Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

Para-nitrophenyl-phenol.

ICT absorption in para-nitrophenyl-phenol promotes an electron from a nonbonding orbital on the oxygen to the nitro group, producing a ¹(nπ^∗) charge transfer excited state (Fig. 1).

Analysis of ground-state absorption spectra for a series of organic bases shows that the ICT absorption band at 335 nm systematically shifts to lower energy as the O─H bond is weakened through hydrogen bonding, ultimately leading to deprotonation. Hydrogen-bond formation with bases ranging from t-butylamine (pK_a = 10.7) to pyridine (pK_a = 5.25) in DCE result in incremental shifts in the ICT absorption band with increasing base strength from λ_max = 335 nm (no base) to a limiting value of 354 nm for t-butylamine. For t-butylamine, K_A = 104 ± 10 M^-1 at 23 ± 2 °C with adduct formation > 90% complete with 0.3 mM para-nitrophenyl phenol and 90 mM base. The latter were the experimental conditions used in the transient experiments.

Complete deprotonation of the phenol occurs upon addition of tetra-n-butylammonium hydroxide, which further red shifts the ICT absorption to λ_max = 466 nm. The intermediate proton transfer state, A─O^-⋯H⁺─:B, cannot be accessed experimentally; however, DFT calculations suggest that its ICT absorption lies at 410 nm between the absorptions for the hydrogen-bonded (354 nm) and deprotonated forms of the phenol (466 nm). The position of the ICT absorption provides a spectroscopic indicator of the location of the proton in the ground state, ranging from 335 nm (A─O─H) to 354 nm (A─O─H⋯:B) to 410 nm (A─O^-⋯⁺H─B) to 466 nm (A─O^-). Examination of the ground-state absorption spectrum (Section I.B, SI Appendix) shows no evidence of significant concentrations of either deprotonated anion (A─O^-) or proton transfer adduct (A─O^-⋯⁺H─B). These observations are consistent with both states lying energetically above the hydrogen-bonded adduct.

The acidity in the ICT excited state is enhanced by approximately 9 pK_a units, resulting in a large driving force for proton transfer after photoexcitation. We have used a combination of femtosecond transient absorption and coherent Raman spectroscopies to probe the structure of the optically prepared state and the dynamics that take place following photon absorption. Details regarding observations and analysis can be found in Sections I.C and III of the SI Appendix.

Femtosecond transient absorption spectra of the hydrogen-bonded adduct show evidence for three excited state absorption bands between 400 and 700 nm. The spectra obtained one picosecond following excitation of the free para-nitrophenyl-phenol and the hydrogen-bonded adduct (1) are shown in Fig. 3.

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Fig. 3.

Transient absorption spectra at early pump-probe delay times of 4-hydroxy-4′-nitro-biphenyl (nitrophenyl-phenol) (Upper, right) and the hydrogen-bonded adduct with t-butylamine (TBA) (Lower, right) in 1,2 dicholoro-ethane (DCE). All spectra were observed 1.0 ps after photoexcitation. The free phenol spectrum was obtained after photoexcitation at 355 nm. Two spectra for the hydrogen-bonded adduct are shown, collected at 355 nm and the other at 388 nm. The diagram at the left illustrates the band assignments made by correlation of a wide array of experimental observations (see SI Appendix). The dashed gray and blue arrows indicate transitions that are not observed experimentally.

Excitation of free phenol at 355 nm in DCE results in an absorption band with (labeled S_a in Fig. 3). It is assigned to an excited state absorption from the singlet ¹nπ^∗ ICT state—i.e., ¹(^-·A─O^·─H⁺)^∗. The singlet undergoes rapid (τ ∼ 3 ps) intersystem crossing to the corresponding ³nπ^∗ state, ¹(^-·A─O^·─H⁺)^∗ → ³(^-·A─O^·─H⁺)^∗, giving rise to the red absorption band at , denoted T_a. The triplet character of the red absorption was verified by transient EPR measurements at 77 K in a toluene glass (SI Appendix) and the intersystem crossing time observed here is comparable to that observed for nπ^∗ states in nitrobenzenes (13). The T_a band reaches its maximum amplitude within a few ps and then decays, without a change in spectral position on a time scale of 2–3 μs.

The singlet and triplet bands are also observed in the transient absorption spectra of the hydrogen-bonded adduct (A─O─H⋯:B), Fig. 3. These absorptions depend upon excitation wavelength and exhibit complex temporal evolution including changes in both the amplitude and position of the bands. Our analysis of the transient spectra, which is discussed in detail in the SI Appendix, correlates observations of the spectral evolution from fs to μs, at two different excitation wavelengths, 355 nm and 388 nm.

The singlet and triplet absorptions observed following 388 nm excitation of the adduct (S_c and T_c) are both shifted to higher energy relative to the free phenol, with λ_max at 395 nm and 570 nm, respectively. While the singlet absorption decays with τ ∼ 4.5 ps, the triplet absorption shows no appreciable evolution during the first 300 ps after excitation.

With 355 nm excitation, the singlet band is shifted to higher energy relative to the unassociated phenol with λ_max at 410 nm (Fig. 3). The triplet band appears at λ_max = 620 nm at early pump-probe delays, similar to the phenol itself (Section I.C.1, SI Appendix).

Careful examination of the singlet absorption reveals that the red and blue edges decay with different kinetics, consistent with two unresolved absorptions with different decay characteristics (Section 1.C.2, SI Appendix). The decay on the lower-energy side is kinetically correlated with the growth of the triplet absorption at 630 nm (τ ∼ 1.5 ps), a clear indication that it arises from the singlet ICT state, ¹(^-·A─O^·─H⁺⋯:B)^∗. The higher-energy side decays more slowly with τ ∼ 4.5 ps. This is identical to the lifetime of S_c, suggesting that it and the unresolved higher-energy contribution in the 355 nm spectrum arise from the same transition. Taking this contribution to have the same position and spectral width as S_c enables a deconvolution of the two unresolved absorptions and suggests that the lower-energy absorption (S_b) has its λ_max at 420 nm. These two bands (S_b and S_c) are depicted by the dashed lines beneath the singlet absorption band in the 355 nm spectrum.

The shift of the singlet absorption from 440 nm in free para-nitrophenyl-phenol (S_a) to 420 nm when hydrogen bonded to the amine (S_b) indicates that the excited state absorption in the ICT state is also sensitive to the location of the proton, shifting to the blue as the O─H bond weakens. With this observation the appearance of the feature at λ_max ≈ 395 nm is consistent with a ¹nπ^∗ ICT state that has a considerable degree of proton transfer character—i.e., ¹(^-·A─O^·⋯⁺H─B)^∗. The proton in this configuration is located at the same nuclear configuration as in the ground state, by application of the Frank–Condon approximation, but is transferred to the base.

This systematic blue shift with O─H bond weakening is also observed in the triplet absorption. Peaking at 630 nm in the free phenol, ³(^-·A─O^·─H⁺)^∗, this band is observed to shift to higher energy as the interaction with the proton is weakened, first through hydrogen bonding ³(^-·A─O^·─H⁺⋯:B)^∗, 620 nm, then proton transfer to the base ³(^-·A─O^·⋯⁺H─B)^∗, 570 nm, and ultimately with deprotonation to the anion ³(^-·A─O^·)^∗, 530 nm (Section I.C.2, SI Appendix). The transition energies of the ground and excited state absorptions for the different protonation states of the nitrophenyl-phenol chromophore are summarized schematically in Fig. 3.

Based on our band assignments, the transient spectra at early times suggest that there are two spectroscopically accessible states corresponding to the configurations ¹(^-·A─O^·─H⁺⋯:B)^∗ and ¹(^-·A─O^·⋯⁺H─B)^∗. They are illustrated in the energy-coordinate diagram shown in Fig. 2. While both states appear to be populated by 355 nm excitation (as evidenced by the presence of both S_b and S_c), 388 nm excitation seems to produce only the latter. The absence of the ¹(^-·A─O^·─H⁺⋯:B)^∗ absorption feature following 388 nm excitation suggests that, at the ground-state geometry, ¹(^-·A─O^·⋯⁺H─B)^∗ is lowest in energy, as depicted in the figure. It is interesting to note that similar ultrafast proton transfer has been observed in related measurements on the green fluorescent protein (GFP). In GFP ultrafast proton transfer was attributed to excitation to a second state underlying the electronic absorption band, but the nature of the transition was unclear (10).

In the para-nitrophenyl-phenol adduct, the two initial singlet states have distinctly different dynamical signatures. Photoexcitation to ¹(^-·A─O^·─H⁺⋯:B)^∗ results in delayed proton transfer following intersystem crossing to the triplet, ³(^-·A─O^·─H⁺⋯:B)^∗. Excitation to the lower-energy surface at 388 nm gives rise to prompt appearance of protonated base—i.e., (^-·A─O^·⋯⁺H─B)^∗ in both the singlet and triplet spectra.

The presence of (^-·A─O^·⋯⁺H─B)^∗ at the earliest observable pump-probe delay times (200 fs) is consistent with a photoinduced electron-proton transfer (photo-EPT) process. However, given the time scale of the experiment, we cannot rule out the presence of an ultrafast but delayed proton transfer process that occurs during the 200 fs excitation pulse solely from the transient absorption data. The key question is whether direct photoexcitation to the ICT-EPT state occurs from the ground-state configuration.

Coherent Raman measurements provide a direct probe of excited state vibrational modes at the Franck–Condon geometry. Similar to spontaneous resonance Raman spectroscopy, the amplitudes of the vibrational resonances reflect geometric distortions between the ground and excited states (i.e., Franck–Condon factors). However, application of a broadband field initiates nuclear motion on both the ground and excited state surfaces.

Details involved in the acquisition and analysis of the Raman signals are provided in the SI Appendix. A typical coherent Raman spectrum obtained from the para-nitrophenyl-phenol adduct is shown in Fig. 4. In this representation, the spectra contain positive and negative going signals, which are described for a general system by Eq. S8 in the SI Appendix. For the systems discussed here, the signals with positive signs correspond to nuclear coherences in the ground electronic state, while the negative going signals are associated with nuclear coherences in the excited electronic state.

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Fig. 4.

Coherent Raman spectrum of the nitrophenyl-phenol/t-butylamine hydrogen-bonded adduct. The positive going feature is the Raman response of the O─H mode in the ground electronic state. The higher frequency negative going feature is an excited state vibrational resonance assigned to the ⁺H─N mode.

The spectrum of the nitrophenyl-phenol adduct shows the ν(O─H) ground-state band at 2,850 cm^-1 and a band at 2,970 cm^-1 that is assigned to an excited state vibrational coherence based on the sign of the signal. ICT excitation of the para-nitrophenyl-phenol weakens the O─H bond and shifts its vibrational frequency to lower energy, suggesting that the O─H mode is not the origin of the excited state resonance. Based on the shift to higher energy, the new band is assigned to the ν(N─H⁺) mode in the ICT adduct. The 2,970 cm^-1 frequency is consistent with ν(N─H⁺) stretching vibrations observed in protonated amines (14), which range from 3,100 to 3,300 cm^-1.

From a Frank–Condon perspective, excitation to the ICT-EPT state is a vertical process along the coupled nuclear coordinates including the proton transfer coordinate. The formation of the protonated base in the optical excitation process is remarkable because it implies that there is redistribution of electron density both in the phenol and at the base. The rearrangement of electron density in intramolecular examples is expected because both the proton donor and acceptor are part of the same electronic framework. The electronic redistribution at the base in the intermolecular adduct is noteworthy, pointing to extensive electronic coupling across the hydrogen bond.

Hydroxy-Coumarin.

ICT excitation of 7-hydroxy-4-(tri-fluoromethyl)-coumarin (HFC) promotes a largely nonbonding electron from the hydroxyl group to a π^∗ orbital with considerable carbonyl character, increasing the acidity of the OH proton by approximately 14 pKa units (7, 8). Evidence of ground-state H-bonding between the coumarin and 1-methyl-imidazole (1-MeIm) base in toluene is observed as a shift in the ICT absorption spectrum from 330 nm to 342 nm upon adduct formation. For 1-methylimidazole, K_A = 2,100 M^-1 with adduct formation of 73% at 2 mM base concentration.

We have used the emissive properties of the coumarin to probe coupled electron-proton transfer following ultrafast excitation. In the absence of an external base, hydroxy-coumarins emit weakly with an emission maximum in toluene of 403 nm. The deprotonated form is strongly emissive with λ_max = 506 nm. Intense emission is also observed upon the addition of hydrogen-bonding bases, and the 1∶1 adduct with 1-MeIm (A─O─H⋯:B) emits at 459 nm (15). The blue-shifted emission in the adduct relative to the anion can be attributed to H-bond stabilization of the ground-state anion by complex formation with the protonated base.

The appearance of the emission at 459 nm is thus an indicator of proton transfer in the excited state. Femtosecond transient absorption methods were used to monitor the growth of stimulated emission at 459 nm (Fig. 5A). Based on this result, approximately 75% of the emission appears on a time scale short relative to the instrument response time (< 1 ps) consistent with rapid proton transfer.

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Fig. 5.

(A) Transient stimulated emission data for the hydroxyl-coumarin 1∶1 adduct with 1-MeIm (2 mM) in toluene following 355 nm excitation. The open circles display the laser excitation pulse reacting with the solvent nuclear response in toluene. Gray points represent the stimulated emission data at 465 nm; only data points outside the instrument response are shown. (B) Coherent Raman spectrum of the hydroxyl coumarin 1∶1 adduct with 1-MeIm (2 mM) in toluene.

The coherent Raman spectrum for the coumarin adduct is shown in Fig. 5B. Like para-nitrophenyl-phenol, the ν(O─H) Raman response of the ground state is observed at 2,900 cm^-1, whereas an excited state vibrational resonance is observed near 3,050 cm^-1. Based on the shift to higher energy, the new band can be assigned to ν(N─H⁺) in the ICT adduct ¹(^-·A─O^·⋯⁺H─B)^∗.