Dear Muhammad,

The following recent published article covers the answer to your question.

DOI: 10.1039/C5SC01945A (Edge Article) Chem. Sci., 2016, 7, 655-665

Excited-state intramolecular proton-transfer reaction demonstrating anti-Kasha behavior†

Huan-Wei Tseng‡ a, Jiun-Yi Shen‡ a, Ting-Yi Kuo a, Ting-Syun Tu a, Yi-An Chen a, Alexander P. Demchenko *b and Pi-Tai Chou

We report unusual photophysical properties observed on two newly designed 3-hydroxychromone derivatives exhibiting the excited-state intramolecular proton transfer (ESIPT) reaction. The efficiency of ESIPT reaction is greatly enhanced upon excitation with high energy quanta to Sn (n > 1) levels in low-polarity solvents. Based on detailed analyses of excitation and emission spectra as well as time-resolved emission kinetics we derive that conditions, in which this phenomenon contradicting Kasha's rule is observed, are quite different from that for observation of anti-Kasha emission.

Conclusions

In this study, we strategically designed and synthesized a series of new thiophenyl 3-hydroxychromone derivatives 3-HTCA and 3-HTC-DiCN. They exhibit classical ESIPT reaction and allow observing the switching between the two reactant and product emissive forms of this reaction in a kinetically controlled regime. For 3-HTCA and 3-HTC-DiCN, we describe unusual behaviour in a display of ESIPT reaction on excitation at shorter wavelengths than the S1 band. The fluorescence intensity is re-distributed from the band belonging to initially excited normal reactant (N*) species to a reaction product phototautomer (T*) band demonstrating an increased rate of ESIPT reaction. Accordingly, the excitation spectra deviate from absorption spectra in a short-wavelength region in opposite directions when recorded at N* and T* emission bands. Moreover, a dramatic decrease of fluorescence quantum yield of N* emission and correspondent rise of T* emission quantum yield is observed.These results are in apparent contradiction to Kasha's rule. They deserve proper explanation and specifying the conditions in which such phenomenon can be observed. The sub-nanosecond time-resolved experiments demonstrate that no equilibrium between the normal form and the tautomer form is achieved during fluorescence lifetime when 3-HTCA and 3-HTC-DiCN species are excited to the S1 state. This indicates the presence of a kinetic barrier in exergonic ESIPT reaction, and it is this barrier that is probably removed or diminished on excitation to higher energy states. Applying femtosecond fluorescence up-conversion technique we obtained direct proof that the rate of overall ESIPT reaction, monitored by the normal and/or tautomer emission, is excitation wavelength dependent. This allows fast SNn → STm ESIPT to compete successfully with SNn → SN1 internal conversion. The origin of this acceleration may be related to the vanishing of solvent-reorganization barrier for the ESIPT reaction.27,30 In contrast to SN1 → ST1 charge transfer coupled ESIPT that is subject to solvent induced barrier, the SNn states of studied compounds must have different distribution of electronic density.33 This distribution may better fit excited tautomer states such that both SNn and STm possess similar equilibrium polarization, minimizing this barrier. Verification of this hypothesis needs very sophisticated computation for the congested higher energy states. The relevant theoretical approach is in progress. Evidently, the uniqueness and difference of 3-HTCA and 3-HTC-DiCNfrom previously reported ESICT/ESIPT coupled molecules27,30 lie in that the efficiency of the lowest SN1 → ST1 ESIPT process is less than unity and there is no reversibility between SN1 and ST1. In comparison, ESIPT pathways in the higher lying states have higher efficiency than that of the SN1 → ST1 ESIPT process. Accordingly, one can see the increase of the tautomer emission upon higher energy excitation in the steady-state manner, a phenomenon of anti-Kasha's rule.

In summary, using the series of dyes 3-HTCA and 3-HTC-DiCN with different electron acceptor substituents in the thiophenyl fragment we demonstrate that the switching between Kasha and anti-Kasha behaviour can be modulated in a quite delicate way by appropriate molecular design. The exploration of anti-Kasha photochemistry opens an unexpectedly broad range of possibilities of selective control on photoreactions by changing the excitation energy not only in single-photon but also in multiple-photon modes. It allows reducing the losses in energy dissipating internal conversion processes that follow the excitation by high energy quanta and presently look unavoidable in photonic devices. This can make more efficient the utilization of photoexcitation energy, which is demanding in many applications.

To view the full paper, please use the following link:

http://pubs.rsc.org/en/content/articlehtml/2016/sc/c5sc01945a

The following is another publication that provides proof for Excited state intramolecular proton transfer ESIPT :

http://www.ncbi.nlm.nih.gov/pubmed/22954380

J Org Chem. 2013 Mar 1;78(5):1811-23. doi: 10.1021/jo301456y. Epub 2012 Sep 24.

Excited state intramolecular proton transfer (ESIPT) from phenol to carbon in selected phenylnaphthols and naphthylphenols.

Basarić N1, Došlić N, Ivković J, Wang YH, Veljković J, Mlinarić-Majerski K, Wan P.

Author information

Abstract

ESIPT and solvent-assisted ESPT in isomeric phenyl naphthols and naphthyl phenols 5-8 were investigated by preparative photolyses in CH3CN-D2O, fluorescence spectroscopy, LFP, and ab initio calculations. ESIPT takes place only in 5 (D-exchange Φ = 0.3), whereas 6-8 undergo solvent-assisted PT with much lower efficiencies. The efficiency of the ESIPT and solvent-assisted PT is mainly determined by different populations of the reactive conformers in the ground state and the NEER principle. The D-exchange experiments and calculations using RI-CC2/cc-pVDZ show that 5 in S1 deactivates by direct ESIPT from the OH to the naphthalene position 1 through a conical intersection with S0, delivering QM 14 that was detected by LFP (τ = 26 ± 3 ns). ESIPT to position 3 in 5 is possible but it proceeds from a less-populated conformer and involves an energy barrier on S1. In solvent-assisted PT to naphthalene position 4 in 5, zwitterion 17 is formed, which cyclizes to stable naphthofuran photoproducts 9-12. The regiochemistry of the deuteration in solvent-assisted PT was correlated with the NBO charges of the corresponding phenolates/naphtholates 5(-)-8(-). Combined experimental and theoretical data indicate that solvent-assisted PT takes place via a sequential mechanism involving first deprotonation of the phenol/naphthol, followed by the protonation by H2O in the S1 state of phenolate/naphtholate. The site of protonation by H2O is mostly at the naphthalene α-position.

Hoping this will be helpful,

Rafik

Dear Muhammad,

The following recent published article covers the answer to your question.

DOI: 10.1039/C5SC01945A (Edge Article) Chem. Sci., 2016, 7, 655-665

Excited-state intramolecular proton-transfer reaction demonstrating anti-Kasha behavior†

Huan-Wei Tseng‡ a, Jiun-Yi Shen‡ a, Ting-Yi Kuo a, Ting-Syun Tu a, Yi-An Chen a, Alexander P. Demchenko *b and Pi-Tai Chou

We report unusual photophysical properties observed on two newly designed 3-hydroxychromone derivatives exhibiting the excited-state intramolecular proton transfer (ESIPT) reaction. The efficiency of ESIPT reaction is greatly enhanced upon excitation with high energy quanta to Sn (n > 1) levels in low-polarity solvents. Based on detailed analyses of excitation and emission spectra as well as time-resolved emission kinetics we derive that conditions, in which this phenomenon contradicting Kasha's rule is observed, are quite different from that for observation of anti-Kasha emission.

Conclusions

In this study, we strategically designed and synthesized a series of new thiophenyl 3-hydroxychromone derivatives 3-HTCA and 3-HTC-DiCN. They exhibit classical ESIPT reaction and allow observing the switching between the two reactant and product emissive forms of this reaction in a kinetically controlled regime. For 3-HTCA and 3-HTC-DiCN, we describe unusual behaviour in a display of ESIPT reaction on excitation at shorter wavelengths than the S1 band. The fluorescence intensity is re-distributed from the band belonging to initially excited normal reactant (N*) species to a reaction product phototautomer (T*) band demonstrating an increased rate of ESIPT reaction. Accordingly, the excitation spectra deviate from absorption spectra in a short-wavelength region in opposite directions when recorded at N* and T* emission bands. Moreover, a dramatic decrease of fluorescence quantum yield of N* emission and correspondent rise of T* emission quantum yield is observed.These results are in apparent contradiction to Kasha's rule. They deserve proper explanation and specifying the conditions in which such phenomenon can be observed. The sub-nanosecond time-resolved experiments demonstrate that no equilibrium between the normal form and the tautomer form is achieved during fluorescence lifetime when 3-HTCA and 3-HTC-DiCN species are excited to the S1 state. This indicates the presence of a kinetic barrier in exergonic ESIPT reaction, and it is this barrier that is probably removed or diminished on excitation to higher energy states. Applying femtosecond fluorescence up-conversion technique we obtained direct proof that the rate of overall ESIPT reaction, monitored by the normal and/or tautomer emission, is excitation wavelength dependent. This allows fast SNn → STm ESIPT to compete successfully with SNn → SN1 internal conversion. The origin of this acceleration may be related to the vanishing of solvent-reorganization barrier for the ESIPT reaction.27,30 In contrast to SN1 → ST1 charge transfer coupled ESIPT that is subject to solvent induced barrier, the SNn states of studied compounds must have different distribution of electronic density.33 This distribution may better fit excited tautomer states such that both SNn and STm possess similar equilibrium polarization, minimizing this barrier. Verification of this hypothesis needs very sophisticated computation for the congested higher energy states. The relevant theoretical approach is in progress. Evidently, the uniqueness and difference of 3-HTCA and 3-HTC-DiCNfrom previously reported ESICT/ESIPT coupled molecules27,30 lie in that the efficiency of the lowest SN1 → ST1 ESIPT process is less than unity and there is no reversibility between SN1 and ST1. In comparison, ESIPT pathways in the higher lying states have higher efficiency than that of the SN1 → ST1 ESIPT process. Accordingly, one can see the increase of the tautomer emission upon higher energy excitation in the steady-state manner, a phenomenon of anti-Kasha's rule.

In summary, using the series of dyes 3-HTCA and 3-HTC-DiCN with different electron acceptor substituents in the thiophenyl fragment we demonstrate that the switching between Kasha and anti-Kasha behaviour can be modulated in a quite delicate way by appropriate molecular design. The exploration of anti-Kasha photochemistry opens an unexpectedly broad range of possibilities of selective control on photoreactions by changing the excitation energy not only in single-photon but also in multiple-photon modes. It allows reducing the losses in energy dissipating internal conversion processes that follow the excitation by high energy quanta and presently look unavoidable in photonic devices. This can make more efficient the utilization of photoexcitation energy, which is demanding in many applications.

To view the full paper, please use the following link:

http://pubs.rsc.org/en/content/articlehtml/2016/sc/c5sc01945a

The following is another publication that provides proof for Excited state intramolecular proton transfer ESIPT :

http://www.ncbi.nlm.nih.gov/pubmed/22954380

J Org Chem. 2013 Mar 1;78(5):1811-23. doi: 10.1021/jo301456y. Epub 2012 Sep 24.

Excited state intramolecular proton transfer (ESIPT) from phenol to carbon in selected phenylnaphthols and naphthylphenols.

Basarić N1, Došlić N, Ivković J, Wang YH, Veljković J, Mlinarić-Majerski K, Wan P.

Author information

Abstract

ESIPT and solvent-assisted ESPT in isomeric phenyl naphthols and naphthyl phenols 5-8 were investigated by preparative photolyses in CH3CN-D2O, fluorescence spectroscopy, LFP, and ab initio calculations. ESIPT takes place only in 5 (D-exchange Φ = 0.3), whereas 6-8 undergo solvent-assisted PT with much lower efficiencies. The efficiency of the ESIPT and solvent-assisted PT is mainly determined by different populations of the reactive conformers in the ground state and the NEER principle. The D-exchange experiments and calculations using RI-CC2/cc-pVDZ show that 5 in S1 deactivates by direct ESIPT from the OH to the naphthalene position 1 through a conical intersection with S0, delivering QM 14 that was detected by LFP (τ = 26 ± 3 ns). ESIPT to position 3 in 5 is possible but it proceeds from a less-populated conformer and involves an energy barrier on S1. In solvent-assisted PT to naphthalene position 4 in 5, zwitterion 17 is formed, which cyclizes to stable naphthofuran photoproducts 9-12. The regiochemistry of the deuteration in solvent-assisted PT was correlated with the NBO charges of the corresponding phenolates/naphtholates 5(-)-8(-). Combined experimental and theoretical data indicate that solvent-assisted PT takes place via a sequential mechanism involving first deprotonation of the phenol/naphthol, followed by the protonation by H2O in the S1 state of phenolate/naphtholate. The site of protonation by H2O is mostly at the naphthalene α-position.

Hoping this will be helpful,

Rafik

Many Thanks, Rafik.

Dear Muhammad: 

Another way to prove ESIPT, although on a much more qualitative scale is to calculate the Stokes shift from absorption and emission spectra. In principle, for molecules exhibiting tautomer emission the shift should be greater than 10000 cm-1, strongly hinting that ESIPT is taking place