Ultrafast Photophysics of the Isolated Indole Molecule - The Journal of

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Ultrafast Photophysics of the Isolated Indole Molecule
lvaro Peralta Conde, Virginia Ovejas, Fernando Casta~no, and Asier Longarte* Ra
ul Montero, A Departamento de Química-Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV/EHU), Apart. 644, 48080 Bilbao, Spain ABSTRACT: The relaxation dynamics of the isolated indole molecule has been tracked by femtosecond time-resolved ionization. The excitation region explored (283
243 nm) covers three excited states: the two ππ* Lb and La states, and the dark πσ* state with dissociative character. In the low energy region (λ > 273 nm) the transients collected reflect the absorption of the long living Lb state. The La state is met 1000
1500 cm
1 above the Lb origin, giving rise to an ultrafast lifetime of 40 fs caused by the internal conversion to the lower Lb minimum through a conical intersection. An additional ∼400 fs component, found at excitation wavelengths shorter than 263 nm, is ascribed to dynamics along the πσ* state, which is likely populated through coupling to the photoexcited La state. The study provides a general view of the indole photophysics, which is driven by the interplay between these three excited surfaces and the ground state.

I. INTRODUCTION Indole, the aromatic ring of the aminoacid tryptophan, has appeared in the last decades as one of the most important biological chromophores. It provides to peptides, proteins, and some neurotransmitters the ability of absorbing light around 300 nm, in the near UV part of the sun light spectrum. Consequently, the photochemical and photophysical processes triggered by its electronic excitation have attracted a great interest. At the same time, the extensive use of tryptophan as fluorescent probe, motivated by the sensitivity of its emission to the molecular environment, has stimulated numerous studies to gain a better understanding on the indole electronic structure.1
3 Two singlet electronic excitations have been theoretically predicted and experimentally identified to appear in the low energy portion of the electronic spectrum of indole. These two excited states have ππ* character and are named Lb (S1) and La (S2) following Platt’s4 nomenclature.5
12 In the isolated molecule Lb is the lowest excited state, with its origin located at 35 233 cm
1. The exact location of La has been a matter of discussion, but recent works place the origin about 1800 cm
1 over Lb.13,14 The different dipole moment of these states (1.86 and 5.86 for Lb and La, respectively)15,16 permits a preferential stabilization of La in polar solvents, resulting in an inversion of the energy ordering. The sensitivity of the La state fluorescence of tryptophan to the environment has been widely used to probe the structure and dynamics of proteins (see ref 17 and references therein). More recently, the existence of a dark πσ* type excitation at relatively low energies, resulting in the promotion of an aromatic electron to the 3s orbital of the nitrogen atom, has been addressed.9,10,18 The πσ* state presents a dissociative surface along the N
H bond, crossing the La, Lb and ground states as the bond stretches.18 The couplings between these surfaces give rise to a variety of relaxation channels that determine the complex photophysical and photochemical behavior of the molecule. The internal conversion (IC) and hydrogen detachment mediated by r 2011 American Chemical Society

πσ* states has been postulated as a general photochemical mechanism, common to many aromatic species containing heteroatoms, as aniline,19,20 phenol21,22 or pyrrole.23,24 In particular, the key role of these processes in the photostability of biologically relevant molecules as aminoacids25 or DNA basis26 has been substantiated in numerous works. For the indole ring ab initio calculations locate the πσ* state at a vertical excitation between 4.8 and 5.05 eV, whereas La and LB are predicted to be reached in the 4.3
5.05 eV range.9,10,12 Different experimental studies in the time27 and frequency domain28
30 have collected evidence on the presence of the πσ* surface, and its spectroscopic and dynamic consequences. In the work by Nix et al.,30 kinetically resolved fast hydrogen fragments attributed to dissociation along the πσ* surface are detected following photoexcitation in the 263
256 nm range. The existence of this ultrafast dissociation channel is confirmed in the timeresolved experiments carried out by Iqbal and Stavros,27 measuring two lifetimes of 100 and 195 fs for the H fragment formation after excitation with 200 nm. However, previous time-resolved experiments have not succeed in detecting the dynamical signature of relaxation processes involving the πσ* state, directly on the parent indole molecule,31 precluding a simultaneous characterization of the competing relation channels. The nature of the Lb/La state coupling and its relationship with the fluorescent properties of the molecule has also received considerable attention from a theoretical and experimental point of view.32,33,13,14 Ab initio calculations predict a conical intersection (CI) connecting both surfaces at about 2000 cm
1 above the Lb minimum with a geometry very close to that of the La Special Issue: Femto10: The Madrid Conference on Femtochemistry Received: August 12, 2011 Revised: October 11, 2011 Published: November 03, 2011 2698

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Figure 1. Decays recorded in the 283
269 nm excitation range by probing with the 1305 pulses. The circles represent the experimental data, and the red solid line is the best multiexponential fit obtained. The black solid, blue dashed, and gray dashed-dotted lines correspond to the individual exponential components used in the fitting. The nonresonant ionization signal of ethylene (dashed line and black circles) used to determine the instrumental function and zero delay is shown in (a).

minimum,13 whereas the frequency resolved experiments have provided a detailed characterization of the strong vibronic interaction between both states.33,13 However, to the best of our knowledge, no dynamical sign of the coupling has been reported up to date in the indole molecule. Herein, we present a time dependent study with femtosecond resolution of the processes involved in the relaxation of the indole molecule, after excitation in the 283
243 nm region, which covers the location of the S1(Lb), S2(La), and S3 (πσ*) states. The employed experimental methods based on time delayed multiphoton ionization provide a high sensitivity to the different dynamic phenomena. The transients recorded reflect the ultrafast coupling of the Lb/La states at excitation energies where the minimum of the La is predicted to lie. The signature of the relaxation along the repulsive πσ* surface is also identified at excitation energies above 263 nm. The collected set of data provides a detailed view of the photophysical and photochemical events following the excitation of the indole molecule, allowing us to balance the relative importance of the different relaxation channels.

II. EXPERIMENTAL SETUP The evolution of electronically excited indole molecules has been followed by time delayed multiphoton ionization. The measurements were carried out in an experimental system described in detail elsewhere,34 with a time-of-flight (TOF) mass spectrometer. The vapor of indole (Aldrich 99%) heated at 50 °C in equilibrium with 2 bar of Ar is expanded through a pulsed electromagnetic valve to form a supersonic expansion. The collimated beam interacts with the femtosecond pulses generated by a Ti:saphire oscillator-regenerative amplifier system (1 KHz, 4.0 mJ, 35 fs pulses at 800 nm). The pump pulses were produced in the 283
243 nm interval by the second harmonic of the sum interaction of the idler or signal of an OPA (optical parametric

amplifier), and the 800 nm fundamental beam. After different wavelengths (400, 800, and 1305 nm) were tried to achieve maximum selectivity, the output of a second OPA system at 1305 nm was chosen to probe the molecule, in all cases but the longer time transients. The probe beam intensities were estimated to be (0.7
1)  1012 and (3.5
6)  1012 W/cm2 for the 800 and 1305 nm wavelengths, respectively. The pump intensity was adjusted to avoid saturation effects and contribution from higher electronic excited states. The nonresonant ionization signal of ethylene, 1 + 40 (50 ) at 800 (1305) nm, was collected simultaneously to the indole ion mass to establish the zero delay time Δt = 0 and the cross-correlation function. The latter yielded values around 100 fs for the different excitation wavelengths. The relative polarization of pump and probe beams was kept at magic angle, except for the anisotropy measurements.

III. RESULTS Transients have been collected at the indole mass channel (117 uma) following excitation at different wavelengths in the 283
243 nm range, and further multiphoton ionization with 1305 or 800 nm (long-term and anisotropy decays) radiation. The measurements covered two different time scales that extend up to 3 and 140 ps. Figure 1 summarizes the decays collected at excitation energies in the 283
269 nm interval, while probing in all cases with 1305 nm radiation. The transient a, corresponding to excitation at the Lb state fundamental band, is modeled by the convolution of the femtosecond pulses with a nanoseconds living state, not showing any sign of ultrafast dynamics. At energy excesses between 750 and 1800 cm
1 above the Lb origin (Figure 1b,c), a rising ∼200 fs feature (τ2) is noticeable in the transients. When the excitation energy reaches 1400 cm
1 above the origin (Figure 1c,d), a second component (τ1) is required to fit the signal. This lifetime has an initial value of 42 ( 9 fs at 269 nm that 2699

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Figure 2. Polarization anisotropy decay as a function of the delay between the 243 pump and the 1305 nm probe.

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Figure 4. Long-term components recorded at the indicated excitation wavelengths with the 1305 nm probe.

The r(t) signal, shown in Figure 2, rises up to a value of 0.1 in 20 fs, matching the lifetime τ1 observed in the magic angle transients at the same excitation energy. Figure 3 summarizes the transients collected at pump wavelengths from 260 to 243 nm. This region is predicted to cover the vertical excitation energy of the S3 πσ* state. The transients collected contain two lifetimes apart from the long-range component that extends up to picoseconds. In addition to the τ1 lifetime mentioned above and related with the La/Lb coupling, a τ3 ∼ 400 fs constant appears at a threshold wavelength of 263 nm, becoming more pronounced at higher energies. This component is associated with the relaxation of the system along the πσ* repulsive surface, postponing a detailed discussion of the process to the next section. Long-term transients, extending up to 140 ps, were also recorded in the whole range of excitation, while probing with 800 nm radiation. The results, collected in Figure 4, yield a lifetime (τ4) that shortens from several nanoseconds at 260 nm, up to 150 ps at 243 nm. This lifetime reflects the relaxation of the electronically excited system to a location where the ionization probability drops to zero.

Figure 3. Transients collected in the 260
243 nm wavelength region while probing with the 1305 nm radiation. Circles and red line correspond to the experimental data and multiexponential fit, respectively. The black solid, blue dashed, and gray dashed-dotted lines are the individual exponential components with the indicated values. The inset in panel (b) shows the τ3 component in detail.

shortens up to 22 ( 9 fs at 243 nm. As will be discussed below, both constants are thought to be related with the La/Lb electronic coupling. To gain additional information on the process, the time dependent polarization anisotropy function r(t), was obtained by recording transients with parallel and perpendicular relative polarizations of pump and probe beams, at 243 and 800 nm excitation and ionization wavelengths, respectively.

IV. DISCUSSION To assign the observed dynamical features, it is necessary to revisit some of the theoretical and experimental evidence collected to date on the indole electronic structure. Three singlet excited states are predicted to lie in the range of excitation energies explored in this work: the two ππ* excitations of A0 symmetry, Lb and La, and the πσ* character state of A00 symmetry.9,10 Different energy ordering of the minima and the vertical excitation energies are found in the literature, depending on the type of calculations applied. A critical analysis on the reliability of these methods is beyond our capabilities and the scope of this paper, because we will ground the discussion on those well established aspects. In principle the most accurate data should be offered by the CASSCF/CASPT29,10 and the CC212 calculations. A considerable discrepancy on the relative energy of the ππ* and the πσ* vertical excitations is found between both methods. Whereas the CC2 overestimates the energy of the ππ* transitions, the CASSCF/CASPT2 underestimates the πσ* excitation.12 However, the results from these two methodologies agree on placing the vertical excitation energies to reach the above-mentioned three states in a gap of roughly 0.75 eVs, yielding an energy difference of less than 1000 cm
1 between the Lb and the La minima. Therefore, whereas the Lb/La energy 2700

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2000 cm
1 interval, the location of the πσ* is not so firmly established. Numerous experimental works have focused on this region of indole electronic spectrum.35,36,7,37,38,14 The first observed vibronic feature at 35 233 cm
1 has been unambiguously assigned to the Lb r S0 fundamental transition. Although the origin of the La state has not been precisely identified, the vibronically resolved spectra becomes diffuse about 1500 cm
1 over the Lb origin, which has been interpreted as the onset of the La state absorption.35
37,13,14 The work by Nix et al.30 provides a solid hint on the location of the πσ* state by detecting the formation of fast hydrogen atoms, attributed to N
H bond dissociation on this surface, at a threshold excitation wavelength of 263 nm, 2800 cm
1 above the Lb origin. A. λExc > 263 nm. The transients covering this region are summarized in Figure 1. At excitations below 278 nm they do not show any exponential component, being fitted by a steplike function that represents the population at the nanoseconds living Lb state.7 A discussion on the final relaxation of the Lb state is postponed to the end of this section. Two femtosecond components are noticeable at higher excitation energies, τ1 = 40
20 fs and τ2 ∼ 200 fs. The latter is a rising exponential present in the range 750
1800 cm
1 above the Lb origin, whereas τ1 is a decaying component occurring when the excitation energies exceeds 1400 cm
1. We associate both constants with the La/Lb coupling, because they appear in the region where the vibronic spectrum gets more diffuse and finally disappears. The coupling of both states is believed to occur through a CI located close to the La minimum, which yields an extremely short lifetime for this state. The τ2 component reflects a gain in the ionization cross-section and may originate from dynamics in the lower part of the cone formed by the Lb/La states. At 278 nm the 35 fs pump pluses employed with ∼420 cm
1 spectral width (fwhm) excite the system in the region of Lb/La mixing. Therefore, the wavepacket initially formed in the Lb may gain La character along the coupling coordinates, inducing the observed enhancement of the ionization efficiency. At slightly higher energies, 1400 cm
1 over the Lb origin, the appearance of the τ1 constant indicates the direct excitation of the La state. The oscillator strengths for La and Lb are 0.123 and 0.045, respectively, the La state will be preferentially populated following photoexcitation at this energy. The prepared La state decays to the lower Lb through the CI with the τ1 constant. This relaxation process seems to be the main channel in the full range of studied energies, which essentially covers the La absorption band. The time dependent anisotropy function points to the same explanation of the τ1 component. The signal (Figure 2), collected at 243 excitation wavelength, shows a rising time component of ∼20 fs, indicating an ultrafast change in the absorption of the probe with respect to that of the pump. This could be caused by the appearance of a resonance in the probe absorption process when the system is ionized from the relaxed Lb state. B. λExc < 263 nm. The transients collected in this region are shown in Figure 2. Essentially, they are characterized by the appearance of a new temporal constant τ3. This decay, with values around 400 fs, starts to be weakly observable in the 265
260 nm interval, becoming more prominent at shorter wavelengths. The observed onset matches very well the threshold energy to detect fast hydrogen atoms reported by Nix et al.30 Consequently, we assign the τ3 lifetime to relaxation dynamics along the πσ* surface. As stated above, this Rydberg nature excitation involving the 3s orbital of the nitrogen atom gains

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repulsive character along the N
H bond. Additionally, at long N
H distances, the πσ* state is coupled to the ground state through a CI promoted by out-of-plane modes of the H atom.12,18 The observed τ3 constant very likely describes the crossing of this intersection that simultaneously populates the ground state and takes the system toward dissociation. Previous time-resolved measurements in the femtosecond time scale by Iqbal and Stavros27 have reported two different lifetimes of 100 and 200 fs, for the formation of H atoms after excitation of the indole molecule at 200 nm, being the fastest channel attributed to the dissociation along the πσ*. The measured lifetime is 4 times shorter than the value reported here for the dynamics on the πσ* surface. The disagreement may be attributed to the different excitation energy employed. The 200 nm wavelength likely excites the system along the ππ* character 1Bb state (following Platt’s4 nomenclature), which is the strongest absorption observed in the indole spectrum centered at 6.02 eVs.5,6 Therefore, the system may follow a different relaxation pathway to reach the πσ*, making very complicated the comparison with the results presented here. In the above-described picture a key point of the problem, such as the pathway followed by the system to reach the πσ* state, remains unsolved, because it seems apparent that the τ3 constant accounts for the dynamics once the system is at the πσ*, but the measured transients do not show any dynamical feature that could reflect the pathway followed to reach this state. A possible approach is to consider that the πσ* state is formed by direct excitation, simultaneously with the intense La absorption. However, taking account of its almost dark character, the observation of dynamics along the πσ* surface on the top of the signal originated by the La would not be possible, unless the ionization cross section was much higher than that of the La state. Although this situation has been invoked previously to explain the dynamics of the πσ* state in aniline,20 it does not seem plausible for the indole molecule, attending to two fundamental reasons: the oscillator strength of the πσ* state is much lower than that of aniline19 and the ionization cross sections of the La and πσ* states of indole are presumably quite similar. The last argument is supported by the fact that both states ionize to the same ion state D0 (7.92 eV),39 and the energy gap between them is expected, as stated above, to be very small. The alternative route to populate the πσ* state is by coupling to the photoexcited La and/or Lb state. In principle calculations predict that the πσ* surface is coupled to both ππ* states through a00 out-of-plane vibrational modes.10,12 La being preferentially populated after excitation in this energy region, the simplest pathway seems the transfer from La through the formed CI. However, we cannot discard more complex routes involving the Lb state. In any case, the question that arises then is whether we are able to identify the dynamical signature of this IC process. Assuming, as stated above, that the τ1 and τ2 lifetimes are related with the La f Lb IC, the ππ*/πσ* has to be a dark process not reflected in the collected measurements. This could be explained by a negligible change in the ionization efficiency along the process, likely caused by the small energy difference between both minima. Alternatively to the above explanation, the ππ*/ πσ* coupling could take place at a rate very similar to that for the La f Lb IC. In this situation, the measured τ1 constant would be, once the threshold excitation energy of the ππ*/πσ* crossing is met, a mixture of the La f Lb IC and the ππ*/πσ* coupling. Assuming that πσ* state is populated through coupling to La, the photophysical picture that emerges for the indole molecule is 2701

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Figure 5. Cartoon of the relaxation processes observed for the indole molecule following excitation in the 263
243 nm interval. τ1, τ3, and τ4 are the lifetimes extracted from the transients of Figure 3. τ? corresponds to the πσ* r La* coupling.

depicted in Figure 5. At excitation wavelengths longer than 273 nm, Lb is the prepared excited state, having a lifetime of the order of nanoseconds. Above this energy, the La state, due to its higher oscillator strength, is preferentially populated. La internally converts in tenths of a femtosecond to the lower Lb state through a CI. At excitation wavelengths between 273 and 263 nm, the coupling between the La state and the πσ* state becomes accessible, populating the latter. For wavelengths shorter than 263, the vibrational energy in the πσ* overcomes the barrier to accede to the repulsive part of the potential, producing the photodissociation of the molecule and the conversion to the ground state, simultaneously. In this range of energy, the relaxation of the excited La state would occur simultaneously through IC to the Lb and the πσ* states; therefore, the fraction of the population reaching each of the states will be determined by the ratio between the time constants of both processes. C. Long-Term Relaxation: τ4. The τ4 constant, which accounts for the relaxation of the long living Lb state, is observable in the full range of studied excitation energies. Whereas at excitation wavelengths below 273 nm the Lb may be populated by direct photoexcitation, at higher energies, the La f Lb IC channel competes efficiently with any other relaxation mechanism. Although a quantification of the population internally converting to the Lb state would require one to know the ionization efficiency from the different surfaces, the collected transient signal seems to indicate that this state is the destination of most part of the excited molecules. The τ4 lifetime shortens from several nanoseconds at the origin of the Lb state, up to the 150 ps value measured with the 235 nm excitation. This observation can be related to the increment of the IC quantum yield derived from the triplet
triplet energy transfer experiments carried out by Sukhodola on indole vapor.40 As has been reported for other aromatic species,41,20 the Lb/S0 IC could be promoted by a CI, which being located at higher energies, would induce a gradual increment of the IC rate, as the vibrational energy excess approaches the crossing point. The involvement of triplet states in the relaxation of the indole molecule has been addressed by Park et al.42 Their electron diffraction measurements at 267 nm excitation reveal an intersystem crossing process with a 6.3 ps lifetime. Our transients do not show any similar lifetime in the explored region of energies or any hint of the participation of triplet states. This relaxation channel could be absent from our measurements due to a very small change in the ionization efficiency along the ISC process.

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V. CONCLUSIONS The reported time domain experiments with femtosecond resolution provide new insights on the rich photophysics of the indole molecule. The electronic relaxation of the molecules is driven for the interplay of at least three singlet electronic states and the gronud state: S1(Lb), S2(La), and S3(πσ*). The ultrafast IC of the La state toward the lower Lb minimum is detected at threshold energies in the range 800
1400 cm
1, above the origin of the Lb. The 40 fs time constant measured points to the involvement of a CI, as has been theoretically predicted. At excitation wavelengths shorter than 263 nm (2800 cm
1 above Lb minimum), the collected transients show an exponential component of ∼400 fs, attributed to dynamics along the πσ* surface. This process can be related with the formation of H atom fragments observed in previous experiments.27,29,30 The formation of the πσ* state in the explored energy region (283
243 nm) likely occurs by coupling to the photoexcited La state. However, the signature of this mechanism cannot be unambiguously identified in the measured transients, not allowing us to establish the location of the crossing point. The photophysics of the indole molecule (beyond the initial 1400 cm
1 of the spectrum corresponding to the Lb state) is dominated by the absorption and subsequent relaxation of the La state. The IC of this state populates the Lb and πσ* surfaces in the femtosecond time scale. Whereas the former shows a lifetime that shortens from nanoseconds to 150 ps with the excitation energy, presumably due to the increasing rate of the IC to the ground state, the latter gives rise to the ∼400 fs dynamics above-mentioned, when the excitation energy is enough to reach the repulsive part of the surface along the N
H coordinate. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone ++34 946018086. Fax: ++ 34 946013500.

’ ACKNOWLEDGMENT We are very thankful to Dr. Dave Townsend for sharing his data with us, and for the helpful discussions and comments on the manuscript. This study was funded by Spanish MICINN under the grant CTQ2010-17749 and Consolider Program “Science and Applications of Ultrafast Ultraintense Lasers” CSD200700013, and by the Basque Government through the “Ayudas para apoyar las actividades de grupos de investigaci
on del sistema universitario vasco” program. The experiments were carried out at the SGIker laser facility of the UPV/EHU. ’ REFERENCES (1) Lakowicz, J. R. In Principles of Fluorescence Spectroscopy; Springer Science+Business Media, LLC: Berlin, 2006. (2) Zhong, D.; Pal, S. K.; Zhang, D.; Chan, S. I.; Zewail, A. H. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 13. (3) Br€am, O.; Oskouei, A. A.; Tortschanoff, A.; van Mourik, F.; Madrid, M.; Echave, J.; Cannizzo, A.; Chergui, M. J. Phys. Chem. A 2010, 114, 9034. (4) Platt, J. R. J. Chem. Phys. 1949, 17, 484. (5) Lami, H. J. Chem. Phys. 1977, 67, 3274. (6) Lami, H. Chem. Phys. Lett. 1977, 48, 447. (7) Bickel, G. A.; Demmer, D. R.; Outhouse, E. A.; Wallace, S. C. J. Chem. Phys. 1989, 91, 6013. (8) Callis, P. R.; Vivian, J. T.; Slater, L. S. Chem. Phys. Lett. 1995, 244, 53. (9) Serrano-Andr
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