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Direct Observation of Thermal Equilibrium of Excited Triplet States of 9,10-Phenanthrenequinone. A Time-Resolved Resonance Raman Study. Venkatraman Ravi Kumar, Nagappan Rajkumar, Freek Ariese, and Siva Umapathy J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b07972 • Publication Date (Web): 18 Sep 2015 Downloaded from http://pubs.acs.org on September 20, 2015

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The Journal of Physical Chemistry

Direct Observation of Thermal Equilibrium of Excited Triplet States of 9,10-Phenanthrenequinone. A Time-Resolved Resonance Raman Study. Venkatraman Ravi Kumar, Nagappan Rajkumar#, Freek Ariese$ and Siva Umapathy* Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore 560012, India

Keywords: Time-Resolved Absorption Spectroscopy, Photochemistry, Solvent Polarity, Intermolecular Hydrogen Bonding, Excited-State Hydrogen Bond Dynamics, Time DependentDensity Functional Theory.

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ABSTRACT:

The photochemistry of aromatic ketones plays a key role in various physico-chemical and biological processes, and solvent polarity can be used to tune their triplet state properties. Therefore, a comprehensive analysis of the conformational structure and the solvent polarity induced energy level re-ordering of the two lowest triplet states of 9,10-phenanthrenequinone (PQ) was carried out using ns-time resolved absorption (ns-TRA), time-resolved resonance Raman (TR3) spectroscopy and Time Dependent-Density Functional Theory (TD-DFT) studies. The ns-TRA of PQ in acetonitrile displays two bands in the visible range and these two bands decay with similar lifetime at least at longer timescales (µs). Interestingly, TR3 spectra of these two bands indicate that the kinetics are different at shorter timescales (ns) while at longer timescales they followed the kinetics of ns-TRA spectra. Therefore we report a real-time observation of the thermal equilibrium between the two lowest triplet excited states of PQ, assigned to nπ* and ππ* of which the ππ* triplet state is formed first through intersystem crossing. Despite the fact that these two states are energetically close and have a similar conformational structure supported by TD-DFT studies, the slow internal conversion (~2 ns) between the T2(13nπ*) and T1(13ππ*) triplet states indicates a barrier. Insights from the singlet excited states of PQ in protic solvents [J. Chem. Phys. 2015, 142, 24305] suggest that the lowest nπ* and ππ* triplet states should undergo hydrogen bond weakening and strengthening, respectively, relative to the ground state and these mechanisms are substantiated by TD-DFT calculations. We also hypothesize that the different hydrogen bonding mechanisms exhibited by the two lowest singlet and triplet excited states of PQ could influence its ISC mechanism.


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1. INTRODUCTION Understanding the photochemistry of aromatic ketones is of fundamental importance because of their diverse applications like photo-polymerization,1,2 sunscreens,3 photostability of drugs4 etc. The photochemistry and photophysics of aromatic ketones have been studied extensively in both theoretical and experimental investigations.5-12 Triplet states are long-lived (typically µs lifetimes) and play a key role in bimolecular photochemistry.6 Their reactivity depends on the electronic character of the lowest triplet state. The two lowest triplet states nπ* and ππ* of aromatic ketones are close to each other in energy. Substituents on the aromatic ring can affect the energy levels while in some aromatic ketones they could even invert them.6,7,13 This is known as “Electronic State Switching”: the electronic character of the lowest triplet state is altered, thereby modifying its reactivity. Alternatively, solvent polarity13 can also be used to switch6 the electronic character of the lowest triplet state by differential solvation of the lowest triplet states, and thus tune its reactivity. The most reactive triplet state towards H-atom abstraction is nπ* because of its electrophilic character.14 An n → π* transition of an aromatic ketone reverses the charges (umpolung) on the C=O group, thus making the oxygen atom electron deficient.14

Figure 1. Molecular structure of 9,10-phenanthrenequinone with atom sequence numbering.


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9,10-Phenanthrenequinone (PQ) is one such aromatic ketone (ortho-quinone) and the molecular structure of PQ is shown in Figure 1. The intersystem crossing (ISC) quantum yield of PQ is almost unity15. Chemically Induced Dynamic Electron Polarization (CIDEP) and TimeResolved Electron Spin Resonance (TRESR) studies on PQ revealed that the ISC occurs between the 11nπ* and 13ππ* states in both non-dipolar and dipolar solvents16, accounting for the high quantum yield of ISC in agreement with El-Sayed’s rule which suggest that the spin-orbit coupling between two states will require a change in the orbital character.17 The CIDEP and TRESR studies also indicated that the lowest triplet state has nπ* and ππ* character in nondipolar and dipolar solvent matrices, respectively. These experimental observations suggest that PQ undergoes electronic state switching with increasing solvent polarity. Phosphorescence studies of PQ in solution by Silva et al.18 indicate that the emitting state is 13nπ* irrespective of the solvent polarity. Their temperature dependent phosphorescence studies revealed that in dipolar solvents the emission is from the thermally populated 13nπ* state, confirming the inversion of states18. Interestingly, in non-dipolar solvents, the two lowest triplet states are almost isoenergetic (~87 cm-1) with 13nπ* being the lowest triplet state, whereas in dipolar solvents the two lowest triplet states are separated by ~2.4 kcal mol-1 (~840 cm-1) with 13ππ* being the lowest triplet state18. Furthermore, the negative activation energy obtained for PQ towards the H-atom abstraction reaction in non-dipolar solvents18 suggests that the two lowest triplet states are almost pure states with no mixing of nπ* and ππ* character in spite of the two lowest triplet states being similar in energy. In summary, it is surprising that there is no mixing of the two states in spite of the fact that the energy difference is only 87 and 840 cm-1 in nondipolar and dipolar solvents, respectively.


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In case of the triplet state of benzophenone the rate constant of H-atom abstraction reaction is insensitive to solvent polarity because the electronic character of the lowest triplet state remains nπ*.9 In the case of PQ, in spite of the inversion of triplet states, the rate constant of H-atom abstraction reaction decreases by a factor of ~20 in dipolar solvents19 which is similar to acetophenone20, suggesting that the reaction can still occur from a thermally populated 13nπ* state. Furthermore, the temperature dependent H-atom abstraction reaction rates of PQ in dipolar solvents21 are also in support of the reaction from the upper triplet state, T2 (13nπ*). The time-resolved absorption (TRA) spectrum of PQ in acetonitrile has two bands in the visible region: at i) 470 and; ii) 680 nm with a shoulder at 630 nm. In non-dipolar solvents, the longer wavelength absorption band was observed only with a high energy flash lamp.22 These bands have the same lifetime at least at the longer timescale (µs) and have been assigned to triplet states19 although in this work we have observed that the kinetics are different at a shorter timescale (ns) which will be discussed in the Results section. However, the electronic character and the vibrational structures of the triplets observed in the time-resolved absorption spectra have not yet been reported. Time-Resolved Resonance Raman (TR3) spectroscopy is a powerful technique that can provide information on the vibrational structure of the transient or intermediate species23-32, especially because of its enhanced sensitivity in the case of resonance with an allowed excited state electronic transition. Furthermore, it can selectively enhance the modes that are coupled to the electronic transition, and thus shed light on the nature of the chromophore involved.23,24,32-34 Time Dependent-Density Functional Theory (TD-DFT) is effective in predicting the equilibrium properties of excited singlet and triplet states, both in terms of accuracy and computational cost34-37 and it has been used to corroborate the experimental results.29-31,38 The main objectives


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of this paper are to determine: i) the electronic structure of the transient species observed in the visible region of ns-time-resolved absorption spectra; ii) the conformational structure of PQ in the two lowest triplet excited states; and iii) the influence of solvent polarity on the energy level re-ordering of the triplet states and its implication for the photophysics and photochemistry of PQ. Furthermore, based on the insights from the intermolecular hydrogen bonding mechanisms39 of the lowest singlet excited states of PQ in dipolar protic solvents13 we predict that the lowest triplet states 13nπ* and 13ππ* could show intermolecular hydrogen bond weakening and strengthening respectively relative to the ground state. In this study, we address the above listed objectives using time resolved absorption and resonance Raman spectroscopy techniques in combination with TD-DFT computational studies. 2. METHODS 2.1. Experimental section. A schematic for the TR3 spectroscopy experimental setup is given elsewhere.23,25 Concisely, the third harmonic (354.7 nm) output of an Nd:YAG Q-switched pulsed laser (INDI, Quanta-ray) was used as excitation pump at a repetition rate of 10 Hz with a 5-8 ns pulse width. The probe wavelengths were obtained from an Optical Parametric Oscillator (Premiscan, MD-ULD 500, Spectra Physics) pumped by the third harmonic (354.7 nm) of another Nd:YAG Q-switch pulsed laser (GCR-250, Quanta-ray) at a repetition rate of 10 Hz with 5-6 ns pulse width; the timing jitter between the lasers was found to be ±10 ns. The pump and probe beams were made collinear before the sample with a dichroic mirror (354.7 nm). Both laser beams were loosely focused at the sample (beam spot size - sub mm) to avoid non-linear effects and their average energies were ≤ 1 mJ/pulse. Furthermore, a linear dependence of the TR3s signal on the probe energies in the range of 0.75-3.00 mJ/pulse was found at different wavelengths of interest, with the pump energy fixed at 1 mJ/pulse. A 90° geometry was used to


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collect the scattered light from the sample as described elsewhere.13 The collected scattered light was then dispersed by two 600 grooves/mm holographic gratings (additive dispersion) of a double monochromator spectrograph (SPEX-1404, 0.85 m and f/7.8), and detected by a liquid N2 cooled back thinned charge coupled device detector (1024 x 256 pixels, Spectrum One, JY Horiba). The two lasers were synchronized and the delay between the pump and the probe pulses was controlled by two delay generators (DG535, Stanford Research Systems Inc.). The arrival of the pulses after the dichroic mirror was monitored by a photo-diode interfaced to an oscilloscope (DPO3054, Tektronix). The sample concentrations used for the time-resolved resonance Raman experiments were ~1 mM. The sample was circulated through a quartz capillary of outer diameter 3.5 mm and inner diameter of 3.0 mm with the help of a gear pump (Cole-Parmer) at a rate of ca. 25 mL/min. The possible accumulation of photoproducts was monitored by recording the absorption spectrum of the solution before and after the experiments and the change was always found to be less than 2%. The recorded Raman spectra were calibrated using solvent peaks and the precision was found to be ± 4 cm-1. The spectral resolution of the spectrograph with 200 µm slit width was ~4-6 cm-1 for the range of wavelengths used in these experiments. All TR3 spectra shown in this work were blank and baseline subtracted as shown in Figure S1 (Supporting Information). The acquisition of TR3 spectra in this work involves the following steps: pump and probe spectra were recorded i) with both positive delay and negative delay (the pump pulse arrives at the sample before the probe pulse for the former and vice-versa for the latter); and ii) probe only spectra were recorded for wavenumber calibration. The negative and positive delay spectra were normalized with respect to the 918 cm-1 peak of the acetonitrile solvent and then the negative delay spectra were subtracted from the positive delay spectra. The subtracted spectra were then


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baseline corrected to obtain the TR3s signal as shown in Figure S1. Self-absorption corrections were carried out as described in our previous work.13 The time-resolved absorption studies of PQ were carried out by a laser flash photolysis setup (LKS.60, Applied Photophysics). Briefly, a third harmonic (354.7 nm) of an Nd:YAG laser (INDI, Quanta-ray) with a pulse width of 5-8 ns was used as excitation pump synchronized to the arc lamp pulse (150 W) which was used as the white light probe. The repetition rate of the laser flash photolysis system was ca. 1 Hz. The timing jitter was sub 100 ns for this system and therefore the data for delays less than 100 ns were not included in this work. A 90° photoexcitation scheme was used and the transmitted white light probe was dispersed by a monochromator and then detected by a five-stage R928 photomultiplier tube (PMT). The bandwidth of the spectrometer was set to 10 nm. An oscilloscope (54520A, Hewlett-Packard) with a sampling rate of 500 MSa/s was used to acquire and digitize the signal from the PMT and was interfaced to a PC for collection and analysis of the data. A suprasil flow cuvette with two optical path lengths (10 mm and 6.5 mm along transmitted light and pump beam directions, respectively) from Hellma Analytics was used and the sample was circulated during the experiments to avoid possible photoproduct accumulation. The pump beam diameter was 10 mm at the sample, while the probe beam was line focused (~1 mm width) at the front edge of the flow cuvette. For both the TRA and TR3 spectroscopy experiments the solutions were purged with high purity argon gas during the experiments to remove dissolved oxygen. All time-resolved absorption and resonance Raman experiments were carried out at room temperature (23°C) unless mentioned otherwise. The temperature dependent TR3 experiments were carried out by heating the solution reservoir through an oil bath on a hot plate (IKA) and the temperature sensor


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was inserted into the reservoir to maintain the prescribed temperature of the solution. The following solvents were used in this work: carbon tetrachloride (CCL), dichloromethane (DCM), chloroform (CHL), benzene (Benz), ethyl acetate (EthAc), acetonitrile (ACN), methanol (MeOH) and iso-propanol (iPrOH). The solvents used were of spectroscopic grade (Merck) and were used as received. The steady state UV-VIS spectrum of PQ in ACN was recorded with a UV-VIS-NIR absorption spectrometer (Lambda 750, Perkin Elmer) using a suprasil cuvette (Hellma Analytics) of 10 mm path length at room temperature. 2.2. Computational section. The calculations presented in this work were performed by Gaussian 09 (G09) suite.40 The ground state geometries were optimized with the B3LYP/631+G(d,p) method in the Integral Equation Formalism Polarizable Continuum Model (IEFPCM) as effected in G09.41-45 The excited state geometries were optimized by the TD-DFT formalism with the above mentioned method. The frequency analyses were carried out at the optimized geometries using the harmonic approximation with scaling factors46 and were found to be real, suggesting that the optimized geometries correspond to (local) minima. To understand the influence of hydrogen bonding on the solute in the ground and the excited states, explicit solvent calculations (supermolecule approach) were performed as described in our previous work.13 The basis set superposition errors for the explicit solvent calculations were determined by the counterpoise method47 as implemented in G09 and were found to be in the range of 2.0-3.5 kJ/mol. The charges were obtained by fitting to reproduce the molecular electrostatic potential at the points selected through the CHELPG scheme48 as implemented in G09. The Potential Energy Distribution (PED) for the normal modes was determined by VEDA 4.0 software.49,50 All calculations were performed with ultrafine grid integration. Gaussview 5.0 and Chemcraft software51 were used as the molecular graphics visualizer.


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3. RESULTS Earlier, it had been reported that the energy levels of the two lowest triplet states of PQ were sensitive to solvent polarity. In spite of the electronic state switching in dipolar solvents, PQ triplet showed high reactivity towards H-atom abstraction implying that the reaction is from the thermally populated upper triplet state. On resonant excitation with an allowed electronic transition, Time-resolved resonance Raman (TR3) spectroscopy peaks that are coupled to the chromophore will get enhanced selectively, thereby shedding light on the geometric structure of the transient species and the nature of the electronic transition. Therefore a detailed experimental and computational analysis was carried out and the results are presented in the following sections. 3.1. Time-Resolved Absorption Spectra.

Figure 2. Triplet-triplet absorption spectra of PQ in different solvents, each recorded after a delay of 250 ns from the pump laser pulse. Please refer to the main text for detailed discussions. Time-resolved absorption spectra (TRA) of PQ in the visible range in different solvents are shown in Figure 2. In ACN, the TRA spectrum consists of two bands at 470 nm (band I) and at 680 nm (band II) with a shoulder at 630 nm. In CCL and other non-dipolar solvents band II is not or hardly observed, and band I is blue shifted by ca. 10 nm (~460 cm-1). TRA spectra of PQ in


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ACN with iPrOH as H-atom donor are shown in Figure S2. The band at 390 nm, rising with a time constant of 200 ± 20 ns as shown in the inset of Figure S2, has been assigned to the ketyl radical (semi-quinone radical) of PQ.22 The kinetic trace of band I is also shown in Figure S2. The absorbance (∆A) ratios of band I to band II and shoulder were constant over time in ACN (longer timescales) and therefore the latter are not shown in the inset of Figure S2. Band I and II were long lived with an identical decay time constant of 1.5 ± 0.2 µs, which decreased in oxygenated solutions. These results are in excellent agreement with the existing literature15,19 and hence band I and II were assigned to triplet states. As discussed earlier, in non-dipolar solvents band II was observed only with a high energy flash lamp22. The relative ∆A ratios of band II to I was very sensitive to solvent polarity as seen in Figure 2; for normalized spectra in different solvents see Figure S3. 3.2. Time-Resolved Resonance Raman Spectra.

Figure 3. Steady state (solid line) and triplet-triplet (solid line with circles) absorption spectra of PQ in ACN shown on the same scale for ease of comparison. The arrows depict the pump and probe wavelengths chosen for the time-resolved resonance Raman experiments. The wavelength selection scheme for time-resolved resonance Raman (TR3) spectroscopy of PQ in ACN is illustrated in Figure 3. The pump wavelength was chosen in resonance with the


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steady state absorption spectrum of PQ in ACN and probe wavelengths were chosen in resonance with band I or II, which were assigned to the triplet states of PQ as discussed in the previous section. TR3 spectra of PQ in non-dipolar and protic dipolar solvents were obscured by the fluorescence background of the intermediate (ketyl radical) and hence were not included in this study. TR3 spectra of PQ in ACN with two probe wavelengths: i) 470 nm (at 20 ns delay); and ii) 680 nm (at 250 ns delay) are shown in Figure 4, with the most prominent peaks labelled for each probe wavelength. Many of the vibrational features of the spectra corresponding to these different probe wavelengths were found to be very different from each other. TR3 spectra of PQ with 470 nm as probe wavelength were performed in different solvents of varying polarity and the vibrational frequencies and relative intensities were not sensitive to solvent polarity as shown in Figure S4.

Figure 4. TR3 spectra of PQ in ACN with pump at 354.7 nm and probe at: a) 470 nm (at 20 ns delay) and b) 680 nm (at 250 ns delay). The most prominent peaks are labeled in the spectra and


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the insets depict the mass weighted atomic displacements of the corresponding modes. For the assignment of other peaks, please refer to Table I. All spectra were blank subtracted as described in Figure S1.

Figure 5. Kinetic traces of TR3 spectra of PQ in ACN; a) probe - 470 nm measured at 1328 cm-1 and b) probe - 680 nm measured at 1337 cm-1. The corresponding TR3 spectra are shown in Figures S5 & S6. TR3 spectra of PQ in ACN observed for the two probe wavelengths are shown in Figures S5 and S6. The kinetic analyses on the most prominent peaks are presented in Figure 5. The relative intensity ratios of various other peaks to the most prominent peak were constant over


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time. Both kinetic traces obtained for the Raman intensities at the two probe wavelengths, I470 and I680, were bi-exponential in nature and were fitted using the following equations:

= I + A exp

= I + A exp

+ A exp

(1)

(2)

− A exp

where τ1, τ2 are the fast and slow decay time constants using the 470 nm probe and τ3, τ4 are the fast rise and slow decay time constants using the 680 nm probe respectively. With 470 nm as probe, the initial fast decay time constant (τ1) was found to be 120 ± 20 ns and the slow decay time constant (τ2) was 1.25 ± 0.16 µs. For the 680 nm probe, the initial rise time constant (τ3) was found to be 110 ± 10 ns and the slow decay time constant (τ4) was 1.23 ± 0.08 µs. The initial rise time (τ3) obtained for the 680 nm probe matches the initial fast decay time (τ1) observed for the 470 nm probe within the experimental error. Furthermore, the slower decay time constants (τ2 and τ4) at both probe wavelengths are identical within the experimental error. The temperature dependence of the initial rise kinetics was also measured; the rise becomes faster with increasing temperature as displayed in Figure S7. The TR3 spectra of PQ in ACN with 630 nm as probe are shown in Figure S8. Most of the peaks show the same kinetic behavior as with the 680 nm probe, except for the peak at ~1328 cm-1, which shows a blue shift from 1328 cm-1 (at 50 ns delay) to 1337 cm-1 (at 200 ns delay) as displayed in Figure S8a. To illustrate the observed anomalies of this peak, a Lorentzian deconvolution of this peak at different time delays was determined as shown in Figure S8b. Transient Raman Excitation Profiles (TREPs) of PQ in ACN were determined for the time-resolved absorption band II, as shown in Figure 6. On the left of Figure 6 are the TR3 spectra of PQ with various probe wavelengths in resonance with band II and on the right are the TREPs of selected Raman peaks viz. 1337, 1475 and 1606 cm-1; in the same graphs the


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absorption profiles are shown for comparison. The normalized ∆A on the left ordinate of Figure 6 were squared because resonance Raman intensities are proportional to the square of the

Figure 6. Transient Raman Excitation Profiles (TREPs). Left: Time-resolved Raman spectra of PQ in ACN at 250 ns delay with different probe wavelengths. Right: The Raman excitation profile of a) 1337 b) 1475 and c) 1606 cm-1 modes as highlighted in the left spectra, in comparison with the squared absorption profiles. Please refer to the main text for further discussions. absorption cross-section.13 These Raman peaks followed the time-resolved absorption band II but with different relative intensity ratios as shown in Figure 6. Also, a new peak appears at 1328


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cm-1 with the 630 nm probe (Lorentzian deconvolution of this peak is shown in Figure S8b), which becomes more prominent on the blue side of the absorption band II as shown in Figure 6. 3.3. Geometric and Electronic Structure. The Singly Occupied Molecular Orbitals (SOMOs) of the two lowest triplet states T1 and T2 of PQ in ACN are shown in Figure S9. For the T1 state the SOMOs involve π and π* orbitals while for the T2 state they involve n and π* orbitals. Therefore the T1 state has ππ* character and the T2 state has nπ* character in ACN, which underpins the electronic state switching in dipolar solvents. The optimized structural parameters of the two Table 1. Bond lengths of PQ in S0, T1 and T2 electronic states Bond lengthb (Å) a Structural parameters S0 T1(13ππ*) T2(13nπ*)

a

C1-C2, C9-C11

1.390 1.385

1.385

C1-C6, C11-12

1.400 1.427

1.427

C2-C3, C8-C9

1.405 1.405

1.404

C3-C4, C7-C8

1.419 1.460

1.460

C3-C21, C8-C23

1.474 1.464

1.465

C4-C5, C7-C13

1.405 1.438

1.438

C5-C6, C12-C13

1.395 1.371

1.370

C21-O22, C23-O24

1.225 1.258

1.258

C21-C23

1.550 1.475

1.476

C4-C7

1.486 1.419

1.419

Refer to Figure 1 for atom sequence numbers.

b

All bond lengths were obtained by the B3LYP/6-31+G(d,p) method with an implicit solvent model. The triplet states were calculated in TD-DFT formalism with the above mentioned method. lowest triplet states are tabulated in Tables 1 and S1. The bond lengths and bond angles of both triplet states are very similar. The C21-C23 bond lengths of both triplet states T1 and T2 (1.475


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and 1.476 Å respectively) are shortened relative to the singlet ground state (1.550 Å) and the same results are found for the C4-C7 bond lengths. The carbonyl bonds are elongated in both triplet states; for the other bond lengths and angles in the lowest triplet states refer to Tables 1 and S1. Electrostatic potential (ESP) derived charge analysis was carried out on the two lowest singlet (S1 and S2) and triplet (T1 and T2) excited states with respect to the ground state (S0); the results are shown in Figure S10. For ππ* excitation, the biphenyl ring was considered as an electron donor group and the C=O groups were considered as electron acceptors as shown in Figure S10a. The charge transfer efficiency of an electronic transition52 was calculated using the following equation, = ∆q ! − ∆q " (3) where fCT is the charge transfer efficiency and ∆qi is the charge difference on the donor/acceptor groups between the excited state and the ground state. The charge transfer efficiencies (fCT) calculated for the singlet and triplet ππ* states were calculated to be +0.96 |e| and +0.72 |e| respectively. For an nπ* excitation, the charge partition was taken between one donor group (the oxygen atoms of the C=O groups) and two acceptor groups: i) the carbon atoms of the C=O groups; and ii) the biphenyl group as shown in Figure S10b. The singlet and the triplet nπ* states involve -0.28 |e| and -0.30 |e| charge transferred from the oxygen atoms to the carbon atoms of the C=O groups respectively. The charge transferred from the oxygen atoms to the biphenyl ring was -0.01 |e| and -0.03 |e| for the singlet and the triplet nπ* states, respectively. To help understand the excited state hydrogen bonding mechanism of the T1(13ππ*) and S2(11ππ*) states, TD-DFT calculations with an explicit methanol solvent (supermolecule approach) were performed as shown in Figure S11. PQ forms two types of intermolecular


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hydrogen bonds with protic solvents13 viz. i) linear; and ii) bifurcated. The intermolecular linear hydrogen bond lengths between PQ and methanol were found to be 1.916, 1.796 and 1.829 Å for the S0, S2(11ππ*) and T1(13ππ*) states respectively. Similarly, the intermolecular bifurcated hydrogen bond lengths (major) between PQ and methanol were calculated to be 2.000, 1.815 and 1.859 Å for the S0, S2(11ππ*) and T1(13ππ*) states respectively, please refer to Table S2 for other structural parameters of the hydrogen bonded PQ complex. Furthermore, the bifurcated hydrogen bond binding energies (HBBEs) of PQ for S0, S2(11ππ*) and T1(13ππ*) states were obtained as 15.4, 38.5 and 23.6 kJ/mol, respectively. The supermolecule calculations were not carried out for the S1(11nπ*) and T2(13nπ*) state because they undergo hydrogen bond weakening (dissociative surface) but however in order to understand the hydrogen bond mechanisms in these states, we have performed vertical excitation energy calculations of the PQ molecule and the PQ-MeOH supermolecule embedded in an implicit solvent (MeOH) model. The differences between the vertical excitation energies of PQ-MeOH supermolecule and PQ molecule are given in Table S3: the S1(11nπ*) and T2(13nπ*) states of PQ-MeOH show a blue shift, while the S2(11ππ*) and T1(13ππ*) states of PQ-MeOH show a red shift relative to the PQ molecule. 4. DISCUSSION We aimed to shed light on the electronic nature of the transient species observed in the visible region of the ns-time-resolved absorption spectra, the conformational structure of the two lowest triplet states, the influence of solvent polarity on the energy levels of these triplet excited states and its inference on the photophysics and photochemistry of PQ. To this end we have carried out time-resolved absorption and resonance Raman spectroscopy experiments in combination with TD-DFT calculations of which the results will be discussed in the subsequent sections.


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4.1. Electronic Nature of the Triplet-Triplet Transitions. The time-resolved absorption spectra as shown in Figure 2 and S2 show two bands viz. at i) 470 nm (band I); and ii) 680 nm (band II) with similar lifetimes as discussed previously. This leads to the following three possibilities for these bands: i) the lowest triplet state (T1) is in resonance with two higher triplet states (Tn’ and Tm’); ii) the two lowest triplet states (T1 and T2)

Figure 7. Illustration of energy levels, transitions and solvent polarity effects on the lowest triplet and singlet excited states in non-dipolar CCl (left) and dipolar ACN (right) solvents. Thin arrows represent pump and probe wavelengths, while the thick arrows depict reordering of energy levels with increasing hydrogen bond donating ability (α) of the protic dipolar solvents.


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The bold solid horizontal lines represent the 0,0 energy levels of that particular electronic state, while thin solid horizontal lines correspond to the λmax of that particular electronic state. The energies of the electronic states were obtained from various spectroscopic experiments as in Ref. 13, 18, 21 and the present work. Please refer to the main text for further discussions. are in resonance with the same higher triplet state (Tn’); and iii) two different triplet states (T1 and T2) are in resonance with different higher triplet states (Tn’ and Tm’). The different TR3 spectra with 470 and 680 nm probe wavelengths (see Figure 4) and their kinetic behavior at the initial time scales (τ1 and τ3; see Figure 5) rule out the first possibility. The second option is also not valid because the difference in λmax of the time-resolved absorption bands I and II (6570 cm1

) is very large compared to the energy gap between the lowest triplet states18,21 (~840 cm-1), but

the third possibility matches with our experimental observations. PQ exhibits ISC with a quantum yield of almost unity and the population of the lowest triplet state T1(13ππ*) by ISC from the S1(11nπ*) state is expected to happen on a picosecond timescale16,22 and hence could not be observed within the time resolution of our experiments. From the previous literature on the ISC mechanism for PQ16 and our present TR3 spectroscopy experimental results, the timeresolved absorption band I is assigned to an electronic transition between T1(13ππ*) and Tn’(n3ππ*) states while band II is assigned to an electronic transition between T2(13nπ*) and Tm’(m3nπ*) as summarized in Figure 7 (right hand side for acetonitrile solvent) The temperature dependent phosphorescence and H-atom abstraction reaction studies in solution suggest that the lowest triplet excited states T1 and T2 are in thermal equilibrium18,21. The kinetic analysis shows that the rise time constant (τ3) from the TR3 spectra with 680 nm probe is identical with the faster decay time constant (τ1) of the TR3 spectra with 470 nm probe


20

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at ns-timescales, as shown in Figure 5. A kinetic scheme is proposed as shown in Figure 8. At thermal equilibrium, the relative populations of the lowest triplet excited states are: &(ππ* )3 ,

.

#$% = &(nπ*)3, = . /0

1/0

(4)

where Keq is the equilibrium constant at a particular temperature, while kIC and k-IC are the rate constants for the internal conversion from T2(13nπ*) to T1(13ππ*) states and vice-versa respectively as depicted in Figure 8. Keq can also be obtained from the Boltzmann distribution equation, #$% = exp

-

∆E

34 T

(4)

where ∆E is the adiabatic energy gap between the lowest triplet excited states assuming that the entropies for both triplet states are similar; T is the temperature and kB is the Boltzmann constant. The adiabatic energy gap between the lowest triplet states18,21 was found to be ~840 cm-1 and kBT ~210 cm-1 at 23°C. k-IC is the rate constant for the population of the T2(13nπ*) state through reverse internal conversion. This can be obtained from the time-dependent integrated Raman band intensities of PQ in ACN with 680 nm probe: k-IC = (τ3)-1 = 9 x 106 s-1. Knowing Keq (from equation 4) and k-IC, kIC was found to be ~5 x 108 s-1. A similar kind of behaviour was observed for thioxanthone in protic solvents.12 Subsequently, we propose that after photoexcitation to the singlet manifold of PQ in ACN, ISC from the S1(11nπ*) state first populates the T1(13ππ*) state in accordance with El-Sayed’s rule17 at a ps timescale (in agreement with the findings of Shimoshi et.al.16) and then the thermal equilibrium is established between the two lowest triplet excited states before the depletion to the ground state by ISC,0. As shown in Figure 8, both decay with the same time constants (τ2 and τ4) because the equilibration between them is an order of magnitude faster than their lifetimes.


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Most of the peaks in the TR3 spectra of PQ in ACN with the 630 nm probe feature similar kinetics as for the 680 nm probe except the peak at ~1328 cm-1 as shown in Figure S8. Also, the maximum of this peak apparently undergoes a blue shift from 1328 to 1337 cm-1 with time delay and then remains constant as shown in Figure S8. The Lorentzian deconvolution of this peak to two peaks viz. 1328 cm-1 and 1337 cm-1 is also shown in Figure S8, each following a different temporal behavior. The 1328 cm-1 peak is stronger at shorter timescales while the 1337 cm-1 peak becomes stronger at longer timescales, suggesting that two species are involved. As shown in Figure S8c, the Raman peak at 1328 cm-1 follows a similar kinetic behavior as with the 470 nm probe. On the other hand, the Raman peak at 1337 cm-1 follows the same kinetic behavior as with the 680 nm probe. The energy difference between the Tm’(m3nπ*) and T1(13ππ*) states was

Figure 8. A kinetic scheme proposed to explain the different temporal behaviour of the timeresolved resonance Raman experiments with probe i) 470 nm; and ii) 680 nm for PQ in ACN. Please refer to the main text for detailed discussions. found to be ~15552 cm-1 (~643 nm) as depicted in Figure 7. Therefore, for probe wavelengths shorter than 640 nm, both T1 and T2 are in resonance with the Tm’ state. The Raman peaks at 1328 (v”10a) and 1337 cm-1 (v’10a) which showed anomalous kinetic behavior with 630 nm probe


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are assigned to the T1 and T2 states respectively. This assignment was further confirmed by Figure 6: the peak at 1328 cm-1 gets prominent at wavelengths shorter than 652 nm. Hence we conclude that the shoulder of band II has two contributions: a vibronic transition T2 to Tm’ and a weak 0,0 electronic transition from T1 to Tm’. Despite the fact that the two lowest triplet states are energetically close; the slow IC (~2 ns) between the triplet states is surprising. The similar structure of the two lowest triplet excited states as inferred from Table 1 (bond lengths) and Table S1 (bond angles) could be responsible for the slower IC.53 The TR3 experiments in conjunction with TD-DFT calculations can provide insight into the conformational structure of the lowest triplet states, as will be discussed in the following section. 4.2. Conformational Structure of the Lowest Triplet States. The vibrational assignments of the TR3 spectra of PQ in ACN for both probe wavelengths are given in Table 2. The most prominent peaks, v”10a (1328 cm-1) and v’10a (1337 cm-1) observed for the 470 nm and 680 nm probe wavelengths respectively, were assigned to in-plane HCC bending and C=C stretching (especially the C4-C7 bond length) vibrations calculated from the potential energy distributions. The mass weighted atomic displacements of these modes are shown in the inset of Figure 4. The vibrations are mainly localized in the biphenyl ring for the triplet nπ* state while for the triplet ππ* state the vibrations are delocalized over the molecule. In resonance with 470 nm (band I), the Raman peaks corresponding to biphenyl ring stretching and bending modes are enhanced selectively, while in resonance with 680 nm (band II) the Raman peaks corresponding to the C=O sites (1364 cm-1) and the biphenyl ring (1337 cm-1) are enhanced. This is in agreement with the electronic transition assignments of these bands as discussed earlier. The TREPs of 1337 (v’10a), 1475 (v’7a) and 1606 (v’5a) cm-1 follow the time-resolved absorption band II, therefore suggesting that these modes are enhanced in resonance with the T2 (13nπ*) to Tm’


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Table 2. Tentative vibrational assignments for the lowest triplet states of PQ in ACN Vibrational frequencies (cm-1) Modea PEDd (%) b c Computational Experimental a) T1(13ππ*) v”5a

1560

60 v(C=C) + 19 δ(HCC) + 12 δ(CCC) 1561

v”39b

1554

62 v(C=C) + 22 δ(HCC)

v”7a

1464

1492

47 δ(CCC) + 44 v(C=C)

v”10a

1343

1328

48 δ(HCC) + 34 v(C=C)

v”12a

1250

1206

40 δ(CCC) + 37 v(C=C) + 18 δ(HCC)

v”13a

1181

1146

56 δ(HCC) + 27 v(C=C) + 14 δ(CCC)

v”28a

416

408

78 τ(CCCC) + 13 τ(HCCC)

v”61b

396

385


v”63b

348

354

66 δ(CCC) + 22 v(CC)

v’5a

1594

1606

71 v(C=C) + 21 δ(HCC)

v’6a

1491

1524

60 v(C=C) + 38 δ(HCC)

v’7a

1478

1475

62 v(C=C) + 18 δ(HCC) + 16 δ(CCC)

v’8a

1427

b) T2(13nπ*)

40 v(C=C) + 28 δ(HCC) + 24 v(C=O) 1364

v’9a

1417

64 v(C=O) + 20 δ(HCC) + 17 v(C=C)

v’10a

1348

1337

65 δ(HCC) + 29 v(C=C)

v’11a

1256

1238

60 v(C=C) + 24 δ(HCC) + 16 δ(CCC)

v’14a

1137

1148

68 δ(HCC) + 16 v(C=C) + 12 δ(CCC)

v’15a

1062

1052

50 v(C=C) + 42 δ(CCC)

v’17a

986

1013

54 δ(CCC) + 39 v(C=C)

v’20a

847

807

95 τ(HCCC)

v’21a

771

780

75 δ(HCC) + 17 v(C=C)

v’22a

749

762

64 τ(HCCC) + 20 τ(CCCO) + 16 τ(CCCC)


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v’23a

692

716


v’24a

646

658

86 δ(CCC)

v’27a

452

448

74 δ(CCC) + 23 v(C=C)

v’29a

402

403

48 δ(CCC) + 24 v(C=C) + 24 δ(CCO)

v’30a

383

357

64 δ(CCO) + 24 δ(CCC) + 12 v(C=C)

a

Following Herzberg notations of vibrational mode assignments (Ref. 13).

b

All computations were carried out by the TD-B3LYP/6-31+G(d,p) method with implicit solvent model and were scaled by a factor of 0.9648 taken from Ref. 46. c

TR3 experiments were performed using probe wavelengths: a) 470 nm; and b) 680 nm with a pump wavelength at 354.7 nm. The bold letters represent some of the prominent peaks discussed in the main text. d

Potential Energy Distributions were calculated for the optimized geometry with the TD B3LYP/6-31+G(d,p) method. PED contributions < 10% are not given. ν, stretching; δ, bending; τ, torsion. (m3nπ*) transition. The TD-DFT calculations suggest that the lowest triplet states have a similar structure. To elucidate the geometrical changes upon excitation to the lowest triplet excited states, SOMOs involved in the transitions are shown in Figure S9. The most prominent structural changes in the T1 and T2 states were the C=O, C4-C7 and C21-C23 bond lengths as given in Table 1. In the T2(13nπ*) state, electron density from the n-orbitals is transferred to the antibonding C=O π* orbitals as depicted in Figure S9, resulting in the elongation of the C=O bond. Similarly in the T1(13ππ*) state, the C=O bond lengths are also elongated, but in this case the electron density from the aromatic ring is transferred to the anti-bonding C=O π* orbitals, as can be inferred from Figure S9. The shortening of the C4-C7 and C21-C23 bond lengths suggest that there is an increased electron delocalization between the biphenyl ring and the dicarbonyl groups. The singlet and triplet ππ* states of PQ are non-planar. Unlike its singlet S1(11nπ*) state which is planar,13 the T2(13nπ*) state was found to be non-planar. Upon 11nπ* excitation which has an anti-symmetric combination of spin states, the planar structure of PQ in the relaxed S1


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state can be stabilized by the symmetric combination of the lone pair orbitals of the oxygen atoms (n+ - c1ϕ1 + c2ϕ2, where ϕi is the lone pair orbital of the oxygen atoms) in accordance with Pauli’s exclusion principle. Therefore, we propose that the T2(13nπ*) state (which involves a symmetric combination of spin states) is non-planar. Furthermore, the increase in the rate of k-IC (1/τ3) with increasing temperature as seen in Figure S7 suggests that both k-IC and kIC should be an activated process with a barrier due to a structural distortion required to achieve the crossing between the states.11 So in spite of the similar conformational structures we observe a slow IC between the two lowest triplet states. Additionally, the two lowest triplet states show different charge distributions (Figure S10) and hydrogen bonding mechanisms (Figure S11) therefore we propose that solvent reorganization could also contribute to the barrier. In order to obtain a clearer picture on the intermolecular hydrogen bonding mechanism of the lowest triplet states and its effect on the photophysics and photochemistry of PQ, a charge analysis was carried out on the two lowest triplet excited states using TD-DFT calculations. Additionally, the hydrogen bonding mechanisms were deduced from the supermolecule calculations in the excited state and are presented in the following section. 4.3. Solvent Polarity Induced Energy Level Reordering. The charge analysis suggests that in the T1(13ππ*) state a charge transfer takes place from the biphenyl ring to the C=O sites, which is manifested by a high positive value of fCT, accompanied by an increase in the dipole moment as shown in Figure S10a. In contrast, in the T2(13nπ*) state, a reversal of charge on the C=O sites takes place with a concomitant decrease in the dipole moment as shown in Figure S10b. Thus for two different reasons there is an elongation of the C=O bond length in both triplet states as discussed in the previous section. Furthermore, it is inferred from the charge analysis that the T1(13ππ*) state should undergo excited state hydrogen bond strengthening relative to the S0 state


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which was substantiated by HBBEs and the shortening of the C=O--H intermolecular hydrogen bond length in the T1(13ππ*) state as discussed in the previous section. The T2(13nπ*) state should undergo hydrogen bond weakening with respect to the S0 state owing to a decrease in the electron density at the oxygen atoms of the C=O sites. The differences between the vertical excitation energy of the PQ-MeOH supermolecule and the PQ molecule also support the above proposed hydrogen bond mechanisms for the two lowest triplet states. Consequently, in protic dipolar solvents the two lowest triplet states T1(13ππ*) and T2(13nπ*) should undergo hydrogen bond strengthening and weakening, respectively, relative to the S0 state. Also, the HBBEs of PQ reflect its basic character which, in agreement with the charge analysis, increases as follows: pK(S0) < pK(T1) < pK(S2). The position of band I was only sensitive to dipolarity of the solvent but not to its protic character (ACN and MeOH have similar dipolarity but MeOH has a much higher hydrogen bond donating ability) as shown in Table S4. This suggests that the hydrogen bonding interactions are very similar in the T1 and Tn’ states. The relative ∆A ratio of band II to I was sensitive to solvent polarity but the position of band II was less sensitive to the solvent polarity. In CCl the T1(13nπ*) state is highly reactive and band II was observed only with a high energy flash lamp as described previously. The ACN (aprotic) and MeOH (protic) solvents have similar dipolarity but they differ in their ability to form hydrogen bonds and therefore to understand the influence of hydrogen bonding on the ISC mechanisms, the relative ratios of band II to I were determined as shown in Table S4. In MeOH solvent, the relative ratio of band II to I is increased with respect to ACN solvent. This observation could be rationalized from the HBBEs for various excited states of PQ as discussed previously i.e. the energy gap between the S1(11nπ*)-T1(13ππ*) states increases with a concomitant decrease in the energy gap between the S2(11ππ*)-T2(13nπ*) states


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as depicted in Figure 7. Therefore we propose that in protic solvents, different hydrogen bonding mechanisms exhibited by the nπ*, ππ* singlet and triplet states of PQ could lead to an additional channel for the population of the T2(13nπ*) state. 5. CONCLUSION A comprehensive analysis of the conformational structure and the solvent polarity induced energy level re-ordering of the two lowest triplet states of 9,10-phenanthrenequinone (PQ) has been carried out using time-resolved absorption (TRA), time-resolved resonance Raman (TR3) spectroscopic and time-dependent-density Functional Theory (TD-DFT) computational studies. The TRA spectrum (in the visible region) of PQ in acetonitrile (ACN) has two bands viz. i) 470 nm (band I); and ii) 680 nm (band II), which have been assigned to triplet-triplet absorption of T1(13ππ*)-Tn’(n3ππ*) and T2(13nπ*)-Tm’(m3nπ*) transitions, respectively, based on the kinetic analysis of TR3 experiments probed at 470 nm and 680 nm. These assignments were also in agreement with the selective enhancements of the respective modes coupled to these chromophores in the TR3 experiments. A kinetic analysis of the 1328 cm-1 peak obtained from the TR3 spectra with probe 470 nm in resonance with band I resulted in a bi-exponential kinetics: a fast decay time constant (τ1~120 ns); and a slow decay time constant (τ2~1.25 µs). However, a kinetic analysis of the 1337 cm-1 peak obtained with probe 680 nm in resonance with band II led to a fast rise time constant (τ3~110 ns) and a slow decay time constant (τ4~1.23 µs). The fast decay time constant (τ1) matches the fast rise time constant (τ3) and the longer decay time constants were also similar. These observations indicate a thermal equilibrium between the lowest nπ* and ππ* triplet excited states of which the latter triplet state is formed first through intersystem crossing at ps timescales16. The temperature dependence of the rise time constant (τ3) suggests that both the reverse (k-IC) and the forward internal conversions (kIC) are activated


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processes owing to a barrier associated with structural distortion. So in spite of the similar conformational structures as evidenced by TD-DFT calculations, we observe a slow IC (~2 ns) between the two lowest triplet states. The electrostatic potential (ESP) derived charge analysis reveals that the T1(13ππ*) state involves a charge transfer from the aromatic rings to the carbonyl sites, while the T2(13nπ*) state involves reversal of charges on the carbonyl site (umpolung) with an insignificant amount of delocalization to the aromatic rings. Based on the charge analysis of PQ, differential solvation of the two lowest triplet states will result in an inversion of states from non-dipolar to dipolar solvents, and with increasing solvent polarity they move further apart from each other. Insight from the singlet excited states of PQ in protic dipolar solvents13 suggests that the two lowest nπ* and ππ* triplet states should undergo hydrogen bond weakening and strengthening, which was substantiated by the ESP derived charge analysis and TD-DFT calculations with an explicit protic solvent in the supermolecule approach. Consequently, because of the different hydrogen bonding mechanisms exhibited by the two lowest singlet and triplet excited states; we propose that in protic dipolar solvents the ISC mechanism could be altered owing to their energy level re-ordering. Supporting Information. Subtraction method for TR3 spectra, Time resolved absorption spectra of PQ in ACN, Solvent effects on T-T absorption spectra, TR3 spectra in different solvents, TR3 spectra of PQ in ACN with probe 470 and 680 nm, Temperature dependent TR3 spectra, TR3 spectra of PQ in ACN with probe 630 nm, Molecular orbitals for the lowest triplet states, Bond angles of PQ in different electronic states, Charge analysis of PQ in different electronic states, Hydrogen bonding of PQ in different electronic states, their associated structural parameters, Differences between the vertical excitation energies of the PQ-MeOH supermolecule and the PQ


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molecule and Solvatochromic parameters for time-resolved absorption of PQ. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Present Addresses #Present Address: Department of Physics, The American College, Madurai 625002, India $On leave from: LaserLaB, VU University Amsterdam, The Netherlands Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We thank the Council of Scientific and Industrial Research (CSIR), the Department of Science and Technology (DST) and the Defence Research and Development Organization for financial assistance. V.R.K. acknowledges CSIR for a research fellowship. S.U. acknowledges the DST for a J.C. Bose fellowship and N.R. acknowledges UGC for a D.S. Kothari fellowship. We would like to thank the Supercomputer Education and Research Centre of the Indian Institute of


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Science (IISc) for providing the computing facilities required to carry out the computational work. REFERENCES (1) Yagci, Y.; Jockusch, S.; Turro, N. J. Photoinitiated Polymerization: Advances, Challenges, and Opportunities. Macromolecules 2010, 43, 6245-6260. (2) Albini, A.; Fagnoni, M. Green Chemistry and Photochemistry were Born at the Same Time. Green Chem. 2004, 6, 1-6. (3) Shaath, N. A. Ultraviolet Filters. Photochem. Photobiol. Sci. 2010, 9, 464-469. (4) Cosa, G.; Lukeman, M.; Scaiano J. C. How Drug Photodegradation Studies Led to the Promise of New Therapies and Some Fundamental Carbanion Reaction Dynamics along the Way. Acc. Chem. Res. 2009, 42, 599-607. (5) Computational Photochemistry; Olivucci, M. Ed.; Theoretical and Computational Chemistry, Vol. 16; Elsevier: Amsterdam, 2005. (6) Porter, G.; Suppan, P. Reactivity of Excited States of Aromatic Ketones. Pure Appl. Chem. 1964, 9, 499-505. (7) Porter, G.; Suppan, P. Primary Photochemical Processes in Aromatic Molecules. Part 12.Excited States of Benzophenone Derivatives. Trans. Faraday Soc. 1965, 61, 1664-1673. (8) Scaiano, J. C. Solvent Effects in the Photochemistry of Xanthone. J. Am. Chem. Soc. 1980, 102, 7747-7753. (9) Scaiano, J. C. Intermolecular Photoreductions of Ketones. J. Photochem. 1974, 2, 81-118.


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(10) Leigh, W. J.; Lathioor, E. C.; Pierre, M. J. St.; Photoinduced Hydrogen Abstraction from Phenols by Aromatic Ketones. A New Mechanism for Hydrogen Abstraction by Carbonyl n,π* and π,π* Triplets J. Am. Chem. Soc. 1996, 118, 12339-12348. (11) 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. (12) Villnow, T.; Ryseck, G.; Constapel, V. R.; Marian, C. M.; Gilch, P. Chimeric Behaviour of Excited Thioxanthone in Protic Solvents: I. Experiments. J. Phys. Chem. A 2014, 118, 1169611707. (13) Kumar, V. R.; Rajkumar, N.; Umapathy, S.; Solvatochromism of 9,10Phenanthrenequinone: An Electronic and Resonance Raman Spectroscopic Study. J. Chem. Phys. 2015, 142, 24305. (14) Wagner, P.; Park, B. S. Photoinduced Hydrogen Atom Abstraction Reaction by Carbonyl Compounds. In Organic Photochemistry; Padwa, A. Ed.; Marcel Dekker: New York, 1991; pp 227-366. (15) Bohning, J. J.; Weiss, K.; A Kinetic Study of the Photochemical Reaction of Phenanthrenequinone with Olefins. J. Am. Chem. Soc. 1966, 88, 2893-2898. (16) Shimoshi, H.; Tero-Kubota, S.; Akiyama, K.; Ikegami, Y. Influence of Solvent Polarity on the Excited Triplet States of Nonphosphorescent 1,2-Naphthoquinone and Phosphorescent 9,10Phenanthrenequinone: Time-Resolved Triplet ESR and CIDEP Studies. J. Phys. Chem. 1989, 93, 5410-5414.


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