Supporting Information

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1 Supporting Information Wiley-VCH Weinheim, Germany

2 Ultrafast Solvation of N-Methyl-6-Quinolone Probes Local Infrared Spectrum J. Luis Pérez Lustres, Sergey A. Kovalenko, Manuel Mosquera, Tamara Senyushkina, Wolfgang Flasche, Nikolaus P. Ernsting* [*] Dr. J. L. Pérez Lustres, Dr. S. A. Kovalenko, Dr. T Senyushkina, W. Flasche, Prof. N. P. Ernsting* Department of Chemistry Humboldt University Brook-Taylor Str. 2, Berlin, Germany Fax: +49 () nernst@chemie.hu-berlin.de Prof. Manuel Mosquera Department of Physical Chemistry, University of Santiago de Compostela Santiago de Compostela, Spain 1

3 1. Experimental section Materials. MQ was prepared by refluxing equimolar amounts of 6-hydroxyquinoline (98 %, Aldrich) with methyl iodide in toluene for 48 hours. The iodide and hydroxyl proton were removed with amberlite Ira 42 and CaH 2 in toluene. After filtration the solvent was evaporated. The resulting form, MQ, was flushed with argon, put into liquid nitrogen and dried under high vacuum for 72 hours. The dried form was fully characterized by 1 H- and 13 C-NMR, mass spectrometry and elementary analysis. Protonated MQ has a pk a of 7.2 and the neutral form can be excited selectively in slightly basic media. Optical measurements were performed in spectroscopic grade solvents (Aldrich) or in deionized water at ph = 9 ([NaOH] = 1-5 M) and 298 K. The peak S 1 S extinction coefficient is independent of solvent, and the radiative rate calculated according to Strickler/Berg agrees with the experimental value from quantum yield and lifetime measurements. It follows that the radiative rate should not change significantly during solvation in the investigated liquids. The fluorescence quantum yield in methanol is about 1%. Quantum mechanical calculations. Ground state molecular properties were calculated for fully optimized geometries at the ab initio and DFT levels RHF/6-31+G* and B3LYP/6-311+G**, respectively. Semiempirical AM1 calculations show excellent agreement with the DFT ones in the ground state and they were used to calculate the electronic spectrum (AM1-PECI) and the molecular properties in the S 1 state. Dipole moments in the gas phase are predicted to be µ = 1.2 D (from DFT; 9.14 D with AM1) and µ 1 = 5.9 D in S and S 1, respectively, oriented O N. Pump-supercontinuum-probe spectroscopy. In water, MQ was excited to S 1 with small excess vibrational energy by 5 fs pulses at 432 nm. After a variable delay the range 28 to 69 nm was probed with spectral and time resolutions of 1.5 nm and 3 fs, respectively. In methanol, MQ was excited by 15 fs pulses centered at 54 nm without excess vibrational energy. The time resolution is estimated to be 2 fs in this case and the transient spectra resemble those measured in water. Transient spectra must be time-corrected for the group-delay dispersion of the probe light. We showed that the time correction is identical for the resonant and nonresonant case. [1] All transient data which are presented and discussed have been time-corrected using nonresonant signal from the pure solvent. The experimental scheme is shown in Figure 5. Measurements in water and methanol were performed with different setups. For water, basic pulses were delivered by a multipass 2

4 Titanium:Sapphire (Ti:Sa) laser system (Femtosource+Femtopower, FEMTOLASERS,.6 mj/pulse, 35 fs, 18 Hz). Part of the output drove a collinear optical parametric amplifier (TOPAS, LIGHT CONVERSION) to produce 5 fs pulses at 432 nm. After compression the latter (1 µj) were focused to a spot diameter of ~ 15 µm to excite the sample in flow-cell (.4 mm path length, fused silica windows.2 mm thick). To generate the supercontinuum, 4 fs pulses at 4 nm were generated in a BBO crystal (.2 mm). After compression, 1 µj pulses were then focused into a CaF 2 plate (1 mm) with a thin lens (f = 2 mm, fused silica). The supercontinuum was filtered and split for reference before being imaged onto the sample cell (spot size ~ 1 µm). Transmitted and reference beams were imaged onto the entrance planes of separate homemade spectrographs, dispersed, and registered by photodiode arrays with 512 pixels (S Q, HAMAMATSU). Imaging was always accomplished with reflective spherical optics, combined for zero astigmatism and minimal coma. For methanol, basic pulses were obtained from a regenerative Ti:Sa amplifier (CPA 21, CLARK MXR,.9 mj/pulse, 15 fs, 12 Hz). They pumped a two-stage, non-collinear optical parametric amplifier (NOPA, CLARK MXR) tuned to 54 nm. 1 µj, 15 fs pulses were then split and used for optical pumping and supercontinuum generation. Measurements were performed at parallel, perpendicular and magic angle between the pump and probe polarizations. Baseline corrections were applied to single shots, and one transient spectrum represents the average from 5 consecutive shots. 2. Spectral decomposition The stimulated emission band is needed at every delay time t and therefore each transient absorption spectrum must be decomposed into its three components. Such a procedure is not required when the spontaneous fluorescence band is followed in time instead, and this is why we explain it here, with the methanol measurements as example. When spectral evolution has ceased the BL and SE components are given by the stationary spectra of Figure 6 with a common scaling factor. When BL and SE are subtracted from the transient absorption the stationary ESA spectrum remains. Scaling is found by inspection so that ESA is positive everywhere and has the same intensity as the BL+SE spectrum at those wavelengths where OD is zero. At earlier delays, the BL contribution is identical to the stationary bleach spectrum which was obtained before (vibrational coherence of solute molecules remaining in the ground state may cause spectral oscillations of the BL spectrum which can be recognized in the absorption range, but we do not observe this effect). Only the ESA and SE components are unknown. The former shows only a small solvation shift. Therefore it is reasonable to approximate the ESA by its stationary form and obtain in this way an estimate for the transient SE 3

5 spectrum. The estimate approaches the true SE component by the time it has shifted away from the prominent ESA band at 5 nm (cf. Fig. 2B). Even before it should represent the true SE component with its red part because ESA is small there. The estimate is fitted by a lognormal distribution [2] and the peak frequency ν is adopted as a measure of the free energy gap. The procedure is repeated for every pump-probe delay t: this is the desired time-resolved Stokes shift ν(t). 3. Calculation of the ole solvation relaxation function We assume throughout that the reaction field susceptibility to changes in the solute ole moment can be described by simple continuum theory, [3] i.e. by χ ( ν ) of Eqation (1). It is interesting to compare the (2-sided power) spectral density (SD) of fluctuations ( ν ) (Figs. 3C, 8C, 9C) with C the corresponding χ ( ν ) = ε ( ν ) for transverse polarization of the bulk liquid (Figs. 3A, 8A, 9A). Transformation (1) emphasizes fast processes. When the solute ole moment is suddenly changed at t =, the mean transition frequency relaxes to a new stationary value, and the normalized time behavior is captured by the nonequilibrium relaxation function S ν (t) of Equation (3). It can be reached from χ ( ν ) by inverse Laplace transformation to give the response function Φ 1 ( t ) L { χ ( ν )} (9) followed by integration S ( t ) Φ ( t ) dt = t (1) In order to calculate the solvation relaxation function it is better, from a numerical perspective, to bypass the inverse Laplace transformation of Equation (9) which depends critically on whether χ and satisfy the Kramers-Kronig relationships. Instead it is convenient to take only χ χ, a nonequilibrium quantity, and to step into the equilibrium world via the fluctuation-dissipation theorem. [4] The latter may be written χ ν ) = C ( ν ) (11) ( 4

6 Here real ( ν ) is the odd component (with respect to ν ν ) of the fluctuation SD, C C ( ν ) C ( ν ) + C ( ν ) + C ( t )e iνt dt (12) while ( ν ) is the even component. So far only ( ν ) is known by virtue of Equation (11). But C C the full SD can be constructed by C hν ( ν ) = 1 + coth C ( ν ) k BT (13) 2 where h and k B are Planck's and Boltzmann's constants and T is the temperature. The above procedure is now applied starting from ε( ν ) of pure water, methanol and methan-d3-ol as shown in Figures 3A, 8A, 9A. We use Equation (1) and assume a trial value for n cav. The result is multiplied with a Gaussian in the frequency domain to account for limited time resolution. Then we invoke (11) and use (13). Tracing back by inverse Fourier transformation (12) and taking the real part we obtain the desired relaxation function S ( t ). The Stokes shift and cavity radius must be consistent with the electronic properties of the probe for dielectric continuum theory to apply. This will be shown next. The relevant properties of the solute MQ are the ole moments µ and µ 1 in the ground and excited electronic state, repectively, the polarisability α 1 and cavity radius r. Our experiment provides values for the full Stokes shift, ν = 1 35m 2 cm, and for the cavity refractive index n cav = 2. 3±. 2. Regarding the latter imagine an dense packing of excited MQ molecules without interaction between them and let each molecule occupy a volume equivalent to a sphere with radius r. The number density becomes N 3 = 3 4πr (14) and the Clausius-Mosotti equation reads [3] 5

7 4πNα 3 1 n = n 2 cav 2 cav 1. (15) + 2 Figure 1 shows how the molecular polarizability α 1 in the excited state and cavity radius r are obtained. The data in the figure refer to MQ in water. r is calculated with Equations (1, 8) as a function of ν and n cav (from experiment) and µ (from quantum-mechanical calculations) while µ remains variable. Equations (14, 15) then give α 1 from r and n cav. Black lines show the calculations for different values of n cav when ν ranges from 285 to 325 cm -1 and µ from 3.5 to 5 D; the red line corresponds to optimal n cav = 2.3. Green dots show the same calculation when the optimal values of n cav and ν are kept and only µ is varied. The magenta dots are obtained when ν is varied instead and the optimal 1 values for the other two parameters are kept. The confidence range (defined by δ ν = m 2 cm, δn cav = ±.2 and δ µ = m 1 D ) is represented by the gray area. The mean values α 1 = 2 Å 3 and r = 3.3 Å can be read from the graph. These values are in good agreement with quantum mechanical calculations ( DFT α =2.1 Å 3 AM 1, r =3.35 Å). 4. On the oscillations in methanol To support the experimental results for MQ in methanol and methan-d 3 -ol we include kinetic traces at wavelengths in the stimulated emission band and compare them with the pure solvent signal in Figure 11. The signal oscillations for MQ solutions (red curves) and for the pure solvent (blue) are of different nature. Even though frequencies are identical, amplitudes and their spectral distribution are characteristically different: (i) MQ and pure solvent oscillations have comparable intensity around 65-7 nm whereas the former are stronger in the wings of the emission band ( λ probe < 65 nm or λ probe > 7 nm). After subtraction of the solvent signal the oscillations are therefore minimal around the SE maximum and relatively large in the blue and red (not shown). (ii) The phase of the oscillations in the MQ signal is wavelength-dependent. As seen in the left panels of Figure 11, for example, there is a phase shift of about π in the red curves upon going from 6 to 8 nm. This is also seen in the right panels, going from 65 to 85 nm, although at 85 nm the situation is not so evident due to higher noise level. This behavior indicates spectral modulation of the emission band. 6

8 (iii) The wavelength dependence of the phase in the solvent signal is minor. Compare for example the blue traces at 6, 7 and 8 nm. Crests appear at the same delays during the first 5 fs. Hence, systematic errors in the subtraction of solvent signal may modulate the SE amplitude but not its peak position or mean frequency. These observations, taken together, ascertain that the signal modulation which remains after solvent subtraction captures the oscillation in the S ν (t) function. Interestingly, experimental S ν (t) oscillates more intensely than calculated S (t) for methanol and methan-d 3 -ol (Figs. 4B and C). The disagreement between experiment and theory may point to local field or specific solvation effects. Another reason may be the quantum nature of the high-frequency solvent mode (compared to the classical treatment so far) which becomes noticeable when exciting in the far wing of the absorption band. [5] Uncertainties of cavity shape or of dielectric data may also be responsible. Spectral densities had to be constructed with ε( ν ) from different experiments so that the relative weights of slow and fast processes are not precisely known. For instance, the dielectric dispersion of methan-d3-ol was constructed from its optical constants measured by ATR and transmission together with the Debye contribution of methanol, since low-frequency experimental data are not available for the deuterated solvent. [1] S. A. Kovalenko, A. L. Dobryakov, J: Ruthmann, N. P. Ernsting, Phys. Rev. A 1999, 59, [2] M. L. Horng, J. A. Gardecki, A. Papazyan, M. Maroncelli, J. Phys. Chem. 1995, 99, [3] C. J. F. Böttcher, P. Bordewijk, Theory of Electric Polarization, Elsevier, Amsterdam, 1996, Vol. 1. [4] C. H. Wang, Spectroscopy of Condensed Media, Academic Press, London, 1985, pp [5] Y. Georgievskii, C. P. Hsu, R. A. Marcus, J. Chem. Phys. 1998, 18,

9 Delay Ti:Sa Oscillator-Amplifier BS OPA Frequency Doubling Chopper Lens PDA λ / 2 CaF 2 BS Pump Super Continuum Probe Sample Cell Lens Reference Spectrograph PDA Signal Spectrograph Figure 5. Experimental setup for femtosecond transient absorption spectroscopy with supercontinuum probing. 8

10 ~ 3-1 ν / 1 cm ε / 1 3 ltr mol -1 cm -1 Abs 298 K pump SE 298 K 2 ESA methanol, 298 K.1 14 ps OD / bleach SE λ / nm Figure 6. Optical spectra of MQ in methanol. (A) Stationary Absorption and Stimulated Emission. The pump spectrum (dashed line) overlaps only with the red wing of the absorption band. (B) Femtosecond transient absorption after excitation with 15 fs pulses centered at 54 nm. Spectra between.1 and.9 ps are shown in.2 ps steps and in 4 ps steps between 2 and 14 ps. 9

11 ν ~ 3-1 / 1 cm ESA OD / bleach SE λ / nm Figure 7. Spectral decomposition of transient absorption by MQ in methanol at 59 ps (with pumpprobe polarization at magic angle). The decomposition yields the positive contribution of excited-state absorption (ESA, green) and the negative contributions of bleach (blue) and stimulated emission (SE, red). The SE band is described by a lognormal distribution. 1

12 A ε ε methanol, 298 K ε B χ C ν / cm -1 c Dipolar solvation spectral density 3-1 ν / 1 cm Figure 8. Calculations for the solvation of a molecular ole by methanol, with dielectric continuum theory as in Figure 3. The solvation spectral density C ( ν ) in Panel C is to be multiplied with a Gaussian filter function (dashed) corresponding to 1 fs time resolution (fwhm). 11

13 A ε ε methan- d 3 -ol, 298 K ε B χ C ν / cm -1 c Dipolar solvation spectral density 3-1 ν / 1 cm Figure 9. Calculations for the solvation of a molecular ole by methan-d 3 -ol, with dielectric continuum theory as in Figures 3 and 8. 12

14 n =2.3 cav r / Å Figure 1. Molecular polarizability α 1 in the excited state and cavity radius r for MQ in water (see text). The confidence range is represented by the gray area. The mean values α 1 2 Å 3 and r 3.3 Å are in agreement with quantum mechanical calculations. 13

15 6 nm 65 nm OD / OD / nm 75 nm OD / nm 85 nm time / ps time / ps Figure 11. Kinetic traces for MQ in methanol at wavelengths in the stimulated emission band (red), and pure solvent signal (blue) at the same experimental conditions. Dotted lines are a guide to the eye. Oscillations in the solvent signal are due to Stimulated Raman Scattering. After solvent subtraction (results not shown), the remaining oscillations sample the modulation in the time-dependent Stokes shift of the chromophor. See text. 14