Phototautomerization on the Singlet and Triplet Surface in o

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Phototautomerization on the Singlet and Triplet Surface in o‑Hydroxyacetophenone Derivatives in Polar Solvents Sujan K. Sarkar,† Geethika K. Weragoda,† R. A. A. Upul Ranaweera, and Anna D. Gudmundsdottir* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, United States S Supporting Information *

ABSTRACT: Nanosecond laser flash photolysis of o-hydroxyacetophenone (1a) and 2,4-dihydroxyacetophenone (1b) in ethanol and acetonitrile results in absorption due to triplet biradicals 2a (λmax 430 nm, τ ≈ 3 μs) and 2b (λmax 400 nm, τ ≈ 1 μs), respectively. Triplet biradical 2a intersystem crosses to form Z-3a (λmax 400 nm, τ ≈ 10 μs), whereas 2b forms both Z3b and E-3b (λmax 350 nm, τ ≈ 5 and 72 μs). Quenching studies demonstrate that 3a,b are formed on both the singlet and triplet excited surface of 1a and 1b. In ethanol at 77 K, ohydroxyacetophenone derivatives 1a and 1b show phosphorescence, as is typical for triplet ketones with (n,π*) configuration. The mechanism for the photoreactivity of 1a,b is supported by density functional calculations.

1. INTRODUCTION Excited-state intramolecular proton transfer (ESIPT) has been studied intensively since Weller first discovered this phenomenon.1 Generally, ESIPT occurs in molecules with a strong intramolecular H bond between two electronegative heteroatoms, forming a H-bonded ring structure. ESIPTs are typically ultrafast and proceed on femtosecond time scales as they are nearly barrierless.2−4 Because ESIPTs are both fast and efficient, they have found their way into a variety of applications such as UV photostabilizers,5,6 scintillators,7,8 luminescent solar concentrators,9 near-UV laser dyes,10 organic light-emitting devices (OLEDs),11 fluorescent imaging probes,12 fluorescent chemosensors,13 and molecular switches.14 Due to the numerous applications that rely on ESIPT, there is a general interest in its mechanistic details. o-Hydroxyacetophenone derivatives have been reported to undergo ESIPT on their singlet excited state to form the corresponding keto tautomers (Scheme 1).4,15,16 The keto

tautomers are not stable as they regenerate their enol tautomers via ground-state intramolecular proton transfer (GSIPT) or by interacting with the solvent (Scheme 2). Scheme 2. Re-enolization of Keto Tautomers via GSIPT and the Solvent

Recent studies show that upon proton transfer, the excited state of keto tautomers can twist or rotate to break the intramolecular H bonds to yield isomers and rotamers of the keto tautomer.3,17 Interestingly, irradiating o-hydroxyacetophenone derivatives in cryogenic temperature forms a nonhydrogen-bonded conformer of the o-hydroxyacetophenone derivatives.2,18−21 Although the mechanism for forming the non-H-bonding conformation of o-hydroxyacetophenone in matrixes has not been fully elucidated, it is likely to take place through ESIPT.21,22 In this article, we report that nanosecond laser flash photolysis of o-hydroxyacetophenone derivatives 1a and 1b in ethanol and acetonitrile results in the formation of E and Z keto tautomers. We used phosphorescence, laser flash photolysis,

Scheme 1. ESIPT of o-Hydroxyacetophenone Dervitives

Special Issue: Photoinduced Proton Transfer in Chemistry and Biology Symposium Received: September 7, 2014 Revised: October 29, 2014

© XXXX American Chemical Society

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dx.doi.org/10.1021/jp509062w | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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structures were optimized using Gaussian09 at the B3LYP level of theory and with the 6-31+G(d) basis set.24,25 Three minimal-energy conformers A, B, and C were optimized for 1a. Conformer A has intramolecular H bonding between the hydroxyl moiety and the ketone, whereas conformers B and C have no intramolecular H bonding and are 14 and 10 kcal/mol higher in energy than A, respectively. Similarly, optimization of 1b yields conformers A, B, and C, with conformer A having intramolecular H bonding and being 15 and 11 kcal/mol more stable than conformers B and C, respectively (Figure 2). Time-dependent density functional theory (TD-DFT) calculations were performed to compute the energy of the first singlet (S1K) and triplet (T1K) excited states of 1a,b. The energies for S1K and T1K of 1a,b are listed in Table 1. Inspection of the molecular orbitals involved in the lowest electronic transition for the S1K of 1a,b indicates that the S1K states of 1aA, and 1b-A have a (π,π*) configuration, whereas the S1K states of 1a-B, 1a-C, 1b-B, and 1b-C have a (n,π*) configuration. The structures of the non-H-bonded conformers B and C of T1K of 1a and 1b were optimized and found to be 64, 69, 67, and 72 kcal/mol above their S0 (Figure 3). The optimized structures of T1K of 1a-B, 1a-C, 1b-B, and 1b-C have prolonged CO bonds, as typically observed for triplet ketones with (n,π*) configurations. Spin density calculations further support this idea as the spin density is localized on the carbonyl oxygen and the phenyl ring. In comparison, the optimized structure of T1K of 1b-A is located 70 kcal/mol above its S0. Spin density calculations show that in T1K of 1b-A, the unpaired spins are delocalized over the entire molecule, and thus, it has (π,π*) configuration (Figure 3). The calculated energies from the optimization of T1K of 1a,b-A and TD-DFT calculations are consistent, whereas the energies obtained from the optimization of the non-H-bonded conformers B and C of T1K of 1a,b are somewhat lower than the energies obtained from the TDDFT calculations. It has been reported, however, that optimization using B3LYP theory underestimates the energies of triplet ketones with (n,π*) configuration.26 It was not possible to optimize T1K of 1a-A because it resulted in triplet biradical 2a-A. The optimized structure of 1,4-biradicals 2a-A and 2b-A, which exhibited intramolecular H bonding, were located 55 and 59 kcal/mol above their corresponding S0, respectively (Figure 4). Spin density calculations show that the unpaired electrons are delocalized over the entire molecule. Similarly, the optimized structures of 1,4-biradicals 2a-B, 2aC, 2b-B, and 2b-C were located at 55, 53, 57, and 55 kcal/mol above their corresponding S0 (Figure 4). Spin density calculations show that the unpaired electrons are less delocalized; they are mainly localized on the O atom on the phenyl ring and on the ketyl carbon atom. The Z and E keto tautomers of 1a and 1b were also optimized. The E ketomer is more stable than the non-Hbonded Z ketomer (Figure 5) as the lone pairs on the oxygen atoms are not in close proximity with one another. As highlighted above, the Z ketomers with intramolecular H atom bonding were not stable and optimized to their corresponding S0 conformers of 1a,b. The calculated stationary points on the singlet and triplet surface of 1b-C, 1b-A, and 1a-C are shown in Figure 6, respectively. The transition-state barrier between T1K of 1a and 2a could not be optimized, nor the T1K of 1a, as it yielded triplet biradical 2a-A. In comparison, the transition-state barrier

and density functional theory (DFT) calculations to elucidate the mechanism for forming the keto tautomers.

2. RESULTS 2.1. Product Studies. Irradiation of both o-hydroxyacetophenone derivatives 1a and 1b in argon- and oxygen-saturated CDCl3 solution with a high-pressure mercury arc lamp through a Pyrex filter for 1 day did not reveal the formation of any stable products (Scheme 3). The photolysis of 1a and 1b was followed by 1H NMR and GC-MS spectroscopy. Scheme 3. Product Studies for 1a and 1b

2.2. Phosphorescence. The phosphorescence spectra of 1a and 1b were obtained at 77 K in ethanol glasses. The phosphorescence spectra showed a well-resolved vibrational feature typical of triplet ketones with (n,π*) configurations (Figure 1).23 The (0,0) bands were located at 413 and 411 nm

Figure 1. Phosphorescence spectrum of a 0.1 mM solution of 1a and 1b in ethanol with the excitation wavelength set at 266 nm.

for 1a and 1b, which corresponds to triplet energies of 69 and 70 kcal/mol, respectively. Thus, phosphorescence verifies that the irradiation of 1a and 1b results in the formation of their triplet excited states. Yoshihara et al. reported similar phosphorescence of 1a in ethanol and concluded that in polar solvents, 1a exists mainly in conformers in which there is no intramolecular H bonding as the molecule is stabilized by intermolecular H bonding with the solvent.15 Because 1a and 1b display phosphorescence characteristic of (n,π*) triplet ketones, we theorize that the triplet ketones can undergo intramolecular H transfer to form the corresponding triplet biradicals 2a,b, which intersystem cross to form keto tautomers of 1a,b, as shown in Scheme 4. 2.3. Calculations. We performed DFT calculations to aid in characterizing the triplet reactivity of 1a and 1b. All of the B



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Scheme 4. Possible Mechanisms for Forming Keto Tautomers 3 from Various Rotamers of 1a,ba

a

It is also possible that sequential excitation (blue arrow) of keto tautomers 3 results in formation of different rotamers.

The calculations demonstrate that the H transfers on the triplet excited state of 1a and 1b to form biradicals 2a and 2b are highly feasible. The calculated rotational barriers for biradicals 2a and 2b around their Ph−CO bond are displayed in Figure 7. The rotational barrier for the non-H-bonded conformers of 2b is less than that for 2a. The rotational barriers for 2a-B rotating into 2a-C are 2 and 4 kcal, whereas the barriers for 2b-B rotating into 2b-C are 1 and 2 kcal/mol. 2.4. Laser Flash Photolysis. We performed laser flash photolysis (Excimer laser, 308 nm, 17 ns)27 of 1a and 1b to detect the excited triplet states and intermediates formed upon irradiation of 1a and 1b. Laser flash photolysis of 1a in argonsaturated acetonitrile and ethanol produced broad transient absorption between 350 and 500 nm with λmax ≈ 430 nm. The intensity of the transient absorption in acetonitrile and ethanol was similar. We assigned this transient absorption to 2a, Z-3a and E-3a based on the similarity between the observed transient spectra and their calculated absorption spectra. TDDFT calculations predicted the most intense electronic transitions above 300 nm for 2a between 340 and 420 nm

Figure 2. H-bonded (A) and non-H-bonded conformers B and C of 1a and 1b (B3LYP/6-31+G(d)). The relative energies are given in parentheses in kcal/mol.

for T1K of 1b-A forming biradical 2b-A was optimized and is almost isoenergetic with T1K of 1b-A. Thus, the barrier for H transfer is very small on the triplet excited surface of 1a and 1b. C



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Table 1. Energies of S1K and T1K of 1a and 1b in kcal/mol phosphorescence wavelength (nm) S1K of 1a S1K of 1b T1K of 1a T1K of 1b

412 411

conformer

energy

TD-DFTa (A)

69 70

90 (π,π*) 95 (π,π*) 69 72

optb (A)

TD-DFTa (B)

− 70 (n,π*)

82 (n,π*) 85 (n,π*) 70 73

optb (B)

TD-DFTa (C)

optb (C)

64 (n,π*) 67 (n,π*)

87 (n,π*) 90 (n,π*) 76 75

69 (n,π*) 72 (n,π*)

a Energies were obtained from TD-DFT (B3LYP/6-31G+(d)) calculations. bEnergies were obtained from optimization (DFT/(U)B3LYP/6-31G +(d)) calculations.

Information). Laser flash photolysis of 1a in oxygen-saturated acetonitrile results in a less intense and somewhat narrower transient spectrum (Figure 8B). Laser flash photolysis of 1b in argon-saturated acetonitrile also resulted in a broad transient spectrum with λmax ≈ 350 nm and ∼400 nm. We assign the transient absorption at 400 nm to 2b and the absorption at 350 nm to Z-3b and E-3b based on the TD-DFT calculations that predicted that the most significant electronic transitions for 2b would be between 360 and 420 nm, whereas for Z-2b and E-2b, the significant transitions would be at 363 ( f = 0.1122) and 363 nm (f = 0.1086), respectively (Figure 9). Laser flash photolysis of 1b in oxygen-saturated acetonitrile reduced the absorption due to 2b near 400 nm (Figure 9B). The kinetic traces obtained at 420 nm from laser flash photolysis of 1a in argon-saturated acetonitrile are best fitted as a biexponential decay that yields rate constants of 3.4 × 105 (∼3 μs) and 1.0 × 105 s−1 (∼10 μs, Figure 10). In oxygensaturated acetonitrile, the faster decay is fully quenched, whereas the slower one is unaffected. The decay is first-order and yields the same rate constant as in argon-saturated acetonitrile (1.0 × 105 s−1). Thus, we assign the faster rate constant to biradical 2a, which is expected to be sensitive to oxygen and the slower one to Z-3a, which will not react with oxygen. Analysis of the kinetics obtained by laser flash photolysis of 1b showed that at 400 nm at a shorter time scale, the absorption in argon-saturated acetonitrile could be fitted as biexponential decay with rate constants of 9.7 × 105 (∼1 μs) and 1.8 × 105 s−1 (∼5.5 μs, Figure 11A). The absorption does not decay all the way to the ground state as there is a residual absorption. In oxygen-saturated acetonitrile, the faster component is quenched while the slower one is not affected; the decay is first-order and yields similar rate constant as those in argon-saturated acetonitrile (2.1 × 105 s−1), and thus we assign the slower one to Z-3b and the faster one to 2b. The kinetics at 350 nm in argon-saturated acetonitrile showed exponential decay on a short time scale due to the decay of 2b (1.0 × 106 s−1). On a longer time scale, the decay can be fitted as exponential with a rate constant of 1.4 × 104 s−1 (τ ≈ 72 μs). We assign the long-lived intermediate to E-3b. In oxygen-saturated acetonitrile, the absorption due to 2b becomes too short-lived to be observed with our laser flash apparatus (time resolution of 17 ns). We observe that the absorption at 350 nm grows with a rate constant of 2.1 × 105 s−1 and assign it to the recovery or GSIPT of the Z-3b. At longer time scales, the decay of E-3b is observed with the same rate constant as that in argon-saturated solution at 350 nm. The laser flash photolysis of 1a and 1b demonstrate that both form triplet biradicals upon irradiation, and the 2a intersystem crosses to form only E-3a, whereas 2b yields both E- and Z-3b.

Figure 3. Spin densities (in parentheses) of the optimized structures of T1K of 1a and 1b. It was not possible to optimize the T1K of 1a-A as it yielded biradical 2a-A. All energies are compared with respect to the corresponding ground-state conformers A, B, and C of 1a and 1b.

Figure 4. Spin densities (in parentheses) of the optimized structures of the 1,4-biradical of 2a and 2b calculated using the DFT/B3LYP/631+G(d) basis set. All energies are given with respect to their corresponding S0 conformers.

Figure 5. Optimized structures of Z and E enols 3a and 3b and their energies in parentheses, given in kcal/mol, calculated using the DFT/ B3LYP/6-31+G(d) basis set. All energies are given with respect to 1aC or 1b-C, as appropriate.

for Z-3a at 380 nm ( f = 0.1731) and for E-3a at 375 nm (f = 0.1692) (Figure 8), which is consistent with the observed spectra. There is a negative absorption at 325 nm due to depletion of the starting material (Figure S1, Supporting D



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Figure 6. Calculated stationary points on the triplet surface of 1b-C, 1b-A, and 1a-C. The energies of the S1K were obtained from the TD-DFT calculations, whereas the other energies were obtained by DFT optimization calculations using the (U)B3LYP/6-31+G(d) basis set. Energies are in kcal/mol.

2.5. Quenching Studies. We utilized a triplet quencher, isoprene, to determine how much of the transient absorption is formed from the T1K of 1a,b. Isoprene quenches the T1K of 1a,b but not 2a,b as they have similar triplet energy as isoprene (60 kcal/mol).28 Also, isoprene does not quench keto tautomers 3a,b as they are singlets. Because T1K of 1a,b are too short-lived to be detected directly with our nanosecond laser flash apparatus, the isoprene quenching of T1K of 1a,b consequently results in reduced absorption due to biradicals 2a,b and keto tautomers 3a,b. As expected, the intensity of the absorption at 420 nm decreased with the addition of isoprene without affecting the lifetimes of 2a,b and 3a,b (Figure 12A). It was possible to fully quench the absorption due to biradicals 2a,b, whereas the quenching of the absorption due to 3a,b reached saturation at a higher concentration of isoprene. Therefore, the quenching studies verify that the T1K of 1a,b is the precursor to 2a,b and some of the keto tautomers 3a,b, whereas some of the keto tautomers must be formed directly from S1K of 1a,b.

The Stern−Volmer plot showed a straight line with a slope of 0.2 M−1for 1a and an intercept of 1. Using the slope of the straight line, the lifetime of T1 of 1a can be determined to be between 0.02 and 0.2 ns by assuming that the quenching is diffusion-controlled and the kq is between 109 and 1010 M−1 s−1. Similarly, the lifetime of T1K of 1b was determined from a Stern−Volmer plot to be between 0.25 and 0.025 ns.

3. DISCUSSION The energies obtained from the phosphorescence of 1a and 1b fit well with the calculated energies for the non-H-bonded conformers of T1K of 1a and 1b. In addition, the calculations support that the T1K of 1a and 1b have (n,π*) configurations, which is consistent with the vibrational feature of the observed phosphorescence spectra. In comparison, the optimized structure of T1K of 1b-A has a (π,π*) configuration and should yield phosphorescence with a different vibrational structure from what is observed. Thus, it is reasonable to surmise that the photochemistry of 1a and 1b derives from conformers E



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Figure 7. Calculated rotational barriers for 2a and 2b.

Figure 9. Transient UV−vis spectrum of 1b in (A) argon-saturated acetonitrile and (B) oxygen-saturated acetonitrile. Calculated electronic transitions for (C) triplet biradical 2b and (D) ketomer 3b using the TD-DFT/B3LYP/6-31+G(d) basis set in acetonitrile.

Figure 10. Kinetic trace at 420 nm in argon and oxygen-saturated acetonitrile obtained from laser flash photolysis of 1a.

The photochemistry of 1a,b depends on how the environment affects the intramolecular H bonding. For example, Su et al. showed that 1a undergoes ultrafast ESIPT on the singlet surface in the gas phase,16 and Yoshihara et al. showed that in nonpolar solvent,15 the intramolecular H bonding is efficient enough for ESIPT to occur, whereas in polar solvents, the photochemistry of 1a,b is best described as H transfer on the excited state of the singlet and triplet excited state of the ketone. Thus, the reactivity of 1a,b arises from the excited ketones with (n,π*) configuration. Because Z-3a,b do not exhibit strong intramolecular H bonding in acetonitrile or ethanol but undergo intermolecular H bonding with the solvent, they have lifetimes of several microseconds. The intermolecular H bonding hinders GSIPT or the 1,5-H shift to regenerate the starting material. The photochemistry of the non-H-bonded conformers of 1a and 1b is similar to the photochemistry of o-alkylacetophenone derivatives 4 (Scheme 5).29−31 For example, the irradiation of o-methylacetophenone results in the formation of photoenols Z-6 and E-6 through the triplet excited state of the ketone, as

Figure 8. Transient UV−vis spectrum of 1a in (A) argon-saturated acetonitrile and (B) oxygen-saturated acetonitrile. Calculated electronic transitions for (C) biradical 2a and (D) ketomer 3a using the TD-DFT/B3LYP/6-31+G(d) basis set in acetonitrile.

stabilized not by strong intramolecular H bonding but by intermolecular H bonding with the solvent. We also theorized that the non-H-bonded conformers of 3a,b are formed within the laser pulse due to ESIPT on the singlet surface of intramolecular H bonded conformers 1a-A and 1b-A and that they are re-excited to form biradicals 2a,b and keto tautomers 3a,b. However, the isoprene quenching studies verify that the precursors to biradicals 2a,b are T1K of 1a,b rather than the singlet excited states of 2a,b. F



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are typically much longer lived than the E photoketo tautomers. It is, however, possible that the E photoketo tautomers can be stabilized further for use in synthetic applications.

4. CONCLUSION In conclusion, we have demonstrated that in polar solvents, the irradiation of 1a and 1b results in H transfer on their triplet excited state to form triplet biradicals 2a and 2b. The biradical 2a intersystem crosses to form Z-3a, whereas biradical 2b forms both Z-3a and E-3b. In addition, some of the keto tautomers must be formed from S1K of 1a,b. We have demonstrated that in polar solvents, 1a,b yield photoketo tautomers. The photochemistry of 1a,b in polar solvents is similar to the photoenolization of o-methyl-substituted acetophenone derivatives, and thus o-hydroxyacetophenone derivatives have the potential to be utilized in applications such as synthesis and as photoremovable protecting groups. 5. EXPERIMENTAL SECTION 5.1. Materials. 1a and 1b were purchased from SigmaAldrich Co. LLC, St. Louis, Missouri, U.S.A., and used as received. 5.2. Calculation. DFT calculations were performed using Gaussian09 at the B3LYP level of theory and with the 631+G(d) basis set24,25 to optimize the ground-state structures, excited states, intermediates, and photoproducts. TD-DFT42−44 calculations were also performed with the B3LYP level of theory and 6-31+G(d) basis set to locate the excited singlet and triplet states of the ground-state minimal energy conformer. All transition states were confirmed to have one imaginary vibrational frequency by the analytical determination of the second derivative of the energy with respect to the internal coordinates. Intrinsic reaction coordinate calculations were used to verify that the transition state correlates with the products and the triplet excited state of the starting material.45,46 5.3. Phosphorescence. Phosphorescence spectra were obtained in a Horiba Jobin-Yvon fluorolog with 5 nm as the emission slit. All of the spectra were taken in ethanol matrixes at 77 K with 100 ms integration. 1a and 1b were excited at (or near) their ground-state UV−visible peak maximum above 250 nm. 5.4. Laser Flash Photolysis. Laser flash photolysis was performed using an excimer laser (308 nm, 17 ns).27 Stock solutions of 1a and 1b were made both in acetonitrile with spectroscopic-grade solvents. The concentration of the stock solutions was such that the ground-state absorbance of the solutions was between 0.3 and 0.8 at 308 nm. A 48 mm long quartz cuvette with a 10 mm × 10 mm cross section was used in laser flash photolysis study. Approximately 2 mL of the stock solutions was added in the cuvette and purged with argon or oxygen for the required time in each case. For isoprene studies, each cuvette was purged with argon prior to the addition of isoprene.

Figure 11. Kinetic trace obtained from laser flash photolysis of 1b (A) at 400 nm in argon- and oxygen-saturated acetonitrile, (B) at 350 nm in argon- and oxygen-saturated acetonitrile (shorter time window), and (C) at 350 nm in oxygen-saturated acetonitrile (longer time window).

Figure 12. (A) Kinetic traces at 420 nm in argon-saturated acetonitrile at various isoprene concentrations. (B) Stern−Volmer plot from quenching biradical 2a with isoprene.

shown in Scheme 5. The lifetimes of the Z and E enols depend on the ability of the solvent to stabilize the photoenols. For example, a solvent that can H bond with the hydroxyl groups make the Z-6 longer-lived by hindering the GSIPT (1,5-H shift) that regenerates 4. In comparison, E-6 regenerates the starting materials through the solvent and has a lifetime on the order of seconds. Because E photoenols are typically very stable or long-lived intermediates, they have been used in organic synthesis32 and as photoremovable protecting groups or phototriggers.29−31,33−41 The most significant difference between the photoenols from 1a,b and the photoketo tautomers from 4 is that E photoenols Scheme 5. Photoenolization of o-Methylacetophenone

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ASSOCIATED CONTENT

S Supporting Information *

Ground-state UV/visible spectra, molecular modeling, and transient spectra in argon saturated ethanol. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

S.K.S. and G.K.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation and the Ohio Supercomputer Center. A.D.G. is grateful to Dr. K. Kristinsson for his technical support.

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