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

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


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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 ⎠

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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.

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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.

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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

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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


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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).

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

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