Introduction
1.2. Excited State Intramolecular Proton Transfer
1.2.1. Factors Affecting ESIPT
PT highly depends on the nature and hydrogen bonding capability of the solvent.4,5,42,66,67
Since in most of the ESIPT dyes intramolecular hydrogen bond forms a stable cyclic ring ESIPT is generally poorly dependent on viscosity. However, there has been one evidence of the strong effect of viscosity on the rate of ESIPT in polar environments.68 Studies have also been done on ESIPT in room temperature ionic liquids (RTILS), which are organic salts that are in the liquid phase at room temperature.69 RTILs possess solvation dynamics in a wide time range, and thus it is of interest to investigate the dynamics and kinetics involved in the proton transfer. The effects of solvent environment within a wide variety of pure and mixed homogeneous and microheterogeneous solvents on ESIPT in different molecules have been studied68,70-72 and a number of theoretical studies from semi- empirical to density functional theory calculations have also been performed.34,52-54,73-78
Apart from the environmental factors substitution also affects the ESIPT. The effect of solvent and substitution are briefly discussed here.
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1.2.1.1. Effect of Intermolecular Hydrogen Bond
The cyclic intramolecular hydrogen bonded ring is the prerequisite for ESIPT reaction. Therefore, intermolecular hydrogen bonding with solvent molecules is the major factor that hampers the ESIPT reaction.9,79-81 Therefore, the relative intensities of the normal and proton transferred bands of ESIPT molecules vary depending on the molecular structures and the nature of the environment surrounding the molecules mainly polarity and hydrogen bonding capacity of the solvent molecules.82 The ratio of the two emissions can range from unity up to the complete disappearance of one of the forms. The ESIPT is more favored in non-polar solvents than in protic or polar solvents.83,84
Scheme 1.11. IPT in 3-hydroxyflavone.
In polar or protic solvents like dimethylsulfoxide, methanol and water the cyclic intramolecular hydrogen bond in ESIPT molecules breaks and intermolecular hydrogen bond is formed between the proton donor/acceptor and the solvents molecules. This leads to the formation of solvated enol form resulting in the significant rise of the normal band at the cost of suppressed tautomer band. Such perturbation is greater in molecules with five-membered cyclic intramolecular hydrogen bonded ring structures as in 3-hydroxyflavone due to weak nature of intramolecular hydrogen bond.9 As discussed in Section 1.1.1 3-hydroxyflavone which has ππ* absorption band at 335 nm in non-polar solvent 2-methylbutane exhibits a single emission large Stokes’ shifted tautomer band13,14 However in methanol, in addition to green tautomer band, a violet emission from enol form is observed (Scheme 1.11).85
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X N
O H
X = NH, O, S
cis-Enol
N
Y
O H X = O, S
trans-Enol
N
N H O
H
trans-Enol Scheme 1.12. Rotameric forms of 2-(2'-hydroxyphenyl)benzazoles.
2-(2'-Hydroxyphenyl)benzazoles exists in two rotameric forms cis-enol and trans- enol (Scheme 1.12). cis-Enol forms hydrogen bond between the hydroxyl group and imidazole nitrogen atom and is responsible for ESIPT. cis-Enol is more stable than trans-enol due to its relatively stronger hydrogen bond. However the effect of protic solvent is less than that of five-membered cyclic intramolecular hydrogen bonded ring structures.
1.2.1.2. Effect of Substitution
Studies show that the fluorescence of ESIPT molecules is affected by substitution in terms of spectra position and quantum yield. Hence, introducing a suitable substituent in a specific position of the molecular framework will result in obtaining a compound with desired properties which can be utilized where ESIPT molecules find applications. Since ESIPT prone molecules find vast applications as laser dyes, probes, sensors, photostabilizers, and molecular devices, it is important to alter molecular structure to get the desired properties. One way is to involve modification of the molecular framework by substituents thus affecting the basicity and/or acidity of the reaction centers. On the other hand substitution within the molecular skeleton can be treated as selective perturbation, and its action can be helpful in elucidation of the reaction mechanisms. A few examples of substitution effects are briefed below:
Salicyl Derivatives
Since the pioneering work on the ESIPT reaction in methyl salicylate by Weller,86 a vast number of works have been performed on the molecule and its derivatives to study the fundamental aspects and to explore the applications of ESIPT. Substitution of methoxy group at position 5 in salicylic acid and at positions 3 or 4 or 5 in methyl salicylate results in large Stokes shift of the phototautomer
O H
O O
R
R = H, Salicylic acid R = Me, Methyl salicylate
1
2 3 4 5
6
Scheme 1.13. Salicylic acid and methyl salicylate.
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emission in nonpolar solvents (Scheme 1.13).86,87 Further, substitution at position 5 also causes a decrease in the intensity of the phototautomer emission with an increase in the intensity of the fluorescence from the primary form.
The effect of methyl and methoxy substitution on the fluorescence of salicylic acid at position 5 has also been studied in supersonic molecular beams.88 The phototautomer emission of methyl substituted compound is strongly red shifted as high as 10,000 cm-1. However, for methoxy substituted compound the fluorescence is red-shifted only 3000 cm-1 indicating the ESIPT reaction is suppressed which is due to the electron donating effect of the methoxy group at the para position to the hydroxy group.
The effect of substitution on ESIPT in salicyldehyde and its derivatives was also studied (Scheme 1.14).89 Replacing R and R' by electron withdrawing group decreases the quantum yield of the phototautomer compared to the parent molecule. But substitution with an electron donating group increases the quantum yield.
O H
O H R'
O H
O R
R = -CF3, -CHCl3, -CH2Cl (Withdrawing)
R = -CH3, -C2H5, -OCH3, -OH, -NH2 (Donating)
R' = -NO2, -CN (Withdrawing) R' = -Cl, -CH3, -OCH3 (Donating)
Scheme 1.14. Derivatives of salicyldehyde.
2-Amino-3-naphthoic acid and its methyl ester (Scheme 1.15) also show excited state prototropism similar to those of salicylates.90 The conformer I undergoes ESIPT to give the tautomer emission while the conformer II gives the normal emission. The conformer II exists as a closed ring structure in non-polar solvent and an open structure in protic solvents.
N
O
OH H H
Conformer I
N
O
O H H
H
Conformer II Scheme 1.15. 2-Amino-3-naphthoic acid.
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Hydroxybenzo[h]quinoline
Spectral tuning is also displayed by derivatives of 10-hydroxybenzo[h]quinoline with different electron donating and accepting substituents (Scheme 1.16).12 The frontier molecular orbitals of the enol form are delocalized over the whole molecule.
However, in the PT keto form the highest
occupied molecular orbital (HOMO) is localized at the cyclohexa-2,4-dienone (Part A) and the lowest unoccupied molecular orbital (LUMO) at pyridine (Part B). Therefore, substituting electron withdrawing group at Part A moiety decreases the energy of HOMO while an electron donating group in Part B raises the LUMO both of which result in increase in transition energy of the keto form. Substituting an electron donating group to Part A decreases the transition energy of the keto form.
Chromones and Flavones
Klymchenko et al. have shown that modulation of solvent dependent dual emission and sensing properties of 3-hydroxychromones can be obtained by the proper choice of subsitutents on the 3-hydroxychromone framework (Scheme 1.17).91 The ESIPT rate of 3- hydroxyflavone, the phenyl ring substituted derivative of 3-hydroxychromone at position 2, is slowed down compared to that of the parent molecule.92 The ortho aryl substitution to hydroxyl group enhances the electron donating ability thus lowering the transfer ability of hydrogen atom. When the phenyl ring is replaced by the bigger naphthyl ring, the rate decreases further.
O
O
O H 1
2 3 5 4
6 7
8
3-Hydroxychromone
O
O
O H 3-Hydroxyflavone Scheme 1.17. 3-Hydroxychromone and 3-hydroxyflavone.
3-Hydroxyflavone displays laser action.9,42 However, in 2-methyl-3- hydroxychromone the lasing activity cannot be achieved.93 The authors explained this
N H O
B
A Scheme 1.16. Hydroxybenzo[h]quinoline.
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observation in terms of fluorescence quenching due to molecular aggregation and transient parasitic absorption of the phototautomeric species. Besides, in flavonols (such as quercetin, and 3,5,7,3',4'-OH flavone), where 5-OH and 3-OH groups are simultaneously present, the hydrogen bond between the carbonyl oxygen and hydroxyl group of C-5 position interferes with that of carbonyl oxygen and hydroxyl group of C-3 position.94 This prevents the ESIPT and consequently lowers the fluorescence quantum yield.
Azoles and Benzazoles
2-(2'-Hydroxyphenyl)benzimidazole (HPBI) exhibits single largely Stokes shifted tautomer emission in nonpolar hydrocarbons and dual emission consisting of normal and tautomer emissions in protic solvents.95 But its analogues 2-(2'-hydroxyphenyl)benzoxazole (HPBO) and 2-(2'-hydroxyphenyl)benzthiazole (HPBT) emit dual emission even in non-polar hydrocarbons.67,95-100 The normal emission is due to the trans-form (Scheme 1.12). Similar behavior is observed in their 2-hydroxynaphthyl derivatives also (Scheme 1.18).101,102
N
X H O
X = NH, O, S