Electronic spectra and excited state dynamics of ...

Electronic spectra and excited state dynamics of pentafluorophenol: Effects of lowlying πσ∗ states Shreetama Karmakar, Deb Pratim Mukhopadhyay, and Tapas Chakraborty Citation: The Journal of Chemical Physics 142, 184303 (2015); doi: 10.1063/1.4919950 View online: http://dx.doi.org/10.1063/1.4919950 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/142/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Experimental investigation of the Jahn-Teller effect in the ground and excited electronic states of the tropyl radical. Part II. Vibrational analysis of the A ̃ E 3 ″ 2 - X ̃ E 2 ″ 2 electronic transition J. Chem. Phys. 128, 084311 (2008); 10.1063/1.2829471 Conformational effects on vibronic spectra and excited state dynamics of 3-fluorobenzoic acid dimer J. Chem. Phys. 121, 5261 (2004); 10.1063/1.1778383 Theoretical study of the low-lying excited singlet states of furan J. Chem. Phys. 119, 737 (2003); 10.1063/1.1578051 Hydride stretch infrared spectra in the excited electronic states of indole and its derivatives: Direct evidence for the 1 πσ * state J. Chem. Phys. 118, 2696 (2003); 10.1063/1.1536616 Ab initio calculations of low-lying electronic states of vinyl chloride J. Chem. Phys. 116, 7518 (2002); 10.1063/1.1466828

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THE JOURNAL OF CHEMICAL PHYSICS 142, 184303 (2015)

Electronic spectra and excited state dynamics of pentafluorophenol: Effects of low-lying πσ∗ states Shreetama Karmakar, Deb Pratim Mukhopadhyay, and Tapas Chakrabortya) Physical Chemistry Department, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

(Received 4 February 2015; accepted 28 April 2015; published online 11 May 2015) Multiple fluorine atom substitution effect on photophysics of an aromatic chromophore has been investigated using phenol as the reference system. It has been noticed that the discrete vibronic structure of the S1 ← S0 absorption system of phenol vapor is completely washed out for pentafluorophenol (PFP), and the latter also shows very large Stokes shift in the fluorescence spectrum. For excitations beyond S1 origin, the emission yield of PFP is reduced sharply with increase in excess vibronic energy. However, in a collisional environment like liquid hydrocarbon, the underlying dynamical process that drives the non-radiative decay is hindered drastically. Electronic structure theory predicts a number of low-lying dark electronic states of πσ∗ character in the vicinity of the lowest valence ππ∗ state of this molecule. Tentatively, we have attributed the excitation energy dependent non-radiative decay of the molecule observed only in the gas phase to an interplay between the lowest ππ∗ and a nearby dissociative πσ∗ state. Measurements in different liquids reveal that some of the dark excited states light up with appreciable intensity only in protic liquids like methanol and water due to hydrogen bonding between solute and solvents. Electronic structure theory methods indeed predict that for PFP-(H2O)n clusters (n = 1-11), intensities of a number of πσ∗ states are enhanced with increase in cluster size. In contrast with emitting behavior of the molecule in the gas phase and solutions of nonpolar and polar aprotic liquids, the fluorescence is completely switched off in polar protic liquids. This behavior is a chemically significant manifestation of perfluoro effect, because a very opposite effect occurs in the case of unsubstituted phenol for which fluorescence yield undergoes a very large enhancement in protic liquids. Several dynamical mechanisms have been suggested to interpret the observed photophysical behavior. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4919950]

I. INTRODUCTION

Phenol is the simplest aromatic alcohol and a photo-acid. In aqueous solution, the acid dissociation constant of the molecule is enhanced by more than six orders of magnitude upon absorption of UV light,1 and this attribute is significant from the viewpoint that a phenolic moiety is the light absorbing chromophore of the aromatic amino acid tyrosine. A unique attribute of tyrosine and other biological molecules having aromatic group, in general, is that the fluorescence lifetime and quantum yield of fluorescence are reduced for excitations at shorter wavelengths.2,3 The behavior indicates opening up of faster relaxation channels with increase in electronic excitation energy, and the attribute has been suggested to be responsible for photo-stability of many important biological molecules in natural environment.4 Therefore, much attention has been paid in recent years to investigate the relaxation dynamics and ensuing photochemistry of phenol on UV excitations to the low-lying electronic states.5–14 Photophysical measurements on phenol have been performed with the isolated molecules,9,15–17 as hydrogen bonded smaller complexes with water and ammonia in the gas phase,5,6,18–23 and also in

a)Email: [email protected]

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condensed media of different polarity and hydrogen bonding abilities.24–29 To interpret different attributes of the electronic relaxation dynamics of phenol, the role of the πσ∗ configuration corresponding to phenolic O–H bond, in addition to traditional concepts of interplay involving the low-lying singlet (S1 and S2) and triplet (T1 and T2) states, has been extensively discussed in recent years.23,24,29–31 Electronic structure calculation reveals energy lowering of this πσ∗ configuration as the O–H bond is made elongated, and a conical intersection of the state with the valence ππ∗(S1) state has been suggested.12,23,30–33 Thus, the possibility for H atom elimination via O–H bond fission following S1 ← S0 electronic excitation has been predicted by theory,30,32 and recently, this has been verified experimentally.11,13 It has also been suggested that the barrier crossing at the conical intersection could occur via a tunneling mechanism.34 However, more investigations are needed to establish parallels and significances of H elimination dynamics in the gas phase with the relaxation processes occurring in condensed phases. The excited state hydrogen elimination dynamics has to compete with the vibronic relaxation by molecular collision and cage effect offered by the molecules of a liquid. It has been shown in a number of recent studies that aromatic systems can have a different kind of low-lying

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but bound πσ∗ state if the C–H hydrogen atoms of the aromatic ring are chemically replaced by fluorine atoms. In the chemistry literature, the phenomenon is known as perfluoro effect.35 Here, the σ∗ orbitals correspond to those of C–F bonds. In the case of benzene chromophore, the perfluoro effect is manifested as new transitions in electronic absorption spectra. Thus, for penta- and hexafluoro benzenes, the new electronic transition, designated in the spectra as C band, is identified at ∼5.85 and 5.36 eV, respectively.36 The other key manifestations of perfluoro effect are lack of vibronic features of the absorption bands, large Stokes shifts of fluorescence spectra, small quantum yields of fluorescence, and also short fluorescence lifetimes.35–40 Furthermore, there are reports that for highly fluorinated benzene and naphthalene derivatives, fluorescence excitation (FE) spectra of the jet cooled samples display long vibronic progressions corresponding to out-of-plane butterfly modes of fluorine atoms.40,41 Such spectral features indicate that in the excited state, the equilibrium geometry of those molecules is largely distorted from planarity. Occurrences of low-lying πσ∗ electronic states for highly fluorinated benzene derivatives have also been suggested from calculation performed by different groups.40,42–45 The consequences of such low-lying πσ∗ states in vibronic coupling and dynamics of inter-electronic relaxations have also been discussed in recent years.42–44 Some disagreements concerning energy ordering of the ππ∗ and πσ∗ states and particularly, the nature of the lowest excited state of hexafluorobenzene, where the perfluoro effect could be the highest among the fluorobenzene family, were found to be dependent on the level of theory used. Thus, calculation performed by Zgierski et al. at configuration interaction singles (CIS)/ timedependent density functional theory (TDDFT) level suggests that the Franck-Condon active lowest excited state (S1) is πσ∗(1E1g) type that lies below the benzenoid ππ∗(S2, 1B2u) state.40 Studzinski et al. combined the experimental timeresolved mass spectrometric method with theoretical calculations to probe the non-radiative dynamics of the low-lying excited states of hexafluorobenzene.45 The authors calculated vertical excitation energies by TDDFT/6-311++** method using several functionals like B3LYP, PBE1PBE, BP86, and HCTH, and the three suggested lowest excited states are πσ∗(S1), ππ∗(S2), and ππ∗(S3). On the other hand, calculations performed recently by Mahapatra and co-workers employing a more advanced theoretical method (equation of motion coupled cluster singles and doubles (EOM-CCSD)) show a reversal in ordering of the two lowest excited states, i.e., πσ∗ turns out to be S2 and the lowest excited state is still the benzenoid ππ∗,42,43 and the energy difference between S1 and S2 is only ∼ 0.4 eV. To interpret the low fluorescence quantum yield and structureless absorption-emission spectral profiles of the molecule, these authors suggested a dynamical mechanism. It involves population transfers from the FranckCondon allowed ππ∗ to the dark πσ∗ state via conical intersections and subsequent emission from S2.43,44 To our knowledge, perfluoro effect has not been investigated in any reduced symmetric electronically perturbed benzene system. Studies on such systems are essential to understand the chemical significances of the effect. We report

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here several interesting photophysical attributes displayed in the gas and liquid phases by pentafluorophenol (PFP) and compare the findings with those of the unsubstituted phenol. The motivations for this study are the following. First, many spectral transitions displayed by fluorobenzenes in the electronic absorption spectra could originate owing to reduction in molecular symmetry. It is well known that transitions to the two lowest excited electronic states of benzene are symmetry forbidden and the bands in the electronic spectrum gain intensity via vibronic coupling mechanism. In the case of a fluorobenzene, the transitions are mostly allowed, and the same is also true for phenol. Therefore, the latter could be an attractive system to look for new electronic transitions in the lower energy region of the absorption spectra as effects of multiple fluorine substitutions. Second, ionic dissociation of the phenolic O–H group in water is enhanced by four orders of magnitude from phenol to PFP.46 Therefore, it is of general interest to see how the propensity of O–H bond dissociation of PFP in the gas phase is altered in comparison with that of phenol. In other words, the system would allow studying the photophysical consequences of interplay of the new low-lying πσ∗ states arising due to multiple fluorine substitutions with the πσ∗(O–H) state. The other objective of the present study is to investigate solvent effects on photophysics of PFP. All the previous studies on perfluoro substitution effects have been performed with only isolated molecules in the gas phase. However, to appreciate the chemical implications of the effect, it is essential to see in what extent the gas-phase manifestations of perfluoro effects are retained in condensed phases. Therefore, in addition to recording gas-phase electronic absorption and emission spectra, photophysical measurements have been performed in liquids of different polarities and hydrogen bonding abilities. For information about the nature of the low-lying electronic states, electronic structure theory method only at a relatively low level of theory has been performed. To understand the solvent effect on electronic spectra of the molecule, particularly in protic liquids, structure and spectra of different PFP-(H2O)n clusters (n = 1-11) have also been calculated. II. EXPERIMENTAL AND THEORETICAL METHODS

Phenol (purity>99.0%) and PFP (>99.0%) were procured from Sigma-Aldrich and were used as supplied. To measure the electronic absorption spectra of vapors of the two compounds, a 10 cm long cylindrical quartz cell was used. The spectra were recorded at laboratory temperature (22 ◦C), i.e., without additional heating of the sample compartment, and the available vapor pressures were sufficient to perform the absorption measurements. An ultraviolet-visible (UV-VIS) spectrometer (Shimadzu, model UV-2410) was used, and the gas absorption cell was placed directly inside the sample compartment of the spectrometer to record the spectra. For spectral measurements in the liquid phase, UV-grade solvents methylcyclohexane (MCH), acetonitrile, methanol, and water were acquired from Spectrochem India Pvt. Ltd. and used as supplied after verifications that they contained no fluorescent impurity. Absorption spectra of the solutions


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were recorded by the same spectrometer using a standard 10 mm cuvette. A fluorescence spectrometer (JobinYvon, FluoroMax-3) was used for recording the fluorescence excitation and emission spectra. Fluorescence spectra in the vapor phase of both the phenols were measured with the available vapor pressures of the solid samples at laboratory temperature. The intensity of the fluorescence as a function of wavelength was corrected taking into account of the instrument response parameters. Quantum chemistry calculations were performed using the Gaussian 09 program package.47 Geometries of PFP and phenol were optimized for the ground state by the density functional theory (DFT)/Coulomb-attenuating method with Becke three parameter hybrid functionals and correlation functional of Lee, Yang, and Parr (CAM-B3LYP)/6-311++** method and for optimization in the lowest excited state, TDDFT/CAM-B3LYP/6-311++** method was used. Vertical excitation energies to six lowest excited states of PFP and its complexes with water were calculated by the same TDDFT/CAM-B3LYP/6-311++** method. III. RESULTS AND DISCUSSION A. Absorption spectra and new electronic transitions

Ultraviolet absorption spectra of PFP and phenol vapors, measured at room temperature within the wavelength range of 200-350 nm, are shown in Figure 1. The contrasts between the two spectra are very pronounced. While the longest wavelength electronic absorption system of phenol is rich with vibronic structures corresponding to different fundamentals and overtones of the aromatic ring modes, the absorption trace of PFP shows no such finer features. The band at the longest wavelength appears as a broad hump with maximum at ∼260 nm. At shorter wavelengths, two band heads are visible at 223 (5.56 eV) and 232 nm (5.34 eV), and presence of a third band at 242 nm (5.12 eV) has been suggested based on spectral fitting (inset). However, at these wavelengths, phenol does not show appreciable absorption. Tentatively, we assign these additional bands to new electronic transitions of PFP that appear as manifestations of the perfluoro effect. Additional experimental evidence in support of this assignment is presented later. At wavelengths shorter than 220 nm, the second ππ∗ absorption system starts appearing, and this feature is common in the spectra of both the phenols. We also note that in spite of different spectral appearances, the first ππ∗ absorption system of PFP spans over almost the same spectral range as that of phenol, and this indicates that five fluorine atom substitutions on aromatic sites induce only a little spectral shift. Consistent with observations, electronic structure calculation predicts additional states in the vicinity of the lowest ππ∗ state, but many of them bear only a little oscillator strength (see below). B. Fluorescence spectra and emitting state of PFP

Fluorescence spectra of the two molecules recorded in the vapor phase at low pressures following excitations of their longest wavelength band systems, λex = 262 nm in both cases, are displayed in part (a) of Figure 2. The most significant

FIG. 1. UV absorption spectra in the vapor phase of (a) phenol and (b) PFP. The inset of the lower panel depicts the spectral fit of the 220-300 nm range of the spectrum of PFP at a blown up scale. The bands labeled II, III, and IV do not have counterparts in the absorption spectrum of phenol, and they have been assigned to new electronic transitions, which originate as manifestation of perfluoro effect.

contrast between the two spectra is large Stokes shift of the fluorescence maximum of PFP compared to that of phenol, which appears at ∼380 and 290 nm, respectively. This contrast in emitting behavior does not alter if a hydrocarbon liquid is used as the medium instead of vapors of the two molecules (shown below in part (b)). A quantitative estimate reveals that the quantum yield of fluorescence of PFP is much smaller compared to that of phenol, and the difference is sharply dependent on the excitation wavelength, which has been depicted below presenting a comparison of their FE spectra. The absence of vibronic structures on absorption and also emission spectral traces indicates that the geometry of PFP in the excited state could be largely distorted in comparison to ground state. Second, although maxima of the lowest ππ∗ absorption profiles of both molecules appear at almost the same wavelength (Figure 1), the emitting state of PFP lies much below that of phenol. The geometries of the two molecules in the ground (S0) and first excited (S1) states were optimized at DFT/CAMB3LYP/6-311++G (d, p) level of theory. A scrutiny of the optimized parameters reveals that unlike phenol, PFP is highly distorted in S1 state (Figure 3). The C–F bonds are distorted out above and below the ring plane, and the ortho C–F bond,


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FIG. 2. Fluorescence spectra of phenol (black) and PFP (red) recorded (a) in the vapor phase by exciting the samples at 262 nm in both cases and (b) in MCH solution (concentration 10−4 M) at room temperature (22 ◦C) upon excitation at 270 nm.

toward which O–H group is oriented, labeled here as C6–F12, is affected the most. In the ground state, the dihedral angle O7–C1–C6–F12 is nearly zero, but it is increased to ∼27◦ in the excited state. The predicted changes for other angular parameters are shown in rows 8-14 of Table I. In addition to angular distortion, the lengths of different C–F bonds are also increased significantly, e.g., C6–F12 bond length in S1 is

FIG. 3. Geometries of PFP in the ground (S0) and first excited (S1) states for optimizations at DFT and TDDFT levels, respectively, for use of 6-311++** basis set in both cases. In S1, the C–F bonds are distorted out-of-plane and the aromatic ring is also deformed significantly.

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increased by ∼0.05 Å, and the changes with respect to other bonds nearly follow the order of their angular distortions. Figure 3 also shows that in S1, the aromatic ring of the molecule is deformed from planarity. In consequence, the vibrational frequencies for the out-of-plane ring bending modes are reduced in the excited state (Table II). According to Franck-Condon rule, all these changes would make the band intensities in the electronic origin region very weak because of smaller vibronic overlaps, and long progressions with respect to several vibrational modes could show up in both the electronic absorption and emission spectra. Theoretically predicted wavelengths and intensities of the low-lying πσ∗ electronic transitions of PFP in the vicinity of the lowest of ππ∗ transition are shown in Figure 4. Vertical electronic transitions corresponding to S0 optimized geometry of the molecule were calculated by TDDFT/CAM-B3LYP/6311++** method. The absorption maximum of the lowest ππ∗ band of the measured electronic spectrum (vapor) appears at a little longer wavelength compared to the predicted wavelength, and the two differ by a factor of only 1.143. Therefore, to make the correspondence between the two types of spectra better, the predicted wavelengths of different transitions are scaled uniformly by this factor (Figure 4). The associated molecular orbitals (MOs) for all the electronic transitions are shown in Figure 5. We notice that the method underestimates the transition energy of the second ππ∗ band. In the predicted spectrum, it appears at 255 nm, whereas in the measured spectrum, the λmax of this band shows up at 210 nm. However, the predicted intensity with respect to the 1st ππ∗ band is consistent with observation. Visualizations of the molecular orbitals involved in different electronic transitions reveal that the lowest two πσ∗ transitions can be corresponded primarily with the antibonding orbitals of C–F bonds and there are small contributions also from the antibonding O–H bond orbital. In the 3rd πσ∗ transition, the contribution of the O–H antibonding orbital is larger. The transition intensities of the former two are very small, but it is significantly large for the latter one near 200 nm. We reiterate the disagreements of theoretical studies that appeared in the literature for vertical transitions (light absorption) to the lowest ππ∗ and πσ∗ states of hexafluorobenzene. Calculations performed by Zgierski et al.40 at TDDFT level show that πσ∗ is the lowest excited state, but the method used by Mondal et al.42–44 (EOM-CCSD) predicts that ππ∗ is the lowest excited state. For occurrence of emission, the theories at both levels suggest that πσ∗ is the emitting state, which explains the large Stokes shift in the measured spectra. As stated above, in the present study on PFP, the energies of the excited states have been calculated by TDDFT method using CAM-B3LYP functional, but the energies could vary if the calculation is performed at a more advanced level. Nevertheless, we see that these predictions can explain the observed photophysical behavior qualitatively. According to Figure 4, the lowest πσ∗ is predicted below the lowest ππ∗, but the transition is definitely forbidden as a comparison with the measured absorption spectrum (Figure 1) reveals. Other two predicted πσ∗ transitions can be corresponded with two of the three new bands at 223, 232, and 242 nm observed in the measured spectrum (vapor). Among these,


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TABLE I. Optimized geometric parameters (selected) of phenol and PFP in S0 and S1 states calculated by DFT and TDDFT/6-311++** methods, respectively. The maximum changes occurring in the S1 state in PFP with respect to the ground state are highlighted with bold letters. Bond length (Å) Phenol S0 O7–H13 C2–H8/C2–F8 C3–H9/C3–F9 C4–H10/C4–F10 C5–H11/C5–F11 C6–H12/C6–F12 C1–O7 O7–C1–C6–H12/O7–C1–C6–F12 H12–C6–C5–H11/F12–C6–C5–F11 H11–C5–C4–H10/F11–C5–C4–F10 H10–C4–C3–H9/F10–C4–C3–F9 H9–C3–C2–H8/F9–C3–C2–F8 H8–C2–C1–07/F8–C2–C1–07 H13–O7–C1–C6

0.9610 1.0825 1.0836 1.0826 1.0835 1.0854 1.3645 0.0007 0.0009 0.0000 0.0003 0.0004 −0.0023 −0.0289

at least one could be predominantly πσ∗(C–F) type, because, for hexafluorobenzene, a recent study has identified only one πσ∗(C–F) transition at ∼7300 cm−1 above the S1 origin that gathers significant oscillator strength,45 and the same is also likely to happen for PFP. As mentioned before, the theory employed here suggests that all the πσ∗ states are mixed with contributions from πσ∗(O–H) orbital. C. FE spectra and excited state dynamics of PFP

The FE spectrum in the vapor phase of PFP within 250350 nm spectral range, recorded for detection at the emission maximum (385 nm), is depicted in Figure 6 (upper panel, part

PFP S1

S0

0.9646 0.9644 1.0803 1.3302 1.0802 1.3287 1.0836 1.3302 1.0806 1.3289 1.0830 1.3434 1.3393 1.3464 Dihedral angle (◦) −0.0286 0.0321 0.0232 −0.0097 0.0044 −0.0009 0.0021 −0.0015 0.0138 0.0009 −0.0144 −0.0205 −0.0376 −0.2153

S1 0.9723 1.3373 1.3443 1.3249 1.3468 1.3789 1.3233 −26.67 39.23 −15.47 −16.19 30.35 −10.00 2.16

(a)). Unlike the absorption spectrum, which is presented again in the lower panel to exhibit the contrast, the FE spectrum looks dramatically different, although both have been measured under the same condition. The three broad humps of the absorption trace do not show up here, instead, a single headed relatively narrow band that has maximum at 278 nm is the only feature of the FE spectrum, and the spectral intensity drops quickly with increase in excitation energy. To authenticate that this observed behavior is not due to any measurement artifact, the spectrum of the unsubstituted phenol measured under the same condition is presented in the right panel (part (b)) of the figure. In this case, the behavior is regular, i.e., all the vibronic bands of the ring breathing progression of the aromatic ring

TABLE II. Vibrational frequencies for low-frequency ring bending modes of PFP in S0 and S1 states calculated by DFT and TDDFT/6-311++** methods, respectively. Experimentally observed infrared (IR) frequencies of some of the modes in the vapor phase and CCl4 solution are also shown. IR frequencies (cm−1) with IR intensity (km mol−1) S0 state Mode no. 1 2 3 4 21 22 23 25 27 29 30 33

Mode description Out-of-plane ring bending Out-of-plane ring bending Out-of-plane ring twisting Out-of-plane C–F and C–O bending C–F and C–O in-plane asymmetric stretching and O–H bending C–F and C–O in-plane asymmetric stretching Ring puckering, C–F and C–O in-plane asymmetric stretching, and O–H in-plane bending Ring C–C symmetric stretching and O–H in-plane bending C–C symmetric stretching and O–H in-plane bending C–C asymmetric stretching C–C asymmetric stretching and in-plane O–H bending O–H stretching

Calculated (unscaled) 134.8 (0.020) 137.6 (0.004) 185.6 (0.0) 213.1 (0.3) 978.8 (68) 1012.8 (67) 1140.8 (37) 1256.0 (37) 1348.4 (14) 1530.3 (100) 1551.5 (78) 3805.0 (34)

Measured solution/vapor-phase

S1 state (calculated, unscaled)

974 (93)/977 (48) 1015 (69)/1017 (50) 1138 (16)/. . .

42.9 73.6 107.5 175.1 808.9 947.9 1015.7

1227 (49)/1226 (21) 1311 (27)/1314 (21) 1517 (100)/1524 (100) 1536 (84)/1541 (78) 3572.6 (38)/3628 (34)

1186.8 1327.3 1402.3 1452.2 3657.9


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FIG. 4. Theoretically predicted (TDDFT/CAM-B3LYP/6-311++** method) wavelengths and oscillator strengths for vertical excitations of PFP to a few low energy states. The natures of the electronic states (ππ∗ or πσ∗) are shown above each bar marking transition wavelengths. The calculated wavelengths are scaled by a factor of 1.143 to match with maximum of the observed ππ∗ transition.

show up in similar fashion both in the absorption (lower trace) and FE (upper trace) spectra. One can infer from these observations that in the case of PFP, some fast intramolecular non-radiative relaxation channel opens up immediately above the electronic origin of the lowest allowed excited state (ππ∗), and efficiency of this relaxation increases with increase in vibronic excitation energy. We assign the maximum of the FE spectral trace as the 000 origin band of the lowest ππ∗ state of PFP and this band position (wavelength) is red-shifted from that of phenol by about 3 nm. We discuss below about the probable chemical origin for such efficient radiationless relaxation process of this molecule. First, in order to characterize how a collisional environment affects the above mentioned radiationless relaxation pathway of PFP, the FE spectrum of the molecule has been measured in an apparently inert hydrocarbon liquid, MCH, at a low molar concentration, ∼10−5 M, and the spectrum is depicted in part (c) of Figure 6. We notice that the medium effect is enormous and unlike the gas-phase behavior, the FE spectrum mimics the absorption spectral trace of PFP in MCH (lower trace). It indicates that hydrocarbon liquid blocks the process responsible for vibronic energy dependent nonradiative dynamics, and the emission intensity as a function of excitation wavelength in this medium follows the absorption spectral trace. Interestingly, such medium dependent excited state behavior is not reflected in the fluorescence emission as shown in Figure 2. The spectrum recorded in hydrocarbon solution looks nearly identical with the gas-phase emission spectrum. Clearly, MCH does not have any effect on the emitting state of PFP. The electronic interaction of MCH with PFP being small, no large shift of the emission spectral maximum is expected. It can therefore be inferred that the emitting state is different from the ππ∗ state accessed by vertical electronic excitation from S0, because, as shown above, the dynamics of the ππ∗ state is profoundly affected by molecular collisions. We suggest that emission occurs from a bound πσ∗ (C–F) as proposed for hexafluorobenzene and which is consistent

FIG. 5. The MOs, labeled by MO numbers, involved in a few low-energy electronic transitions of PFP corresponding to S0 optimized geometry. Calculation was performed at TDDFT/CAM-B3LYP/6-311++** level. Shown also are the calculated transition energies (nm/eV) and oscillator strengths (within brackets).

with predictions of the electronic structure calculations. In MCH, the relaxation of the Franck-Condon state can occur via the two following mechanisms. First, an intramolecular process that originates due to an interplay of the FranckCondon active ππ∗ state with the dark and non-bound πσ∗ (O–H) states. This can lead to excited state dissociation of the O–H bond. It has been mentioned before that a recent photodissociation study has demonstrated occurrence of O–H bond dissociation upon ππ∗ excitation of the unsubstituted phenol.11,13 In the case of PFP, O–H bond dissociation could be more facile due to large lowering of the πσ∗ (O–H) state. According to FE spectral data, there are dark states very close to the electronic origin of the allowed ππ∗ state, which result to disappearance of fluorescence with increase in excitation energy. The second process that occurs in MCH solution is the collisional relaxation of the Franck-Condon state to the emitting bound πσ∗ (C–F) state. In the hydrocarbon liquid, this later process could be more preferred over the former, and


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FIG. 6. Comparison of FE and absorption (abs) spectra of PFP and phenol recorded in the vapour phase and MCH solution (10−4 M). The probe wavelengths for FE spectral recording, both in solution and vapor phases, are 385 and 300 nm, respectively, for PFP and phenol.

as a result, the FE spectral trace mimics the absorption spectral trace in this medium. We discuss below a chemical approach that unearths the hidden electronic states isoenergetic to the optically bright ππ∗ state of PFP. D. New electronic transitions in hydrogen-bonded liquids

The electronic absorption spectra of PFP recorded in four different solvents are presented in Figure 7. In each panel, the measured solution phase spectrum is compared with the vapor phase spectrum to highlight the solvent induced changes. Similarly, the spectral changes observed for phenol in different solvents are also shown. In MCH (part (a)), as pointed out before, the overall appearance of the absorption spectrum of PFP is not different from that of the vapor phase spectrum. The wavelength maximum of the lowest ππ∗ band is shifted from 260 (vapor) to 263 nm (MCH solution) only. The solvent induced shift of λmax does not appear much prominent because of the larger widths of the absorption bands. On the other hand, for phenol, the vibronic bands being narrower, the changes appear more prominent. In acetonitrile, which is a significantly polar but aprotic liquid, the spectrum of PFP (part (c)) still does not look much different, but for phenol, the sharper vibronic bands are broadened with distinct red shifts (part (d)). In contrast, in the two protic liquids, the

spectral changes of PFP are profound (parts (e) and (g)). Although the red shift of the ππ∗ band is nearly similar to what occurs in acetonitrile, several intense new features develop indicating strong interaction of the solute with the protic solvent molecules via hydrogen bonding at its different sites. The band developed at wavelengths longer than 270 nm is certainly the result of such interactions. Below, we discuss the origin of these new solvated features. The spectral band fitting of the measured spectra in different solvents is shown in Figure 8. A comparison of fitted band structures between protic liquids and vapor phase indicates clearly that a new broad feature develops at longer wavelength that has λmax at ∼280 nm (methanol), and the intensity of the lowest ππ∗ band is enhanced significantly. Therefore, the new band cannot be assigned as the solvated ππ∗ transition of a fraction of PFP in methanol. We assign it to a new electronic transition that has gained intensity due to hydrogen bonding between solute and solvent molecules. The lower energy σ∗ orbitals must be the origin for this new band in the absorption spectra. Some of the πσ∗ states which are nearly isoenergic with the lowest ππ∗ state but remained dark in the vapor phase absorption spectrum could gain intensity in the protic liquids. Remarkably, similar changes also occur at shorter wavelengths of the spectrum. We notice that the band heads at 223 and 232 nm of the vapor absorption spectrum are no longer discernable in the protic liquids. Instead, a single


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FIG. 7. Solvent effects on absorption spectral changes of PFP and phenol. In each case (concentration 10−4 M), the corresponding vapor absorption spectrum is presented to highlight the changes.

headed more intense band appears at 225 nm in methanol (part (c)) and at 223 nm in water (part (d)). It is notable that such drastic changes do not occur in the case of phenol, and the spectra in the two protic liquids are similar to that in acetonitrile (parts (f) and (h)). This difference corroborates the suggestion that the new solvent-induced intensity enhanced bands of PFP in the protic liquids are due to contributions of the lower energy σ∗ orbitals, and we provide below additional theoretical support to justify the assignment. At still shorter wavelength, the second ππ∗ band appears with a red shift by several nanometers, and visibly, this band does not contribute to the intensity enhancement of the nearby new absorption bands. In order to understand how the protic solvents induce new transitions in the electronic absorption spectra of PFP, the electronic spectra of small PFP-(H2O)n clusters (n = 1-11) have been calculated by TDDFT method using CAM-B3LYP functional and 6-311++** basis set. The optimized structures

of four such complexes are shown in Figure 9 (part (a)) and the corresponding calculated electronic spectra are shown in part (b) of the figure. In the latter, the electronic transitions of bare PFP are indicated using black bar in each panel and solvent-induced changes are shown using different colors. In the case of the 1:1 complex, water acts only as the H-bond acceptor (panel-1) and phenolic O–H the donor. As a result, a new πσ∗ band at shorter wavelength corresponding to phenolic σ∗(O–H) shows up near 200 nm, along with usual enhancement of the ππ∗ transition. In this complex and also in others up to n = 4, water molecules are clustered only around the phenolic OH group, which do not affect the σ∗(C–F) orbitals. Thus, the relative intensities of the lower energy πσ∗ transitions remain almost unchanged in smaller clusters. With increase in cluster size, the H-bonding network is extended to ortho and meta F sites of PFP (Figure 9, part (a)) and the effects of such interactions are revealed in the spectra depicted in other three panels. In the case of 1:11 complex, the πσ∗


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FIG. 8. Spectral fit of the absorption spectra of PFP (blue-green trace) measured under different conditions. The fitted spectra are depicted as dotted black traces, and the component bands are depicted by continuous red traces. The band positions are indicated by arrows and their respective areas are mentioned in brackets.

transition at the longest wavelength distinctly gains intensity and such predicted changes are consistent with observations. The dramatic absorption spectral changes (Figures 7 and 8) occur only in pure protic liquids, where each solute molecule is fully solvated by many solvent shells. It is shown below that in the condition when 1:1 complex is dominant, the changes in the measured absorption spectra are also small. An interesting effect of complexation of the protic liquids on excited state behavior of PFP is revealed also in the emission spectra. Depicted in Figure 10 (part (a)) is that the fluorescence yield of phenol is significantly enhanced in all the three polar liquids, acetonitrile, methanol, and water. The wavelength of fluorescence maximum also shows a small (∼10 nm) red shift indicating stabilization of the emitting state. In the case of PFP (part (b)), somewhat similar behavior is also noted but only in acetonitrile, where the maximum of the emission spectrum shows a further red shift by ∼15 nm. However, the fluorescence yield in this liquid is smaller compared to that in MCH. The behavior of PFP in the two protic liquids is very different, where the molecule turns out to be totally non-fluorescent. The observations imply that the protic liquids affect the emitting states of phenol and PFP differently, and the difference is consistent with the assignment that the emitting state of PFP is πσ∗ type, whereas for phenol, it is the ππ∗. For disappearance of PFP fluorescence in methanol and water, the following explanations can be given. A straightforward one is further lowering of the emitting πσ∗ state due to hydrogen bonding interactions with solvents that result in faster internal conversion to the ground state. But the other likely possibility is opening up of a new dynamical channel for internal conversion of the solvated molecules to ground state. As explained before that the Frank-Condon

allowed lowest excited state of solvated PFP in protic liquids is a πσ∗ state. In consequence, the decay of the solvated molecules from this new state depends on the nature of the interstate coupling. Second, the most likely reason for the large spectral bandwidth of this transition (Figure 8) is that the solvated geometry of the molecule in this excited state is largely distorted compared to the ground state geometry (Figure 3). Such large scale change in configuration can prompt faster internal conversion. The other possibility is a charge transfer (CT) type transition of solute-solvent complex involving the low-lying σ∗ orbital of PFP and non-bonded electrons of solvents. Below, we present further discussion on this issue. The complexation efficiency of PFP with methanol has been depicted in Figure 11. The spectral traces shown here indicate that the fluorescence intensity of the molecule in MCH (10−4 M) sharply drops with only a little addition of methanol. Thus, for a 1:1 mixing ratio, where the produced complex is primarily 1:1 type,48 the fluorescence intensity drops by more than 90%. This implies clearly that the 1:1 complex itself is a non-emitting species. The spectral fit of the absorption spectrum under this condition is depicted in part (b) of the figure. It is seen that the absorption intensity of the 263 nm monomer band does not show any sign of depletion, but the new band (λmax∼280 nm) that appears with a large spectral bandwidth is barely discernable within the band profile. This observation is consistent with the theoretical prediction that the intensity of πσ∗ transition of the 1:1 complex is barely different from that of the uncomplexed molecule (Figure 9). Furthermore, the spectral attributes displayed here, i.e., large width of the absorption band and disappearance of fluorescence indicate that a CT type process


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FIG. 9. Shown in part (a) are the structures of four different water complexes of PFP. Theoretically predicted (TDDFT/CAM-B3LYP/ 6-311++** method) wavelengths and oscillator strengths for vertical excitations for these complexes have been shown along with that of the PFP monomer (black bars) in the four panels of part (b) of the figure. Colored bars are used for the complexes. The natures of the electronic states (ππ∗ or πσ∗) are also shown. All the calculated wavelengths are scaled by 1.143 for comparison with the monomer spectral positions.

involving nonbonding electrons of methanol and low-lying σ∗ orbitals of PFP mediated by hydrogen bond between the two molecules could be a strong possibility. Further work in this direction is in progress in our lab, and details will be reported elsewhere.

IV. SUMMARY

In this paper, we have presented a photophysical study demonstrating the perfluoro substitution effects on electronic spectra of phenolic chromophore. In the vapor phase at


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FIG. 10. Solvent effects on fluorescence spectra of (a) phenol and (b) PFP. In each solution, the concentration of the solute is same, 10−4 M solutions.

room temperature, while the S1 ← S0 electronic absorption system of the unsubstituted phenol displays discrete vibronic structures, the same for PFP within the same energy range appears completely diffused. Below 6 eV of excitation energy, two new electronic features appear in the absorption spectrum of PFP, which are absent in the spectrum of the unsubstituted phenol. These new bands have been assigned to πσ∗ electronic transition. Although the absorption maximum for the lowest ππ∗ system of PFP is similar to that of phenol, the fluorescence maximum of the former displays a very large Stokes shift, and this emitting behavior remains unaltered in a hydrocarbon liquid. This red-shifted fluorescence has been assigned to an emission from a bound πσ∗(C–F) state lying below the lowest ππ∗ state. In a polar aprotic liquid like acetonirile, the wavelength maximum is shifted to further red. On the other hand, the emission is completely quenched in protic liquids like water and methanol displaying a broad new feature in the absorption spectrum at wavelengths longer than the lowest ππ∗ state. It has been suggested that the new band could be due to a CT type transition of the solute-solvent complex involving lower energy σ∗ orbitals of PFP. The fluorescence excitation spectra manifest an interesting energy dependent dynamical behavior in the excited surface. In the vapor phase, i.e., when the molecules are isolated, the emission is dramatically diminished for a little increase in excitation energy beyond the S1 electronic origin. However, this behavior disappears in a weakly interacting

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FIG. 11. Changes in fluorescence spectrum of PFP in MCH (10−4 M) upon successive addition of trace amounts of methanol (part (a)). Excitation wavelength is 270 nm. The corresponding change in absorption spectrum for 90% quenching of fluorescence due to methanol addition is depicted in part (b) along with a spectral fit to indicate positions of the new bands and their areas.

collisional medium like hydrocarbon liquid, which implies that the underlying dynamical process is hindered in such medium. We propose that the energy dependent non-radiative decay behavior of the isolated molecule originates because of an interplay between the optically bright ππ∗ and a dissociative dark πσ∗ state, and the likely chemical outcome is dissociation of the phenolic O–H bond. In the condensed hydrocarbon liquid, this bond breaking event is likely to be hindered either due to cage-effect of the medium and/or fast collision-induced vibrational relaxation in the excited surface, i.e., faster decay of the Franck-Condon state to the lowest emitting state, and the latter has been assigned to be a πσ∗(C–F) type state. In protic liquids, several new electronic transitions show up in the electronic absorption spectrum, both below and above the lowest ππ∗ state. At shorter wavelengths, the changes are more prominent. We have attributed these changes to brightening up of some of the πσ∗ states owing to intermolecular hydrogen bonding between solute and solvents. Electronic spectra of a series of PFP-(H2O)n clusters (n = 1-11) calculated by electronic structure theory method at DFT/CAM-B3LYP level are found consistent with the suggestion. In protic liquids, the spectral changes occur due to H-bond formation between the solvent OH and C–F of PFP. The possibility for CT type transitions involving the lower energy σ∗ orbitals of PFP moiety in solute-solvent complexes has also been discussed. To our knowledge, such behavior has not been observed for any other phenolic compound. A direct time domain measurement for unraveling more features


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of the excited state dynamics is essential. It is also essential to employ electronic structure and dynamical calculations at higher levels for information about electronic structure of PFP near its low-lying πσ∗ state to understand the origin of the medium dependent non-radiative dynamics.

ACKNOWLEDGMENTS

The authors acknowledge CSIR and DST, Government of India, for partial support to this research. S.K. and D.P.M. also acknowledge CSIR for research fellowships. 1J.

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