Towards Organic Photohydrides: Excited-state Behavior of 10- Methyl-9-phenyl-9, 10-dihydroacridine

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1 Supporting Information Towards Organic Photohydrides: Excited-state Behavior of 10- Methyl-9-phenyl-9, 10-dihydroacridine Xin Yang, Janitha Walpita, Dapeng Zhou, Hoi Ling Luk, + Shubham Vyas, + Rony S. Khnayzer, Subodh C. Tiwari, Kadir Diri, Christopher M. Hadad, + Felix N. Castellano, Anna I. Krylov and Ksenija D. Glusac*, Department of Chemistry, Bowling Green State University, Bowling Green, OH Department of Chemistry, The Ohio State University, Columbus, OH Department of Chemistry, University of Southern California, Los Angeles, CA *Corresponding author kglusac@bgsu.edu Table of content: 1. Hydrogen Detection...S2 2. Photoreduction of Oxygen....S3 3. Photoreduction in the absence of O 2 at neutral ph s4 4. Photoreduction in the absence of O 2 at low ph s5 5. Nanosecond transient absorption kinetics of PhAcrH....S6 6. Computed and experimental absorption spectra PhAcrH-derived species......s8 7. Thermodynamic Parameters S9 1

2 1. Hydrogen detection Shimadzu GC-8A was operated with ultra-high purity argon as carrier gas and 5 Å molecular sieves column (Restek) to separate gas mixtures. This GC was customized with two injection ports, the first for syringe injections, and the second for automatic injections from a 500 µl sample loop directly linked to a Schlenk line. The detector was calibrated against known amounts of H 2 gas. In principle, 500 µl of 10 % H 2 balanced Ar certified gas standard (Praxair) was injected at different pressures using a home-built Schlenk line linked to a pressure gauge. The area under the hydrogen peak was then plotted against the calculated number of the moles of hydrogen injected to get the calibration constant of the detector. This constant was then verified by syringe injections of different volumes of 25% H 2 balanced Ar certified gas standard (Praxair), equilibrated to atmospheric pressure and placed in an airtight vial. The calibration was performed routinely with variation typically below 5 % at a certain Ar flow rate (Figure S1). H 2 (µmol) R 2 = slope = x GC H 2 Area x 10 5 Figure S1. Calibration plot for GC-8A instrument. 2

3 2. Photoreduction of oxygen The photochemical behavior of PhAcrH in the presence of molecular oxygen was studied at ph 0.65 and ph 6 followed by the detection of H 2 O 2 which could be the possible co-product. 1 Figure S2a and b show the changes in the UV/Vis absorption spectra during the course of 300 nm irradiation of aerated PhAcrH in acetonitrile:water mixtures. The decrease of PhAcrH (λ abs =287 nm) is accompanied with the growth of PhAcr + (λ abs =365 and 424 nm), suggesting that the efficient photo-oxidation of PhAcrH takes place. The conversion efficiencies of PhAcrH to PhAcr + at both ph values are ~ 65%. Previous studies of organic hydrides reported similar oxygen reduction behavior. 2-4 Figure S2. UV-Vis absorption changes of 0.08 mm PhAcrH upon irradiation (λ ex = nm) in the presence of O 2. (a) at ph 0.65 (b) at ph 6. Times of spectral collection: ( ) 0 min. ( ) 2 min. ( ) 5 - min. ( ) 10 min. ( ) 15 min. (c) Growth of I 3 absorption at 290 nm and 365 nm after adding excess of I - to the irradiated PhAcrH sample. The triiodide method 5 was used to test for the formation of H 2 O 2, by monitoring the oxidation of iodide to triiodide, which absorbs at 290 nm and 365 nm. The detected amount of I - 3 was significantly lower (~ 6 %) than expected based on the amount of PhAcr + formed (65%). One possible reason for low yield of triiodide could be the reaction of I - 3 with remaining PhAcrH to form PhAcr + and I - as shown below: 6 3

4 (i) PhAcrH + O 2 + H + PhAcr + + H 2 O 2 (ii) H 2 O 2 + 3I - +2H + 2H 2 O + I 3 - (iii) I PhAcrH 3I - + H + + PhAcr + 3. Photoreaction in the absence of O 2 at neutral ph In the absence of oxygen and at neutral ph, photoirradiation of PhAcrH does not lead to the formation of PhAcr +. Instead, the solution becomes cloudy, as can be observed by the formation of the broad tail in the UV/Vis absorption spectra (Figure S3a). The product of this reaction is likely a covalently-linked dimer PhAcr-AcrPh, formed via the following mechanism: (1) PhAcrH + hv PhAcrH.+ + e - (2) PhAcrH.+ PhAcr. + H + (3) 2PhAcr. PhAcr-AcrPh The support for the dimer formation comes from the fact that the MS analysis of the reaction mixture gives a signal at m/z= 540, which corresponds to the mass of the PhAcr-AcrPh dimer (Figure S3b). Furthermore, NMR study of the irradiated solution shows that the ratio of integrated peaks between aromatic hydrogens and the hydrogen at C-9 position is increasing with prolonged irradiation, suggesting a loss of hydrogen at C-9 position which ultimately leads to a dimer formation via same carbon center. In addition, photochemical dimerization of other acridine derivatives was shown to generate dimeric species. 7 4

5 Figure S3. Photolysis of PhAcrH in acetonitrile: water mixture in the absence of O 2 at ph =6 (a) UV-Vis absorption changes of 0.08 mm PhAcrH upon irradiation (λ ex = nm). Times of spectral collection: ( ) 0 min. ( ) 2 min. ( ) 5 min. ( ) 10 min. ( ) 17 min. ( ) 30 min. ( ) 45 min. ( ) 60 min. (b) GC-MS (DIP) of the irradiated product. 4. Photoreduction in the absence of O 2 and at low ph As discussed in the previous section, the irradiation of an oxygen-free solution of PhAcrH at neutral ph does not generate PhAcr +. However, as the solution ph is decreased, the formation of PhAcr + was observed (Figure S4). The UV/Vis absorption spectra of irradiated PhAcrH in the ph= 2-5 range exhibit broad absorption throughout the visible range due to the formation of aggregates of varying size. However, at low ph values (ph<2), the formation of PhAcr + is observed, with its characteristic absorption bands at 365 and 424 nm. These results clearly show that the overall hydride release occurs from the excited PhAcrH at low solution ph. 5

6 Figure S4. UV/Vis absorption changes upon photo irradiation of 0.1 mm PhAcrH in acetonitrile: different ph water (v:v= 1:1) mixtures under Ar purged conditions. 5. Nanosecond transient absorption kinetics of PhAcrH 6

7 Figure S5. Transient absorption kinetics of 0.18 mm PhAcrH in acetonitrile:water mixture (v:v= 1:1), λ exc = 266 nm. (a) and (c): dynamics in the presence ( ) and absence ( ) of O 2 (ph=7); λ probe = 550 nm in (a) and 580 nm in (c); (b) and (d): dynamics for a ph = 0.65 sample in the absence of O 2 ; λ probe = 550 nm in (b) and 580 nm in (d). To investigate the origin of intermediates observed in nanosecond transient absorption spectra of PhAcrH, we investigated the effect of molecular oxygen on their dynamics. Panels a and c show the kinetics of PhAcrH at neutral ph in the presence (blue) and absence of O 2 (red). The 550 nm signal is sensitive to O 2 and is assigned to the T 1 absorption of PhAcrH. This signal exhibits a single-exponential decay at low ph (panel b). In the case of 580 nm signal, only the submicrosecond component is sensitive to O 2. This is consistent with our assignment of 580 nm kinetics to a combination of the triplet (τ 3 ) and the radical cation absorption (τ 4 and τ 5 ). The Ph-Acr + signal at 425 nm was too weak to obtain accurate kinetic constants. Qualitatively, the signal at this wavelength increases as the radical cation signal decreases in intensity. 7

8 6. Computed and experimental absorption spectra PhAcrH-derived species. Figure S6 presents the experimental and calculated absorption spectra of several compounds relevant to our study. The calculations were done using TD-DFT with B3LYP/6-31+G*. The triplet spectrum as calculated using BNL/6-31+G*. All calculated spectra are ~0.3 ev blue-shifted relative to the experimental values, which is consistent with ~ 0.5 ev accuracy of the methodology employed. 8

9 Figure S6. Calculated (red line) and experimental (black line) absorption spectra of different PhAcrH species. (The calculations were performed using TD-DFT/TDA with B3LYP/6-31+G*, except for the T 1 absorption which was computed using BNL/6-31+G*). 7. Thermodynamic Parameters a) Estimation of Gibbs Free Energies from Experimental Values Scheme 2 in the manuscript outlines the G values for the proton reduction by excited PhAcrH in aqueous solution. The values were obtained as follows: (i) (ii) (iii) S 1 energy (81 kcal/mol) was obtained from emission peak of PhAcrH ( 1 PhAcrH * : 352 nm). T 1 energy (69 kcal/mol) was obtained from emission peak of PhAcrH ( 3 PhAcrH * : 413 nm). The energy for the reaction PhAcrH + H + PhAcrH.+ + H. ( G =80 kcal/mol) was obtained as G=- G 1 + G 2, where: PhAcrH.+ + e - PhAcrH G 1 H + + e - H G 2 The value for G 1 was obtained using the reported standard reduction potential E o =0.94 V vs SCE 7 (1.184 V vs. SHE 8 ). It is assumed here that the reduction potential does not change significantly going from acetonitrile (where the experiment was collected) to water (solvent used for this estimate). To obtain the absolute potential, we used E NHE = V. 9 The value for G 2 was obtained using the standard reduction potential in water E o = V vs SHE. 10 (iv) The energy for the reaction PhAcrH PhAcr + H. ( G =69.6 kcal/mol) was estimated to be G =- G 1 + G 2 + G 3, where: PhAcrH.+ + e - PhAcrH G 1 9

10 PhAcrH + PhAcr + H + G 2 H + + e - H G 3 The value for G 2 was obtained from the pk a value of PhAcrH.+. The reported pk a value in acetonitrile is However, this value is expected to change significantly in aqueous environment. To evaluate the pk a in the aqueous solution, we calculated the G for the reaction PhAcrH PhAcr + H in acetonitrile (see text below). We further assume that the G for this homolytic bond breaking will not be significantly changed in the aqueous environment. This assumption is supported by the fact that the calculated G values for the homolytic channel in acetonitrile (67.9 kcal/mol) and water (66.5 kcal/mol) are very similar. Using this assumption, the aqueous pk a value for PhAcrH.+ was evaluated from the G value for the homolytic bond scission. The obtained value is pk a = in water. (v) The energy for the reaction PhAcrH + H + PhAcr + + H 2 was obtained as G = G 1 G G 3 G 4, where: PhAcrH PhAcr. + H G 1 PhAcr + + e - PhAcr G 2 H + + e - ½ H 2 G 3 H + + e - H G 4 The value for G 2 was obtained from the reported reduction potential (E=0.229 V vs. SCE). 7 The value for G 3 was obtained from the absolute potential for the normal hydrogen electrode (E NHE = V). 9 The same expressions were used for the thermodynamic parameters in acetonitrile (Table 1). However, the aqueous G values were replaced by the corresponding parameters in acetonitrile. 7,10 b) Calculation of Gibbs Free Energies from Calculations 10

11 The calculated G values for electron, hydrogen atom and hydride ion release from PhAcrH were evaluated using density functional theory (DFT). The calculations were performed for two compounds (PhAcrH and AcrH 2, Scheme S2). Scheme S2. Structures of model compounds used in DFT calculations. b1. Computational details. The quantities of interest are free energies of the two reactions: homolytic and heterolytic bond cleavage: N R + H (homolytic) N C + H - (heterolytic, hydride release) and reduction potentials of the following reactions: CR + e N C + e R where N represents the organic hydride (PhAcrH or AcrH 2 ), C stands for the corresponding iminium ions (PhAcr + or AcrH + ), R represents the neutral radicals (PhAcr. or AcrH. ), CR represents cation radicals (PhAcrH.+ and AcrH.+ 2 ). The calculation of thermodynamic properties was performed using a Hess cycle consisting of the gas-phase energetics plus respective solvation free energies of all relevant species. The gas phase calculations included the following steps: 1. Ε: electronic energy differences. 11

12 2. H ( E+ H corr ): enthalpy consists of E and enthalpy correction. The dominant component of the latter is zero-point energy (ZPE). 3. G ( E + G corr ): gas-phase free energy consists of E and G corr. The latter includes enthalpy correction ( H) and T S term (entropy correction). 4. Solvation free energy ( G solv ). 5. Standard state correction (work needed to compress the gas-phase species from its molar volume at 1 atm to a molar volume of one liter) G o * = kcal/mol was added to all species that do not have gas-phase standard state (all except electron, in our case). 6. Empirical corrections to account for systematic errors in solvation models and errors due to spincontamination (see section b4). For atoms (H and H - ), G corr was computed as follows: G corr = E trans + k b T - T(S trans + S elec ). For hydride, all quantities are the same as for H atom, except that S elec is zero. The gas-phase thermodynamic quantities for all species were computed using wb97x-d/6-31+g(d,p). 11,12 Electronic energies were computed using wb97x-d/ g(2df,p) at the geometries optimized at the same level of theory. Some species have several isomers; this is discussed in section b2. The solvation free energies were computed using CPCM model 13 with the G(2df,p) basis set. We also consider SM8 model, but found that the agreement with the experiment is worse. For hydride, which is known to be a difficult case for implicit solvent models due to its small size, 14 we used G solv derived using experimental electrochemical data (see section b3). All calculations were performed using the Q-Chem electronic structure package. b2. Equilibrium structures and isomers. For energy calculations, equilibrium geometries were optimized with wb97x-d/6-311(+,+)g(2df,p). In thermochemical calculations, geometries were optimized at wb97x-d/6-31(+)g(d,p), the same level as used for subsequent frequency calculations. 12

13 Neutral PhAcrH has 3 low-lying isomers. The one used in most of the calculations of thermochemistry and spectra is cis2 (Figure S7). The cation radical also has isomers: cis, cis2 and trans. No isomers were found for the cation and radical. The relative electronic energies (kcal/mol) of the isomers are given in Table S1. Figure S7. Optimized structures of AcrH 2 and three PhAcrH conformers. A: AcrH 2 ; B: AcrPhH (cis2); C:AcrPhH (trans); D: AcrPhH (cis). 13

14 de dg gas dg solv Boltzmann factor (gas) Boltzmann factor (solvent) PhAcrH cis PhAcrH cis PhAcrH trans PhAcrH.+ cis PhAcrH.+ cis PhAcrH.+ trans Table S1. PhAcrH and PhAcrH.+ isomers, E and Gs (kcal/mol) using wb97x-d. Es: G(2df,p), Gs: 6-31+G(d,p). For the neutral, Boltzmann populations at T=298 K are also shown. b3. Solvation free energies. Solvation free energy calculation poses the biggest challenge in the calculations. For the homolytic channel, solvent effects are small (no more than 3 kcal/mol for the overall reaction energy in ACN and methanol) and different implicit solvent models generally agree with each other (e.g., CPCM and SM8 are within 0-2 kcal/mol). However, for heterolytic channel solvent effects are expected to be significant. Unfortunately, the PCM models fail dramatically for hydride (giving rise to errors of up to 50 kcal/mol in relevant energetics) because of its small size. To circumvent this problem, we computed G solv values for all species except for H -, for which we combine theoretical and experimental values to estimate G solv, as described below (Table S2). 14

15 System N C R CR AcrH PhAcrH (cis2) (cis2) Table S2. Solvation free energies ( G solv, kcal/mol) computed by CPCM/ G(2df,p) for different species in ACN. G solv (H)=-0.1 kcal/mol. The energy of reaction H + e - H - in the gas phase is well characterized - it is just electron affinity of H atom, EA= au = ev = kcal/mol (EA is positive, G is negative). It is well reproduced by ab initio calculations provided sufficiently large basis set is used (need diffuse functions on H). G(H+e - H - ) = kcal/mol (or kcal/mol if RT= kcal/mol correction for electron is used). As illustrated by Eq. (8) 15, G solv (H - ) is related to the reduction potential of H/H - couple: E 0 (H aq. /Haq - ) = -[ G 0 (H g - ) - G 0 (H g. ) + FE 0 abs (H aq + /H2. g ) - hyd G 0 (H g. ) + hyd G 0 (H aq - )]/F We note that this equation can be used to test computed values of G solv (H - ). For example, using this equation, E 0 =-0.37 V (in acetonitrile) was obtained when using their estimate of hydride solvation energy (-85 kcal/mol, which is quite close to the CPCM value of 91 kcal/mol). 15 This value of E 0 is different from the experimental measurements, i.e., and V. 15 Thus, we use the electrochemical data and the equation above to compute the G solv (H - ). We use our ab initio gas phase free energies and the best reference value of the free energy of the hydrogen electrode H + /H 2 of G(2H+e->H 2 )=4.281 ev (Ref. SHE). 15

16 Using E 0 =-0.62 V, G solv = kcal/mol. Using E 0 =-0.60 V, G solv = kcal/mol. Below we show how calculations are done: Reaction: H+e H - G(H/H - )=-nfe ev = ev= ev= kcal/mol (or ev= kcal/mol using E 0 =-0.62 V). G(H/H - )= G gas (H/H - )+ G s (H - )- G s (H) G s (H - )= G(H/H - )- G g (H/H - ) + G s (H) Thus, the solvation energy of H - : Using E 0 =-0.60 V: Using E 0 =-0.62 V: G s (H - )= kcal/mol (or kcal/mol with RT correction). G s (H - )= kcal/mol (or kcal/mol with RT correction). The range for the H - solvation energy therefore is from to kcal/mol. In calculations below, we use G solv = kcal/mol, to be consistent with E 0 used by the experimentalists in their estimates (-0.60 V). Using this energy, we arrive to the absolute free energy of hydride in acetonitrile G abs (H - /ACN)= kcal/mol. This is quite far (9.8 kcal/mol off!) from the recently reported value ( kcal/mol). 14 However, it is close to another value he cites ( kcal/mol). 16,17 We note that using the value kcal/mol yields the following value for G solv = = kcal/mol. It is useful to compute E 0 for H + e - H - reaction using this solvation energy: G(H+e H - /ACN) = = kcal/mol = ev. Using the RT correction, G = kcal/mol = ev Thus, E 0 (H/H - )= = V. This is obviously too far from both experimental values for E 0. b4. Empirical corrections. There are two major sources of systematic errors in computed G s. The first one is coming from PCM calculations of solvation energies. It is partially due to inability to account for changing the van 16

17 der Waals radius in charged species. 18 The net result is that the solvation energy of cations seems to be overestimated ( G is too negative). Thus, for all charged species a correction E s needs to be added to their G solv and, consequently, their G in solution. The second cause of imbalance in the above equations is due to open shell character of some species. Since we use UDFT (as we should be using), we may accumulate some additional stabilization of doublet species due to additional spin-contamination. Thus, a positive correction, E os, should be added to E gas (and, consequently, to their G and H in the gas phase and in the solution). The magnitude of these correction is determined from the differences between computed and experimental redox potentials for the AcrH 2 system. 19 Our rational is that all other molecules we investigate are homologically similar to AcrH 2, thus, our empirical corrections should be transferrable. Our computed corrections are: E s =0.20 ev = kcal/mol E os =0.05 ev=1.153 kcal/mol Thus, for G of each C + e - R reaction, we subtract 0.25 V (or add 0.25 V to E 0 ). For each CR+e - N reaction, we subtract 0.15 V (or add 0.15 V to E 0 ). Consequently, for E (... dh... dg) of each N R + H reaction, we add 0.05 ev=1.153 kcal/mol. For G s ( G in solution) of each N C + H - reaction, we add 0.20 ev=4.612 kcal/mol. b5. Summary of computed redox potentials and thermochemical quantities. Compound Rxn E 0, calc E 0, exp AcrH 2 CR + e - N ,21 C + e - R ,21 PhAcrH CR + e - N , ,20 17

18 C + e - R Table S3. Calculated and experimental redox potentials (V, vs SHE). Compound N R+H N C + H - AcrH PhAcrH Table S4. Calculated Gibbs free energies (kcal/mol, in ACN) for the homolytic and heterolytic bond cleavage. References: (1) Fukuzumi, S.; Ishikawa, M.; Tanaka, T. J. Chem. Soc., Perkin Trans , (2) Fukuzumi, S.; Kondo, Y.; Mochizuki, S.; Tanaka, T. J. Chem. Soc., Perkin Trans , (3) Fukuzumi, S.; Yorisue, T. J. Chem. Soc., Perkin Trans , (4) Fukuzumi, S.; Patz, M.; Suenobu, T.; Kuwahara, Y.; Itoh, S. J. Am. Chem. Soc. 1999, 121, (5) Fukuzumi, S.; Kuroda, S.; Tanaka, T. J. Am. Chem. Soc. 1985, 107, (6) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1990, 29, 653. (7) Anne, A.; Fraoua, S.; Hapiot, P.; Moiroux, J.; Saveant, J. M. J. Am. Chem. Soc. 1995, 117, (8) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298, 97. (9) Isse, A. A.; Gennaro, A. J. Phys. Chem. B 2010, 114, (10) Handoo, K. L.; Cheng, J. P.; Parker, V. D. J. Am. Chem. Soc. 1993, 115, (11) Chai, J. D.; Head-Gordon, M. J. Chem. Phys. 2008, 128, (12) Chai, J. D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, (13) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, (14) Muckerman, J. T.; Achord, P.; Creutz, C.; Polyansky, D. E.; Fujita, E. Proc. Natl. Acad. Sci. U. S.A., 2012, 109, (15) Kelly, C. A.; Rosseinsky, D. R. Phys. Chem. Chem. Phys. 2001, 3, (16) Kovács, G.; Pápai, I. Organometallics 2006, 25, 820. (17) Nimlos, M. R.; Chang, C. H.; Curtis, C. J.; Miedaner, A.; Pilath, H. M.; DuBois, D. L. Organometallics 2008, 27, (18) Sviatenko, L.; Isayev, O.; Gorb, L.; Hill, F.; Leszczynski, J. J. Comput. Chem. 2011, 32, (19) Konezny, S. J.; Doherty, M. D.; Luca, O. R.; Crabtree, R. H.; Soloveichik, G. L.; Batista, V. S. J. Phys. Chem. C. 2012, 116,

19 (20) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J. J. Am. Chem. Soc. 1993, 115, (21) Hapiot, P.; Moiroux, J.; Saveant, J. M. J. Am. Chem. Soc. 1990, 112,