Spectroscopy and photophysics of 1-phenylisatin and

Optical Materials 15 (2000) 131±141

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Spectroscopy and photophysics of 1-phenylisatin and oxindole Prakriti Ranjan Bangal, Sankar Chakravorti * Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India Received 3 December 1999; accepted 9 February 2000

Abstract The absorption and photoluminescence characteristics of 1-phenylisatin (PI) and oxindole (OI) were studied in dierent solvents at 300 K. In the basic solution a new absorption band system found for PI has been attributed to a structural change leading to formation of new species. Dual ¯uorescence for PI and OI in dierent solvents was explained as normal 1 B2u ® 1 A1g transition and the other well-structured band as an anomalous band. This large Stokes shifted anomalous band of PI in water and alcohol was explained as ¯uorescence emission from a state formed due to solute±solvent relaxation producing a sort of critical inversion. Studies in dierent solvents including a b-cyclodextrin solution revealed that in suitable active medium PI has all the properties to be potential laser dye. Ó 2000 Elsevier Science B.V. All rights reserved.

1. Introduction Spectroscopic studies of bichromophoric organic systems and their photophysics and photochemistry have been an engaging ®eld of research for a decade [1±4]. Depending upon the length between the two chromophores they exhibit various properties in their excited states, such as twisted intramolecular charge transfer [1], intramolecular excimer formation, intramolecular exciplex formation [2] and other dierent intramolecular phenomena. It is well documented that the rotation of the more ¯exible part of a bichromophoric molecule acts as a main ¯uorescence quencher of the system [3]. To design an eciently ¯uorescing system, for example a laser dye, one should essentially control the internal * Corresponding author. Tel.: +91-33-476-4971; fax: +91-33473-2805. E-mail address: [email protected] (S. Chakravorti).

rotation of the molecule and hence the formation of twisted intramolecular charge transfer and formation of intramolecular exciplex [4] by rigidizing the system. Out of the dierent approaches, the reduction of interchromophoric distance is one of the best ways to make a system rigid. One can easily convert a bichromophoric system to an apparent monochromophoric system. Phenylisatin (PI) and oxindole (OI) are this type of fused molecules that apparently look like a monochromophoric system but contain two chromophores. For PI and OI the two chromophores are benzene and amidic (>N±ÇC±ÇO) and benzene and (±N±ÇO), respectively. Indole and its dierent derivatives are a family of compounds that have peculiar photophysical properties. It is well documented that the spectroscopy and photophysics of most indole derivatives is largely in¯uenced by the nature and position of dierent substituents as well as with changed environments [5,6]. Perhaps a close

0925-3467/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 0 0 ) 0 0 0 1 1 - 2

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P.R. Bangal, S. Chakravorti / Optical Materials 15 (2000) 131±141

similarity of this type of compounds with tryptophan, a biologically important molecule, has led to intensive and extensive studies for quite some time [7±10]. An unusually large Stokes shift of emission spectra of dierent indole derivatives in polar solvents mainly created a lot of controversy over the subject which ®nally settled down to appropriate labeling of emitting state as 1 La in polar solvents [8]. On this backdrop we want to investigate the room temperature luminescence and absorption of PI and OI, a diketo and monoketo derivative of fused bichromophoric system, respectively, along with semi-empirical calculations. Also, by taking advantage of environment-dependent spectral change we report the modulation of the photophysics of the above compounds within the interior of the b-cyclodextrin (b-CD) hydrophobic cavity. 2. Experimental The compounds PI and OI (Aldrich Chemical) were sublimed several times under reduced pressure to obtain pure samples and melting points were checked before use. The b-CD (Aldrich Chemical) was used as supplied. The solvents, ethanol (EtOH), acetonitrile (ACN), sulfuric acid and NaOH (E. Merck, spectroscopic grade) were used as supplied, but only after checking the purity ¯uorimetrically in the wavelength range of interest. The methylcyclohexane (MCH) and carbon tetrachloride (CCl4 ) (E. Merck, spectroscopic grade) were distilled (dry) and used after checking for any emission in the required wavelength range. For aqueous solutions, de-ionized Milli Pore water was used. The absorption spectra at 300 K were recorded with a Shimadzu absorption spectrophotometer model UV-2101PC, and the ¯uorescence spectra were obtained with a Hitachi F-4500 spectro¯uorimeter. Spectrum correction was performed to enable measuring a true spectrum by eliminating instrumental responses such as wavelength characteristics of the monochromators or detectors. Fluorescence lifetime measurement was performed by using a single photon counting ¯uorimeter (Edinburgh Instrument) [14,15]. For emission

measurement, the sample concentration was maintained at 10ÿ5 M in each case in order to avoid aggregation problems. To nullify the contribution of impurity, if any, we used several very low dilutions of the sample and also checked the solvent in a control experiment. The quantum yields were determined by using the secondary standard method with recrystallized naphthalene in cyclohexane (Uf 0:23) described elsewhere [11].

3. Results and discussion 3.1. Absorption spectra The absorption spectrum of PI is composed of two well-separated poorly structured bands (Fig. 1) with peaks at two dierent wavelengths 290±300 and 246 nm, respectively (Table 1). The absorption spectrum of OI consists of two broad bands lying in the same region of wavelength of PI (Table 1). The molar absorption coecient (Table 1) of each band for both the molecules gave an indication

Fig. 1. Absorption spectra of PI ( ± ) and OI (± ±) in dierent environments. (a) In ethanol, (b) in ethanol and 10% 0.1 N H2 SO4 and (c) in ethanol and 10% 0.1 N NaOH. (a0 ) In ethanol, (b0 ) in ethanol and 10% 0.1 N H2 SO4 and (c0 ) in ethanol and 10% 0.1 N NaOH.


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Table 1 Absorption data of PI and OI at 300 K in dierent solvents Solvent

1-Phenylisatin

Oxindole

emax (M)

kmax (nm)

MCH

15424 1067

246 295

9595 1079

244 283

Ethanol

20988 1055

245 297

17830 1096

248 281

Ethanol 10% 0.1 N H2 SO4

21988 1126

245 295

17810 1089

246 280

Ethanol 10% 0.1 N NaOH

22978 3236 10231

253 286 390

19283 1083

244 292

ACN

20938 1015

245 297

14830 1096

246 280

CCl4

14438 987

244 294

11342 1012

248 281

H2 O

22467 1065

244 295

12339 1167

244 280

H2 O b-CD

22471 1069

245 297

12341 1169

246 281

emax (M)

kmax (nm)

M dm3 molÿ1 cmÿ1 .

that the higher energy band of PI is of pp* character and the lower energy band of np* character and the two bands of OI might be of pp* and np* character, respectively. The two pp* bands for both molecules are associated with the electronic transitions 1 B1u ¬ 1 A1g and 1 B2u ¬ 1 A1g of benzene, respectively. Absence of any solvent-dependent character in absorption spectra towards hypsochromic (blue shift) or hypochromic shift (decrease in intensity) [12] in dierent solvents including in acidic solution for both the molecules indicates that the ionization of carbonyl group has little eect on the energy of the states responsible for the absorption. In the basic solution a large change in the absorption spectra of PI is observed: a strong new band in the 390 nm region appears along with a little red shift and hyperchromic eect in other higher energy bands (Fig. 1). OI shows only a hyperchromic eect with little red shift of absorption bands. The appearance of a new long wavelength band of PI may be explained in the following way:

Benzilic acid rearrangement

Through benzilic acid rearrangement [13], the PI molecule is converted into carboxylic acid in the presence of NaOH having absorption band in the range of 350±400 nm (like cinnamic acid) [14,15]. The formation of four-membered lactum in basic medium might have resulted hyperchromic eect in PI in basic solution. However, in basic solution

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OI takes part in Aldol condensation reaction in the following way, which is responsible for the hyperchromic eect of both the bands and a red shift of the absorption band:

The absorption spectra of PI and OI in a water solution are of similar structure as in ethanol or the other solution stated earlier. In an aqueous solution, PI shows two bands peaking at 244 and 295 nm and OI also shows two bands peaking at 244 and 280 nm. The addition of b-CD in the water solution produces only minor changes in both cases in absorption spectra (Table 1). In both the cases the molar absorption coecient slightly increases shifting all peaks position to the red by 1±2 nm. This slight increase in the molar absorption coecient in most cases are barely outside the experimental errors. 3.2. Fluorescence emission spectra At room temperature, both the molecules PI and OI show structured ¯uorescence spectra very similar to indole and its derivatives [16,17], but according to the intensity they are not similar to the above derivatives. The ¯uorescence quantum yield of PI in dierent non-polar and polar (aprotic) solvents and the ¯uorescence quantum yield of OI in all solvents is nearly 10 times less than that of indole derivatives. In the ethanol solvent, the molecule PI at room temperature shows prominent dual ¯uorescence depending upon the excitation wavelength (Fig. 2(a)). When PI is excited by 290 nm, it ¯uoresces weakly as indole (quantum yield 0.005, Table 2), which is due to 1 B2u ® 1 A1g transition of benzene, and it is termed as normal emission. The lifetime of this normal emission varied from 4 to 5 nanoseconds depending on solvents. However, when PI is excited in the higher energy absorption band (i.e., 250 nm S2 ) then dual ¯uorescence appears; one is the weak normal ¯uorescence that has been observed earlier and the other is anomalous long wavelength wellstructured ¯uorescence (second band). The

Fig. 2. (a) Excitation dependent corrected ¯uorescence emission spectra of PI in ethanol. ( ± ) Excitation wavelength of (1) 250, (2) 255, (3) 260, (4) 270, (5) 275, (6) 280 and (7) 290 nm, respectively; (± ±) corrected ¯uorescence emission spectra of PI in MCH at room temperature. (b) Excitation spectra of PI in EtOH solvent (1) emission wavelength 415 nm, (2) emission wavelength 350 nm, respectively, (3) excitation spectra of PI in MCH solvent, emission wavelength monitored at 350 nm.

anomalous ¯uorescence band has a strong solventdependent character, i.e., it appears only in aqueous and hydroxylic solution. Unfortunately, we did not have the facility of lifetime measurement with 250 nm excitation but the dierence in solvent dependence on the two bands somewhat con®rms the dual emission. It is noteworthy here that different indole derivatives show large red shifted structureless band [8] in dierent polar solvents. On increasing the excitation energy the second band intensity increases while the intensity of the ®rst normal band decreases, but below 275 nm excitation the second band vanishes totally. A distinct isoemissive point (a common point between two components of emission, i.e., a point of equilibrium) is observed at 400 nm, quite indicative of an equilibrium condition of population density between the excited emissive states. Excitation spectra of PI in these two solvents have been recorded in Fig. 2(b). In ethanol solvent the two


135

Table 2 Fluorescence data of PI and OI in dierent solvents at 300 K Solvent

1-Phenylisatin 1

a b

kfl (nm)

Oxindole 2

kfl (nm)

1

ufl

2

ufl

1

kfl (nm)

2

kfl (nm)

ufl

MCH ACN CCl4

335 350 350

± ± ±

0.005 0.008 0.006

± ±

335 355 345

± ± ±

0.002 0.002 0.002

EtOH

355

416 438 466 490

0.004

0.026

358

415a 438b 467b

0.001

H2 O

±

416 438 466 490

±

0.25

±

±

H2 O b-CD

350

416 438 466 490

0.005

0.19

345 358

±

0.001

EtOH 10% 0.1 N H2 SO4

345 358

±

0.01

±

347 360

±

0.009

EtOH 10% 0.1 N NaOH

350

416 438 466 490

±

0.08

360

407

0.015

Very weak. Very very weak.

excitation spectra are clearly dierent. The excitation spectrum of higher energy ¯uorescence band has good resemblance with the absorption spectrum while the excitation spectrum of the lower energy ¯uorescence band has no similarity with the absorption bands. On the other hand, the excitation spectrum of PI in MCH is nearly identical to the absorption spectrum. This discrepancy in the solvent-dependent excitation spectra points to the fact that there is a special type of interaction in the excited state between PI and the ethanol solvent [8]. It is also important to note here that similar excitation spectra could be observed in water (and also water NaOH) solution as in ethanol, when monitored at 415 nm. Now these two dierent excitation spectra indicate that the origins of the two ¯uorescence bands are dierent. In this situation, the presence of isoemissive point in excitation-dependent ¯uorescence spectra indicates that the population of excited state, respon-

sible for anomalous ¯uorescence is at the cost of depopulation of the excited state that is responsible for normal ¯uorescence. Hence, a non-radiative transition seemed to be present here to establish the equilibrium between those two states. Fig. 3 shows the room temperature phosphorescence spectra of PI and OI in solid state (in powder form) on two excitations at 250 and 290 nm. Both the compounds show the phosphorescence band of similar structure and intensity. However, in case of OI the phosphorescence is red shifted compared to that of PI. We checked the room temperature phosphorescence in solid state by purging the sample with N2 gas and found that the lifetime for PI in N2 atmosphere is 37 ms whereas in O2 atmosphere it is 17 ms. The nature of the spectrum remains basically the same in N2 and O2 atmosphere, only the intensity changes a little. The phosphorescence lifetime of OI was measured to be 14 ms in O2 atmosphere. Possibly,

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Fig. 3. Room temperature phosphorescence spectra of PI and OI in solid and EtOH solvent. Phosphorescence spectra of PI in solid state (a) on excitation at 250 nm, (b) on excitation at 290 nm and (c) in ethanol solvent on excitation at 290 nm. Phosphorescence spectra of OI in solid state (a0 ) on excitation at 250 nm and (b0 ) on excitation at 280 nm, respectively.

the presence of hetero-atom (O) in both the molecules facilitates the appearance of phosphorescence at room temperature [11]. It is important to note that PI in ethanol solvent shows room temat 450 nm. perature phosphorescence with kmax ph The lifetime (15 ms) of the phosphorescence band of PI in ethanol solvent is little less than that in the solid state. This phosphorescence of PI in ethanol solvent attains nearly the same intensity on excitation at large wavelength region (250±350 nm). The phosphorescence quantum yields in all cases are very low (10ÿ4 ). Here it may be mentioned that the excitation at 250 nm produces both normal and lower energy anomalous ¯uorescence but the position of lower energy ¯uorescence overlaps phosphorescence band. In the aqueous solution, PI shows only the anomalous ¯uorescence for both excitations 250 and 290 nm. The intensity as well as quantum yield of ¯uorescence band decreases with decreasing excitation energy. The highest intensity is observed for 250 nm excitation and it decreases nine times on 280 nm excitation (Fig. 4). In the aqueous solution, the quantum eciency of anomalous band is 10 times more than that in alcoholic solution. At room temperature, the ethanol solution of OI produces no such strong dual ¯uorescence. On excitation at lower energy (290 nm), it shows very weak normal ¯uorescence that is similar to that of indole derivative [16,17]. However, higher energy excitation (250 nm) produces a comparatively in-

Fig. 4. Excitation dependent corrected ¯uorescence emission spectra of PI in aqueous solution. The excitation wavelength of (1) 250, (2) 255, (3) 260, (4) 265, (5) 270, (6) 275, (7) 280, (8) 285, (9) 290, (10) 295, (11) 300, (12) 305 and (13) 310 nm, respectively.

tense normal ¯uorescence band with a prominent shoulder nearly in the overlapping zone of wavelength of second ¯uorescence of PI but in aqueous solution no ¯uorescence could be observed for OI. In ACN solvent PI rather shows normal ¯uorescence (S1 ) with higher quantum yield than other solvents (3.5 times greater than that in ethanol). However, in presence of ethanol in ACN, PI shows dual ¯uorescence on excitation by 250 nm. Fig. 5 shows the emission spectra of PI in ACN as a function of ethanol concentration. On increasing ethanol concentration, the intensity of the anomalous ¯uorescence band increases whereas no

Fig. 5. Fluorescence emission spectra of PI in ACN as function of ethanol concentration. (1) ACN and 15% EtOH, (2) ACN and 13% EtOH, (3) ACN and 11% EtOH, (4) ACN and 8% EtOH, (5) ACN and 6% EtOH, (6) ACN and 4% EtOH, (7) ACN and 2% EtOH, (8) ACN and 0.5% EtOH and (9) ACN and 0.0% EtOH. (kexc 250 nm).


remarkable change is noticed for normal ¯uorescence. It makes quite clear that the presence of ethanol in ACN makes a propitious condition to populate the excited molecules that are responsible for the second ¯uorescence. A similar type of dual ¯uorescence is observed in carbontetrachloride (CCl4 ) that has been concluded to be due to the presence of a trace of water in CCl4 . Because, after making vacuum distillation of CCl4 , PI does not produce any dual ¯uorescence in this solvent. So here water has the same attribute as ethanol in ACN for the dual ¯uorescence in CCl4 . In an acidic medium (ethanol in presence of 0.1 N H2 SO4 ) PI produces only normal ¯uorescence on excitation to both 250 as well as 290 nm but the intensity or quantum yield is three times more for the latter excitation. Step-wise increase of acid in the solution increases the normal ¯uorescence quantum yield accordingly and ®nally it increases three times than that of neutral alcoholic solution. The same type of result was observed in aqueous acidic solution. In aqueous acidic solution, the total quantum yield decreases as a result of appearance of normal ¯uorescence and decreasing intensity of second ¯uorescence. Here the second ¯uorescence does not vanish totally. The same type of experiment was carried out with OI. In ethanol, the presence of H2 SO4 increases the normal ¯uorescence of OI by three times making a weak shoulder vanish. In an aqueous acidic solution, OI shows very weak normal ¯uorescence that ceases to ¯uoresce in neutral aqueous solution. In basic (0.1 N NaOH) ethanol solution, PI shows strong dual ¯uorescence on excitation to absorption bands, 250 and 290 nm. The intensity of second ¯uorescence band (anomalous ¯uorescence) increases with base concentration whereas the ®rst band (normal ¯uorescence) intensity remains constant. In a basic aqueous solution, PI shows such strong anomalous ¯uorescence on excitation at 250 nm that the normal ¯uorescence seems to vanish in the same scale, but when it is excited by the 290 nm, the normal ¯uorescence gets the prominence and the second band (anomalous ¯uorescence) appears as a shoulder. In the case of OI, basic ethanol solution shows dual ¯uorescence on excitation at 250 and 290 nm. With increasing base concentration, the second band

137

(410 nm) intensity increases while the ®rst band intensity remains unaltered. Finally, for certain base concentrations ( above 10% of 0.1 N NaOH in solution), the second band intensity increases so high that the ®rst band is nearly covered by the envelope of the second band. 3.3. Fluorescence due to supramolecular complex In the earlier section, we have already seen that PI in aqueous solution does not produce normal ¯uorescence: it produces strong anomalous ¯uorescence in the longer wavelength region. However, the interesting point is that in aqueous b-CD solution it shows dual ¯uorescence with reduced quantum yield on excitation at 250 nm. As the concentration of b-CD decreases, the intensity of ®rst band decreases while the second band intensity increases (Fig. 6). Finally, the ®rst band vanishes and the second band attains the same intensity as aqueous solution. Now, in absorption spectra, not much of new information could be observed due to insertion of the molecule into the b-CD cavity. However, in ¯uorescence spectra, a dramatic change could be observed with a clear isoemissive point at 396 nm. In contrast to the change in the ground state, the ¯uorescence intensity of the ®rst band is increased and that of the second band is dramatically

Fig. 6. Corrected ¯uorescence emission spectra of PI ( ± ) as a function of b-CD concentration in aqueous solution. The concentration of b-CD for (1) 0 M/l, (2) 8:125 10ÿ4 , (3) 1:625 10ÿ3 , (4) 3:25 10ÿ3 , (5) 6:5 10ÿ3 and (6) 1:3 10ÿ2 M, respectively. (kexc 250 nm). Corrected ¯uorescence emission spectra of OI (áááá) as a function of b-CD concentration in aqueous solution. The concentration of b-CD for (1) 1:3 10ÿ2 , (2) 6:5 10ÿ3 , (3) 3:25 10ÿ3 , (4) 1:625 10ÿ3 , (5) 8:125 10ÿ4 and (6) 0 M, respectively. (kexc 250 nm.)

138


decreased 6±10 times for the highest concentration of cyclodextrin used, without any signi®cant shift in wavelength. This spectral change, i.e., appearance of normal ¯uorescence and decrease of anomalous ¯uorescence intensity, is attributed to the hydrophobic nature within b-CD cavity. At the same time, the anomalous ¯uorescence intensity at the highest concentration of b-CD (0.01 M) is greater than in ethanol showing that water molecules might be accessible to the probe molecule in the b-CD cavity. Most complexation studies assume a 1:1 stoichiometry between b-CD and the guest of interest. However, this is not always true and works have been reported where two cyclodextrins can encapsulate a single molecule [18±22]. This factor is important because interpretation of the results in studies involving complexation may vary signi®cantly depending on the stoichiometry. Using the Benesi±Hildebrand plot one can estimate the value of dissociation constant K from ¯uorescence data [23]. In case of simple 1:1 complex, the equilibrium can be written as K

Fig. 7. 1=I ÿ I0 vs. 1=CD plot.

other solvents and in the lowest concentration of b-CD the ¯uorescence vanishes as in pure water solution. It is important to note that in the b-CD cavity OI shows ¯uorescence with sucient amount of blue shift compared to the ¯uorescence spectra in ethanol solvent. This implies that the b-CD cavity provides an environment like non-polar molecule, at least for the case of OI, where there is no signi®cant change in excited state geometry.

S CD$SCD:

4. Origin of second ¯uorescence (anomalous) band

For this equilibrium one can obtain the following expression of the Benesi±Hildebrand double reciprocal plot:

In general, ¯uorescence does not occur in compounds that have np* as the lowest excited singlet [24] state because almost total intersystem crossing takes place from the lowest np* singlet state to a triplet state from which phosphorescence occurs. From the absorption spectra of PI, it has been concluded that the lowest singlet state is np*. PI and OI both have shown the common phenomena a weak ¯uorescence (normal), a strong phosphorescence at 77 K [25]. The ¯uorescence quantum yields of the higher energy band (350 nm ¯uorescence band) of both the compounds are higher in aprotic solvents (ACN, MCH, CCl4 ) than that in protic solvents, and in extreme case, i.e., in aqueous solution no ¯uorescence could be observed in the wavelength region of 300±400 nm. Since hydrogen bond formation acts as a non-radiative channel [26] we may expect a hydrogen bond formation of those compounds with protic solvents. What we have gathered is that PI behaves similar to indole derivatives except that of an

1 1 1 ; I ÿ I0 KI1 ÿ I0 CD I1 ÿ I0 where I0 , I1 denote ¯uorescence intensities of the probe molecule in bulk water and in the complex, respectively, I the ¯uorescence intensity at a given CD concentration and K is the association constant. Fig. 7 shows the double reciprocal plot for complexation of a PI with b-CD. A straight line describes the plot, as it should follow the above equation. From the slope and intercept of linear plot of the 1 I ÿ I0 vs. 1 CD, we compute the value of the association constant K, which is found to be 74.4 dm3 /M for PI and 25.3 dm3 /M for OI. Fig. 6 also shows the ¯uorescence spectra of OI in aqueous b-CD solution as a function of b-CD concentration. For the highest concentration of bCD, the intensity and the quantum yield of the ¯uorescence are the same as those in ethanol and


anomalous band and the problem of the second ¯uorescence, i.e., anomalous ¯uorescence, is largely centered on the nature of the solvent required. It has been noted that water or hydroxylic solvents (ethanol) are necessary for this anomalous ¯uorescence. Judging from the properties [27±29] of organic laser dyes one may conclude that the second ¯uorescence band of PI might have a similar property of dye molecules of lasing in active medium. To explain the excitation-dependent ¯uorescence spectra of PI, a schematic representation of the energy levels of PI molecule has been proposed (Scheme 1), specially in water and ethanol, which has been strongly corroborated from absorption point of view. When PI is excited to the lowest singlet state S1 (np*), intersystem crossing occurs, weak ¯uorescence with little room temperature phosphorescence is observed. Now on excitation to the S2 (pp*) state, two phenomena occur spontaneously. Some excited molecules come down to S1 state (internal conversion) and get deexcited like the previous way and other excited molecules come to Sm state and produce a sort of ``critical inversion''. The lower energy ¯uorescence emission is then achieved. This Sm ¯uorescencent state is probably achieved in the same way as solvent±solute relaxation [7,30]. It seems that the La state is heavily stabilized as compared to indole derivatives. Strangely enough, on slight lowering the excitation energy ``critical inversion'' is not achieved properly and lower energy emission dies out with a little increase in normal ¯uorescence and that too happens in particular media. Possi-

Scheme 1. Schematic representation of the dierent energy levels and dissipative paths of PI in water and alcoholic medium.

139

bly, room temperature phosphorescence at 450 nm in ethanol solvent quenches the emission. It is to be noted that for ecient lasing system quenching of the triplet state is essential. The proposed state and lower energy triplet state are lying in the range of vibrational crossover in ethanol solvent that is the major reason for production of less ecient anomalous ¯uorescence. The reason for PI not showing ¯uorescence in solid state is the same. In water solution of PI, probably the energy dierence DE between Sm and T1 is greater than that in ethanol, which in turn reduces the energy dierence of T1 and S0 . This increase in DE decreases the possibility of vibrational crossover and quenching of Sm by T1 . At the same time, the radiative phosphorescence decreases or vanishes in a water solution because of T1 lying so close to S0 . These cumulative eects might have helped to increase the anomalous ¯uorescence in water than in ethanol. This schematic model is valid only for water and alcoholic solutions that are the typical active media for organic dye laser. The enhancement of the second band emission, i.e., the anomalous ¯uorescence, might be due to structural change of PI molecule in the presence of NaOH, as we have seen earlier that the addition of NaOH augments the second band emission. This enhancement could be due to formation of the fourmembered lactum. However, in acid medium no such lactum formation is found and only the increase of lowest triplet state energy is found. Due to this increment of lowest triplet state energy the intersystem crossing is enhanced to produce phosphorescence on excitation to S2 . Again, decrement of second band emission and increment of normal ¯uorescence in the b-CD cavity could be explained by the hydrophobic environment inside of b-CD cavity. Since the probe molecule faces nearly the same environment [31] inside the cavity like ethanol, the same type of spectral nature is expected from the PI and b-CD inclusion complex and these are the observed results described earlier. Moreover, all cases where second band appears are of the same vibronic structure. No solvent dependence or solvent polarity eect could be observed towards spectral shift or structural change of ¯uorescence, which indicates that the responsible state for anomalous emission could

140


strongly be coherent in nature, i.e., the band position and vibronic structure in this case is strongly determined by the compound itself and not on the solvent character. From the foregoing observations and discussion on the anomalous emission we can conclude that PI has all the properties of a laser active substance at room temperature to produce organic dye laser in water or alcohol.

5. Semi-empirical calculation In order to get ground and excited state optimized geometry, dipole moments in ground and excited states and excited state electronic states of PI molecule, the help of MOPAC (Version 5) package with AM1 Hamiltonian [32±34] was taken. In several occasions [35±37] it was found that the AM1 method is suitable and easy for handling the excited state calculations in respect of above. Precise geometry optimization was obtained by MOPACÕs NLLSQ gradient optimization criterion. Fig. 8 shows the energy minimized geometry in ground and excited state of PI molecule. In this energy minimized con®guration the molecule is slightly deviated from the ¯at con®guration, i.e., the phenyl ring makes an angle of 27° with the main group of PI. So the angle of twist of phenyl ring around CN bond is 27°. However, in excited state (Fig. 8), the optimized geometry changes the twist angle 5° more than that in ground state. So

Fig. 8. Optimized ground state geometry of PI; optimized excited state geometry of PI.

the angle of twist of phenyl ring around CN bond in excited state is 32°. Semi-empirical calculation provides the ground and excited state dipole moments to be 2.33 and 4.20 Debye, respectively. The rearrangement of charge distribution is not expected because the change of ground state to excited state dipole moment is not large. So it is not possible for this molecule to form the intramolecular charge-transfer state. CI calculation corroborates the singlet state arrangement that is observed in absorption spectra.

6. Conclusion The present investigation evinces dierent features of absorption and photoluminescence studies of PI and OI in dierent solvents at 300 K. In a basic solution, a new absorption band system for PI has been attributed to a structural change leading to formation of new species. The dual ¯uorescence of PI in dierent solvents has been concluded as normal 1 B2u ® 1 A1g transition and the other well-structured band as anomalous band. A simple scheme shows the normal ¯uorescence of PI arise from ®rst excited singlet (S1 ) to ground state singlet. The anomalous band of PI appears only in water and alcohol and has also been concluded to arise from a singlet state that is created from second excited singlet S2 after large Stokes shift. Whereas OI shows the normal photophysical behaviour like aromatic carbonyl compounds. Quantum chemical calculations show no evidence from an intramolecular chargetransfer form and also these calculations provide that PI is not planar in ground or excited states. Photophysical studies in dierent solvents including a b-CD solution con®rm that the anomalous ¯uorescence of PI only appears in a hydrophilic medium. Since the operation of organic dye laser is based upon ¯uorescent transitions in large molecules typically in water and alcohol solution, we can conclude that PI has all the properties of a potential laser dye in active mediums like water and ethanol considering the ¯uorescence quantum yield of anomalous band (Table 2).


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