Slow excited state phototautomerization in 3-hydroxyisoquinoline

Photochemical &Photobiological Sciences

PAPER

Cite this: Photochem. Photobiol. Sci.,2014, 13, 929

Received 9th October 2013,Accepted 10th March 2014

DOI: 10.1039/c3pp50356a

www.rsc.org/pps

Slow excited state phototautomerization in3-hydroxyisoquinoline

Neeraj Kumar Joshi,†a Priyanka Arora,a Sanjay Panta and Hem Chandra Joshi*b

In the present work we report the spectral and photophysical properties of 3-hydroxyisoquinoline in

various protic/aprotic solvents. Our steady state and time resolved fluorescence data indicates that in the

monomer form of 3HIQ phototautomerization can take place in the excited state through excited state

intramolecular proton, while as per earlier suggestions phototautomerization in 3HIQ occurs in dimer or

complex (in the presence of acetic acid) form. Moreover, we find rather slow tautomerization (occurring

on the nanosecond scale). It is found that proton transfer occurs both in the ground as well as excited

states and is controlled by the polarity of the solvent.

1. Introduction

For the aromatic molecules which contain proton donor andacceptor sites, transfer of protons from the donor to the accep-tor group in the excited state can occur in the gas phase andalso in the condensed phase depending upon the surroundingmedium/solvent.1–5 In general, ESIPT reactions are believed tobe very fast (occurring on the femtosecond scale);6–8 however,in some cases slower ESIPT has been reported.9 In the case ofaprotic solvents or in the gaseous phase, direct excited stateintramolecular proton transfer (ESIPT) can occur. On the otherhand, in protic solvents the mechanism of proton transferbetween distant groups can become feasible if bridging by thesolvent is facilitated.10–12

3-Hydroxyisoquinoline (3HIQ) belongs to the class offamily in which proton donor and acceptor groups are presentin the same moiety. In 3HIQ (Scheme 1a), the hydrogen (H18)attached to oxygen (O17) has two possible choices – either itcan migrate to the adjacent carbon (C10) to form its keto struc-ture (Scheme 1b) (keto–enol tautomerization) or attach withnitrogen (N16) to form its lactam form (Scheme 1c) (lactim–

lactam tautomerization). It can be mentioned that the ketoform is energetically less favoured in the ground state.13

The first report on the lactim–lactam tautomerization of3HIQ was published by Evans et al.13 in 1967, in which theyreported that in the ground state, 3HIQ can exist in the pure

lactam form in water whereas in diethyl ether it exists in thelactim form. In other solvents including non-polar solvents(benzene, carbon tetrachloride, and chloroform) and otherprotic (ethanol)/aprotic (DMSO) polar solvents, 3HIQ exists asa mixture of lactim and lactam. Wei et al. found that in non-polar solvents, at low concentrations (<10−4 M) only the lactimform is present and essentially ESIPT is absent.14 However,with an increase in concentration or with the addition ofacetic acid, signatures of the lactam form are found.14 Further-more, they had not observed any rising portion when thedecay was monitored at the lactam emission, indicating ratherfast excited state tautomerization.

Furthermore, the mechanism of tautomerization in the firstexcited state of 3HIQ as well as in 1 : 1 mixtures of water andacetic acid was theoretically studied at the CIS/6-31G(d) levelby Ramos et al.15 They proposed that in the presence of a guestmolecule, proton transfer is a tunnelling process. An energybarrier of 51.1 kcal mol−1 was predicted for tautomerization of

Scheme 1 Various forms of 3HIQ.

†Present address: Molecular Photoscience Research Center, Kobe University,1-1 Rokkodai, Nada, Kobe 657-8501, Japan.

aPhotophysics Laboratory, Department of Physics, DSB Campus, Kumaun University,

Nainital 263002, Uttarakhand, IndiabInstitute for Plasma Research, Laser Diagnostics Division, Bhat, Near Indira Bridge,

Gandhinagar 382428, Gujarat, India. E-mail: [email protected];

Tel: +91-7923962056

This journal is © The Royal Society of Chemistry and Owner Societies 2014 Photochem. Photobiol. Sci., 2014, 13, 929–938 | 929

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the isolated 3HIQ in the ground state, while the barrier waslowered to ∼40 kcal mol−1 in the 1 : 1 mixture with acetic acid.

Gerega et al. investigated the stabilities of lactim andlactam forms experimentally and theoretically.16 They did notobserve the lactam form in the experimental IR spectra in Armatrices but it was theoretically found to be ∼30 kJ mol−1

higher in energy than the lactim at the QCISD/cc-pVDZ andQCISD-(T)/cc-pVDZ levels based on the geometries at theB3LYP/ccpVDZ level. Their results reveal links between aroma-ticity and tautomerism. Recently, Pan et al. studied the singlephoton ionization of the gas phase 3HIQ by means of VUV syn-chrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer.17 Their work was focused onthe spectroscopy of the cationic form of 3HIQ (lactim+ andlactam+). They inspected the equilibrium geometrical para-meters of 3-hydroxyisoquinoline and its cations at the PBE0/6-311+G(d,p) level and observed that lactam+ is more stable ascompared to the lactim+ by 0.27 eV.

It is evident from the above discussion that the tauto-merism of 3HIQ is quite complex. Also, most of the studies on3HIQ, however, focus on theoretical calculations14–17 in thegaseous phase16,17 and in nonpolar solvents.14 In order to gainfurther insight into the tautomerization in the excited state(from the view point of temporal behaviour), in the presentwork, we have undertaken a systematic study by consideringaprotic as well as protic solvents. The important observation isthat the proton transfer resulting in lactim/lactam transform-ation takes place both in the ground state as well as in theexcited state. Unlike earlier observations, clear signatures ofESIPT are noticed in solvents taken for the study indicatingthat the solvent polarity induced ESIPT. Moreover, we havealso observed slow excited state photo-tautomerization for thissystem which was not noticed earlier.

2. Experimental section2.1. Materials

3HIQ (obtained from Aldrich, 99.0%) was tested for their fluo-rescence purity and used as such. All the solvents used wereeither of spectroscopic grade or were checked for their fluo-rescence purity. Dehydrated solvents were used in the study.The content of water is 0.05% in MeOH, 0.1% in DEE and0.05% in ethyl acetate. Deionized water was used in the study.Freshly prepared pH solutions were used for steady state aswell as time-resolved measurements. pH of the solution wasmonitored with a digital pH meter (Model: pHep, Accuracy:±0.1, Hanna Instruments).

2.2. Instrumentation

Steady state absorption spectra, at room temperature, wererecorded using a dual beam JASCO V-550 spectrophotometer.The excitation and emission spectra were recorded using aJASCO FP – 777/Varian Cary Eclipse spectrofluorimeter anddata were analyzed using the related software. Fluorescencedecay times were recorded with the help of the Edinburgh-199

time domain spectrometer and analyzed using the TCC – 900and FAST software. The excitation source was a thyratron-gatedhydrogen filled nanosecond flash lamp. The lamp profile wasmeasured at the excitation wavelength using a Ludox scatterer.The pulse width of the ns flash lamp was about ∼1.0 ns with arepetition rate of 30 kHz. A pulsed LED (340 nm and 375 nm,JobinYvon) and a pulsed diode laser (405 nm) were also usedas excitation sources. The pulse width of the LED was about∼0.6 ns with a repetition rate of 1 MHz. The pulse width of thediode laser was about ∼0.3 ns with a repetition rate of 1 MHz.The time correlated single photon counting (TCSPC) techniquewas used to collect the decay curves. Care was taken in dataanalysis to differentiate between the mono-exponential and bi-exponential fits by judging (goodness of fitting) reduced chi-square (χ2) values, standard deviations and weighted residuals.Time-resolved fluorescence decay curves were analyzed bydeconvoluting the observed decay with the instrumentresponse function (IRF) to obtain the intensity decay functionrepresented as a sum of discrete exponentials:

Iðτ; tÞ ¼Xi

αie�t=τi ð1Þ

where I(t ) is the fluorescence intensity at time t and αi is theamplitude of the ith lifetime such that ∑iαi = 1.

Quantum yields (Q.Y., φ) were determined using quininesulfate in 0.5 M H2SO4 (φref = 0.55) as the reference and usingthe following relation:

φ ¼ Aref Isns2

AsIrefnref 2� φref ð2Þ

where Is and Iref are the integrated fluorescence intensities,and As and Aref are the absorbances, and ns

2 and nref2 are the

refractive indices of the sample and reference, respectively.Radiative lifetimes were estimated by the well-known

Strickler–Berg formula18 as follows:

1τr

¼ 2:88� 10�9 � n2 �ÐIvdvÐ Ivv3dv

�ðεvdðln vÞ ð3Þ

where τr is the radiative lifetime, n is the refractive index of thesolvent, I is the fluorescence intensity, ε is the molar extinctioncoefficient (in M−1 cm−1) and v is the wavenumber (in cm−1).Using the radiative lifetime and quantum yield, the fluo-rescence lifetime (τ) is calculated by the formula as follows:

φ ¼ τ

τr; thus τ ¼ φ� τr ð4Þ

2.3. Estimation of the ground and excited state dipolemoments

Geometry optimization and the ground state dipole moments(µg) of various observed species of 3HIQ are estimated with the6-31G* basis set at the DFT-B3LYP level. The excited statedipole moment is estimated at the CIS level using the 6-31G*basis set. All the computations are performed using theGaussian 03 program.

Paper Photochemical & Photobiological Sciences

930 | Photochem. Photobiol. Sci., 2014, 13, 929–938 This journal is © The Royal Society of Chemistry and Owner Societies 2014

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3. Results and discussion

Steady state and time resolved experiments for 3HIQ indifferent polar aprotic and polar protic solvents were carriedout at 298 K. Steady state data are summarized in Table 1.

3.1. Absorption study

3.1.1. In polar protic solvents. In water (pH = 7.0), 3HIQexhibits an absorption maximum at 395 nm (Fig. 1), whichdoes not depend on the concentration. The observed absorp-tion peak in water matches well with the reported value byEvans et al.13 We also recorded spectra in acidic (pH = 1.0) andbasic (pH = 13.0) solutions and the spectral features are shownin Fig. 2. They show distinct features as compared to thatobserved at pH = 7 and hence the presence of cations as wellas anions can be ruled out in water. Absorption maxima atpH 1 and pH 13 can be attributed to cationic (protonated) andanionic (deprotonated) forms, respectively. In analogy with ref.13, it can be believed that in water (pH 7) the absorption

maximum is due to the lactam form (Scheme 1c). The lactamform may be described as a combination of lactam(Scheme 1c-i) and zwitterionic resonance structures(Scheme 1c-ii). In the ground state, the lactam resonance struc-ture is likely to best represent the electronic structure of 3HIQ.It is possible also because water has exceptional proton donat-ing and accepting properties and the proton can migratethrough the proton wire in water.19,20 This suggests that 3HIQexists in a lactam form (Scheme 1c) near pH 7. At the sametime, from the study of the prototropic behaviour of hydroxy-quinolines, Mason et al.21 have also reported that at certainpH values, zwitterions should be present.

In alcoholic solvents, i.e. methanol (MeOH), ethyl alcohol(EtOH) and butanol (BuOH), the absorption spectra of 3HIQare shown in Fig. 1. In all alcoholic solvents, similar absorp-tion spectra were observed. In MeOH, at sufficiently low con-centrations (O.D. < 0.08, ∼2 × 10−5 M), two absorption bandsat ∼340 nm and ∼405 nm are observed (Fig. 1). In all alcoholicsolvents, the longer wavelength absorption band (peak centredat 405 nm) has a broad profile with vibronic structural featuresat ∼425 nm.Fig. 1 Absorption spectra of 3HIQ in protic solvents.

Fig. 2 Absorption and emission spectra of 3HIQ at pH 1, 7 and 13.

Table 1 Physical properties of the solvents and photophysical parameters of 3HIQ in different solvents. The fluorescence quantum yield (Q.Y.) wasmeasured using eqn (2)

Solvents ENTb αb βb λabs/nm λem/nm

Q.Y.a

(V band)Q.Y.a

(BG band)

DEE 0.117 0.0 0.47 340 380, 517 0.12 0.06EA 0.228 0.0 0.45 340, 425 378, 517 0.06 0.04DCM 0.309 0.10 0.13 340, 420 372, 516 0.09 0.06MeOH 0.586 0.69 0.80 345, 406 381, 500 0.14 0.17EtOH 0.654 0.86 0.75 345, 406 381, 500 0.09 0.09BuOH 0.762 0.98 0.66 345, 406 380, 500 0.12 0.13Water (pH = 7) 1.00 1.17 0.47 395 485 0.11Acid (pH = 1) — — — 362 485 0.13Base (pH = 13) — — — 372 470 0.32

a In the solvents where dual emission bands are observed, the V band corresponds to the normal emission band (higher energy) and the BGband corresponds to the large Stokes shifted emission band (lower energy). b ENT is a microscopic solvent polarity parameter, α is the hydrogenbond donating ability of the solvent, and β is the hydrogen bond accepting ability of the solvent. Values were taken from ref. 24–26.

Photochemical & Photobiological Sciences Paper


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Due to the very low concentration of the solute, here wediscard any self-association of 3HIQ as the origin of the405 nm absorption band (which is relatively red shifted com-pared to water). Moreover, we did not observe any change inthe 405 nm band with the increase in concentration, whichrules out its origin in aggregates. In analogy with the literature,the 405 nm band can be attributed to the lactam form of 3HIQand the ∼340 nm band is due to the lactim form.13

3.1.2. In polar aprotic solvents. The absorption spectrumof 3HIQ in diethyl ether (DEE) exhibits a maximum at∼340 nm (Fig. 3), which also resembles the reported value byEvans et al.13 On the other hand in other aprotic solvents viz.ethyl acetate (EA) and dichloromethane (DCM), in addition tothe ∼340 nm absorption band, in the longer wavelength regiona broad absorption with a peak centred at ∼425 nm wasnoticed (Fig. 3). In EA and DCM, similar to alcoholic solvents,the longer wavelength broad absorption band also possesses avibronic structural feature at ∼405 nm. As in the case of alco-holic solvents, this band can be assigned to the presence ofthe lactam form in the ground state. Interestingly this bandappears when the solvent polarity is increased (absent in thecase of DEE). The cause of the appearance of this band will bediscussed in a later section.

Evans et al.13 have reported that in DEE only the lactimform exists in the ground state with the absorption band at∼340 nm, while in other solvents different ratios of the lactamto the lactim were observed. Our results are in conformity withthe suggestions of Evans et al.13

However, according to Wei et al., in cyclohexane (CH) atonly higher concentrations of 3HIQ (1.2 × 10−4 M or higher), anew absorption band at 420 nm was observed and they attribu-ted this band to the keto–enol complex.14 Here it can bementioned that at low concentrations (<10−4 M; the used con-centration in our study) we did not observe the long wave-length absorption band in DEE. In our study the concentrationof 3HIQ in DEE was ∼10−5 M and it is unlikely that therewould be a case of self-association.

3.2. Fluorescence study

3.2.1. In polar protic solvents. In water, the emissionmaximum was independent of the excitation wavelength, andthe emission maximum is observed at ∼485 nm (Fig. 4). Theshape of the excitation spectrum is also found to be indepen-dent of the monitored emission wavelength and it resemblesthe absorption spectrum (Fig. 3).

For further investigation on the excited state dynamics inwater, fluorescence decays were recorded for the entire emis-sion profile with λex = 340 nm, 375 nm and 405 nm. No signifi-cant changes were noticed in fluorescence decay curvesirrespective of any excitation/monitored emission wavelength.All these fluorescence curves were best fitted with the singleexponential decay function and the recovered decay time (τ) is∼6.6 ns. The lifetime calculated from eqn (4) also comes to5.1 ns (Table 4), which is in agreement with it within experi-mental limitations and also limitations of the Strickler–Bergformula.18 It can be noted here that the decay is mono-expo-nential at pH = 1 and pH = 13 also with the respective decaytimes of 6.9 ns and 16.8 ns. The calculated (from eqn (4))decay times in pH 1 and pH 14 are 5.5 ns and 18.7 ns, respecti-vely, which are again in agreement within limitations. Thisindicates that in water at pH = 7, single species of 3HIQ existsboth in the ground as well as in the excited state. As men-tioned earlier, and from this observation, it is likely that thelactam form is the emitting species and has a decay time of∼6.6 ns.

Emission profiles of 3HIQ in alcoholic solvents are shownin Fig. 5. Spectral features of emission spectra in all alcoholicsolvents are very similar. In alcoholic solvents at λex = 335 nm,a large Stokes shifted blue green (BG band) band at ∼500 nmalong with a normal band at ∼380 nm assigned hereby asviolet (V band) was observed (inset of Fig. 5). On a careful lookat the BG band, a hump (vibronic structure) is also observed at∼475 nm, in all alcoholic solvents. No effect of the excitationwavelength on the peak position of V and BG bands wasnoticed (a case of MeOH is shown in the inset of Fig. 4). Atfirst glance (inset Fig. 4), it appears that the absorption at

Fig. 4 Excitation and emission spectra of 3HIQ in water, λem = 490 nm(a), 460 nm (b), 530 nm (c), 590 (d) and λex = 390 nm (i), 405 nm (ii),345 nm (iii), 430 (iv).

Fig. 3 Absorption spectra of 3HIQ in aprotic solvents.



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∼340 nm (in MeOH) results in V and BG bands while the BGband originates due to the longer wavelength broad absorptionband (∼405 nm).

To know the origin of these V and BG bands, excitationspectra were monitored corresponding to these emissionbands (Fig. 6). Interestingly, when emission was monitored forthe BG band, in excitation spectra (Fig. 6) two bands werenoticed at ∼340 nm and ∼405 nm, which are identical to theabsorption spectrum of 3HIQ in MeOH (Fig. 1). Furthermore,when the excitation spectrum was monitored for the V band,only a band at ∼340 nm was observed (Fig. 6).

From Fig. 5 and 6, it can be suggested that the lactim form(which absorbs at ∼340 nm) emits as a V band (∼380 nm) as anormal Stokes shifted emission and also undergoes an excitedstate intramolecular proton transfer reaction (ESIPT) in theexcited state to form a lactam. Here we rule out the possibilityof any other species being the origin of the BG band as theexcitation spectra for the BG band are identical. Therefore, itcan be inferred that they originate from the lactim form(through ESIPT) and also direct excitation of the lactam form.

Dipole moments for both lactim and lactam forms weredetermined theoretically (DFT-B3LYP/6-31G*). It is found thatthe ground state dipole moment for the lactam form (6.2 D) isconsiderably higher as compared to the lactim form (1.9 D). Inthe excited singlet (S1) state, there is an increase in the dipolemoments for both lactim and lactam forms which are 2.6 Dand 7.5 D, respectively.

To investigate the mechanism of transformation fromlactim to lactam forms in the excited state, fluorescence decaysfor the entire emission profile were recorded at λex = 340 nmand 405 nm (in MeOH). Decay data are summarized inTable 2. Fluorescence decay curves are shown in Fig. 7.

In MeOH, at λex = 340 nm, in the wavelength region 380 nmto 440 nm, two decay components 0.2 ns and 4.3 ns areobserved and beyond 460 nm a rise time (∼4 ns) with a decay-ing component of ∼6.2 ns is observed. This rise time can beclearly visualized in fluorescence decay curves (Fig. 7). On theother hand, at λex = 405 nm, a decay time of ∼6.2 ns isobserved, which remains constant regardless of the emissionwavelength. As the decay at shorter emission wavelengths fitswith two exponentials and at longer wavelengths a rising com-ponent is observed, at first sight it may appear that there is anexcited state reversible reaction. However, looking at the com-

Fig. 6 Excitation spectra of 3HIQ in MeOH.

Fig. 5 Emission spectra of 3HIQ in different alcoholic solvents. (Inset:emission spectra of 3HIQ at different excitation wavelengths in MeOH).

Table 2 Fluorescence decay data of 3HIQ in MeOH

λex/nm λem/nm τ1a τ2

a α1c α2

c χ2 b

340 380 0.19 (0.021) 4.33 (0.014) 0.57 0.43 1.02420 0.18 (0.014) 4.32 (0.016) 0.58 0.42 1.01440 0.21 (0.017) 4.80 (0.031) 0.53 0.47 1.01450 0.26 (0.019) 6.20 (0.037) 0.40 0.60 1.03465 3.75 (0.081) 6.50 (0.027) −0.43 1.0d 1.01475 3.86 (0.051) 6.37 (0.046) −0.53 1.0d 1.04500 4.34 (0.067) 6.14 (0.035) −0.66 1.0d 1.00530 4.48 (0.042) 6.21 (0.058) −0.72 1.0d 1.00

405 475 — 6.19 (0.025) — 1.0 1.03500 — 6.18 (0.047) — 1.0 1.00550 — 6.18 (0.026) — 1.0 1.01

aDecay time (τ) is in ns. Standard deviation is given in parentheses.b χ2 is the chi-square value for best fit either for single exponential fitor for double exponential fit. c α’s are relative amplitudes. d Scaled to 1.

Fig. 7 Fluorescence decay profile of 3HIQ in MeOH.



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ponents it is evident that the shorter component observed forthe V band (∼0.2 ns) is not present at longer wavelengths (BGband) as a rising or decaying component. Hence it appearsunlikely that there is a reversible reaction. In the case of revers-ible reaction, V and BG bands would have identical com-ponents. V should show two decay components and BG shouldshow one rising and one decay component. Moreover, theamplitude corresponding to this component decreases whilegoing towards longer wavelengths. Hence the shorter decaytime of ∼0.2 ns can be attributed to the lactim form whichdoes not convert to the lactam form, i.e., the anti-lactim form(Scheme 1d). In time resolved experiments care was taken toavoid scattered light, therefore experimental artefacts cannotbe considered for the shorter component. It can be mentionedthat in this form, the OH proton should be in the anti-configur-ation with respect to nitrogen (Scheme 1d), which is hydrogen-bonded to traces of water.22 Another possibility for short decaycomponents (∼0.2 ns) would be due to efficient internal con-version via hydrogen bonding, assuming that the OH anti-form is not hydrogen bonded. Here it can be mentioned thatthe absence of this short component in non-polar CH may beeither due to its very short value or due to the low quantumyield in this solvent.

It can also be argued that conversion from lactim to lactamforms may be a two-step process i.e., dissociation of hydrogenfrom the OH group and in the second step association (gemi-nate recombination) with nitrogen to form a lactam. However,we did not notice signatures of anions (dissociation of hydro-gen) in the spectra or in the decay behaviour. It is likely thatthe second step in the process of association may be very fastand hence we could not observe the anion. In such a case, ofcourse, the rate limiting step will be the first one, i.e. thedissociation process. Moreover, a power law dependence23 isnot noticed in the decay curves of both lactam and lactimemissions.

In fact, the other decay component (4.3 ns) matches withthe rise time observed at longer wavelengths. Hence this com-ponent should correspond to those lactim conformers whichconvert into lactams. The longer decay component observedfor BG emission should correspond to the intrinsic decay time(∼6.2 ns) of the lactam form. Interestingly, from Fig. 6 andTable 2, there is clear evidence of slow excited state conversionbetween lactim to lactam forms. Using eqn (4), the calculatedlifetime of the lactam and lactim form turns out to be 7.5 nsand 3.6 ns, respectively (Table 4). These values are ingood agreement with the assumptions and limitations ofeqn (3).18

In MeOH, the rate of tautomerization between lactim andlactam forms is estimated by assuming that there is no tauto-merization in the case of cyclohexane (CH) at low concen-trations.14 The decay time at low concentrations in CH is 6.62ns, which corresponds to the intrinsic decay time of the lactimform (as no tautomerization is observed).14 Here we can con-sider that the appearance of the rise time at longer wave-lengths is mainly due to tautomerization. As the rise time doesnot correspond to the tautomerization time (but to the decay

time of the precursor), the rate of tautomerization can be esti-mated by

ktaut ¼ 1τrise

� 1τlactim

ð5Þ

which comes about 0.8 × 108 s−1.In the foregoing discussion we see that the rise time

decreases with the solvent polarity. The larger rise time in thecase of MeOH indicates the role of intermolecular hydrogenbonds in slowing down the photoreaction (ESIPT) in proticmedium. If the alcohol really helped the reaction, the rise timeobserved for the phototautomer emission would have beenshorter. Moreover, since methanol is polar, this should havealso stabilized the product and led to a shorter rise time.However, a longer rise time indicates that due to intermole-cular hydrogen bond formation with MeOH, the reaction getsdelayed (slowed down).

3.2.2. In polar aprotic solvents. The fluorescence spectrumof 3HIQ in DEE is depicted in Fig. 8. At λex = 335 nm, a normalfluorescence band at ∼380 nm (V band) and the large Stokesshifted emission (lower energy emission) at ∼520 nm (BGband) with a hump at ∼485 nm are observed. The observedfluorescence maxima are found to be independent of the exci-tation wavelength (Fig. 8). It is also interesting to note that onexcitation beyond 370 nm, no emission was observed. Theabsence of fluorescence on longer wavelength excitation(beyond 370 nm) rules out the possibility of the availability ofthe ground state lactam form or self-associated form of 3HIQ.Thus, it is an indication that the origin of V and BG bands isonly in the lactim form. It is to be noted here that at low con-centrations in CH we could also notice only the V band in con-formity with the results of Wie et al.14 Thus, it is clearlyevident that the presence of the BG band in DEE indicates therole of ESIPT facilitated by solvents. It appears that theincrease in polarity brings about the BG band in the emission.

For confirming the above fact, excitation spectra (in DEE)were also monitored at different emission wavelengths. Exci-

Fig. 8 Emission spectra of 3HIQ in DEE.



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tation spectra (Fig. 9) monitored at different emission wave-lengths are identical and match with its absorption spectrum.From Fig. 9, three facts are very clear, (i) the origin of V andBG bands is in the lactim form only, (ii) large Stokes shiftedbands (BG) do not originate from the ground state lactamform and, (iii) there is an excited state reaction which resultsin a large Stokes shifted emission band (BG).

Had there been some other ground state species (likelactam and/or self-associated forms of 3HIQ) as the origin ofthe BG band, there should have been some signatures of thoseground state species and they should have appeared in theexcitation spectra. On the basis of steady state observations inDEE, it can be concluded that only the lactim form exists inthe ground state which absorbs at ∼340 nm, and upon photo-excitation, the lactim form emits at ∼380 nm. Moreover, in theexcited state it undergoes an excited state reaction (ESIPT)which results in large Stokes shifted emission giving rise tothe BG band. Also, the observed rise time (Fig. 10, Table 3) onmonitoring the emission at the BG band is clear and firmproof of excited state reaction in DEE (discussed in detail

later). These results are worth noting and are in sharp contrastto the previous reports stating that for the occurrence of anexcited state reaction (tautomerization) in 3HIQ,14,15 either aguest molecule is required or it should be self-associated(dimer).

In the case of DEE, the excited state reaction is the excitedstate intramolecular proton transfer (ESIPT), which results inthe lactam form in the excited state. The decay data (Table 3)clearly indicate a slow ESIPT from lactim to lactam forms.Lactim to lactam transformation can occur via ESIPT reactionin DEE where only lactim (monomer) exists in the groundstate.

At λex = 340 nm, in the emission wavelength region 380 nmto 460 nm, two decay components with ∼0.3 ns and ∼3.0 nsare observed (Table 3). At longer emission wavelengths a risingcomponent of ∼3.1 ns is observed. These results indicate thatESIPT takes place to form lactam from lactim. Again one of thecomponents (∼3.0 ns) is identical to the rise time observed forthe BG band, indicating that two types of lactim forms arepresent – one of those forms which undergo ESIPT and has alonger decay time (corresponding to rise time for BG) andthe other form which does not undergo ESIPT (decay time∼0.3 ns). The calculated lifetime (using eqn (4)) of the lactimform in DEE is 2.4 ns (Table 4), which seems to be consistentwith the experimental value of lactim.

Fig. 9 Excitation spectra of 3HIQ in DEE.

Fig. 10 Fluorescence decay profile of 3HIQ in DEE.

Table 3 Fluorescence decay data of 3HIQ in DEE, λex = 340 nm

λem/nm τ1a τ2

a α1c α2

c χ2 b

380 0.28 (0.018) 2.63 (0.021) 0.30 0.70 1.01400 0.26 (0.017) 2.67 (0.024) 0.27 0.73 1.05440 0.27 (0.021) 2.65 (0.019) 0.20 0.80 1.02450 0.38 (0.023) 2.96 (0.022) 0.12 0.88 1.03460 — 4.34 (0.018) — 1.0 1.02470 2.97 (0.031) 4.21 (0.026) −0.76 1.0d 1.01480 3.20 (0.028) 3.70 (0.024) −0.96 1.0d 1.04500 3.30 (0.026) 3.80 (0.020) −0.89 1.0d 1.02525 3.10 (0.029) 3.70 (0.021) −0.96 1.0d 1.01


Table 4 Value of the radiative lifetime (τr), the calculated lifetime (τcal)and the experimental value of lifetime (τexp) of different species atdifferent media

Solventλabs/nm

Assignedspecies

ε/M−1

cm−1 Q.Y.a τrb/ns τcal

c/ns τexpd/ns

pH 1 360 Cation 3950 0.13 42.0 5.5 6.9pH 7 395 Lactam 3200 0.11 46.6 5.1 6.6pH 13 370 Anion 2480 0.32 58.6 18.7 16.5MeOH 340 Lactim 2820 0.14 25.4 3.6 4.3

406 Lactam 3790 0.17 44.3 7.5 6.2DEE 340 Lactim 4100 0.12 20.3 2.4 3.1

a From eqn (2). b From eqn (3). c From eqn (4). d From eqn (1).



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The shorter component should correspond to the lactimform in which the OH proton is in the anti-configuration withrespect to nitrogen22 as mentioned earlier. The tautomeriza-tion rate from eqn (5) comes out to be 1.8 × 108 s−1.

Furthermore, the emission spectrum of 3HIQ in EA andDCM is shown in Fig. 11. In both solvents (EA/DCM), at λex =335 nm, similar to DEE, V (∼375 nm) and BG (∼517 nm)bands are observed. The slight blue shift in V and BG com-pared to DEE may be due to the difference in the polarity ofthe solvents. Similar to DEE, the emission spectrum of 3HIQin EA/DCM is independent of the excitation wavelength, butin contrast to DEE, on excitation at longer wavelengths(>370 nm), an emission spectrum with the BG band isobserved. On excitation at longer wavelengths, it appears thatthere is direct excitation of ground state lactam tautomersgiving rise to the emission from the BG band. It is similar situ-ation which was observed in MeOH. This lactam conformer islikely to come from ground state intramolecular proton trans-fer (GSIPT).

To probe the situation, excitation spectra at various emis-sion wavelengths were monitored in EA and DCM. In boththe solvents, excitation spectra exhibit similar behaviour.Excitation spectra of 3HIQ in EA are shown in Fig. 12. Thefluorescence decay profile is shown in Fig. 13.

Like MeOH and unlike DEE, EA/DCM possesses the lactamform in the ground state. Unlike DEE, it appears that due tothe increased polarity of EA and DCM, the ground state intra-molecular proton transfer (GSIPT) mechanism may be respon-sible for the observed lactam form. Here it is worthmentioning that the ground state dipole moment of thelactam form is considerably larger and hence its presence isnoticeable in solvents of higher polarity. Similar to DEE andMeOH, ESIPT is observed. Rise time and excitation spectrasupport this observation.

However, the rise time observed in the case of EA is smallerthan in the case of DEE. The estimated tautomerization rate

from eqn (5) is ∼5 × 108 s−1. It is likely that the ESIPT reactionmay become faster in the case of EA and hence shows ashorter rise time (Table 5) and hence a higher tautomerizationrate. Schematically it can be seen that the polarity brings

Table 5 Fluorescence decay data of 3HIQ in EA

λex/nm λem/nm τ1a τ2

a α1c α2

c χ2 b

340 375 1.25 (0.021) 7.0 (0.152) 0.91 0.09 1.03400 1.27 (0.027) 7.3 (0.127) 0.90 0.10 1.01430 1.24 (0.019) 6.5 (0.113) 0.90 0.10 1.01485 1.44 (0.028) 4.12 (0.071) −0.81 1.0d 1.06525 1.41 (0.023) 4.54 (0.088) −0.88 1.0d 1.02

405 485 — 4.42 (0.092) — 1.0 1.01525 — 4.42 (0.103) — 1.0 1.02550 — 4.42 (0.087) — 1.0 1.02


Fig. 11 Emission spectra of 3HIQ in EA and DCM.Fig. 12 Excitation spectra of 3HIQ in EA.

Fig. 13 Fluorescence decay profiles of 3HIQ in EA.



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about a decrease in barrier and thus facilitate GSIPT as well asfaster ESIPT (Fig. 14).

4. Conclusion

We have systematically investigated the spectral and timeresolved properties of 3HIQ in solvents with varying polaritiesand proton donating/accepting abilities. In the present studywe have observed lactim–lactam tautomerization in the mono-meric form, in contrast to earlier suggestions that it isobserved only when it is present as a dimer or complexed withacetic acid. Moreover, we observe rather slow tautomerizationoccurring on the nanosecond scale. In contrast to aproticsolvents, it can be anticipated that in alcoholic solvents, forproton translocation in the excited state, solvent moleculesmay slow down the ESIPT reaction due to intermolecularhydrogen bonds.

Acknowledgements

Prof. Akihide Wada, Molecular Photoscience Research Center,Kobe University, Kobe, Japan is greatly acknowledged for criti-cal reading of the manuscripts and providing the necessaryfacility for steady state measurements. Prof. (retd.) SatoshiHirayama, Kyoto Institute of Technology, Matsugasaki, Kyoto,606-8585, Japan is acknowledged for discussion and suggestions.

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Slow excited state phototautomerization in 3-hydroxyisoquinoline