Unusual photobehavior of benzophenone triplets in hexafluoroisopropanol. Inversion of the triplet character of benzophenone

Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 1–8

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Unusual photobehavior of benzophenone triplets in hexafluoroisopropanol. Inversion of the triplet character of benzophenone A. Lewandowska-Andralojc a,∗ , G.L. Hug a,b , G. Hörner c , T. Pedzinski a , B. Marciniak a a b c

Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA Institut für Chemie, Technische Universität Berlin, 10623 Berlin, Germany

a r t i c l e

i n f o

Article history: Received 23 April 2012 Received in revised form 11 June 2012 Accepted 16 June 2012 Available online 26 June 2012 Keywords: Benzophenone Hexafluoroisopropanol Hydrogen-bonding Triplet inversion Electron transfer

a b s t r a c t In the present work a systematic study of the photophysical and photochemical behavior of unsubstituted benzophenone (BP) in acetonitrile–hexafluoroisopropanol (ACN–HFIP) mixtures was undertaken. Using laser flash photolysis, steady-state absorption and emission techniques, our results indicate that the lowest triplet excited state of BP can possess both n,␲* and ␲,␲* character, contrary to the previous assumption that the triplet of BP has pure n,␲* character. In ACN, the BP triplet has an n,␲* configuration with its typical triplet–triplet absorption spectrum having a maximum at 520 nm. However, in HFIP this band with max = 520 nm is accompanied by a new band with a maximum at 380 nm, which is attributed to a triplet of ␲,␲* character. Furthermore, changes in the character of the triplet state of BP were also detected by the measurement of the triplet quenching rate constants by 2-propanol (good hydrogen donor) in ACN (kq = 3.4 × 106 M−1 s−1 ) and HFIP (kq = 8.6 × 104 M−1 s−1 ). In contrast to 2-propanol, the reactivity of an electronically excited BP molecule toward anisole was greatly enhanced by changes in the nature of the solvent. The quenching rate constant was three orders of magnitude larger in HFIP than in ACN. In addition, the quenching mechanism itself appears to change on going from charge transfer in pure ACN to efficient full electron transfer in ACN–HFIP mixtures having a large (>60%, v/v) HFIP content. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The behavior of carbonyl compounds in their excited triplet states has been one of the most fundamental and important subjects in photochemistry. The nature of a carbonyl’s lowest excited triplet state (n,␲* , ␲,␲* ) and hence the electronic distribution in this state is of decisive importance for the reactivity of an excited carbonyl compound. It is well-known that aromatic ketones with lowest ␲,␲* triplet states are normally much less reactive, regarding hydrogen abstraction, than those having lowest n,␲* states [1–4]. While the specific electronic configuration of a carbonyl’s lowest triplet state is believed to be less significant in regard to electron-transfer-mediated reactions [5], there is an almost three orders of magnitude difference in the rate constant of H-abstraction from 2-propanol by benzophenone in a T1 (n,␲* ) state (kq = 1 × 106 M−1 s−1 ) and 4-phenylbenzophenone in a T1 (␲,␲* ) state (kq = 5 × 103 M−1 s−1 ) [6]. The superior reactivity of n,␲* -configured triplets toward H-abstraction reflects the electron deficiency on the carbonyl oxygen, which is the source of this state’s

∗ Corresponding author. Tel.: +48 61 829 1327; fax: +48 61 829 4367. E-mail address: [email protected] (A. Lewandowska-Andralojc). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2012.06.010

radical-like reactivity. Well-defined criteria for triplet-state differences in reactivity can be obscured because of vibronic mixing of closely lying n,␲* and ␲,␲* states. Such a mixed state will undergo the same processes as a pure n,␲* state but with rate constants that are limited by the amplitudes of mixing. Probably the most intensely studied carbonyl compound is benzophenone (BP). The lowest S1 and T1 states of BP have been shown to have n,␲* configurations in almost all kinds of solvents, including nonpolar and polar, as well as hydrogen-bonding solvents [4,7–9]. To obtain an inversion of n,␲* and ␲,␲* states in unsubstituted benzophenone, Rusakowicz et al. applied acidic media such as 85% H3 PO4 and 98% H2 SO4 [9]. In these latter cases, however, the inversion was attributed to the presence of protonated BP triplets. Inversion of the BP triplet state was also reported to occur in BP adsorbed on metal oxides (MgO, SiO2 , and Al2 O3 ) at 77 K where the energy levels of the excited states were systematically changed with increasing surface acidity, resulting finally in a cross-over from n,␲* -type phosphorescence to ␲,␲* -type phosphorescence [10]. This triplet-state inversion was accompanied by a complete loss of the vibronic bands in the phosphorescence spectra and by a significant increase in the phosphorescence lifetimes. Based on studies of the phosphorescence of benzophenone in the mixed solvent of 2,2,2,-trifluoroethanol (TFE) and water, Hamanoue et al.

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Scheme 1. State diagram of benzophenone at 77 K and mechanism for populating the lowest triplet state [6].

proposed that benzophenone forms complexes with water and TFE that may be of mixed n,␲* –␲,␲* character [11]. To our knowledge, there is no other report stating that the triplet state of unsubstituted benzophenone can possess other than pure n,␲* character. Thus, BP is the textbook example of a compound having pure n,␲* triplet state character (Scheme 1). The state diagram for benzophenone that is believed to operate in all kinds of solvents with the exceptions of the solvents discussed above is presented in Scheme 1. In contrast, our analysis in this work indicates that two triplet states (n,␲* and ␲,␲* ) of BP are in equilibrium in fluid HFIP solutions. This study establishes a thorough extension of previous laser flash photolysis work that showed unusual spectroscopic features of the BP triplet state in hexafluoroisopropano [12]. These features, not studied in detail at that time, served as the starting point of the present work as they appeared to be connected with the change in the character of the lowest triplet state. Besides characteristic changes in the steady-state and time-resolved spectra of BP in HFIP, the smaller reactivity of the T(␲,␲* ) state toward hydrogen abstraction serves as an indicator of switching between T1 (n,␲* ) and T1 (␲,␲* ) states with a change in the solvent. The present work was undertaken to gain further insight into the BP triplet-state character in acetonitrile–hexafluoroisopropanol (ACN–HFIP) mixtures. Our interest was focused on establishing the dependence of the photophysical and photochemical properties of the BP triplet state on the solvents’ mixture ratios. For this purpose, steady-state absorption and emission measurements and nanosecond flash photolysis of BP were performed in several ACN–HFIP mixtures. To determine how the medium composition influences the photochemical triplet-state properties, the quenching rate constants by donors (DH) that are known to quench triplet states via different mechanisms (hydrogen abstraction (HAT), electron transfer (ET) and charge transfer (CT)) were investigated (Scheme 2). The sharp decrease in the reactivity in the H-atom abstraction of triplet BP from 2-propanol (2-PrOH) between the solvents ACN and HFIP was observed, which is in agreement with the established finding that the ␲,␲* triplet state has reduced reactivity toward Hatom abstraction. Interestingly, our results have shown that when anisole (AN) was used as a quencher, not only the quenching rates were dramatically solvent dependent, but also the mechanisms change from being charge transfer in non-protic solvents to being a full electron-transfer-initiated process in protic solvents. The data

obtained for BP in ACN were used as the reference for the BP triplet state of pure n,␲* character. 2. Experimental 2.1. Materials Chemicals: anisole, benzophenone, triethylamine (TEA), and naproxen were purchased from Aldrich. Solvents for time-resolved spectroscopy and steady-state measurements were of the highest available analytical grade and were used without further purification. Acetonitrile, dichloromethane (DCM) were purchased from Merck, and 1,1,1,3,3,3-hexafluoro-2-propanol was purchased from Aldrich. 2.2. Steady state measurements UV–vis spectra were measured at room temperature using a Cary 300 Bio Varian spectrophotometer. The concentration of BP, used here for the study of the n → ␲* transitions of the benzophenone chromophores around 340 nm, was 2.5 × 10−3 M. The phosphorescence spectra were recorded with a Perkin-Elmer MPF3 spectrofluorimeter. The respective spectra were measured at room temperature in different mixtures of the solvents: ACN–HFIP. The phosphorescence spectra were recorded in the 350–600 nm range with an excitation wavelength of exc = 340 nm. Concentrations were set in the millimolar range, corresponding to absorbances, A, at 340 nm of 0.2, and samples were deoxygenated with high-purity argon for 30 min prior to the measurements.

Scheme 2.

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Fig. 1. Solvent dependence of the UV–vis absorption spectrum of BP: (2.5 × 10−3 M); inset (5 × 10−5 M) at room temperature in different ACN–HFIP mixtures; volumetric percentage of HFIP: (1) 0%, (2) 40%, (3) 50%, (4) 80%, and (5) 100%.

2.3. Nanosecond laser flash photolysis The set up for the nanosecond laser flash photolysis (LFP) experiments and its data acquisition system have previously been described in detail [13]. LFP experiments employed a pulsed Nd:YAG laser (355 nm, 5 mJ, 7–9 ns) for excitation. Transient decays were recorded at individual wavelengths by the step-scan method with a step distance of 10 nm in the range of 320–700 nm as the mean of 10 pulses. Samples for LFP were deoxygenated with highpurity argon for 15 min prior to the measurement. Rectangular quartz cells (1 cm × 1 cm) were used. Experiments were performed with freshly prepared solutions at room temperature (295 ± 1 K). The transient absorption spectra of BP in pure HFIP were additionally measured at 313 K and 273 K. This temperature variation was achieved using a temperature-controlled cell holder (Quantum Northwest, model TC 125). The concentrations of the benzophenone derivatives were set in the millimolar range to keep the optical densities at 355 nm between 0.3 and 0.4. 3. Results and discussion 3.1. Absorption and emission spectra The UV absorption spectra of benzophenone were recorded in different ACN–HFIP mixtures and, in agreement with the known strong hydrogen-bonding ability of HFIP [14], they revealed a strong solvent dependence (Fig. 1). H-bonding effects on the photophysics and photochemistry of carbonyl compounds have been traditionally explained in terms of the local electron density at the carbonyl oxygen [15]. When a BP n,␲* state is populated, the electronic density of the non-bonding lone pairs on the carbonyl oxygen (Scheme 1) is diminished relative to their electronic density in the BP ground state. Hence, the H-bonding strength of HFIP to the carbonyl oxygen should be greatly reduced in an n,␲* excited state relative to that in the ground state. This results in a larger energy gap between an n,␲* state and the ground state on H-bonding. On the other hand, when a BP ␲,␲* state is populated, the electronic density of the non-bonding lone pairs on the carbonyl oxygen should not change significantly relative to the ground state. Thus, the energy gap between a BP ␲,␲* state and the BP ground state would not be expected to change significantly upon H-bonding to the carbonyl. However, a general solvent-effect red-shift could appear because of a stabilization due to the ␲,␲* state being more

3

Fig. 2. Effect of the addition of HFIP on the absorption spectra of benzophenone in ACN (3 × 10−3 M). Inset: plot of absorbance data at 310 nm, according to Eq. (1).

polarizable than the ground state since the unexcited ␲ electron is more tightly held than the excited one. In line with this picture, the absorption of the n → ␲* band around 340 nm showed a continuously increasing blue shift with an increasing amount of HFIP in the ACN–HFIP mixture. In pure HFIP the typical shape of the n → ␲* band was covered by the strong ␲ → ␲* transition around 250 nm, which experienced a significant red shift on going from ACN to pure HFIP (Fig. 1, inset). This red shift of the ␲ → ␲* band amounts to as much as 12 nm in HFIP with respect to ACN, corresponding to a stabilization of the singlet ␲,␲* state relative to the ground state by 22.0 kJ mol−1 . These data are in accord with previous findings [16–19] where the most important contribution to the observed solvatochromism was attributed to the effect of specific solute–solvent interactions resulting from the formation of hydrogen bonding between the solute molecule and the protic solvent. In order to characterize the hydrogen bonding of the solvent molecules with benzophenone in a quantitative manner, the equilibrium constants K for the formation of 1:1 hydrogen-bonded solvent–solute complexes were extracted from the spectral effects of the protic additive, HFIP, to BP solutions in ACN on the n → ␲* band of BP around 310 nm. The absorption spectra of BP in dilute ACN solution were measured in the presence of varying amounts of HFIP (Fig. 2). This figure exhibits the typical blue shift of an n → ␲* absorption band in hydrogen-bonding (HB) media. The analysis followed the procedures invented by Mataga and Tsuno [20,21] which is summarized in Eq. (1). [1 − (A0 /A)] = −K + c0

ε  A c 0 ε

K

A

(1)

Herein, εc and ε denote the molar absorption coefficients of the complex and the free molecules, respectively, whereas A0 and A denote the measured absorbance at a given wavelength in the absence and presence of the HB-additive, and c0 is the total concentration of the HB additive, which has to be much greater than the concentration of BP. The plot of the absorption data in terms of Eq. (1) for HFIP is linear (inset of Fig. 2). Therefore, it was concluded that benzophenone and HFIP form 1:1 ground-state complexes in ACN in the concentration range under study. From the intercept of the plot in Fig. 2, an equilibrium constant K = 3.5 ± 0.7 M−1 was obtained for H-bonding with HFIP in ACN. The validity of the extracted equilibrium constants was cross-checked by plotting data from several

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Fig. 3. Phosphorescence emission of BP at room temperature in different ACN–HFIP mixtures; volumetric percentage of HFIP: (1) 0%, (2) 40%, (3) 50%, (4) 80%, and (5) 100%. The spectra were obtained following excitation at 340 nm, time delay: 0.01 ms. Inset: phosphorescence decay traces at 420 nm of BP at 77 K in (a) HFIP, (b) ACN, exc = 355 nm.

wavelengths. The value of the equilibrium constant indicates that for the concentration of HFIP, which corresponds to a 30% volumetric ratio of HFIP to ACN, already more than 90% of the molecules of BP are complexed with HFIP in the ground state. H-bonding interaction between the solvent molecule and BP was recently confirmed by density functional theory calculations for TFE as a solvent. The calculations showed that hydrogen bonding plays an important role both in the ground state and in the excited state of BP [18]. It is well established that when the lowest triplet state of a benzophenone is an n,␲* state, the phosphorescence spectra have a well-defined vibrational progression with a period of ca. 1600–1800 cm−1 which indicates the participation of the >C O vibration in the radiative process [7,12,16,23,24]. On the other hand, the ␲,␲* phosphorescence spectra of the benzophenones are broad, and their vibrational structure is lost. In addition the ␲,␲* phosphorescence of benzophenones is normally weaker [22,23]. At 298 K, the phosphorescence spectra of BP were obtained in different ACN–HFIP mixtures with matched absorbances at the excitation wavelength for all solutions. As shown in Fig. 3 the shapes of the phosphorescence spectra in mixed solvents of HFIP and ACN change gradually with an increase in the HFIP content. First, at this temperature the phosphorescence intensity strongly decreased with increasing amounts of HFIP and appeared to shift to higher energy. Secondly, the increase in HFIP concentrations was accompanied by the disappearance of the typical “five-fingered” pattern of the n,␲* benzophenone chromophores. Similar types of changes in the phosphorescence spectrum of BP have also been observed in ethanol–water and trifluoroethanol (TFE)–water mixtures [11]. While the behavior in ACN was well accommodated by a dominantly n,␲* configured triplet state, the phosphorescence spectra obtained at high HFIP concentration were in agreement with these general features for ␲,␲* state phosphorescence of benzophenones. Phosphorescence decay dynamics at 77 K vary with solvent composition (inset of Fig. 3). A rate constant of 220 s−1 was extracted from monoexponential decay in ACN which is similar to the value reported by Mittal et al. for the BP phosphorescence decay in methylcyclohexane [22]. In HFIP the decay was biexponential, with rate constant values of 135 s−1 and 45 s−1 . This kind of dual phosphorescence has already been reported for a few of the aromatic carbonyl compounds, including

Fig. 4. Normalized transient absorption spectra obtained during laser flash photolysis at 355 nm of deoxygenated solutions of BP in different ACN–HFIP mixtures; volumetric percentage of HFIP: (1) 0%, (2) 40%, (3) 80%, (4) 90%, and (5) 100%. Inset: normalized decay profiles of the transient absorption monitored at 380 nm and 520 nm in pure HFIP.

benzophenones [11,22,24]. Dual components in the phosphorescence decay indicate that the two emitting states (n,␲* and ␲,␲* ) are populated and that the two states are not in thermal equilibrium at 77 K. Hamanoue et al. [11] also observed the dual exponentiality of the phosphorescence decay of BP in TFE–water mixtures and assigned the shorter component (270 s−1 ) to the lowest triplet state of free benzophenone, with normal n,␲* character, and the long-lived (63 s−1 ) component to the lowest triplet state of a complex BP–H2 O–TFE with mixed n,␲* /␲,␲* character. Based on the general nature of the phosphorescence spectrum and decay of the benzophenones, the phosphorescence observed in this work can be understood by assigning the lowest triplet state of BP in ACN to the n,␲* state but by assigning the lowest triplet state of BP in HFIP as having both ␲,␲* and n,␲* character which, based on the data above, might be mixed or in thermal equilibrium (but see below). Triplet-state inversion in a strongly hydrogen-bonding solvent, as indicated above, is a rare finding for BP itself, but it is not unusual for benzophenone derivatives. Such effects on the relative positions of the n,␲* and ␲,␲* states are especially obvious, when electrondonating groups, such as OH, OCH3 , and NH2 , are substituents in the aromatic rings of benzophenone [23,25–27]. Notably with respect to the following section, changes in the triplet–triplet transient absorption spectra in different media have been used as one of the indicators of the inversion of the electronic nature of the lowest lying triplet state in the works on substituted BPs [22,23,25,26]. 3.2. Nanosecond laser flash photolysis: photophysical characterization In order to compare the triplet–triplet absorption spectra of BP in different ACN–HFIP mixtures, LFP experiments were carried out with excitation at 355 nm. Transient absorption spectra of BP in pure ACN up to ACN–HFIP (2:3, v/v) were identical (Fig. 4). These spectra matched those previously reported for the BP triplet state [2]. They are characterized by absorption maxima at 325 (not shown) and 525 nm, in addition to a long-wavelength tail above 600 nm. Upon addition of amounts of HFIP to acetonitrile higher than 80 percent by volume, the spectra still showed a band with a maximum at 520 nm, but a new broad absorption band appeared with a maximum at 380 nm (Fig. 4). In pure HFIP the ratio of the absorbance at 380 nm to the absorbance at 520 nm was equal to

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Table 1 Summary of solvent parameters and rate constants for quenching of the BP triplet state by donors. Solvent

εa

˛Kamlet

ACN HFIP DCM

35.9 16.7 8.93

0.19 1.96 0.3

a b

kq (M−1 s−1 ) 2-PrOH

TEA

AN

3.4 × 106 8.6 × 104 1.9 × 106

4.1 × 109 (4.1 × 109 )b 2.2 × 106 2.9 × 109

4.5 × 106 2.8 × 109 7.5 × 106

At 25 ◦ C. CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 76th ed., 1995. Literature data [1].

1.75 in comparison to 0.26 in pure ACN (Fig. 7). Long lifetimes of the transient(s) in the microsecond range were obtained for all ACN–HFIP mixtures, and, importantly, the kinetic traces at 380 nm and 520 nm coincided with each other (Inset of Fig. 4). In addition both bands with maximum at 380 and 520 nm, obtained at high concentrations of HFIP, were efficiently quenched with the same kinetics by oxygen and via energy transfer to naproxen. Thus, the 380 nm band is also assigned as a BP triplet state. The band centered at 520 nm matched with the previously reported n,␲* triplet, whereas the 380 nm band can be assigned to the ␲,␲* triplet. This new transient absorption band did not arise due to the presence of some impurities in the solvent, because then it should have grown gradually with the increase of HFIP concentration, and the kinetic profiles would not have decayed with the same rate constant. In addition, the origin of the new band cannot be attributed to the absorption of the n,␲* complex of triplet state of BP hydrogen bonded to HFIP because it would have appeared already at small concentration of HFIP whereas the new absorption appears only at HFIP concentrations higher than 5.7 M (ACN–HFIP (2:3, v/v)). It is worth mentioning that the increase in the intensity of the absorbance around 380 nm was also observed for BP in trifluoroethanol, but the effect was less pronounced. We postulate that, due to a small energy gap between the BPtriplet states, both states are populated in solutions of high HFIP concentration. To test this hypothesis, we performed LFP experiments with BP in HFIP at temperatures of 313 K and at 273 K which is close to the freezing point of HFIP. It was expected, according to the Boltzmann distribution, that decreasing the temperature should reduce the population of the upper state of the two closely lying triplet states. The data (Fig. 5) show that the change in temperature influences the ratio of the absorbances at 380 nm and 520 nm. At the temperatures of 313 K and 273 K, the ratios of the absorbance at 380 nm to that at 520 nm were found to be 1.75 and 2.17,

Fig. 5. Normalized transient absorption spectra obtained during laser flash photolysis at 355 nm of deoxygenated solutions of BP in HFIP at 273 K and 313 K.

respectively. This also confirms that the two types of triplet states (n,␲* , ␲,␲* ) were detected simultaneously. The energy levels of these two kinds of triplets are very close to each other, and in this solution, thermal equilibrium was established between these excited triplet levels. According to the Boltzmann distribution, the energy gap between two triplet states must be <4 kJ mol−1 to allow for significant populating of both states. In addition, the intensity of the band with maximum at 520 nm, that is assigned to the n,␲* triplet state, decreased with the lowering of the temperature. This, in turn, identifies the lower-lying triplet as the one with ␲,␲* character. 3.3. Nanosecond laser flash photolysis: photochemical behavior The nature of the lowest excited triplet state is of decisive importance in the reactivity of a carbonyl compound; e.g., the reactivity of the n,␲* state toward hydrogen abstraction is much higher than that of ␲,␲* triplets. Thus, after characterization of the photophysical properties of the triplet state of BP in HFIP, the photobehavior toward quenching processes was studied. It was accomplished by a comparison of characteristic quenching rate constants of the BP-triplet state in HFIP with respective values in the nonprotic solvents ACN (polar) and DCM (nonpolar). The quenchers (2-PrOH, TEA, and AN) were chosen such as to probe the intrinsic reactivity of the triplet states by a broad spectrum of different quenching mechanisms: hydrogen, electron and charge transfer, respectively (Scheme 2). In order to determine the bimolecular quenching rate constants kq for the triplet quenching of BP, the decay of the BP triplet was monitored in the presence of increasing concentrations of quencher. The underlying pseudo-first order rate constants kobs were extracted from the experimental absorption decay profiles at 630 nm since, in the absence of significant amounts of the BP anion radical BP•− or in the case of very fast protonation of BP•− , the wavelength region around 630 nm is specific for the absorption of the BP triplet state and therefore appropriate for its kinetic studies. Under our experimental conditions, all the time profiles could be adequately fitted to first-order exponential decay functions. The resulting quenching rate constants that are summarized in Table 1 exhibit a striking divergence among the chosen solvents for all of the quenchers. Photoreduction of BP by TEA, which is known to be a good electron donor, was studied as an example of pure electrontransfer quenching. The transient spectra recorded in HFIP and ACN, after complete triplet decay, showed the presence of hydrogenatom-transfer derived products, which is in agreement with the Fessenden et al. observations (transient absorption spectra in Supplementary information). The kinetics deviated from our initial expectation that the quenching rate constants would not differ in ACN and HFIP. In contrast, the quenching rate constant of the BP triplets in HFIP of 2.2 × 106 M−1 s−1 was found to be more than three orders of magnitude smaller than that in ACN (4.1 × 109 M−1 s−1 ). For this quenching reaction and this pair of solvents, the role of the medium in the observed drastic reduction of the reactivity can be attributed to the change in the properties of the electron donor and

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Fig. 6. Transient absorption spectra obtained during laser flash photolysis at 355 nm of deoxygenated solutions of (a) benzophenone (2.5 × 10−3 M) and (b) benzophenone and anisole (2.8 × 10−3 M) in ACN–HFIP (1:9, v/v), time delays after flash (from top to bottom): (a) 0.4, 1, 2, 4 ␮s and (b) 40, 200, 400, 1000 ns; inset: normalized decay profiles of the transient absorption monitored at 520 nm, 540 nm and 430 nm for the quenching of BP by anisole.

not the BP triplet state. The slow rate of quenching in HFIP is due to a strong H-bonding interaction between TEA and HFIP. The availability of an electron in the nonbonding orbital of the nitrogen in TEA is reduced by the overlap of that orbital with the H atom from the alcohol moiety of HFIP. Our experimental findings are in accord with the data reported by Devadoss and Fessenden [28]. In their nanosecond experiment, the BP triplet quenching rate constants by TEA in TFE (1.8 × 107 M−1 s−1 ) was two orders of magnitude smaller than the value obtained when ACN was used as a solvent. When 2-PrOH was used as a hydrogen donor, the ratio between the quenching rate constants measured in ACN vs HFIP or DCM vs HFIP were ca. 40 and 20, respectively. Thus, the reactivity decreased sharply in HFIP solutions. This result supports the contention that T1 (␲,␲* ) is the dominant electronic configuration for the lowestlying triplet BP in HFIP solutions, because it is known that n,␲* to ␲,␲* inversion of aryl ketones produces a significant decrease in their rates of quenching by hydrogen donors [4,25,29]. In contrast to 2-PrOH and TEA, when anisole was used as the quencher for triplet BP, the quenching rate constant was almost three orders of magnitude larger in HFIP than in ACN or DCM (Table 1). This finding is in agreement with our recent observation of enhanced photoactivity of BP in protic media toward quenching by anisole [18], and it parallels the results from a related study by Miranda et al. [26]. In our system, the gain in the reactivity in HFIP was accompanied by a switch in the quenching mechanism itself. Recently, we found some evidence that the overall appearance of the BP triplet quenching by AN changes from charge-transferinduced nonreactive quenching in nonprotic solvents to reactive ET-quenching in protic solvents, concomitant with efficient freeradical formation in the protic solvents [18]. This was interpreted as due to a decrease in the free energy of the ET and the free energy of activation, both as an effect of the specific H-bonding of the BP chromophore by protic solvents and additives. The results in non-protic solvents were compatible with unproductive quenching via a charge-transfer state, which is known to operate for anisole [30–32]. Importantly, AN and BP do not form a ground-state charge-transfer complex in either ACN or HFIP. This was checked by measuring appropriate UV–vis spectra. These UV–vis spectra of the BP and AN system are the sum of the UV–vis spectra of BP and AN. Accordingly, the triplet quenching by anisole in ACN solution, which was studied in detail recently [18], was not connected with any formation of free radical-ion products that might signify that a full ET occurred. In contrast, the significant spectral evolution seen in HFIP signals that there was efficient formation of

quenching-derived transients at 540 and 430 nm that were stable on the microsecond timescale (Fig. 6b and inset). Taken as a reference, only minor spectral evolution, probably due to H-abstraction from the solvent itself, was evident in the absence of AN (Fig. 6a). The transients in the presence of AN are assigned as the anisole cation radical (max = 430 nm) [33] and the BP ketyl radical (max = 540 nm) [34]. The predominance of the BP ketyl radicals in the transient spectra points to an ET reaction from the anisole to the BP triplet which is followed by the protonation of the BP radical anion by the proton donor HFIP. It is noted that the complete absence of spectral contributions of the intermediately formed radical anion points to very fast protonation from the protic solvent. In order to get deeper insight into the solvent dependence, the anisole quenching of triplet BP was subjected to a detailed investigation of solvent mixtures: ACN–HFIP. That is, the kinetics of the reaction and the product ratios (ketyl radical, anisole radical cation) observed during the quenching of the BP triplets were quantified in solvent mixtures of ACN and HFIP. The quantum yields for the formation of the primary transient photoproducts during quenching of triplet BP by anisole were found to be ˚ ≈ 0.0 in ACN and reached ˚ ≈ 1.0 in ACN–HFIP (1:4, v/v). These quantum yields were determined by relative actinometry with optically matched solutions of BP in ACN. The concentration of anisole was sufficiently large to quench the BP triplets almost totally (>90%). It is noted that our approach rests on the assumption that the absorption properties of the transients (ketyl radical and anisole cation) do not vary significantly with the nature of the solvent. As a consequence, the derived quantum yields carry a relatively high intrinsic uncertainty. Irrespective of the uncertainty in the absolute values, it becomes evident that the undesired recombination in the photoinduced electron transfer (PET) model system benzophenone/anisole can be largely suppressed by the action of hydrogen-bonding solvents even of low polarity (HFIP, ε = 16.7). This finding was not limited to HFIP as a solvent but appears to be a common feature in protic solvents, e.g. in TFE [18] or water [35]. In the ACN–water mixtures, the triplet quenching rate constants were found to increase by three orders of magnitude with an increase of the water content. The same was observed for the water-content driven increase of the free-radical yields present after complete triplet decay. A general triplet quenching pathways in protic solvents with an anisole as a quencher is summarized in Scheme 3. Interestingly, the quenching rate constant increases by one order of magnitude when going from ACN–HFIP (2:3, v/v) to ACN–HFIP (1:4, v/v) which coincides with the first noticeable

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7

Scheme 3.

changes in the transient absorption spectra of the BP triplets (Fig. 7). Actually, there appears to be a correlation between the spectral characteristics and the photochemical activity in the complete set of ACN/HFIP solvent systems studied. The influence of the solvent nature on the reaction mechanism might be attributed to the energetics of the electron-transfer quenching as we recently discussed in terms of the Marcus theory of adiabatic electron transfer [18]. In nonprotic solvents the ET between anisole and triplet-excited BP is strongly hindered and thermodynamically uphill by ca. +0.6 eV [18]. In protic solvents the energetics of the ET process are much more favorable for a full ET to take place. A significant contribution of solvation to the ET energetics stems from an increase in the triplet energies (Fig. 3), which became evident from the blue-shifted phosphorescence spectra at high HFIP contents. In addition specific solvation via H-bonding is well established to affect strongly the reduction potentials of aromatic ketones. Protic solvents can, in fact, shift reduction potentials by up to 300 mV [36–39], an effect, which is causally connected to the stabilization of the incipient negative charge by a hydrogen bond. At the same time the oxidation potential of anisole is lowered in a protic environment [40,41]. These cooperative redox-potential shifts will certainly favor the ET step thermodynamically in protic solvents, as the respective free-energy changes are of the order of 0.5 eV. Our results show that specific solute–solvent interactions change the thermodynamics for electron transfer from endergonic in ACN to exergonic in strongly protic solvents. Interestingly, the dependence of the ratio of the absorbance at 380 nm to that at 520 nm on the molar fraction of HFIP, which reflect the changes of the triplet state character, is in accord with the changes of the quenching rate constants (Fig. 7). In particular, the higher reactivity correlates with the increased ␲,␲* character of the BP triplet state. On one hand, this observation could be interpreted in terms of a higher intrinsic reactivity of ␲,␲* configured triplet states toward ET reactions. It was reported recently by Miranda et al. that quenching rate constants of 4-methoxybenzophenone and 4,4dimethoxybenzophenone by 1,4-dimethoxybenzene, similar to our work, increased by one order of magnitude in aqueous medium in comparison to the quenching rate constants in an organic solvent. In addition, it was shown that for the 4-methoxy and 4,4-dimethoxy derivatives of benzophenone that the ␲,␲* state is predominant

Fig. 7. Plot of the reaction rate constants of anisole with benzophenone () and ratio of transient absorption at 380 nm and 520 nm () vs molar fraction of HFIP in ACN–HFIP mixtures.

in aqueous medium while the n,␲* state is enhanced in nonpolar solvents [26]. On the other hand, the majority of experimental findings, rather point to minor effects of the configuration of the lowest triplet state on the ET reactivity [6,42]. Rather, as discussed above, the redox potentials associated with the ET change from being unfavorable in ACN to somewhat favorable in protic solvents in the triplet BP reaction with anisole. Furthermore, the blue-shift of the triplet excited states on going to HFIP will also favor the ET thermodynamics of this reaction. Although these free-energy changes are of the order of 0.5 eV, such changes can account for orders of magnitude changes in ET rate constants when the system under consideration is in the region of GET ≈ 0 on Marcus or Rehm–Weller curves of log kq vs GET [43–46].

4. Conclusions In the present work it was established that the lowest triplet excited state of BP can possess both n,␲* and ␲,␲* character, contrary to the previous assumption that the triplet of BP has always just n,␲* character. Using HFIP as the solvent, the two types of triplets (n,␲* , ␲,␲* ) can be simultaneously detected by transient absorption spectroscopy. In ACN–HFIP mixtures their relative contributions become strongly dependent on the solvent mixing ratios. Triplet–triplet absorption from the lowest-lying ␲,␲* triplet (max 380 nm) dominated at large HFIP concentrations, whereas triplet–triplet absorption from the n,␲* state (max 520 nm) was enhanced in nonprotic solvents. As a consequence of these changes in the character of the lowest-lying triplet states of BP, the photochemical properties of the triplet states of benzophenone were seen to be very sensitive to solvent characteristics. The reactivities toward different types of quenchers were compared through the quenching rate constants in non-protic and protic solvents. The sharp decrease in the reactivity in the H-atom abstraction of triplet BP from 2-PrOH between the solvents ACN and HFIP can thus be attributed to the change in the character of the lowest-lying triplet state from n,␲* (in ACN) to ␲,␲* (in HFIP). This conclusion relies on the established finding that the ␲,␲* triplet state has reduced reactivity toward H-atom abstraction. In contrast, we showed that the reactivity of an electronically excited BP molecule toward anisole can be greatly enhanced by changes in the nature of the solvent. The exact role of protic solvents on rate constants and quantum yields of radical formation in the one-electron oxidation of anisole by triplet BP in protic solvents depends on several energetic factors. However, even small changes in these factors can lead to the large changes in the observed kinetics and yields when the freeenergy changes are in the region of GET ≈ 0 as is the case in the BP/anisole system investigated in this work. Such significant differences between the quenching rate constants kq in non-protic and protic solvents have been observed before in a chemically related system, namely for the intramolecular H-atom transfer between phenols and BP [47,48]. In summary, in this work we showed that inversion of the triplet states of BP can be effected by the action of a strong H-bonding interaction. Furthermore, in the BP/anisole system we demonstrated that H-bonding greatly enhance a triplet state’s electron acceptor character. Thus observed changes of photophysical as well photochemical properties of the triplet state of BP can be attributed the effects of H-bonding by the strong H-bond donor HFIP.

8

A. Lewandowska-Andralojc et al. / Journal of Photochemistry and Photobiology A: Chemistry 244 (2012) 1–8

Acknowledgements We are grateful for financial support from the Foundation for Polish Science (“Mistrz” Programme for B. M) and for general support from the Faculty of Chemistry of AMU. This is document no. NDRL 4922 from the Notre Dame Radiation Laboratory which is supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, United States Department of Energy through grant number DE-FC02-04ER15533.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jphotochem.2012.06.010.

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