Chimeric Behavior of Excited Thioxanthone in Protic Solvents: I

Article pubs.acs.org/JPCA

Chimeric Behavior of Excited Thioxanthone in Protic Solvents: I. Experiments T. Villnow,† G. Ryseck,† V. Rai-Constapel,‡ C. M. Marian,‡ and P. Gilch*,† †

Institut für Physikalische Chemie and ‡Institut für Theoretische Chemie und Computerchemie, Heinrich-Heine-Universität Düsseldorf, Universitätstrasse 1, D-40225 Düsseldorf, Germany ABSTRACT: The photophysics of thioxanthone (TX) dissolved in methanol (MeOH) and 2,2,2,-trifluoroethanol (TFE) was studied by time-resolved fluorescence and absorption spectroscopy. The spectrally integrated stimulated emission is seen to lose amplitude within ∼5−10 ps. This is much shorter than the fluorescence lifetimes of the compound (2.7 ns for MeOH and 7.6 ns for TFE). The initial reduction in amplitude is attributed to reversible intersystem crossing between the primarily excited 1ππ* and a triplet 3nπ* state. The latter one is energetically slightly (∼0.02 eV) above the former one. Addition of the quencher 1-methylnaphthalene (1-MN) reduces the fluorescence lifetime and yields triplet excited 1-MN, giving further evidence for the equilibrium of singlet and triplet excitations. The depopulation of these two states on the nanosecond time scale results in the rise of the lowest triplet state, a 3ππ* state. Temperature dependencies attribute this to an activated internal conversion process between the two triplet states. Kinetic and energetic parameters derived from the experimental data will be compared with quantum chemical results in the accompanying paper [Rai-Constapel, V.; Villnow, T.; Ryseck, G.; Gilch, P.; Marian, C. M. J. Phys. Chem. A 2014, DOI: 10.1021/jp5099415]. emit fluorescence and act as a triplet energy donor. The behavior was attributed to a rapidly established equilibrium between the bright 1ππ* and the dark 3nπ* state. This requires the two states to be isoenergetic within kBT. High-level quantum chemical computations17 on TX in MeOH show that the adiabatic energies of these two states are indeed equal within the computational error. The mechanism further requires that this equilibrium prevails for a rather long time (∼2 ns). This implies that the IC between the 3nπ* state and the lowest triplet (a 3ππ* state) occurs on that time scale. In light of the energetic vicinity of these two states,17 this is surprising. Here, the equilibration and its prevalence will be inspected closer by means of time-resolved fluorescence and transient absorption spectroscopy. We focus on TX in two protic solvents, methanol (MeOH) and 2,2,2-trifluoroethanol (TFE), because in these solvents, the singlet 1nπ* state is energetically well above the other states of interest.17 This will simplify the kinetic analysis. The analysis will afford rate constants, quantum yields, and equilibrium constants. In the accompanying paper, many of these quantities will be derived from quantum chemical computations. Beyond the particular processes looked upon, the study will show how close experiment and theory are when it comes to spin-forbidden processes in organic compounds.

1. INTRODUCTION Aromatic carbonyls and, in particular, thioxanthone (TX) and its derivatives are utilized in a wide range of applications, such as triplet sensitizers,1,2 intramolecular antenna in photolabile protecting groups,3,4 enantioselective catalysts in photoactivated synthesis,5 photoinitiators of polymerization,6,7 and antitumor agents.8,9 Many of these applications rely on triplet excitation of TX. Several studies, both experimental10−16 and theoretical,10−12,17 have dealt with the photoinduced processes resulting in the triplet excitation. These studies also aimed at the surprisingly large effect of the solvent on the fluorescence lifetime and the quantum yield.18,19 In a combined quantum chemical and femtosecond transient absorption study on TX in aprotic polar and nonpolar solvents by Angulo et al., two solvent-dependent rate constants could be obtained.10 The faster process (rate constants in the range of 8−183 ns−1) was assigned to intersystem crossing (ISC) between the primarily excited singlet (1ππ*) and a triplet state (3nπ*). The slower process (4−32 ns−1) was assigned to the internal conversion (IC) between the 1ππ* and 1nπ* state from where it can deactivate to the ground state via a conical intersection. The very low fluorescence quantum yields in less polar solvents were attributed to fast ISC, and the easily accessible conical intersection. In polar solvents, this intersection was believed to be far less accessible due to the stabilization of the 1ππ* state. At least in protic solvents, experiments employing a TX moiety as an intramolecular sensitizer lead to a different picture.20 Two separate experiments showed that TX dissolved in MeOH exhibits chimeric behavior. It can simultaneously © XXXX American Chemical Society

Received: October 1, 2014 Revised: November 13, 2014

A

dx.doi.org/10.1021/jp5099393 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

Article

Figure 1. Steady-state absorption and fluorescence data on TX dissolved in MeOH (black solid lines and dots) and TFE (red dashed lines and dots). (Left) Absorption and fluorescence spectra at room temperature (20 °C). The absorption coefficient ϵ is plotted versus wavelength. The fluorescence signal is proportional to the photon flux per wavelength interval. For the acquisition of the fluorescence spectrum, the excitation was tuned to 370 nm. To ease the comparison, the respective fluorescence spectra were scaled to match the amplitudes of the absorption spectra. (Right) Fluorescence quantum yields ϕfl as a function of temperature.

2. MATERIALS AND METHODS TX (≥97%) was obtained from Sigma-Aldrich, 1-methylnaphthalene (1-MN) (≥97%) from Acros Organics, and 5-amino-2nitrobenzotrifluoride (≥98%) from Fluka. Coumarin 102 was provided by Radiant Dyes. MeOH (Merck KGaA, Darmstadt, Germany) and 2,2,2-TFE (Sigma-Aldrich) were of spectroscopic grade. All chemicals were used as supplied. Absorption spectra were recorded using a two-beam absorption spectrometer (Perkin Elmer, Lambda19) with a high linearity and dynamic range. Emission spectra were obtained by a fluorescence spectrometer (HORIBA Scientific, FluoroLog-3) in an orthogonal collecting geometry without polarization selectivity. For the emission spectra, the optical density was kept below 0.05 to avoid excessive inner filter effects. All fluorescence spectra were corrected for the solvent background and for the spectral sensitivity of the setup. Fluorescence quantum yields were obtained by comparison with a sample of known quantum yield (Coumarin 102, ϕfl = 0.9521). All measurements were performed using 1 cm quartz cuvettes (Hellma Analytics, 111-QS). The general features of the time-resolved fluorescence setup are given in ref 22. Briefly, the sample was excited by ∼180 fs laser pulses, and the resulting emission was spectrally and temporally resolved by the combination of a spectrograph and a streak camera system operating in the photon counting mode. The excitation pulses with a center wavelength of 388 nm are generated by frequency doubling of the output of a laser amplification system (Clark CPA 2001, 775 nm, 1 kHz) in a nonlinear crystal (BBO). The excitation light was focused with a fused silica lens ( f = 50 mm) onto a fused silica flow cell (Hellma 137.108) with path lengths of 200 (for TX dissolved in MeOH) and 500 μm (for TFE as the solvent). The excitation beam impinged the flow cell under an angle of 15° to avoid residual excitation light from reaching the detector. At the sample, the beam diameter (fwhm) amounted to ∼60 μm and the energy per pulse to 10 nJ. The emitted light was collected by an achromatic lens (f = 80 mm, Bernhard Halle Nachfl. GmBH, OUV4.20) and coupled into a spectrograph (Princeton Instruments, Acton Series SP2356, 50 lines per mm grating blazed for 600 nm) by a second achromatic lens ( f = 75 mm, Edmund optics). Using an entrance slit set to 20 μm, the spectral resolution was ∼3 nm. The relative polarization of the excitation and the emission light were set to the magic angle

using wire grid polarizers (MOXTEK, PPL04), suppressing contributions due to rotational diffusion. The exit focal plane of the spectrometer was imaged on the photocathode of the streak camera system (Hamamatsu, C5680-24 C). The time window was set to 500 ps by the fast single-sweep unit (Hamamatsu M5677-01), yielding a time resolution of ∼13 ps in continuous sweeping mode. Additional measurements with a time window of 20 ns and a time resolution of ∼200 ps were performed to independently get information on the fluorescence decay on the nanosecond time scale. The optical density for TX dissolved in MeOH and TFE amounted to 0.2 and 0.5, respectively. Then, 105 frames with an acquisition rate of 150 Hz were accumulated for each measurement. All data were corrected for the spectral sensitivity of the instruments. The transient absorption setup was described in detail elsewhere.23−25 Briefly, part of the output of a 1 kHz Ti:Sa laser amplifier system (Coherent, Libra-F-1K-HE-230) was fed into the two branches of a pump−probe experiment. In the pump branch, the fundamental of the laser/amplifier system was frequency doubled to yield 400 nm pulses. At the sample location, the energy of these pump pulses amounted to ∼1 μJ, and the focal diameter (fwhm) was ∼160 μm. Another part of the fundamental was used to generate probe pulses by continuum generation in an eccentrically moved CaF2 disk. The probe pulse energies were on the order of tens of nanojoules. The probe beam had a diameter of ∼40 μm at the sample. Pump and probe beams were spatially overlapped in a fused silica flow cell with a thickness of 500 μm (Hellma, customer-specific) with their relative polarization set to the magic angle. The temporal width (fwhm) of the instrumental response function (IRF) of the setup was ∼200 fs. Two scans with 164 delay line settings were recorded, 75 of those in equidistant steps on a linear scale from −2 up to 1 ps and the remaining 89 on a logarithmic one up to 3.3 ns. At every setting of the delay line, signals from 2000 laser shots (1000 pump on) were averaged. The flow rate in the sample cell was such that the solution in the volume excited was exchanged in between laser shots. The overall amount of sample (>10−5 mol TX) was large enough to render contributions of photoproducts negligible. In the transient absorption experiments, the temperature was controlled by a thermostatic bath (Julabo Paratherm FT1) connected to both the sample container and the mount holding the flow cell. The temperature-dependent measurements began and ended at room temperature, thereby B



Article

Figure 2. Time-resolved fluorescence data on TX dissolved in MeOH (top panel) and TFE (lower panel) at room temperature (20 °C). Data was obtained using a streak camera. The femtosecond excitation was tuned to 388 nm. The contour representations in the center give an overview. Time traces at indicated detection wavelengths are plotted on the left. Transient fluorescence spectra at indicated detection times after photoexcitation are shown on the right.

amounts to 0.12; in TFE, it is higher by a factor of 3 (0.42). This is in line with earlier reports.18 The fluorescence quantum yields ϕfl prove to be temperature-dependent. They decrease with increasing temperature. The relative effect is more pronounced for TX in MeOH (∼3 times). With the absorption and fluorescence spectra as input, a Strickler−Berg analysis29,30 yields radiative rate constants krad of 4.07 × 107 s−1 (MeOH) and 3.96 × 107 s−1 (TFE). The fluorescence decay of the two samples was investigated using a streak camera. Unfortunately, our Kerr gate setup,31 which provides a better time resolution, cannot cope with long-lived (∼1 ns) fluorophores. Setting the time window of the streak camera to 20 ns allows one to trace the longest decay components (data not shown). Time constants τ3 of 2.65 (TX in MeOH) and 7.6 ns (TX in TFE) are derived and are in good agreement with earlier results.18 With the intention to see signatures of the ISC process, this time window was set to 400 ps (data in Figure 2). For either sample, a fluorescence decay on the 10 ps time scale is observed in the high-frequency part of the spectrum. Because the magic angle condition was applied, this decay should not be due to rotational diffusion. Cross-checking with parallel and perpendicular polarization yielded longer rotational diffusion times τrot of 20 ps (MeOH, dynamic viscosity η = 0.591 mPa·s32) and 70 ps (TFE, η = 1.74 mPa·s33). In accordance with Stokes− Einstein−Debye theory,21 rotational diffusion time scales linearly with viscosity. The decay in the high-frequency part is, however, accompanied by a rise in the low-frequency part. In other words, the fluorescence spectra red shift with time (cf. Figure 2, right). Describing the data sets with a multiexponential trial function (see the Materials and Methods section) requires two components. The one with the time constant τ3 has already been mentioned. The respective DAS (see Figure 3) match the

excluding signal distortion by long-term drifts. Signals of the neat solvent were subtracted after suitable scaling. The scaling factor was given by (1 − 10−Aex)/(2.3·Aex). Hereby, it was assumed that the solvent signal originates from a two-photon absorption of the pump and probe light; see, for example, ref 26. To account for small differences of the absorption Aex at the excitation wavelength, data sets were multiplied by (1 − 10−Aex)−1. Both time-resolved experiments yield signals S(λ,t) as a function of detection wavelength λ and delay time t. The signals were analyzed using two approaches. Time constants τi and the respective decay-associated spectra (DAS) ΔAi(λ) were obtained by a global fitting routine using the below trial function27 S(λ , t ) = IRF ⊗

∑ ΔAi(λ)e−t/τ

i

i

(1)

IRF⊗ stands for the convolution with the instrumental response function. Where appropriate, also spectral integrals were computed and studied as a function of time (see the Experimental Results section).

3. EXPERIMENTAL RESULTS TX in MeOH and TFE exhibits a lowest-energy absorption band peaking at around 370 nm with an absorption coefficient ϵ of ∼6000 M−1 cm−1 (see Figure 1). A solvatochromism is observed. In MeOH (dielectric constant of 33.028), the spectrum peaks at 377 nm, and in TFE (dielectric constant of 27.6828), the maximum is located at 389 nm. The peak absorption coefficient of TX in MeOH is about 95% of that for TX in TFE. The dependence of the fluorescence quantum yield ϕfl is more pronounced. For MeOH and room temperature, it C



Article

Figure 3. DAS retrieved from the time-resolved fluorescence data given in Figure 2. The left panel refers to TX in MeOH, and the right one refers to TX in TFE. The respective time constants are given next to the spectra. Data in the shaded areas are not reliable due to low sensitivity or scattering of excitation light.

Figure 4. Femtosecond transient absorption on TX dissolved in MeOH (top panel) and TFE (lower panel) at room temperature (21 °C). The femtosecond excitation was tuned to 400 nm. The contour representations in the center give an overview. Time traces at indicated detection wavelengths are plotted on the left. Transient difference spectra at indicated delay times after photoexcitation are shown on the right.

steady-state fluorescence spectra (cf. Figure 1). The time constant τ2 equals 13 ps for TX in MeOH and 27 ps for TX in TFE. The spectra associated with this time constant ΔAfl,2 are positive for the high-frequency part describing the decay in this range. The negative contribution in the low-frequency part stands for a rise. To clarify whether this process goes along with a reduction of the population of the radiative state, a spectral integral

∞

∫−∞

ΔA fl,2 (ν)̃ ν 3̃

dν ̃

(2)

was computed. For this integral, the DAS as a function of the wavenumber ΔAfl,2(ν̃) was divided by ν̃3. This division accounts for the frequency dependence of the spontaneous emission (see, e.g., Parson30). The integrals are zero within error margin for both samples. This strongly suggests that the time constant τ2 is not associated with the ISC process. As will be further D



Article

experiment. In the range of the SE, the respective DAS (Figure 5) for the time constants τ1 and τ2 resembles the one for τ2 in the fluorescence data (cf. Figure 3). Note that because SE is a negative quantity and the fluorescence a positive one, the DAS are inverted. Thus, both time constants seem to parametrize spectral shifts due to dielectric relaxation (see the Discussion). The DAS ΔA3 describes the spectral changes going along with the “overall” decay of the fluorescence. Accordingly, the properly scaled fluorescence spectra match a part of these spectra (see Figure 5). A pronounced positive feature at 670 nm describes the decay of ESA. The negative band at 600 nm points to the rise of a species absorbing there. Indeed, the offset spectrum ΔA∞ features a strong band at 600 nm. Overlaying this spectrum with one from nanosecond spectroscopy20 (see Figure 5) gives clear evidence that the lowest triplet state of TX (a 3ππ* state17) is the carrier of this spectrum. From the magnitude of this spectrum the triplet quantum yield ϕT was determined as follows. For the lowest triplet being the only transient species, ΔA∞(λ) is given by

elaborated in the Discussion, the underlying process is dielectric relaxation. Further insight into the photophysics is acquired by transient absorption spectroscopy. In the experiment, the excitation was tuned to 400 nm, thereby addressing the lowest (allowed) transition with very little excess energy. The data sets taken at room temperature are depicted in Figure 4. The data on TX in MeOH matches our earlier results.20 Because of the larger spectral coverage with the present setup, data on the sample are given once more. Qualitatively, TX in MeOH and TFE behaves similarly, and the data will be described jointly. Around time zero, a positive difference signal due to excited-state absorption (ESA) peaking at ∼340 nm is observed. With reference to the absorption spectrum (Figure 1), the negative contribution at around 390 nm can be assigned to ground-state bleach (GSB). The negative band further to the red is due to stimulated emission (SE). A strong positive band due to ESA at around 660 nm is seen. On the 10 ps time scale, a pronounced shift of the SE is discernible. Consistent with that, the ESA at around 660 nm changes in shape. Within nanoseconds, the SE decays, and a band peaking at ∼600 nm is seen to rise. Analyzing the data sets with a multiexponential trial function requires three exponential terms and an offset. The timeresolved fluorescence data already suggest the use of two terms. Inspection of early delay times in the transient absorption data shows that an additional term is required to avoid systematic deviation. The offset accounts for species silent in the fluorescence experiment. For TX in MeOH, the time constants are τ1 = 0.9 ps, τ2 = 13 ps, and τ3 = 2400 ps (all time constants are compiled in Table 1). The constants τ2 and τ3 are in good agreement with the fluorescence data. For the analysis of the TX in TFE data, the τ3 value from the fluorescence experiment was relied upon. The limited time range (up to 3 ns) of the setup renders its determination difficult. The constants for TX in TFE are τ1 = 2.5 ps, τ2 = 24 ps, and τ3 = 7600 ps (fixed). The value for τ2 is close to the one from the fluorescence

ΔA∞(λ) = ϕT ·Δϵ T(λ) ·c*·d

The difference absorption coefficient ΔϵT(λ) = ϵT(λ) − ϵGS(λ) is difficult to determine independently. In the spectral region of the GSB, it may be replaced by the negative absorption coefficient of TX (−ϵGS(λ)). At around 380 nm, the GSB dominates the DAS ΔA∞(λ) (cf. Figure 5), but there is some contribution of the triplet absorption (ϵT(λ)). To obtain the pure GSB contribution, the absorption spectrum of TX was fitted into the DAS ΔA∞(λ). The resulting GSB amplitudes ΔAGSB relate to the triplet yield, which refers to the 3ππ* state via ΔA GSB = −ϕT ·ϵmax ·c*·d

ϕfl

τ1/ps

τ2/ps

288 293 303 313 323

0.126 0.118 0.096 0.080 0.065

0.8 0.9 0.9 0.7 0.4

15 13 11 10 8

288 293 303 313 323 331 338

0.429 0.421 0.399 0.379 0.357

2.7 2.5 1.8 1.2 0.8 0.5

25 24 19 13 11 9

τ3/ps

MeOH 2500 2400 2100 1800 1600 TFE 7600

τISC/ps

ΔIISC

ϕT

5.5 5.3 5.4 5.1 5.5

0.31 0.30 0.34 0.32 0.33

0.57 0.60 0.62 0.63 0.65

9.9 9.1 10.5 7.2 8.9 8.5

0.19 0.19 0.20 0.20 0.19 0.22

(4)

ϵmax is the peak absorption coefficient of TX. The product of the initial concentration of excited molecules c* and the effective path length d was obtained by relying on a reference sample. With the same settings, 5-amino-2-nitrobenzotrifluoride dissolved in acetonitrile was investigated. Around time zero, it exhibits a clearly discernible GSB (ΔAGSB,r), which is related to c*·d via (ϵmax,r is 3600 M−1 cm−1)

Table 1. Temperature Dependence of the Photophysical Parameters of TXa T/K

(3)

ΔA GSB,r = −ϵmax,r ·c*·d

(5)

Combining eq 4 and 5 yields ϕT =

ΔA GSB ϵmax,r · ΔA GSB,r ϵmax

(6)

A cross-check in which c*·d is obtained from the pulse parameters yields values identical within 5%. A value for ϕT of 0.6 for TX in MeOH was determined to be in good agreement with a value determined via thermal lens spectroscopy (0.56 ± 0.08).15 For TX in TFE, the value (0.55) is also in accordance with values obtained using other techniques.13,15,34 In relation to the ISC process, we now have a closer look at the transient absorption data for early delay times (Figure 6). The set of spectra shows that on the time scale of dielectric relaxation, indicated by the shift of the SE, the ESA at around 680 nm decreases in amplitude. At ∼590 nm, a slight increase of the ESA is observed. This suggests that on the 10 ps time scale, dielectric relaxation and population changes occur. To extract the population change, a procedure similar to the one given in ref 35 was adopted. The difference absorption ΔA(ν̃,t)

0.55

0.317

The fluorescence quantum yield ϕfl was obtained by steady-state spectroscopy. The triplet quantum yield ϕT was derived from the offset amplitudes ΔA∞(λ) as described in the Experimental Results section. The time constants τ1−3 were obtained from the transient absorption spectra (see Figures 4 and 5) by a global fitting routine (cf. the Materials and Methods section). The time constant of the ISC τISC and the relative intensity change of the SE ΔIISC were computed from time-dependent spectral integrals (cf. eq 7). a

E



Article

Figure 5. DAS retrieved from the femtosecond transient absorption data given in Figure 4. The panel on the left refers to TX in MeOH, and the one on the right refers to TX in TFE. The respective time constants are given next to the spectra. The appropriately corrected and scaled (to the SE) stationary fluorescence spectrum is overlaid (blue dots). The offset spectrum ΔA∞ for MeOH is overlaid with the triplet spectrum of TX in MeOH obtained by nanosecond spectroscopy (red dashed line).20

the reduction of τ2 is also observed. The limited time window precludes the investigation of the behavior of τ3 (see above). Where applicable, for all temperatures, values for time constants τISC and τ1−3 and the triplet yields were determined. The results are summarized in Figure 8 and Table 1. Within the error margin, no temperature dependence of τISC is observed. The reduction of the spectral integral I on the 10 ps time scale shows that the bright 1ππ* state loses population. Quenching experiments can ensure that a triplet state receives that population.20,36−40 For TX in MeOH, such an experiment has already been conducted. Here, we supply data on TX in TFE and have a closer look at the quenching in both solvents. 1-MN serves as a quencher. The energy of its excited singlet state (3.9 eV)41 is well above that of TX in MeOH (3.07 eV) and TFE (2.98 eV) (energies deduced from Figure 1; see also ref 13), excluding singlet energy transfer. The triplet energy of 1-MN is close to the one of the lowest triplet energy of TX.37 Thus, the transfer from an upper triplet state should be exergonic. For the quenching to compete with the intrinsic decay of the 3nπ* state, a high 1-MN concentration should be applied.42 For high concentrations, the dielectric properties of the solvents will be affected on which TX is very sensitive. For the concentration chosen (0.5 M), the polarity effect is not pronounced. The spectral positions of the SE and the GSB are identical for transient data in the presence and absence of 1MN (Figure 9). 1-MN strongly affects the lifetime of the SE. For TX in MeOH, it is reduced to 140 ps (70 ps for TX in TFE). The decay of the SE goes along with the rise of an absorption band at ∼420 nm. This band is characteristic for the

as a function of wavenumber ν̃ and delay time t was spectrally integrated I (t ) =

∫ν̃

νmax ̃

min

ΔA(ν ̃, t ) dν ̃ ν̃

(7)

The division by ν̃ accounts for the frequency dependence of the SE.30 The spectral range, defined by ν̃min and ν̃max, was restricted to the one of SE (cf. Figure 6). In this range, SE and GSB contribute predominantly to the signal. The contribution of the GSB, which is constant up to ∼500 ps, was subtracted (see Figure 6). Plotting the integral I as a function of the delay time reveals a decay on the 10 ps time scale. Single-exponential fits yield time constants and relative amplitudes for the decay. For TX in MeOH, the time constant equals 5 ps, and the relative amplitude (ΔIISC) is 0.33. For TX in TFE, the time constant is somewhat longer (9 ps), and the relative amplitude amounts to 0.2. The relative decrease in signal is of the same magnitude as the reduction of the ESA at ∼660 nm. The decay of I for later delay times is merely due to the overall decay of the fluorescence. As expected from stationary fluorescence, the transient absorption of TX is sensitive to the temperature (Figure 7). The most obvious effect for TX in MeOH is a decrease of the time constant τ3 with temperature (2500 ps for 288 K and 1600 ps for 323 K). Also, the time constant τ2 goes down with temperature (15 ps for 288 K and 8 ps for 323 K). Inspecting time traces and relying on fits show that not only are τ2 and τ3 affected but also the offset. It increases with temperature, giving evidence for a rising triplet yield ϕT (Figure 7). For TX in TFE, F



Article

Figure 6. Early time behavior of the transient absorption. Transient absorption spectra of TX in MeOH from 0.5 (black solid line) to 20 ps (red solid line) in 0.5 ps steps (gray solid lines) are plotted in the upper left graph. The arrows indicate the development of the bands with increasing time. The area from 750 to 780 nm shaded in gray is subject to large white light fluctuations and therefore does not represent the actual band shape. In the lower left panel, the temporal evolution of the spectrally integrated SE is given. The integration boundaries are chosen from zero crossing to zero crossing represented by the dashed lines in the upper graph. The contribution of the GSB is marked in blue. This contribution was subtracted from the integral. The x-axis is plotted linearly up to 50 ps, and from there on, it is plotted logarithmically. On the right, the same representation is shown for TX in TFE. The steps between the different transient spectra in the upper graph are 1 ps, going from 1 up to 35 ps.

Figure 7. Temperature dependence of transient absorption on TX dissolved in MeOH (top panel) and TFE (lower panel). The femtosecond excitation was tuned to 400 nm. The contour representations on the left correspond to the lowest temperature (15 °C for MeOH and 16 °C for TFE), and those in the center correspond to the highest (48 °C for MeOH and 58 °C for TFE). On the upper right, time traces for the highest and lowest temperature and a detection wavelength of 600 nm are plotted. The bullets represent experimental data, the solid lines are the result of the global fit, and the dashed lines are extrapolations based on these fits. Due to the long fluorescence lifetime of TX in TFE, a different representation was chosen. At the selected wavelength of 425 nm, the temperature dependence of both the earlier and the later decay times can be seen.

G



Article

Figure 8. Temperature dependence of the time constants τISC, τ2, and τ3 and the triplet quantum yield ϕT. The parameters τ2, τ3, and ϕT were obtained by global analysis of data sets such as the ones given in Figure 7. The values for τISC were obtained by spectral integration as described in Figure 6. The left panels refer to TX dissolved in MeOH, and the right ones refer to TX in TFE.

triplet state of 1-MN.43−45 Whereas for TX in MeOH the band persists for more than 3 ns and defines the shape of the offset spectrum, in TFE, it is seen to decay within 520 ps. The offset spectrum of TX in TFE thus does not show the triplet band of 1-MN but the distinct signature of the 3ππ* state, which is not visible in MeOH. The quenching experiments clearly indicate a

partial triplet population. The details of the energy-transfer processes occurring are the subject of further investigations.

4. DISCUSSION The time-resolved experiments on TX in MeOH and TFE presented here showed that a portion of the excited TX H



Article

Figure 9. Impact of 1-MN on the transient absorption of TX. (Left) No 1-MN added (data sets are identical to the ones given in Figure 4). (Right) 0.5 M 1-MN added. The femtosecond excitation was tuned to 400 nm. The upper panels refer to TX dissolved in MeOH, and the lower ones refer to TX in TFE.

Dielectric relaxation is expected to conserve the fluorescence and SE spectra scaled for the “trivial” frequency dependencies.50 The time-resolved fluorescence spectroscopy has shown that the process with time constant τ2 retains the reduced fluorescence intensity. However, in the transient absorption data, the reduced SE exhibits a decay with a time constant τISC that is distinct from the time constants parametrizing mostly dielectric relaxation. We note that the DAS ΔA1 and ΔA2 also hold information on the depopulation of the 1ππ* state. Adding the spectra and computing the respective integrals also yield a signal reduction. The ISC and ensuing processes will now be treated in terms of a kinetic scheme (Figure 10). As a start, the electronically excited states involved are reiterated. The lowest excited singlet state in alcohols is a 1ππ* state.17 Its adiabatic energy amounts to 3.07 (MeOH) and 2.98 eV (TFE) (see Figure 1 and ref 13). Because in the experiments the transition to this state is addressed, higher excited singlet states need not be considered here. The lowest triplet state of TX in alcohols is a 3ππ* state. Its adiabatic energy as deduced from phosphorescence spectroscopy15 and quenching36,37 equals ∼2.8 eV. Quantum chemistry17 predicts that in alcohols, the 3nπ* state is higher in energy and approximately isoenergetic with the 1ππ* state. Presently, no direct spectroscopic confirmation of this is available. Including the ground state, four electronic states are thus considered in our kinetic modeling. Which processes interconnect these states? The primarily excited 1ππ* state may radiatively decay to the ground state (rate constant krad). It may undergo ISC to either the 3nπ* or the 3ππ* state. With reference to El-Sayed’s rule,51,52 we neglect the latter process for the present kinetic modeling. Quantum chemistry17 suggests El-Sayed’s forbidden transitions to occur in apolar solvents. In the accompanying paper,47 the relevance for protic

population undergoes ISC on the 10 ps time scale. On this time scale, also dielectric relaxation occurs,46 and its spectroscopic signature obscures the one of ISC. We will, thus, briefly discuss the dielectric relaxation before turning to the ISC kinetics. Quantum chemistry47 and spectroscopy48 predict that 1ππ* excitation increases the dipole moment of TX. The solvent molecules adapt to this higher dipole moment and thereby lower the energy of the excited state. The fluorescence lifetime of TX in MeOH (2.4 ns) and TFE (7.6 ns) is much longer than the characteristic times of the dielectric relaxation. Thus, only relaxed molecules contribute to the steady-state fluorescence spectrum. The Stokes shift (cf. Figure 1) then holds data on the energy associated with dielectric relaxation. It amounts to 3430 cm−1 for TX in MeOH and 3080 cm−1 for TX in TFE (see Figure 1 and refs 13 and 49). Following Horng et al.,50 we separate the intramolecular contribution to the Stokes shift by subtracting the shift for TX in a nonpolar solvent. In n-hexane, the shift amounts to 1130 cm−1,49 which leaves 2300 (MeOH) and 1950 cm−1 (TFE) for the dielectric relaxation. The shifts of the SE during the first tens of picoseconds have amplitudes of similar magnitude but are smaller (700 cm−1 for MeOH and 1100 cm−1 for TFE). This is due to the multiexponential nature of dielectric relaxation.46 A part of the relaxation occurs within ≤100 fs; this part is “missed” here. This also explains why two time constants τ1 and τ2 are required to handle the data. Their magnitudes are matching values for the intermediate and long part of the relaxation. Finally, theory predicts that time constants for longer components of the relaxation should scale with solvent viscosity. Indeed, the temperature dependence of τ2 behaves like the solvent viscosity. Increasing the temperature by 20 °C, τ2 of TX in MeOH decreases by 25% (TX in TFE: 45%; cf. Figure 8) and the viscosity of MeOH by 24%32 (TFE: 40% for 15 °C33). I



Article

K=

k [3nπ *] = ISC kReISC [1ππ *]

(10)

It can be deducted from the time-dependent band integral I(t) (cf. Figure 6) K=

I0 − Ieq Ieq

(11)

Ieq is the amplitude of the integral after the equilibration. It is proportional to the population of the 1ππ* state. I0 is the amplitude at the time zero, and the difference I0 − Ieq is proportional to the 3nπ* population. The quantum yields are given by

Figure 10. Kinetic scheme relied on in the data analysis.

3

1 1 K K = k rad + kIC + kISC,0 τ3 1+K 1+K 1+K

1 ·k rad·τ3 1+K

(12)

ϕT =

K ·kIC·τ3 1+K

(13)

ϕISC,0 =

solvents will be further scrutinized. As the ππ* and the nπ* state are presumably close to isoenergetic, forward (kISC) and backward (kReISC) processes are considered. Because excited TX finally ends up in the 3ππ* state, a process connecting this state with either the 1ππ* or the 3nπ* state is required. We consider an IC process between the two triplet states most likely (see the accompanying paper47). The quantum yields ϕfl and ϕT do not sum to unity. An additional channel must, thus, exist. This could either be an IC process, 1ππ* → S0, or an ISC process, 3 nπ* → S0. Because of the equilibrium of the 1ππ* and the 3 nπ* state, we cannot decide which process actually occurs. For the kinetic modeling in the following, the ISC path is assumed to dominate. The plausibility of the assumption will be discussed later on. Relating the rate constants in the scheme with the experimental time constants and yields is simplified by the time constants τISC and τ3 being orders of magnitude apart. Thus, to a good approximation, the equilibrium between the 1 ππ* and 3nπ* state is first established, and then, the two states are depleted on a much longer time scale. With this approximation, the following relations are obtained 1 = kISC + kReISC τISC (8) 1

ϕfl =

K ·kISC,0·τ3 1+K

(14)

The elementary rate parameters resulting from this set of equations and the experimental data are compiled in Table 2. As expected, the radiative rate constants are not temperaturedependent. The values obtained (∼7 × 107 s−1) from the kinetic analysis is larger than the Strickler−Berg one ((∼4 × 107 s−1). The time constant τ3 for TX in MeOH and at room temperature obtained by transient absorption spectroscopy (2.4 ns) is shorter than the fluorescence value (2.7 ns). Inserting the latter one gives a somewhat better match. The equilibration constant K = (kISC/kReISC) turns out to be temperatureindependent in the range covered. It equals 0.57 for TX in MeOH and 0.32 for TX in TFE. Provided that entropy differences may be neglected and only one triplet substate gets populated (see ref 17), K can be related to the difference in adiabatic energy ΔE K = e−ΔE / kbT

(15)

For TX in MeOH, a value of 0.014 eV results; for TX in TFE, it is slightly larger (0.028 eV). Therefore, indeed, the two states are close to isoenergetic, and K should be approximately constant in the small temperature range covered. Because K is close to unity, the rate constants kISC and kReISC are of the same magnitude (1011 s−1). The sums of forward and backward rate constants are very similar for both solvents. The difference in overall fluorescence lifetime τ3 between TX in MeOH and TFE is due to two contributions. For established equilibrium, the relative populations of the 3nπ* are lower for TX in TFE. This

(9)

whereby the equilibrium constant K is defined by

Table 2. Temperature Dependence of the Elementary Rate Constants for TXa T/K

krad/s−1

kISC/s−1

kReISC/s−1

kISC,0/s−1

kIC/s−1

MeOH × × × × ×

107 107 107 107 107

288 293 303 313 323

7.9 7.7 7.2 7.0 6.4

293

7.3 × 107

6.9 6.9 6.9 6.9 6.9

× × × × ×

1010 1010 1010 1010 1010

1.2 1.2 1.2 1.2 1.2

× × × × ×

1011 1011 1011 1011 1011

3.3 3.2 3.7 4.4 4.9

× × × × ×

108 108 108 108 108

6.3 6.9 8.1 9.6 1.1

× × × × ×

108 108 108 108 109

TFE

a

2.7 × 1010

8.3 × 1010

1.5 × 107b

3.0 × 108

Values were obtained from parameters compiled in Table 1 and eqs 8−10. bThe value is not well-defined; see the text. J



Article

Photokinetics of the Release of Nucleosides. Chem.Eur. J. 2008, 14, 6490−6497. (5) Müller, C.; Bauer, A.; Bach, T. Light-Driven Enantioselective Organocatalysis. Angew. Chem., Int. Ed. 2009, 48, 6640−6642. (6) Fouassier, J.-P. Photoinitiation, Photopolymerization, And Photocuring: Fundamentals and Applications; Hanser: Munich, Berlin, Germany, 1995. (7) Balta, D. K.; Arsu, N.; Yagci, Y.; Jockusch, S.; Turro, N. J. Thioxanthone-Anthracene: A New Photoinitiator for Free Radical Polymerization in the Presence of Oxygen. Macromolecules 2007, 40, 4138−4141. (8) Corbett, T. H.; Panchapor, C.; Polin, L.; Lowichik, N.; Pugh, S.; White, K.; Kushner, J.; Meyer, J.; Czarnecki, J.; Chinnukroh, S. Preclinical Efficacy of Thioxanthone SR271425 against Transplanted Solid Tumors of Mouse and Human Origin. Invest. New Drug. 1999, 17, 17−27. (9) Stevenson, J. P.; DeMaria, D.; Reilly, D.; Purvis, J. D.; Graham, M. A.; Lockwood, G.; Drozd, M.; O’Dwyer, P. J. Phase I/ Pharmacokinetic Trial of the Novel Thioxanthone SR233377 (WIN33377) on a 5-Day Schedule. Cancer Chemother. Pharmacol. 1999, 44, 228−234. (10) Angulo, G.; Grilj, J.; Vauthey, E.; Serrano-Andrés, L.; RubioPons, Ò .; Jacques, P. Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing. ChemPhysChem 2010, 11, 480−488. (11) Pandey, R.; Umapathy, S. Time-Resolved Resonance Raman Spectroscopic Studies on the Triplet Excited State of Thioxanthone. J. Phys. Chem. A 2011, 115, 7566−7573. (12) Pandey, R.; Umapathy, S. Simultaneous Detection of Two Triplets: A Time-Resolved Resonance Raman Study. J. Phys. Chem. A 2012, 116, 8484−8489. (13) Krystkowiak, E.; Maciejewski, A.; Kubicki, J. Spectral and Photophysical Properties of Thioxanthone in Protic and Aprotic Solvents: The Role of Hydrogen Bonds in S1-Thioxanthone Deactivation. ChemPhysChem 2006, 7, 597−606. (14) Morlet-Savary, F.; Ley, C.; Jacques, P.; Wieder, F.; Fouassier, J. P. Time Dependent Solvent Effects on the T1−Tn Absorption Spectra of Thioxanthone: A Picosecond Investigation. J. Photochem. Photobiol., A 1999, 126, 7−14. (15) Allonas, X.; Ley, C.; Bibaut, C.; Jacques, P.; Fouassier, J. Investigation of the Triplet Quantum Yield of Thioxanthone by TimeResolved Thermal Lens Spectroscopy: Solvent and Population Lens Effects. Chem. Phys. Lett. 2000, 322, 483−490. (16) Ley, C.; Morlet-Savary, F.; Jacques, P.; Fouassier, J. P. Solvent Dependence of the Intersystem Crossing Kinetics of Thioxanthone. Chem. Phys. 2000, 255, 335−346. (17) Rai-Constapel, V.; Salzmann, S.; Marian, C. M. Isolated and Solvated Thioxanthone: A Photophysical Study. J. Phys. Chem. A 2011, 115, 8589−8596. (18) Dalton, J. C.; Montgomery, F. C. Solvent Effects on Thioxanthone Fluorescence. J. Am. Chem. Soc. 1974, 96, 6230−6232. (19) Burget, D.; Jacques, P. Dramatic Solvent Effects on Thioxanthone Fluorescence Lifetime. J. Lumin. 1992, 54, 177−181. (20) Wö ll, D.; Laimgruber, S.; Galetskaya, M.; Smirnova, J.; Pfleiderer, W.; Heinz, B.; Gilch, P.; Steiner, U. E. On the Mechanism of Intramolecular Sensitization of Photocleavage of the 2-(2Nitrophenyl)propoxycarbonyl (NPPOC) Protecting Group. J. Am. Chem. Soc. 2007, 129, 12148−12158. (21) Lakowicz, J. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006. (22) Dittmann, M.; Graupner, F. F.; Maerz, B.; Oesterling, S.; de Vivie-Riedle, R.; Zinth, W.; Engelhard, M.; Lüttke, W. Photostability of 4, 4-Dihydroxythioindigo, a Mimetic of Indigo. Angew. Chem., Int. Ed. 2014, 53, 591−594. (23) Laimgruber, S.; Schachenmayr, H.; Schmidt, B.; Zinth, W.; Gilch, P. A Femtosecond Stimulated Raman Spectrograph for the Near Ultraviolet. Appl. Phys. B: Lasers Opt. 2006, 85, 557−564.

state is the active one in terms of depopulation of the two states. Furthermore, the rate constants kIC and kISC,0 are lower for TX in TFE. All rate constants (for the same process and the two solvents) are within 1 order of magnitude. The exception is kISC,0, which is significantly lower for TX in TFE. The term ϕISC,0 = 1 − ϕfl − ϕT enters the computation of kISC,0. For TFE, this term is close to zero. Therefore, it is particularly sensitive to experimental uncertainties, and the low kISC,0 value for TFE is not reliable. The magnitude of the rate constant kISC,0 (108 s−1) is rather large for an ISC process to the ground state. However, the process is El-Sayed-allowed, and for other compounds, comparable values were reported.53,54 As stated above, also an IC process, 1ππ* → S0, might account for the depletion of the kinetically coupled states. For MeOH, the analysis clearly shows that the temperature dependence of the overall fluorescence lifetime τ3 and the fluorescence quantum yield ϕfl is due to the temperature effect on kIC. In the range covered, it changes by a factor of ∼2. An Arrhenius analysis affords an activation energy of 0.11 eV and a pre-exponential factor of 2.7 × 1010 s−1. Thus, in part, the slow IC allowing for the chimeric behavior of TX is due to an activation. Still, the pre-exponential factor is low for an IC process. For IC processes, often factors approaching vibrational frequencies (1013 s−1) are reported.55 In summary, the experimental study on the chimera has afforded a time constant τISC of 5−10 ps with which singlet and triplet excitations equilibrate. Because in the equilibrium the singlet predominates, this process is difficult to trace. An activated IC process to the lowest triplet state defines the nanosecond fluorescence lifetime and the fluorescence quantum yield. In the accompanying paper,47 the rate constants kISC and kReISC, the energetics, and spectroscopic signatures will be derived from quantum chemistry and compared with those presented here.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +49-211-81-12803. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft (Projects GI349/3-2, GI349/5-1, and MA1051/12-1) is gratefully acknowledged. We thank our former Bachelor student Steffen Pfeiffer for experimental support and Wolfgang Zinth (LMU München) for access to the streak camera.

■

REFERENCES

(1) Herkstroeter, W. G.; Lamola, A. A.; Hammond, G. S. Mechanisms of Photochemical Reactions in Solution. XXVIII. Values of Triplet Excitation Energies of Selected Sensitizers. J. Am. Chem. Soc. 1964, 86, 4537−4540. (2) Klán, P.; Solomek, T.; Bochet, C. G.; Blanc, A.; Givens, R.; Rubina, M.; Popik, V.; Kostikov, A.; Wirz, J. Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy. Chem. Rev. 2013, 113, 119−191. (3) Wöll, D.; Smirnova, J.; Pfleiderer, W.; Steiner, U. E. Highly Efficient Photolabile Protecting Groups with Intramolecular Energy Transfer. Angew. Chem., Int. Ed. 2006, 45, 2975−2978. (4) Wöll, D.; Smirnova, J.; Galetskaya, M.; Prykota, T.; Bühler, J.; Stengele, K.-P.; Pfleiderer, W.; Steiner, U. E. Intramolecular Sensitization of Photocleavage of the Photolabile 2-(2-Nitrophenyl) Propoxycarbonyl (NPPOC) Protecting Group: Photoproducts and K



Article

(24) Schmierer, T.; Ryseck, G.; Villnow, T.; Regner, N.; Gilch, P. Kasha or State Selective Behavior in the Photochemistry of OrthoNitrobenzaldehyde? Photochem. Photobiol. Sci. 2012, 11, 1313−1321. (25) Bay, S.; Villnow, T.; Ryseck, G.; Rai-Constapel, V.; Gilch, P.; Müller, T. J. J. The Ugi Four-Component Reaction Route to Photoinducible Electron-Transfer Systems. ChemPlusChem 2013, 137−141. (26) Lorenc, M.; Ziolek, M.; Naskrecki, R.; Karolczak, J.; Kubicki, J.; Maciejewski, A. Artifacts in Femtosecond Transient Absorption Spectroscopy. Appl. Phys. B: Lasers Opt. 2002, 74, 19−27. (27) Satzger, H.; Zinth, W. Visualization of Transient Absorption DynamicsTowards a Qualitative View of Complex Reaction Kinetics. Chem. Phys. 2003, 295, 287−295. (28) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1997; Vol. 78. (29) Strickler, S.; Berg, R. Relationship between Absorption Intensity and Fluoresence Lifetime of Molecules. J. Chem. Phys. 1962, 37, 814− 822. (30) Parson, W. W. Modern Optical Spectroscopy, with Exercises and Examples from Biophysics and Biochemistry, Student ed.; Springer Verlag: New York, 2009. (31) Schmidt, B.; Laimgruber, S.; Zinth, W.; Gilch, P. A Broadband Kerr Shutter for Femtosecond Fluorescence Spectroscopy. Appl. Phys. B: Laser Opt. 2003, 76, 809−814. (32) Shukla, D.; Singh, S.; Parveen, S.; Gupta, M.; Shukla, J. Thermophysical Properties of Binary Mixtures of Methanol with Chlorobenzene and Bromobenzene from 293 to 313 K. Int. J. Thermophys. 2008, 29, 1376−1384. (33) Palepu, R.; Clarke, J. Viscosities and Densities of 2,2,2Trifluoroethanol + Water at Various Temperatures. Thermochim. Acta 1989, 156, 359−363. (34) Lai, T.-i.; Lim, E. Photophysical Behavior of Aromatic Carbonyl Compounds Related to Proximity Effect: Thioxanthone. Chem. Phys. Lett. 1980, 73, 244−248. (35) Mora, A. K.; Murudkar, S.; Singh, P. K.; Gowthaman, N.; Mukherjee, T.; Nath, S. Ultrafast Excited State Dynamics of 1Nitropyrene: Effect of H-Bonding. J. Photochem. Photobiol., A 2013, 271, 24−30. (36) Rodrigues, J. F.; Silva, F. d. A. d.; Netto-Ferreira, J. C. Laser Flash Photolysis Study of the Photochemistry of Thioxanthone in Organic Solvents. J. Braz. Chem. Soc. 2010, 21, 960−965. (37) Bieri, O.; Wirz, J.; Hellrung, B.; Schutkowski, M.; Drewello, M.; Kiefhaber, T. The Speed Limit for Protein Folding Measured by Triplet−Triplet Energy Transfer. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 9597−9601. (38) Hochstrasser, R. M.; Lutz, H.; Scott, G. W. The Dynamics of Populating the Lowest Triplet State of Benzophenone Following Singlet Excitation. Chem. Phys. Lett. 1974, 24, 162−167. (39) Högemann, C.; Vauthey, E. Investigation of the Competition between Electron and Energy Transfer in the Quenching of Aromatic Ketones in the Triplet State Using Picosecond Transient Grating Spectroscopy. J. Phys. Chem. A 1998, 102, 10051−10059. (40) Heinz, B.; Schmidt, B.; Root, C.; Satzger, H.; Milota, F.; Fierz, B.; Kiefhaber, T.; Zinth, W.; Gilch, P. On the Unusual Fluorescence Properties of Xanthone in Water. Phys. Chem. Chem. Phys. 2006, 8, 3432−3439. (41) Perkampus, H.-H.; Sandemann, I.; Timmons, C. UV Atlas of Organic Compounds: UV Atlas Organischer Verbindungen; Verlag Chemie: Weinheim, Germany, 1966. (42) Satzger, H.; Schmidt, B.; Root, C.; Zinth, W.; Fierz, B.; Krieger, F.; Kiefhaber, T.; Gilch, P. Ultrafast Quenching of the Xanthone Triplet by Energy Transfer: New Insight into the Intersystem Crossing Kinetics. J. Phys. Chem. A 2004, 108, 10072−10079. (43) Holroyd, R.; Capellos, C. Yields and Reactions of Lowest Excited Singlet and Triplet States in the Radiolysis of Naphthalene and 1-Methylnaphthalene. J. Phys. Chem. 1972, 76, 2485−2491. (44) Melhuish, W. Low-Energy Triplet−Triplet Transitions in Naphthalene and Some Monosubstituted Derivatives. J. Chem. Phys. 1969, 50, 2779.

(45) Wang, X.; Kofron, W.; Kong, S.; Rajesh, C.; Modarelli, D.; Lim, E. Transient Absorption Probe of Intermolecular Triplet Excimer of Naphthalene in Fluid Solutions: Identification of the Species Based on Comparison to the Intramolecular Triplet Excimers of CovalentlyLinked Dimers. J. Phys. Chem. A 2000, 104, 1461−1465. (46) Bagchi, B.; Jana, B. Solvation Dynamics in Dipolar Liquids. Chem. Soc. Rev. 2010, 39, 1936−1954. (47) Rai-Constapel, V.; Villnow, T.; Ryseck, G.; Gilch, P.; Marian, C. M. The Chimeric Behaviour of Excited Thioxanthone in Protic Solvents: II Theory. J. Phys. Chem. A 2014, DOI: 10.1021/jp5099415. (48) Abdullah, K. A.; Kemp, T. J. Solvatochromic Effects in the Fluorescence and Triplet−Triplet Absorption Spectra of Xanthone, Thioxanthone and N-Methylacridone. J. Photochem. 1986, 32, 49−57. (49) Burget, D.; Jacques, P. Interprétation des Effets de Solvants sur la Fluorescence de la Thioxanthone en Fonction de Paramètres Solvatochromiques π*, α et β. J. Chim. Phys. 1991, 88, 675−688. (50) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. Subpicosecond Measurements of Polar Solvation Dynamics: Coumarin 153 Revisited. J. Phys. Chem. 1995, 99, 17311−17337. (51) El-Sayed, M. Spin−Orbit Coupling and the Radiationless Processes in Nitrogen Heterocyclics. J. Chem. Phys. 1963, 38, 2834− 2838. (52) Klán, P.; Wirz, J. Photochemistry of Organic Compounds: From Concepts to Practice; Wiley: Chichester, U.K., 2009. (53) Garner, A.; Wilkinson, F. Laser Photolysis Studies of the Triplet State of Xanthone and Its Ketyl Radical in Fluid Solution. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1010−1020. (54) Harriman, A.; Mills, A. Photochemistry of Anthraquinone-2,6disodium Sulphonate in Aqueous Solution. Photochem. Photobiol. 1981, 33, 619−625. (55) Michl, J.; Bonačić-Kouteckỳ, V. Electronic Aspects of Organic Photochemistry; Wiley: New York, 1990.

L


Chimeric Behavior of Excited Thioxanthone in Protic Solvents: I

Recommend Documents