Excited-State Proton Transfer in Indigo - The Journal of Physical

Feb 21, 2017 - ... it is a molecule charged with mysticism, whose synthesis is linked to the genesis of modern chemical ...... Christian PetermayerHen...
0 downloads 0 Views 4MB Size
Article pubs.acs.org/JPCB

Excited-State Proton Transfer in Indigo J. Pina,† Daniela Sarmento,† Marco Accoto,†,‡ Pier Luigi Gentili,‡ Luigi Vaccaro,*,‡ Adelino Galvaõ ,§ and J. Sérgio Seixas de Melo*,† †

CQC, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italia § Centro de Química Estrutural, Instituto Superior Técnico (IST), Universidade de Lisboa, 1049-001 Lisboa, Portugal ‡

S Supporting Information *

ABSTRACT: Excited-state proton transfer (ESPT) in Indigo and its monohexyl-substituted derivative (Ind and NHxInd, respectively) in solution was investigated experimentally as a function of solvent viscosity, polarity, and temperature, and theoretically by time-dependent density functional theory (TDDFT) calculations. Although a single emission band is observed, the fluorescence decays (collected at different wavelengths along the emission band using time-correlated single photon counting (TCSPC)) are biexponential, with two identical decay times but different pre-exponential factors, which is consistent with the existence of excited-state keto and enol species. The femtosecond (fs)-transient absorption data show that two similar decay components are present, in addition to a shorter (1010 s−1) and intra-

Figure 7. Arrhenius-type plots of the logarithm of kESPT vs the reciprocal of temperature for (A) indigo and (B) N-hexylindigo in ⎛ 1 ⎞ 2MeTHF solutions, together with the plot of ln⎜ T(K) − 1⎟vs 1/T in ⎝ ϕF ⎠ methylcyclohexane.

activation energy maintained the same order of magnitude (decreased from 10.8 to 7.1 kJ mol−1; see Section S5). These values are consistent with those reported for the radiationless deactivation of indigocarmine in water (3.5 kJ mol−1) or DMSO (8.8 kJ mol−1), which were attributed to H-bond breaking, whereas the low value in water suggests the involvement of intermolecular H-bonds.21 However, it should be stressed that the ESPT mechanisms in water and DMSO are probably different because the mechanism in water is probably dominated by acid−base processes in the solvent cage. With NHxInd, the activation energy for the proton transfer reaction was also obtained from the steady-state fluorescence data as a function of temperature by applying eq 4.46 In this case, it is considered that only one nonradiative rate constant is important (kIC), Ea is its activation energy, and the preexponential factor A0, its amplitude. 2314

DOI: 10.1021/acs.jpcb.6b11020 J. Phys. Chem. B 2017, 121, 2308−2318

Article

The Journal of Physical Chemistry B

Figure 8. Reaction profile for ESIPT in Ind (left) and NHxInd (right). IFC excitation; IIadiabatic solvent relaxation; IIIS1 minimum; IV saddle point; VENOL; VI, VIIsecond ESIPT and potential rotation around the central carbon−carbon bond.

Figure 9. Saddle point imaginary normal modes for Ind on the left and NHxInd (here replaced by a N-methyl).

molecular (>1011 s−1) excited-state processes and should not be viewed as proof for the dominance of an intra- versus intermolecular process.47 Although previous theoretical investigations point to a SPT mechanism17 rather than a concerted proton transfer involving two protons, they disagree regarding the associated energy barrier, suggesting the value of 2.2 kJ mol−1, which indicates a barrierless process.16,42 In the present work, the TDDFT PES was calculated adiabatically using the static dielectric constant of the solvent, whereas for excitations, only the fast component (essentially, the square of the refractive index) was used. Figure 8 shows the reaction profile for Ind and NHxInd. To decrease computing time, a methyl group was used for alkylation of the N−H rather than the hexyl group. However, this is not expected to make any significant difference. In Figure 8, the activation energy was corrected for the zero point vibrational energy. Because the reaction coordinate is roughly collinear with an N−H stretching normal mode, this correction is about 1/2hνRM, where νRM is the frequency of the reactive mode. The data in this figure supports a SPT mechanism for indigo. Analysis of the data shows that upon excitation: (i) Ind and NHxInd decay to a relaxed FC state of the rapidly formed keto tautomer (2) and (3) with energies at 640 nm (Ind) and 699 nm (NHxInd); (ii) this then undergoes single ESPT, with Ea

equal to 12.3 kJ mol−1 (Ind) and 3.1 kJ mol−1 (NHxInd), leading to the enol form. In the current calculations, the enol form is slightly lower in energy (red-shifted) relative to the experimental values (Figures 1, 4, 5, and 6). Indeed, as can be seen for both compounds from Figure 1, depending on the solvent, the emission of the keto band ranges from 631 to 655 nm for indigo and from 660 to 701 nm for NHxInd, in clear agreement with the data in Figure 8. However, the same does not happen with the enol resulting from the single excited-state proton transfer (SESPT), where the energy minimum is found with energies lower than those of the red part of the emission bands of the two compounds. From the temperature dependence of the fast decay and fluorescence quantum yields, both of which mirror the ESPT, it can be seen that the process involves an energy barrier close to ∼11 kJ mol−1 in the case of indigo, whereas with N-hexylindigo this value is ∼5 kJ mol−1. The relatively low activation energy barriers for proton transfer make tunneling a competitive mechanism compared with reaction over the barrier. An Eyring equation modified with a tunneling enhancement factor (tef), eq 5, was used with the TDDFT calculations to predict kESPT values. 2315

DOI: 10.1021/acs.jpcb.6b11020 J. Phys. Chem. B 2017, 121, 2308−2318

Article

The Journal of Physical Chemistry B

Figure 10. (A) Energy profile for the rotation around the C−C bond; (B) geometry of the closest point to the seam of the CI (left): geometry at the seam (right) (the OH stretching was exaggerated in the picture for readability).

k = tef

molecular process only becoming visible in dried nonprotic organic solvents.

kBT ΔS # / R −ΔH # / RT e e h

2 1 ⎛ hνi ⎞ tef = 1 + ⎜ ⎟ 24 ⎝ kBT ⎠



ASSOCIATED CONTENT

S Supporting Information *

(5)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b11020. Synthetic details for the synthesis of NHxInd (S1), fluorescence decays in 2MeTHF and 2MeTHF/water (S2), together with additional fs-TA (S3) experimental and TDDFT (S4) details and the kESPT obtained with eq 2 but with the value of τMC from the dependence of the longer decay component (τ3 in Table 3) with temperature (S5) (PDF)

νi is the imaginary frequency on the saddle point, see Figure 9. With this model and eq 2, a kESPT of 1.2 × 1011 s−1 for Ind was computed, which is virtually identical to the experimental value. In NHxInd, reversibility prevents the computation of an ESPT rate, as the back reaction and deactivation through a different CI, as described above, become competitive. Finally, it should be noted that, after the first proton transfer, indigo can deactivate by moving along the branching space coordinates of a CI, closing the energy gap between S0 and S1. This can be achieved either by further movement toward a second ESPT, which is geometrically close to a CI, or by rotation around the central C−C bond, although these processes are never reached, as shown experimentally by the absence of a second proton transfer in indigo and the lack of solvent viscosity dependence of the fast component (see Table 3). NHxInd has only the later path for deactivation. In fact, the torsional normal mode that starts this rotation was calculated to be only 16 cm−1, which is compatible with that of a very shallow PES energy profile along this mode. Rotation around the central C−C bond brings the system close to the CI (less than 0.4 eV), which is of the sloped type, whereas O−H stretching allows the system to reach the seam of the CI, see Figure 10.



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.V.). *E-mail: [email protected] (J.S.S.D.M.). ORCID

J. Sérgio Seixas de Melo: 0000-0001-9708-5079 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We acknowledge funding by Fundo Europeu de Desenvolvimento Regional (FEDER) through Programa Operacional Factores de Competitividade (COMPETE) and the Università degli Studi di Perugia. The Coimbra Chemistry Centre is supported by the Fundaçaõ para a Ciência e a Tecnologia (FCT), Portuguese Agency for Scientific Research, through the Project PEst-OE/QUI/UI0313/2014. CQE is also supported by FCT through the project UID/QUI/00100/2013. This work was performed under the project “SunStorage Harvesting and storage of solar energy”, with reference POCI-01-0145-FEDER-016387, funded by the European Regional Development Fund (ERDF), through COMPETE 2020 - Operational Programme for Competitiveness and Internationalisation (OPCI), and by national funds, through



CONCLUSIONS In summary, the study of a mono-alkylated indigo (NHxInd) and indigo (Ind) under identical conditions provided clear experimental evidence for the following statements: (i) the ESPT mechanism involves a SPT in both indigo and N-hexylindigo; (ii) an additional deactivation channel is competitive with ESPT, as seen by different values of the activation energy obtained for NHxInd from steady-state and time-resolved data; (iii) the ESPT process is likely to involve both intra- and inter-molecular mechanisms, with the intra2316

DOI: 10.1021/acs.jpcb.6b11020 J. Phys. Chem. B 2017, 121, 2308−2318

Article

The Journal of Physical Chemistry B

(19) Elsaesser, T.; Kaiser, W.; Lüttke, W. Picosecond Spectroscopy of Intramolecular Hydrogen-Bonds in 4,4′,7,7′-Tetramethylindigo. J. Phys. Chem. 1986, 90, 2901−2905. (20) Costa, A. L.; Gomes, A. C.; Pillinger, M.; Gonçalves, I. S.; Seixas de Melo, J. S. An Indigo Carmine-Based Hybrid Nanocomposite with Supramolecular Control of Dye Aggregation and Photobehavior. Chem. Eur. J. 2015, 21, 12069−12078. (21) Nagasawa, Y.; Taguri, R.; Matsuda, H.; Murakami, M.; Ohama, M.; Okada, T.; Miyasaka, H. The Effect of Hydrogen-Bonding on the Ultrafast Electronic Deactivation Dynamics of Indigo Carmine. Phys. Chem. Chem. Phys. 2004, 6, 5370−5378. (22) Iwakura, I.; Yabushita, A.; Kobayashi, T. Why Is Indigo Photostable over Extremely Long Periods? Chem. Lett. 2009, 38, 1020−1021. (23) Murov, S.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry; M. Dekker Inc.: New York, 1993. (24) Seixas de Melo, J. S.; Pina, J.; Dias, F. B.; Maçanita, A. L., Experimental Techniques for Excited State Characterisation. In Applied Photochemistry; Evans, R. C., Douglas, P., Burrow, H. D., Eds.; Springer: Netherlands, 2013; pp 533−585. (25) Pina, J.; Seixas de Melo, J.; Burrows, H. D.; Maçanita, A. L.; Galbrecht, F.; Bunnagel, T.; Scherf, U. Alternating BinaphthylThiophene Copolymers: Synthesis, Spectroscopy, and Photophysics and Their Relevance to the Question of Energy Migration Versus Conformational Relaxation. Macromolecules 2009, 42, 1710−1719. (26) Striker, G.; Subramaniam, V.; Seidel, C. A. M.; Volkmer, A. Photochromicity and Fluorescence Lifetimes of Green Fluorescent Protein. J. Phys. Chem. B 1999, 103, 8612−8617. (27) Pina, J.; Seixas de Melo, J. S.; Eckert, A.; Scherf, U. Unusual Photophysical Properties of Conjugated, Alternating Indigo-Fluorene Copolymers. J. Mater. Chem. A 2015, 3, 6373−6382. (28) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347−1363. (29) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange− Correlation Functional Using the Coulomb-Attenuating Method (Cam-B3lyp). Chem. Phys. Lett. 2004, 393, 51−57. (30) Klessinger, M. The Origin of the Color of Indigo Dyes. Dyes Pigm. 1982, 3, 235−241. (31) Bauer, H.; Kowski, K.; Kuhn, H.; Luttke, W.; Rademacher, P. Photoelectron Spectra and Electronic Structures of Some Indigo Dyes. J. Mol. Struct. 1998, 445, 277−286. (32) Lüttke, W.; Hermann, H.; Klessinger, M. Theoretically and Experimentally Determined Properties of Fundamental Indigo Chromophore. Angew. Chem., Int. Ed. Engl. 1966, 5, 598−599. (33) Zoon, P. D.; Brouwer, A. M. A Push-Pull Aromatic Chromophore with a Touch of Merocyanine. Photochem. Photobiol. Sci. 2009, 8, 345−353. (34) Seixas de Melo, J. S.; Serpa, C.; Burrows, H. D.; Arnaut, L. G. The Triplet State of Indigo. Angew. Chem., Int. Ed. Engl. 2007, 46, 2094−2096. (35) Laws, W. R.; Brand, L. Analysis of Two-State Excited-State Reactions. The Fluorescence Decay of 2-Naphthol. J. Phys. Chem. 1979, 83, 795−802. (36) Zhou, P.; Hoffmann, M. R.; Han, K.; He, G. New Insights into the Dual Fluorescence of Methyl Salicylate: Effects of Intermolecular Hydrogen Bonding and Solvation. J. Phys. Chem. B 2015, 119, 2125− 2131. (37) Chai, S.; Zhao, G.-J.; Song, P.; Yang, S.-Q.; Liu, J.-Y.; Han, K.-L. Reconsideration of the Excited-State Double Proton Transfer (Esdpt) in 2-Aminopyridine/Acid Systems: Role of the Intermolecular Hydrogen Bonding in Excited States. Phys. Chem. Chem. Phys. 2009, 11, 4385−4390. (38) Li, G.-Y.; Zhao, G.-J.; Liu, Y.-H.; Han, K.-L.; He, G.-Z. Td-Dft Study on the Sensing Mechanism of a Fluorescent Chemosensor for Fluoride: Excited-State Proton Transfer. J. Comput. Chem. 2010, 31, 1759−1765.

FCT. The FCT is also acknowledged for a post-doctoral grant to J. Pina (ref SFRH/BPD/108469/2015). The research leading to these results has received funding from LaserlabEurope (grant agreement no. 284464, EC’s Seventh Framework Programme).



REFERENCES

(1) Melo, M. J.; Ferreira, J. L.; Parola, A. J.; Seixas de Melo, J. S. Photochemistry for Cultural Heritage. In Applied Photochemistry: When Light Meets Molecules; Bergamini, G., Silvi, S., Eds.; Springer International Publishing: Cham, 2016; pp 499−530. (2) Seixas de Melo, J. S.; Rondão, R.; Burrows, H. D.; Melo, M. J.; Navaratnam, S.; Edge, R.; Voss, G. Spectral and Photophysical Studies of Substituted Indigo Derivatives in Their Keto Forms. ChemPhysChem 2006, 7, 2303−2311. (3) Seixas de Melo, J.; Moura, A. P.; Melo, M. J. Photophysical and Spectroscopic Studies of Indigo Derivatives in Their Keto and Leuco Forms. J. Phys. Chem. A 2004, 108, 6975−6981. (4) Rondão, R.; Seixas de Melo, J.; Melo, M. J.; Parola, A. J. ExcitedState Isomerization of Leuco Indigo. J. Phys. Chem. A 2012, 116, 2826−2832. (5) Rondão, R.; Seixas de Melo, J. S.; Voss, G. Characterization of the Excited States of Indigo Derivatives in Their Reduced Forms. ChemPhysChem 2010, 11, 1903−1908. (6) Moreno, M.; Ortiz-Sanchez, J. M.; Gelabert, R.; Lluch, J. M. A Theoretical Study of the Photochemistry of Indigo in Its Neutral and Dianionic (Leucoindigo) Forms. Phys. Chem. Chem. Phys. 2013, 15, 20236−20246. (7) Blackburn, R. S.; Bechtold, T.; John, P. The Development of Indigo Reduction Methods and Pre-Reduced Indigo Products. Color. Technol. 2009, 125, 193−207. (8) Rondão, R.; Seixas de Melo, J. S.; Bonifácio, V. D. B.; Melo, M. J. Dehydroindigo, the Forgotten Indigo and Its Contribution to the Color of Maya Blue. J. Phys. Chem. A 2010, 114, 1699. (9) Domenech, A.; Domenech-Carbo, M. T.; del Rio, M. S.; Pascual, M.; Lima, E. Maya Blue as a Nanostructured Polyfunctional Hybrid Organic-Inorganic Material: The Need to Change Paradigms. New J. Chem. 2009, 33, 2371−2379. (10) Dejoie, C.; Martinetto, P.; Tamura, N.; Kunz, M.; Porcher, F.; Bordat, P.; Brown, R.; Dooryhée, E.; Anne, M.; McCusker, L. B. Crystal Structure of an Indigo@Silicalite Hybrid Related to the Ancient Maya Blue Pigment. J. Phys. Chem. C 2014, 118, 28032− 28042. (11) Tilocca, A.; Fois, E. The Color and Stability of Maya Blue: Tddft Calculations. J. Phys. Chem. C 2009, 113, 8683−8687. (12) D’Elboux Bernardino, N.; Brown-Xu, S. E.; Gustafson, T. L.; Araujo de Faria, D. L. Time Resolved Spectroscopy of Indigo and of a Maya Blue Simulant. J. Phys. Chem. C 2016, 120, 21905−21914. (13) Rondão, R.; Seixas de Melo, J. S. Thio-Mayan-Like Compounds: Excited State Characterization of Indigo Sulfur Derivatives in Solution and Incorporated in Palygorskite and Sepiolite Clays. J. Phys. Chem. C 2013, 117, 603−614. (14) Doménech-Carbó, A.; Doménech-Carbó, M.; Valle-Algarra, F.; Domine, M.; Osete-Cortina, L. On the Dehydroindigo Contribution to Maya Blue. J. Mater. Sci. 2013, 48, 7171−7183. (15) Seixas de Melo, J.; Rondão, R.; Burrows, H. D.; Melo, M. J.; Navaratnam, S.; Edge, R.; Voss, G. Photophysics of an Indigo Derivative (Keto and Leuco Structures) with Singular Properties. J. Phys. Chem. A 2006, 110, 13653−13661. (16) Cui, G. L.; Thiel, W. Nonadiabatic Dynamics of a Truncated Indigo Model. Phys. Chem. Chem. Phys. 2012, 14, 12378−12384. (17) Yamazaki, S.; Sobolewski, A. L.; Domcke, W. Molecular Mechanisms of the Photostability of Indigo. Phys. Chem. Chem. Phys. 2011, 13, 1618−1628. (18) Kobayashi, T.; Rentzepis, P. M. On the Picosecond Kinetics and Photostability of Indigo and 6,6′-Dimethoxyindigo. J. Chem. Phys. 1979, 70, 886−892. 2317

DOI: 10.1021/acs.jpcb.6b11020 J. Phys. Chem. B 2017, 121, 2308−2318

Article

The Journal of Physical Chemistry B (39) Rondão, R. J.; Seixas de Melo, J. S.; Schaberle, F. B.; Voss, G. Excited State Characterization of a Polymeric Indigo. Phys. Chem. Chem. Phys. 2012, 14, 1778−1783. (40) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. Subpicosecond Measurements of Polar Solvation Dynamics: Coumarin 153 Revisited. J. Phys. Chem. 1995, 99, 17311−17337. (41) Iwakura, I. The Experimental Visualisation of Molecular Structural Changes During Both Photochemical and Thermal Reactions by Real-Time Vibrational Spectroscopy. Phys. Chem. Chem. Phys. 2011, 13, 5546−5555. (42) Iwakura, I.; Yabushita, A.; Kobayashi, T. Transition State in a Prevented Proton Transfer Observed in Real Time. Bull. Chem. Soc. Jpn. 2011, 84, 164−171. (43) Dittmann, M.; Graupner, F. F.; Maerz, B.; Oesterling, S.; de Vivie-Riedle, R.; Zinth, W.; Engelhard, M.; Lüttke, W. Photostability of 4,4′-Dihydroxythioindigo, a Mimetic of Indigo. Angew. Chem., Int. Ed. Engl. 2014, 53, 591−594. (44) Iwakura, I.; Yabushita, A.; Kobayashi, T. Kinetic Isotope Effect on the Proton-Transfer in Indigo Carmine. Chem. Phys. Lett. 2010, 484, 354−357. (45) Seixas de Melo, J. S.; Costa, T.; de Castro, C. S.; Maçanita, A. L. Photophysics of Fluorescently Labeled Oligomers and Polymers. In Photochemistry; Albini, A., Ed.; The Royal Society of Chemistry, 2013; Vol. 40. (46) Meech, S. R.; Phillips, D. Photophysics of Some Common Fluorescence Standards. J. Photochem. 1983, 23, 193−217. (47) Shizuka, H.; Machii, M.; Higaki, Y.; Tanaka, M.; Tanaka, I. Excited-State and Ground-State Intramolecular Proton-Transfer Reactions of 6-(2-Hydroxy-5-Methylphenyl)-S-Triazines in Poly(Methyl Methacrylate). J. Phys. Chem. 1985, 89, 320−326.

2318

DOI: 10.1021/acs.jpcb.6b11020 J. Phys. Chem. B 2017, 121, 2308−2318