Supplemental Information: Photobasicity in. Quinolines: Origin and Tunability via the. Substituents Hammett Parameters

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1 Supplemental Information: Photobasicity in Quinolines: Origin and Tunability via the Substituents Hammett Parameters Eric Driscoll, Jonathan Ryan Hunt, and Jahan M Dawlaty University of Southern California dawlaty@usc.edu To whom correspondence should be addressed 1

2 The Förster cycle and its approximations In addition to the Förster cycle it is also possible to obtain pka values via a fluorescence titration. However, in cases where one species has a low quantum yield, the titration does not result in an accurate pka. The issues associated with this are detailed by Lasser and Feitelson. 1 There are also possible quenching effects that may interfere with getting reliable pka values when titrating in extremely basic conditions (ph > 15). For these reasons, we chose to use the Förster cycle to calculate pka. Although the Förster cycle is a thermodynamic cycle and is analytically correct, several practical assumptions need to be made to use it. These are discussed in detail by Weller 2 and Ireland. 3 Formally, the Förster cycle is defined as H H = hc( ν base 00 ν acid 00 ) (1) and the approximation that H H = G G (2) is made so that equilibrium constants can be related to the spectroscopic observables. Using the relation G = k B T ln(k a ) = k BT log(e) pk a (3) we arrive at the form of Förster cycle equation used to calculate change in pk a upon excitation. pka pk a = log(e)hc ( ν 00 base ν 00 acid k b T ) (4) Assuming T = 298 K and collecting constants, Eq. 4 can be expressed as pk a = ν cm -1 (5) The necessary approximation in Eq. 2 is that S S = 0, which is a good assumption if the 2

3 entropy of protonation of the excited state is similar to that of the ground state. Choosing a set of Hammett parameters Hammett parameters were developed to quantitatively capture a functional group s ability to donate or withdraw electrons. They were derived empirically from measuring the effect a functional group had on the equilibrium of a reference reaction, e.g. the σ p constants were determined from the acid-base equilibrium of para substituted benzoic acid. 4 When using them to describe the effect on system significantly different than the reference reactions, it is not clear which set is the most appropriate to use. It is also possible that an excited state equilibrium may resemble a different reference reaction more closely than the ground state equilibrium. In practice, correlations are typically made with several sets of parameters for comparison. We found that for ground and excited state quinoline correlations to other parameters did not qualitatively change our conclusions and chose to present σ p in the main body of the paper. The other correlations and the reaction constants are listed below with the Hammett parameters from reference 5 and our pk a measurements. Table S.1: Hammett parameters and thermodynamic measurements R pk a pka σ m σ p σ + σ NH MeO H Br Cl CN

4 Table S.2: Reaction constants and correlations σ ρ ρ R 2 R 2 * σ m σ p σ σ Charge Density Analysis There has been much discussion about intramolecular charge transfer (ICT) and its role in photoacidity. Calculations of the Mulliken charges on the oxygen in 2-naphthol have illustrated that although both ROH and RO undergo partial ICT upon excitation, it is the stabilization of conjugate base after proton transfer which is the greater contributor. 6 Experiments, in particular Stark shift measurements, 7 on hydroxy pyrene trisulfonic acid (HPTS), are congruent with the latter notion, but also show that the effect can be reversed (acid form has greater ICT) for the cationic photoacid APTS. 8 We find that in substituted quinolines the calculated Mulliken charge on the heteroatom is roughly correlated with pk a and pka (figure S.1). ICT occurs upon excitation on both the free base and conjugate acid causing a negative excess charge to build on the nitrogen. Unlike 2-naphthol, 9 the amount of optical ICT for both forms is nearly equivalent. The results suggest that optically induced ICT in RQ prepares the initial charge density difference on the heteroatom to initiate proton transfer. After proton transfer, the charge density on the heteroatom is reduced to values far lower than the ground state RQ because of sharing electrons in the newly created bond with the proton. To better visualize the ICT process and the substituent effect on it, electron density difference maps were calculated for Q, AQ, 1-naphthol, and 5-cyano-1-naphthol (figure S.2). The changes in density show a general migration of electrons from the distal ring towards hydrogen bond accepting nitrogen in quinoline. Similarly, the direction of electron migration is reversed in the case of 1-4

5 Figure S.1: Calculated Mulliken charge on the heterocyclic nitrogen in 5-substituted quinolines when in the ground state (o), excited state (x), protonated (red), and deprotonated (blue) as a function of experimentally measured pk a. naphthol. Addition of electron withdrawing groups to the distal ring in 1-naphthol resulted in the creation of photoacids with negative pka values We predict that further substitution of the distal ring in quinoline will result in even larger pk a values than the ones presented in this paper. However, the use of an amine in the 8 position may interfere with the acid-base equilibrium via intramolecular hydrogen bonding. Electron density difference (EDD) maps showed an increase in density on the heterocyclic nitrogen upon optical excitation (figure S.2). Similarly, a decrease in density on the phenolic oxygen upon excitation in 1-naphthol derivatives is computed. Experimentally, the addition of an electron donating (withdrawing) group in the 5 position will enhance photobasicity (photoacidity) in quinolines (1-naphthols). The computational model shows that optical excitation does push (pull) electrons from the functional group towards the proton acceptor (donor) site of the molecule. The larger sensitivity of the excited state pka to the Hammett parameter is likely due to the larger polarizability of the excited state charge density. A diffuse and polarizable excited state is likely to respond more sensitively to the electron withdrawing strength of a substituent than the more 5

6 confined ground state charge density. Figure S.2: Comparison of the influence of the substituent groups on photoacidity and photobasicity. Electron density difference maps show the S 1 electron density minus the S 0 electron density. Green (red) indicates an increase (decrease) in density upon excitation. Electron donating (withdrawing) groups are shown to enhance photobasicity (photoacidity). 6

7 Absorption and Emission Spectra 5-aminoquinoline Figure S.3: Absorption spectra (left) and titration curve (right) for 5-aminoquinoline in aqueous solution. Figure S.4: Emission spectra of 5-aminoquinoline in aqueous solution. 7

5-methoxyquinoline Figure S.5: Absorption spectra (left) and titration curve (right) for 5-methoxyquinoline in aqueous solution. Figure S.6: Emission spectra of 5-methoxyquinoline in various solvents.

8 5-methoxyquinoline Figure S.5: Absorption spectra (left) and titration curve (right) for 5-methoxyquinoline in aqueous solution. Figure S.6: Emission spectra of 5-methoxyquinoline in various solvents. λ ex = 340 nm Photoacid-like behavior of MeOQ In acidic solution, dual emission from MeOQH + is observed, if the higher lying 1 L b state is excited. Excitation spectra monitored at the MeOQ* and MeOQH+* peaks were distinct. Explanation for this is proton release to form MeOQ*, in which case it may be temping to make the conclusion that neutral MeOQ is a photobase and cationic MeOQH+ is a photoacid. One may calculate a pka for the photoacid as well using the Förster cycle in the way we have above. However, the assumptions inherent in the Förster cycle no longer hold in the case of this potential 8

9 Figure S.7: Excitation spectra of 5-methoxyquinoline in acidic aqueous solution shows two distinct species corresponding to the protonated and deprotonated forms. A solvent Raman peak is labeled with R". Figure S.8: Excitation dependent emission in excess acid suggests photoacid-like behavior. Exciting the higher lying 1 L b state releases the proton to form the neutral MeOQ* species. A solvent Raman peak is labeled with R". 9

10 photoacid. There is no quasi-equilibrium established between the 1 L b state of the cation and the 1 L a state of the neutral species. The quasi-equilibrium that is established is between the netural 1 L a state and the cationic 1 L a state. A more succinct way of explaining this is that reversible proton transfer is part of the internal conversion process in MeOQH+*. We cannot make the same separation of timescales argument (excite state lifetime >> than IC) for two IC processes since it is reasonable to assume that they occur with similar timescales. We expect similar behavior with AQ since it has the same state ordering as shown for MeOQ, however evidence of dual emission is inconclusive due to its low quantum yield. 10

11 Quinoline Figure S.9: Absorption spectra (left) and titration curve (right) for quinoline in aqueous solution. Figure S.10: Gaussian fits of the absorption spectrum of quinoline. 11

12 Figure S.11: Emission spectra of quinoline in two solvents. 5-bromoquinoline Figure S.12: Absorption spectra (left) and titration curve (right) for 5-bromoquinoline in aqueous solution. 12

13 Figure S.13: Gaussian fits of the absorption of 5-bromoquinoline. Figure S.14: Emission spectra of 5-bromoquinoline in acidic and basic aqueous solution. 13

14 5-chloroquinoline Figure S.15: Absorption spectra (left) and titration curve (right) for 5-chloroquinoline in aqueous solution. Figure S.16: Gaussian fits of the absorption of 5-chloroquinoline. 14

15 Figure S.17: Emission spectra of 5-chloroquinoline in various solvents. 5-cyanoquinoline Figure S.18: Absorption spectra (left) and titration curve (right) for 5-cyanoyquinoline in aqueous solution. 15

16 Figure S.19: Gaussian fits of the absorption of 5-cyanoquinoline. Figure S.20: Emission spectrum of 5-cyanoquinoline in acidic solution. 16

17 5-nitroquinoline Figure S.21: Absorption spectra of 5-nitroquinoline in acidic and basic solution. References (1) Lasser, N.; Feitelson, J. Excited State pk Values from Fluorescence Measurements. Journal of Physical Chemistry 1973, 77, (2) Weller, A. Fast Reactions of Excited Molecules. Progress in Reaction Kinetics and Mechanism 1961, 1, (3) Ireland, J. F.; Wyatt, P. A. H. Acid-Base Properties of Electronically Excited States of Organic Molecules. Advances in Physical Organic Chemistry 1976, 12, (4) Anslyn, E.; Dougherty, D. Modern Physical Organic Chemistry; University Science, (5) Hansch, C.; Leo, a.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chemical Reviews 1991, 91, (6) Agmon, N.; Rettig, W.; Groth, C. Electronic Determinants of Photoacidity in Cyanonaphthols. J. Am. Chem. Soc. 2002, 124, (7) Silverman, L. N.; Spry, D. B.; Boxer, S. G.; Fayer, M. D. Charge Transfer in Photoacids Observed by Stark Spectroscopy. Journal of Physical Chemistry A 2008, 112,

18 (8) Spry, D. B.; Fayer, M. D. Charge Redistribution and Photoacidity: Neutral Versus Cationic Photoacids. Journal of Chemical Physics 2008, 128, (9) Agmon, N. Primary Events in Photoacid Dissociation. Journal of Molecular Liquids 2000, 85, (10) Clower, C.; Solntsev, K. M.; Kowalik, J.; Tolbert, L. M.; Huppert, D. Photochemistry of "Super" Photoacids. 3: Excited-State Proton Transfer from Perfluroalkylsulfonyl-Substitutued 2-Naphthols. Journal of Physical Chemistry A 2002, 106, (11) Tolbert, L. M.; Solntev, K. M. Excited-State Proton Transfer : From Constrained Systems to Super Photoacids to Superfast Proton Transfer. Acc. Chem. Res. 2002, 35, (12) Solntsev, K. M.; Clower, C. E.; Tolbert, L. M.; Huppert, D. 6-Hydroxyquinoline-N-oxide : A New Class of Super Photoacids. Journal of the American Chemical Society 2005, 127,

Observation of slow charge redistribution preceding excited-state proton transfer

THE JOURNAL OF CHEMICAL PHYSICS 127, 204501 2007 Observation of slow charge redistribution preceding excited-state proton transfer D. B. Spry and M. D. Fayer a Department of Chemistry, Stanford University,