcitation. 3.2. Femtosecond Fluorescence Up-Conversion Study. Femtosecond time-resolved fluorescence transients were recorded by exciting NPP at 405 nm. The fluorescence signal decays on time scales ranging from a hundred femtoseconds to a few picoseconds depending upon the nature of the solvent. In case of water, the fluorescence transient at 500 nm was fitted to a biexponential function with τ1 = 250 fs (96%) and τ2 = 690 fs (4%) (see Table 1). In acetonitrile as well, NPP decays biexponentially with τ1 = 350 fs (80%) and τ2 = 900 fs (20%) at 490 nm. On decreasing the polarity, the decay time constants increased, Table 1. Fitting Parameters for Fluorescence Transients of NPP in Different Solventsa a1

τ1 (ps)b

a2

τ2 (ps)b

water

0.96

0.25

0.04

0.69

acetonitrile

0.96

0.35

0.04

0.90

ethanol

0.86

0.56

0.14

1.67

butanol

0.81

0.62

0.19

2.00

octanol

0.68

0.65

0.32

2.72

tetrahydrofuran

0.84

0.65

0.16

2.22

solvent

a

Data are described by double-exponential functions, convoluted with the instrument response function. The sum of amplitudes a1 and a2 is normalized to one. b (0.10 ps.

and for tetrahydrofuran (at 500 nm), the respective time constants are τ1 = 650 fs (84%) and τ2 = 2.22 ps (16%). The femtosecond transients are shown in Figure 4a. We also testified the compliance with a few longer chain alcohols as solvent in our measurements as mentioned in Table 1, and transients are shown in Figure 4b. In all alcohols studied, the data was fitted biexponentially. It is also observed that with increase in the length of the carbon chain, the magnitude of the time constant increases. In order to have a vivid knowledge about the ultrafast timedependent dynamics following excitation to the Franck Condon state, the decay transients of NPP in acetonitrile (ACN) were measured at wavelengths ranging over the whole of the emission spectra from 440 to 590 nm. From the fitting parameters, time-resolved intensity- and area-normalized emission spectra (TRES and TRANES) were constructed (Figure 5). 3.3. Quantum Chemical Studies of Ground and Excited States. The ground-state B3LYP/6-311++G(d,p) optimized structure of NPP is characterized by the NO2 dihedral angle of 0.14°. The single-point energy calculations of the ground state were performed for the torsional motion of the NO2 group from its equilibrium planar position with an interval of 10° away from the equilibrium. The corresponding excited-state energies were computed with the TD-DFT framework under the regime of vertical excitations. All the potential energy values were plotted as a function of torsional angle of the NO2 group considering the ground-state optimized energy as the zero energy. The S0, S1, T1, T2, and T3 potential energy surfaces are shown in Figure 6a. The shape of the frontier molecular orbitals of NPP corresponding to the ground state, FranckCondon state, and the relaxed state provide very substantial information regarding the distribution of charge in each state. Figure 7 shows the highest occupied molecular orbital (HOMO) of the B3LYP/ 6-311++G(d,p) optimized ground state, the lowest unoccupied molecular orbital (LUMO) of the vertically excited Franck Condon state using the TD-DFT/B3LYP/6-311++G(d,p) basis, and the LUMO of the excited-state configuration which represents the minimum in the S1 potential energy surface. Vertical excitation energies of the fully optimized NPP structure were also computed in different solvents using TD-PBE0/NE-IEFPCM/6-311 ++G(d,p)||B3LYP/IEFPCM/6-311++G(d,p) level of theory.29,30

Figure 4. Femtosecond time-resolved fluorescence transients of NPP in different solvents: (a) water (red, )), acetonitrile (green, +), ethanol (blue, 3), and tetrahydrofuran (black, O) and (b) ethanol (blue, 3), butanol (pink, ), and octanol (orange, 4). All the traces are fitted by a sum of two exponentials. Solid lines indicate fitted decays convoluted with the instrument response function. 8338



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Figure 5. (a) Few representative femtosecond up-conversion decay transients of NPP in acetonitrile at the following wavelengths: 440 nm (blue, 4), 490 nm (green, O), and 570 nm (red, +). The decay at 570 nm has an initial ∼100 fs rise component followed by an 800 fs decay component. (b) Timeresolved emission spectra (TRES) and (c) time-resolved area-normalized emission spectra (TRANES) of NPP in acetonitrile constructed from the bestfit parameters of fluorescence decays and emission intensity at 0 (O), 0.02 (b), 0.04 (0), 0.06 (9), 0.1 (Δ), 0.2 (2), 0.4 ()), 0.6 ((), 1 (left-pointing triangle), and 1.5 ps (+).

The relative potential energy values of S0, S1, and T1, T2, and T3 in vacuum and four different solvents (THF, ethanol, ACN, and water) are mentioned in Table 2 and also shown in Figure 6b. These single-point potential energy values represent the potential energy of respective states at the FranckCondon nonequilibrium level and not the solvent-relaxed energy values.30 Qualitatively, it was observed that as the polarity of solvent increases from vacuum to water, the energy states are stabilized to different extents in different solvents in such a way that, as we reach to the high dielectric medium water, the S1 and T2 energy states have almost became degenerate.

4. DISCUSSION Steady-state measurements infer that there is an appreciable change in the dipole moment of NPP on photoexcitation because of charge transfer. The fluorescence quantum yield is found to be very small (5 ( 1 104) in acetonitrile suggesting the involvement of nonradiative relaxation pathways. The femtosecond time-resolved fluorescence study reveals very fast decay kinetics in all the solvents studied with two distinct ultrafast time components. Following the previous interpretation on various nitrated polyaromatic hydrocarbons and p-nitroaniline,10,11,17,26

we propose that the biexponential decay of the S1 state in NPP is because of the relaxation of the FranckCondon state through many stages. This relaxation is a consequence of some molecular rearrangements, which direct the excited state to decay extremely fast just after the photoexcitation, subsequently followed by slow decay kinetics. Subpicosecond components in the fluorescent transients are often assigned to processes like internal conversion, charge transfer, solvation, vibrational redistribution, and cooling.25,34,35 The fastest process is the intramolecular vibrational redistribution of high-frequency internal modes developing on a 10 fs time duration which we cannot observe under the prevailing experimental constraints.25,36,37 The processes between 100 fs to a few picoseconds in NPP include intramolecular charge transfer and isc as a consequence of torsional motion of the NO2 group during its equilibration to the intramolecular charge-transfer state.25 Femtosecond fluorescence up-conversion measurements of NPP in acetonitrile have been performed at 11 different wavelengths throughout the emission spectrum, to authenticate the proposition about excited-state intramolecular charge transfer, which exhibit strong wavelength-dependent fluorescence transients. On the blue side of the emission spectrum, the decay is biexponential in nature. The fast time constant constitutes the 8339



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Figure 6. (a) Potential energy surfaces of the ground state (black, b), first singlet excited state S1 (red, 2), and triplet excited states T1 (blue, 2), T2 (green, (), and T3 (orange, 1) plotted as a function of the NO2 group torsional angle in NPP. The ground-state potential energy values are calculated with the basis B3LYP/6-311++G(d,p) for every torsional angle of the NO2 group. The excited-state potential energy values corresponding to every torsional angle are obtained considering vertical excitation using TD-DFT/B3LYP/6-311++G(d,p). (b) Vertical transition energies of fully optimized NPP structure in vacuum and in the presence of various solvent media of different polarities calculated using TD-PBE0/NE-IEFPCM/6-311++G(d,p) B3LYP/IEFPCM/6-311++G(d,p) level of theory.

Figure 7. Frontier molecular orbitals of NPP: (a) HOMO of the B3LYP/6-311++G(d,p) optimized ground state; (b) LUMO of the vertically excited FranckCondon state; (c) LUMO of the excited-state NPP configuration representing the minimum in the S1 potential energy surface.

major part of the relaxation process. For example, at 440 nm, the observed components are τ1 = 60 fs (0.96) and τ2 = 470 fs (0.04%), whereas at the red end, 590 nm, the decay of time constant 520 fs is preceded by a distinct rise component of 80 fs. A few representative decay profiles of NPP in acetonitrile solution are shown in Figure 5a. The decay component at the short-wavelength region accompanied by the fast rise component at the longer wavelength region gives clear evidence of the transition from the LE to CT state. The TRES and TRANES were constructed using the parameters of best fit to the fluorescence decays and the steady-state emission spectra emission (Figure 5, parts b and c).43a,b The TRES exhibits a time-dependent dynamic Stokes shift of 2000 cm1 within 1 ps indicating that the charge-transfer dynamics is very fast in NPP. The emission in the very early time reflects the existence of the LE state, and in the latter time a distinct charge-transfer emission is observed. TRANES clearly shows the presence of the isoemissive point indicating the LE to CT state transformation.

On the basis of previous literature and from our results, we propose that the NO2 torsional motion constitutes the reaction coordinate for the excited-state relaxation dynamics in NPP.21,25,38 The nature of the potential energy surfaces of NPP was calculated as a function of the torsional/twisting angle of the NO2 group (see Figure 6a). It suggests that the vertical excitation of the ground-state optimized geometry leads to the excited singlet state having very high energy, corresponding to the directly excited nonrelaxed nuclear configuration. The ground-state energy of NPP keeps on increasing as a NO2 group dihedral angle changes from its equilibrium geometry. However, the energy of the singlet excited state decreases until the minimum in the first singlet excited-state potential energy surface is reached. That minimum corresponds to the relaxed nuclear configuration and is represented by the NO2 group torsional angle of 90°. Thus, the S1 state is characterized by the presence of a potential well at the torsional angle of the NO2 group, which renders its orientation perpendicular to the phenyl ring relative to its stable planar configuration in the ground state. 8340



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Table 2. Vertical Excitation Energy Values of S1, T1, T2, and T3 of the Fully Optimized NPP Ground-State Structure in Solvents of Different Dielectric Properties solvent

S1 (eV)

T1 (eV)

T2 (eV)

T3 (eV)

S1 T2 (eV)

tetrahydrofuran

3.30

2.27

3.16

3.50

0.14

ethanol

3.26

2.20

3.21

3.54

0.05

acetonitrile

3.25

2.18

3.22

3.55

0.03

water

3.24

2.17

3.23

3.56

0.01

As shown in Figure 6a, the rotation of the NO2 group along the twist coordinate results in the formation of a prominent conical intersection (CI) between the S1 and T3 states. This CI occurs at the torsional angle of ca. 32°. These calculations also govern the presence of another CI between the S1 and T2 states at ca. 70°. On the basis of these observations, we propose that NPP upon photoexcitation to the FranckCondon singlet state loses some energy and moves down the barrierless potential energy surface toward the less energetic relaxed state through the twisting motion of the NO2 group. Moving down the barrierless surface leads to the occurrence of a CI between the singlet state S1 and the triplet states T3 and T2. The CI between the S1 and T3 or T2 states will furnish an essential nonradiative pathway for the excited-state relaxation of S1 to the triplet manifold involving isc. Thus, according to the present TD-DFT calculations, the coordinates for excited-state intramolecular charge transfer and isc channels occur along the barrierless torsional motion of the NO2 group. However, the location of the CI calculated using TD-DFT is not certain. The CI is being predicted between adiabatic potential energy surfaces, whereas the main feature of the CI is its nonadiabatic nature, and hence, this may eventually predict the CI with less certainty.44 It is also important to know that the adiabatic spatially local functionals traditionally used within TD-DFT do not allow for sufficient accuracy to describe long-range charge-transfer transitions as well.45 The frontier molecular orbitals of NPP in the ground and excited states reveal significant information about the distribution of charge density. As shown in Figure 7, the distribution of charge density is altogether different relative to each other. The HOMO of the ground state is characterized by the uniform spread of charge density. However, in the LUMO of the relaxed state the electron density is shifted more toward the NO2 group relative to the LUMO of the nonrelaxed excited state. Such kind of gradient of electron density distribution in the excited state suggests a significant charge transfer from the pyrrolidine moiety to the NO2 group. This charge transfer lies along the twisted coordinate of the NO2 group and is thus known as a TICT process. Apart from TICT, as mentioned earlier, the isc channels also operate along the same torsional coordinate. In order to probe the influence of dielectric properties of the solvent on the rate of isc between the S1 state and triplet state, vertical excitation energies of the fully optimized NPP structure were computed in different dielectric solvents. The relative potential energy values of S0 , S1, T1, T2, and T3 in vacuum and four different solvents are tabulated in Table 2 and also shown in Figure 6b. Qualitatively, it was observed that, as the polarity of solvent increases from vacuum to water, the electronic states are stabilized to different extents in different solvents in such a way that as we reach to the high dielectric medium water, the S1 and T 2 energy states have almost became degenerate. This differential stability, which brings

Scheme 1. Schematic Representation of the Excited-State Relaxation Dynamics of NPP in Acetonitrile

the states close together, increases the rate of isc in polar solvents. On the basis of above discussion, we assign the observed first subpicosecond time component in the fluorescence transients to the TICT from the pyrrolidine moiety to the NO2 group. In nonpolar tetrahydrofuran, the fast component (τ1) is 650 fs. As we increase the polarity from THF to water, τ1 decreases to 250 fs as shown in Table 1. This decrease in the magnitude of the time constant with increase in solvent polarity is ascribed to the stability of the charge-transfer state in the highly polar solvents compared to that in nonpolar solvents and hence substantiates our proposition of assigning the first time component to the charge-transfer process in the excited state. It is also observed that, as we increase the viscosity of the medium from ethanol (1.087 cP) to n-octanol (7.21 cP),33,39 although the polarities are not much different, τ1 increases from 560 to 650 fs, respectively. The change in magnitude of the time constant with change in the viscosity of the medium is accredited to the large-amplitude motion of the NO2 group, which shows viscosity dependence and has been proposed earlier for a number of molecules.4042 While the first component is the TICT of a few hundred femtoseconds, the second time component was assigned to the isc from the S1 state to the triplet manifold (Scheme 1). This process occurs on a time scale of a few picoseconds. In THF, the observed isc time constant (τ2) is 2.2 ps, and as the medium is changed to more polar water, the time constant decreases to 0.69 ps showing a strong dependence on polarity of the solvent. This decrease in the isc time component (τ2) with increasing polarity is due to the decrease in energy spacing between the S1 state and the nearest triplet state with increasing polarity (see Figure 6b). As the energy spacing between the states decreases on increasing the polarity, the molecule relaxes immediately from the CT state to the triplet manifold via isc, which eventually increases its rate. This time component has also shown its dependence on the viscosity of the solvent with τ2 increasing from 1.67 ps in ethanol to 2.72 ps in n-octanol. In general, increase in viscosity hinders the torsional motion of the NO2 group down the barrierless S1 potential surface and delays the occurrence of isc. Thus, the solvent’s role in determining the fluorescence decay behavior may be ascribed to the effect of polarity on the relative energies of the singlet and the triplet manifolds, which changes the degree of spinorbit coupling, as well as the viscosity of the solvents. 8341



5. CONCLUSION In this paper, for the first time, we have directly shown the involvement of the NO2 group rotation as the main coordinate in the excited-state relaxation dynamics of an NLO dye, (S)-()1-(4-nitrophenyl)-2-pyrrolidinemethanol. The steady-state measurements report an appreciable change in dipole moment of 7.55 D in the molecule on photoexcitation. The quantum yield calculations comprehend the nonfluorescent nature of NPP with ΦNPP ∼ 5 ( 1 104 in acetonitrile. Quantum chemical calculations in vacuum predict the presence of few triplet states in between the ground and first singlet excited state with the occurrence of a CI between S1 and the upper triplet states T3 and T2. The femtosecond transients of NPP in different solvents show biexponential decay behavior. The faster time component of a few hundred femtoseconds was assigned to TICT from the pyrrolidine moiety to NO2, and the second time constant of a few picoseconds was attributed to isc from the S1 state to some upper triplet manifold. The excited-state charge-transfer dynamics was authenticated by the construction of TRES and TRANES, which inferred a decay of the LE state and fast rise of the CT state within a few hundred femtoseconds. Solvent polarity is found to have a dominant effect in deciding the relative rates of TICT and isc. High-polarity solvents increase the stability of the charge-transfer state and hence hasten the rate of TICT. The isc rate is also observed to be polarity-dependent, as the increase of polarity decreases the energy separation between the S1 state and the nearest upper triplet state. The effect of using long carbon chain alcohols elucidates the consideration of choosing the torsional motion of the NO2 group as the main relaxation coordinate. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: +91-512-259-7436.

’ ACKNOWLEDGMENT S.R. and R.Y. thank CSIR (Counsel of Scientific and Industrial Research, India) and UGC (University Grants Commission, India), respectively, for awarding fellowships. We thank Professor Kankan Bhattacharyya for providing the femtosecond fluorescence up-conversion facility required for the present study under the Department of Science and Technology, India. This work is financially supported by the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), Government of India. ’ REFERENCES (1) Rizzo, F.; Cavazzini, M.; Righetto, S.; Angelis, F. D.; Fantacci, S.; Quici, S. Eur. J. Org. Chem. 2010, 4004. (2) Zyss, J.; Kelley, P.; Liao, P. F. Molecular Nonlinear Optics: Materials, Physics and Devices; Academic Press: Boston, MA, 1994. (3) Schneider, A.; Guenter, P. Ferroelectrics 2005, 318, 83–88. (4) Marder, S. R.; Kippelen, B.; Jen, A. K.-Y.; Peyghambarian, N. Nature 1997, 388, 845. (5) Facchetti, A.; Beverina, L.; Van der Boom, M. E.; Dutta, P.; Evmenenko, G.; Shukla, A. D.; Stern, C. E.; Pagani, G. A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 2142. (6) Davies, J. A.; Elangovan, A.; Sullivan, P. A.; Olbricht, B. C.; Bale, D. H.; Ewy., T. R.; Isborn, C. M.; Eichinger, B. E.; Robinson, B. H.; Reid, P. J.; Li, X.; Dalton, L. R. J. Am. Chem. Soc. 2008, 130, 10565.

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Femtosecond Excited-State Dynamics of 4-Nitrophenyl

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