Fluorescence studies of some protonated cinchona alkaloids in polymers

Journal of Luminescence 131 (2011) 1550–1555

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Fluorescence studies of some protonated cinchona alkaloids in polymers Neeraj Kumar Joshi a, Ranjana Rautela a, Hem Chandra Joshi b,n, Sanjay Pant a,nn a b

Photophysics Laboratory, Department of Physics, D.S.B. Campus, Kumaun University, Nainital 263002, India Institute for Plasma Research, Laser Diagnostics Division, Bhat, Near Indira Bridge, Gandhinagar, Gujarat 382428, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 November 2010 Received in revised form 3 January 2011 Accepted 5 February 2011 Available online 15 February 2011

In the present paper we report spectral and photophysical properties of two probes viz. cinchonidine and quinidine in two different polymers viz. polyvinyl alcohol (PVA) and polymethylmethacrylate (PMMA). The emission spectra exhibit edge excitation red shift (EERS) for both the probes and multiexponential decays are observed. Presence of various conformers and geometries is suggested to explain the observed results. The photophysical properties and excited state behavior of Cd++ are more sensitive towards the microenvironment of the polymer. & 2011 Elsevier B.V. All rights reserved.

Keywords: Cinchonidine Quinidine Edge excitation red shift (EERS) Polymeric environment

1. Introduction Cinchona alkaloids are photolabile and photo-induced modifications in their case have long been known [1–9]. Besides wellknown importance in drug industries [10–12], cinchona alkaloids are endowed with numerous interesting applications in various fields, e.g. sensors, standard for fluorescence quantum yield, etc. [13–24]. Cinchona alkaloids consist of three basic molecular units—a quinoline ring, a quinuclidine ring (tertiary amine) and a methylenic alcohol group linking the two (Scheme I) [3]. Major cinchona alkaloids are quinine (Q), quinidine (Qd), cinchonine (C) and cinchonidine (Cd). These alkaloids fall into two configuration dependent polarimetric groups, i.e. dextrorotatory and levorotatory. Cinchonine, quinidine and their di-hydro derivatives belong to the dextro group and have a similar configuration. Cinchonidine and Quinine share a different configuration and are levorotatory [25,26]. Cinchonine and cinchonidine have all the structural features of quinidine and quinine, respectively, except that they do not have the methoxy ( OCH3) group at the 6th position. The doubly charged cations of these compounds derived from the acidified aqueous solution (1 N H2SO4) have been used because of the high photostability and good fluorescence quantum yield. Fluorescence studies on cinchona alkaloids has been a thrust area right from the time when quinine sulfate was projected as standard for fluorescence decay time and quantum n

Corresponding author. Tel.: +91 79 23962099. Corresponding author. Tel.: +91 94 11198359. E-mail addresses: [email protected] (H.C. Joshi), [email protected] (S. Pant). nn

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.02.015

yield measurements [16,17]. Later, many workers questioned its utility as fluorescence standard for quantum yield [18–20] and decay time measurements [21–24]. Since then, many studies on the fluorescence features as well as conformation of cinchona alkaloids have been reported [1–9,27–49]. The basic fluorophore unit of all cinchona alkaloids is the quinoline ring, which is known to display close-lying weak n–p* and stronger p–p* transitions in the gas phase in the range of 300–320 nm [47]. However, the n–p* transition is so weak that only the p–p* transition is observed in the condensed phase as well as in rare gas matrices [48]. Moreover, the vinyl group in quinidine, quinine, cinchonine and cinchonidine, which absorbs at 180 nm, is not considered as an active chromophoric group in the near UV–vis region [24]. Upon electronic excitation, the ring nitrogen of quinoline derivatives shows a dramatic increase in its basicity. Indeed the increase is of the order of 5–6 pK units in the aqueous solution. The pK values for quinine, quinidine and 6-methoxy quinoline are 4.34, 4.34 and 5.13 and corresponding pK* are 9.7, 9.9 and 11.6, respectively [2]. An exhaustive temperature dependent fluorescence study of quinine sulfate (QS), quinidine and 6-methoxy quninoline (6 MQ) in acidic aqueous solution was carried out by Pant et al. [24,27] and they suggested that around 160 K, a rapid relaxation process (charge transfer from methoxy group to the quinoline ring) other than the solvent relaxation occurs. Further, their pH dependent time resolved study revealed that 6 MQ undergoes a proton transfer reaction in the excited state at pH¼7, whereas QS and Qd do not exhibit excited state protonation [27]. Recently, excited state protonation in some another cinchona derivatives in aprotic and protic solvents have also been reported by Qin et al. [7] and Qian and Brouwer [8]. Photophysics

N. Kumar Joshi et al. / Journal of Luminescence 131 (2011) 1550–1555

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2. Experimental section 2.1. Materials and polymer film preparation

Scheme 1

Scheme 2

of dications of cinchonine (C++) and cinchonidine (Cd++) in acidic aqueous solution have also been investigated which suggests the existence of two distinct emitting species (conformers) having charge transfer (CT) character at longer wavelengths [5,28]. Further, conformation of cinchona alkaloids plays an important role in biological as well as chemical activity [32,36,38,40]. However, the conformational behavior of cinchona alkaloids is rather complex, as it is remarkably influenced by various parameters such as solvent [30], solute [42–44], self-interaction [45], protonation [36] and the presence of metallic surfaces [46]. Optical methods based on vibrational spectroscopy have been used to characterize the conformation of the cinchona alkaloids and its modification upon molecular interaction. Recently, Scuderi et al. [9] have shown that UV excitation leads to a fragmentation pattern using mass spectroscopy and UV photo-dissociation, which strongly differs from that obtained by collision-induced dissociation. They proposed that the driving force for the reaction is the dramatic increase in the basicity of the quinoline nitrogen upon electronic excitation. Study of photo-induced excited-state dynamical processes in various polymeric rigid media due to different specific interactions between solute and host polymers as well as of various polymer micro-environmental effects have been reported in the literature [31,37,49]. In our previous work [37] we had reported the spectral and photophysical behavior of cinchonine dication (C++) in polymeric medium. In the present study we report the spectral behavior of Cd++ and Qd++ in polymeric media with an aim to explore the behavior of these two probes under similar microenvironmental conditions; hence, to decipher the nature of their interaction with the micro-environment. Interestingly, these two probes reveal some striking differences in their spectral and photophysical behavior, which are ascribed to the presence of different conformers and/or trapped geometries. The molecular structures of Cd++and Qd++ are shown in Scheme II.

Quinidine and cinchonidine (obtained from Aldrich) of 98% purity were tested for their fluorescence purity and used as such. All the polymers were purchased from Sigma-Aldrich, India. All the solvents used were either of spectroscopic grades or were checked for their fluorescence purity. Solute doped polymer films were prepared as described elsewhere [37]. Cd++ doped poly methacrylate (PMMA) films were prepared by dissolving PMMA (average molecular weight 2,00,000) grains in dichloromethane (DCM) and mixing it with desired concentration of Cd++ in 1 N H2SO4 in DCM. The films were obtained by drying the mass in polypropylene dish. Cd++ doped poly vinyl alcohol (PVA) films were prepared by mixing PVA (average molecular weight 1,25,000) grains with desired amount of Cd++ in 1 N H2SO4 solution in water. This mass was spread in polypropylene dish and was dried in an incubator. Qd++ doped films were also prepared by the same method. All the films were prepared for two concentrations, i.e., 0.2 and 0.4 wt%. The average thickness of the polymer films was around 0.5 mm. The molecular structural formulae of different polymer matrices viz. PVA and PMMA are shown in Scheme III. Fluorescence run of undoped films does not show any impurity.

2.2. Instrumentation Steady state absorption spectra, at room temperature, were recorded by dual beam JASCO V-550 spectrophotometer. The excitation and emission spectra were recorded by using JASCO FP—777 spectrofluorometer and data were analyzed by related software. Fluorescence decay times were recorded with the help of Edinburgh—199-time domain spectrometer and analyzed by TCC—900 software. The excitation source was a thyratron-gated hydrogen filled nanosecond flash lamp. Lamp profile was measured at the excitation wavelength using Ludox scatterer. The pulse width was about 1.5 ns with repetition rate of 30 kHz. Time correlated single photon counting (TCSPC) technique was used to collect the decay curves and the resolution of the system was about 200 ps. The number of counts in the peak channel was at least 10,000. Time-resolved fluorescence decay curves were analyzed by deconvoluting the observed decay with the instrument response function (IRF) to obtain the intensity decay function represented as a sum of discrete exponentials: Iðt,tÞ ¼

X

ti ai et= i

ð1Þ

i

where I(t) is the fluorescence intensity at time t and ai is the P amplitude of the ith life time such that iai ¼1. The average decay time has been calculated from the relation:

tav ¼ Sai ti =Sai

ð2Þ

H

H

C

C

H

n

H

H

C

C

H

O H

O

PMMA

PVA Scheme 3

n OMe

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N. Kumar Joshi et al. / Journal of Luminescence 131 (2011) 1550–1555

3. Results and discussion:

Table 1 Excitation dependence emission parameters of Qd++ and Cd++ in PVA and PMMA.

3.1. Steady state results of Qd++ and Cd++

kex (nm)

e

Optical Density

0.8 0.50 0.6

e 0.25

0.4

d

a Absorption

Emission

c

315 325 335 345 355 365 375 385 EERS (cm  1)

0.2

b

Cd++

PMMA

PVA

PMMA

PVA

417 417 417 417 417 419 423 425 450

417 417 417 417 417 419 423 425 450

392 393 394 398 405 410 – – 1120

396 397 401 405 414 424 – – 1670

8.0x103

1.0

Normalized Intensity (arb. units)

a

0.75

Qd++

Intensity (Arb. Units)

Steady state measurements for Cd++ and Qd++ in protic (PVA) and aprotic (PMMA) polymer were carried out at 298 K. UV/vis absorption and excitation dependent fluorescence spectra of Cd++ and Qd++ in protic PVA are shown in Figs. 1 and 2, respectively. Table 1 summarizes the photophysical data of Qd++ and Cd++. It is observed that the absorption maxima for both the probes are independent of solute concentration as well as host polymeric matrices. In PVA, at lex ¼325 nm, fluorescence maxima are observed at 417 and 397 nm for Qd++ and Cd++, respectively (Table 1). On excitation at the red edge of the first absorption band, shift in emission maximum towards longer wavelength, i.e. edge excitation red shift (EERS) is observed for both probes in both polymeric matrices. However, the EERS is larger for Cd++ as compared to Qd++ (Table 1). Magnitude of EERS is found to be dependent on the host polymer matrix in case of Cd++, while in Qd++ it is independent of the polymer matrix (Table 1). The excitation spectra for different emission wavelengths for Cd++ and Qd++ in PVA are shown in Figs. 3 and 4, respectively. In both PVA and PMMA, an isobestic point at  350 nm is observed in case of Cd++ but the excitation does not shift with the change in monitored emission wavelength. It appears that in Cd++, basically there two type

kem (nm)

(a) λem = 380 nm (b) λem = 420 nm (c) λem = 460 nm (d) λem = 500 nm

a b

6.0x103 c

4.0x103

d

2.0x103 0.0

280

300

320 340 Wavelength (nm)

360

380

Fig. 3. Excitation spectra of Cd++ in PVA, monitored at different emission wavelengths.

a

d

2000 0.0 315

350

385

420

455

490

Fig. 1. Absorption and emission spectra of Cd++ in PVA at lex ¼ 325 nm (a), 335 nm (b), 345 nm (c), 355 nm (d), and 365 nm (e).

1.00

d

1.0

Optical Density

0.75

0.8

d 0.50

0.6

c

Absorption

b 0.25

0.4 Emission

a

0.2

0.00

Normalized Intensity (arb. units)

a

0.0 315

350

385

420

b

a

Wavelength (nm)

455

490

Wavelength (nm) Fig. 2. Absorption and emission spectra of Qd++ in PVA at lex ¼ 355 nm (a), 365 nm (b), 375 nm (c), and 385 nm (d).

Intensity (arb. units)

0.00 280

1600 d

1200

c

800

400

300

320

340 Wavelength (nm)

360

380

Fig. 4. Excitation spectra of Qd++ in PVA, monitored at lem ¼ 410 nm (a), 430 nm (b), 450 (c), and 470 nm (d).

of ground state conformers present as closed hydrogen bonded and open conformers. We can rule out the presence of singly protonated species, as it exists only in the doubly protonated form in the presence of acidic medium (1 N H2SO4) [24]. It can be noted that similar behavior is observed in C++ [37].

N. Kumar Joshi et al. / Journal of Luminescence 131 (2011) 1550–1555

The red shifted band that appears in the excitation spectrum can be attributed to that conformer, which has significant charge transfer (CT) character (Fig. 3). However, trapping of the molecule in various geometries in the ground state can be ruled out, as the excitation spectra do not exhibit any shift with the monitored emission wavelength. On the other hand in case of Qd++, the excitation spectra in both polymeric matrices show a gradual shift towards longer wavelengths when the monitored emission is moved towards red (Fig. 4). The shift in excitation spectra with monitored emission in Qd++, points towards various geometric configurations. At the same time there is no change in the magnitude of EERS for both PMMA and PVA which rules out the role of free volume (Table 1). Had there been a free volume effect, PVA should have shown larger EERS because of less free volume. Hence it is likely the cause of EERS in Qd++ can be due to near isoenergetic conformers in the ground state. As reported in earlier works, in fluid media the EERS can be rationalized by the fact that the excitation of the solute is accompanied by a large change in dipole moment and the solvent reorientation time is not fast enough to bring the molecule in the relaxed state during the decay time [31]. On the other hand, in rigid matrices EERS can be attributed to the existence of various trapped geometries, which absorb at different energies. In a fluid medium these conformers interchange rapidly; however, in rigid medium the conformers are trapped in certain geometric configurations. The viscosity required to trap different conformers depends on the structure of the molecule and the amount of free volume in the matrix [50–52]. Moreover, it is well-known that cinchona alkaloids can adopt different conformations. It has been reported that free rotation around the C–C bonds in the linker moiety (cf. Scheme I) leads to the availability of many close iso-energetic conformations [30,32,38,40,41]. Quantum mechanical calculations also predict various possible conformers in cinchona alkaloids by estimating relative stabilities. The dependence of the energy of formation of cinchona on the values of the two central dihedral angles has been calculated for cinchonidine [30,32,40], cinchonine [32,38,41], quinidine [32,38,41], and quinine [32,41]. Most of those calculations have

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identified a number of stable configurations classified as ’’open’’ or ’’closed’’ structures depending on the position of the quinuclidine nitrogen atom with respect to the quinoline ring. Further, it has been reported that protonated form exists only in open structure [36]. Large magnitude of EERS for Cd++ in PVA with no shift in excitation spectra and appearance of a new band for longer emission wavelengths suggest that at least two types of conformers – close and open may be present – which are also in agreement with the findings of theoretical and ab initio calculations [30,32,36–41]. Similar behavior in case of PMMA can also be attributed to the presence of conformers. On the other hand, for Qd++, red shift in excitation spectrum when monitored at longer emission for both PVA and PMMA suggests the presence of various geometric species but almost similar EERS rules out the role of free volume. Of course, the difference in behavior of these two probes can be attributed to varying binding nature as well as flexibility. It can be mentioned here that Qd++ has methoxy group at 6th position unlike in case of C++ and Cd++. It is likely that the presence of methoxy group may inhibit the formation of two ground state conformers (observed in case of Cd++ or C++).

3.2. Time resolved studies To study the excited state dynamics of studied probes, fluorescence decay behavior was studied in protic and aprotic polymers at 298 K. Decay parameters are summarized in Tables 2–5. For Cd++, the fluorescence decays were found to be best fitted with a triple exponential function in all the polymers and throughout the entire emission profile (Fig. 5). This is in contrast to the data obtained in aqueous solutions [5,28], where a bi-exponential decay function was found to be sufficient to fit with the observed decay. It can be seen that the average decay time (tav ) increases for longer wavelengths (Tables 1 and 2). Though the excitation spectra reveal two ground state conformers, the appearance of a third decay component indicates that there should be excited state CT reaction (resulting in a CT state discussed below) besides the normal emitting state with

Table 2 Decay parameters of Cd++ in PVA, lex ¼ 320 nm.

kem (nm)

a

360 390 420 450 480

1.76 3.00 3.27 5.08 4.95

s1 (Standard deviation) (0.094) (0.027) (0.230) (0.464) (0.529)

a

a

6.54 (0.291) 8.5 (0.415) 9.57 (0.417) 11.69 (0.935) 11.09 (0.85)

13.96 17.24 18.53 20.97 20.96

s2 (Standard deviation)

s3 (Standard deviation) (0.220) (0.353) (0.425) (0.754) (0.618)

a1

a2

a3

a

0.14 0.12 0.12 0.14 0.13

0.40 0.48 0.48 0.49 0.48

0.45 0.40 0.40 0.37 0.39

a1

a2

0.14 0.13 0.14 0.16 0.19

0.46 0.43 0.43 0.40 0.44

sav

w23

w22

9.23 11.33 12.39 14.18 14.14

1.01 1.04 1.01 1.03 1.02

1.61 1.32 2.21 2.30 1.71

a3

a

w23

w22

0.40 0.44 0.43 0.44 0.37

10.70 11.13 11.58 11.75 10.37

1.00 1.01 1.09 1.09 1.02

1.69 1.70 2.03 2.10 1.99

a’s are relative amplitudes. w22 and w23 are for two and three exponential fits respectively. a

Decay times (ti’s) are in nanoseconds (ns).

Table 3 Decay parameters of Cd++ in PMMA, lex ¼ 320 nm.

kem (nm)

a

360 390 420 450 480

1.90 1.88 1.93 1.92 1.77

s1 (Standard deviation) (0.085) (0.085) (0.068) (0.078) (0.063)

a

s2 (Standard deviation)

7.44 7.64 7.92 8.02 7.09

(0.252) (0.242) (0.241) (0.375) (0.187)

a’s are relative amplitudes. w22 and w23 are for two and three exponential fits respectively. a

Decay times (ti’s) are in nanoseconds (ns).

a

s3 (Standard deviation)

15.96 17.28 18.40 18.72 18.69

(0.260) (0.218) (0.229) (0.393) (0.211)

sav

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N. Kumar Joshi et al. / Journal of Luminescence 131 (2011) 1550–1555

Table 4 Decay parameters of Qd++ in PVA, lex ¼ 360 nm. a s1 (Standard deviation)

a

s2 (Standard deviation)

a1

a2

a

(nm) 390 420 450 480

5.58 6.95 8.54 9.38

13.32 16.30 18.11 18.99

0.52 0.45 0.41 0.36

0.48 0.55 0.59 0.64

kem

(0.063) (0.082) (0.123) (0.179)

(0.108) (0.113) (0.138) (0.115)

sav

w22

w21

9.29 12.09 14.18 15.53

0.98 1.15 1.12 1.08

9.46 32.34 6.76 5.65

a

w22

w21

a’s are relative amplitudes w21 and w22 are for two and three exponential fits respectively. a

Decay times (ti’s) are in nanoseconds (ns).

Table 5 Decay parameters of Qd++ in PMMA, lex ¼ 360 nm. a s1 (Standard deviation.)

a s2 (Standard deviation.)

a1

a2

(nm) 390 420 450 480

4.80 6.56 7.70 6.80

12.35 15.70 17.41 17.23

0.50 0.48 0.43 0.31

0.50 8.57 1.08 8.85 0.52 11.32 1.07 10.12 0.57 13.23 1.16 8.96 0.69 13.98 1.32 7.98

kem

(0.057) (0.073) (0.097) (0.105)

(0.092) (0.110) (0.118) (0.073)

sav

a’s are relative amplitudes. w21 and w22 are for two and three exponential fits respectively. a

Decay times (ti’s) are in nanoseconds (ns).

closer energies. The absence of rise time can be understood by the fact that because of almost overlapped emission, the contribution from the rising component may be masked. However, we rule out the presence of various trapped geometries for the multicomponent decay as we do not see any shift in excitation spectrum of Cd++. Interestingly in PMMA, the change in average decay time is rather smaller. The longer average decay time in PVA may be due to stabilization of the molecule due to hydrogen bonding. Fluorescence decay of Qd++ are shown in Fig. 6 and it was best fitted with a two exponential function even at the red edge of the emission profile unlike in acidic aqueous solution, where it was mono-exponential [27]. However, the change in tav is less as compared to Cd++ (Tables 3 and 4). As discussed, in fluid medium, in general, the two decay components have been observed for cinchona alkaloids and its analogous compounds [5–8,24,27,29,31]. These have been attributed to the presence of the different conformers. Shorter wavelength emission was supposed to originate from normal state and the charge transfer (CT) state modified by the solvent relaxation process emits at longer wavelengths [5,24,27–29]. The quinuclidine amino group being a good electron donor and the quinoline a reasonable electron acceptor, the possibility of intramolecular charge transfer (ICT) may be considered for Cd++. However, charge transfer from methoxy group to the quinoline ring has also been reported in case of Qd++ [24,27]. As both methoxy and quinuclidine

χ = 1.01

4

χ = 0.98

4 0

0

-4 -4 χ = 1.04

4

χ = 1.15

4 0

0 -4

-4

10000

104

Counts

iii

iv (ii) λ = 390 nm (iii) λ = 420 nm (iv) λ = 450 nm

100

ii

103

iii

Counts

ii

1000

Decay at (i) λ = 360 nm

102

i

Decay at (i) λ = 390 nm (ii) λ = 420 nm iv (iii) λ = 450 nm (iv) λ = 480 nm

i

lamp profile

lamp profile

0

10 20 30 40 50 60 70 80 90 100 110 120 130

0

Time (ns) 4

χ = 1.01

50 75 Time (ns)

100

125 χ = 1.12

0

0

-4

-4 4

4

25

χ = 1.03

4

0

0

-4

-4

Fig. 5. Decay profiles of Cd++ in PVA at different emission wavelengths. The residuals and the w2 values for lem ¼ 360 nm (i), 390 nm (ii), 420 nm (iii), and 450 nm (iv).

χ = 1.08

Fig. 6. Decay profiles of Qd++ in PVA at different emission wavelengths. The residuals and the w2 values for lem ¼390 nm (i), 420 nm (ii), 450 nm (iii), and 480 nm (iv).

N. Kumar Joshi et al. / Journal of Luminescence 131 (2011) 1550–1555

have electron-donating character, both can give rise to a CT state. However, the CT character will depend on their respective electron donating strengths. In short, on comparing the photophysics of these two probes following features are revealed: The bonding characteristics of the studied probes and the nature of the host polymeric matrix give rise to conformational changes and may be responsible for the difference in the steady state and dynamical parameters of these two probes. The photophysical properties and excited state behavior of Cd++ appear to be more sensitive towards the microenvironment of the polymer.

Acknowledgements Authors, N.K.J. and R.R. are thankful to UGC, New Delhi, India for research fellowship under the scheme for the meritorious students in sciences (RFSMS). Financial assistance in the form of DRDO, New Delhi India funded research project sanction letter no. DLS/81/4822/LSRB-180/ID/2009 dated 09 February 2009 is greatly acknowledged. References

[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

[25] [26]

[27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

[1] V.I. Stenberg, E.F. Travecedo, J. Org. Chem. 35 (1970) 4131. [2] S.G. Schulman, R.M. Threatte, A.C. Capomacchia, W.L. Poul, J. Pharma. Sci. 63 (1974) 876. [3] L. Mink, Z. Ma, R.A. Olsen, J.N. James, D.S. Sholl, L.J. Muller, F. Zaera, Top. Catal. 48 (2008) 120. [4] R. Gatti, M.G. Gioia, V. Cavrini, Anal. Chim. Acta 512 (2004) 85. [5] H. Mishra, S. Pant, H.B. Tripathi, J. Fluoresc. 18 (2008) 17. [6] F.G. Sa´nchez, A.N. Dı´az, M.C. Torijas, E.N. Baro, M. Algarra, Anal. Chim. Acta 639 (2009) 67. [7] W. Qin, A. Vozza, A.M. Brouwer, J. Phys. Chem. C 113 (2009) 11790. [8] J. Qian, A.M. Brouwer, Phys. Chem. Chem. Phys. 12 (2010) 12562. [9] D. Scuderi, P. Maitre, F. Rondino, K. Le Barbu-Debus, V. Lepere, A.Z. Rentien, J. Phys. Chem. A 114 (2010) 3306. [10] M. Pesic, T. Andjelkovic, J. Bankovic, I.D. Markovic, L. Rakic, S. Ruzdijic, Invest. New Drug 27 (2009) 99. [11] J.M. Hutzer, G.S. Walker, L.C. Wienkers, Chem. Res. Toxicol. 16 (2003) 450. [12] P. Genne, O. Duchamp, E. Solary, J. Maguette, J.P. Belon, B. Chauffert, Anti Cancer Drug Des. 10 (1995) 103. [13] Z. Ma, F. Zaera, J. Phys. Chem. B 109 (2005) 406.

[37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]

1555

Z. Gong, Z. Zhang, X. Yang, Analyst 122 (1997) 283. A. Dondoni, A. Massi, Angew. Chem., Int. Ed. 47 (2008) 4638. W.H. Melhuish, J. Phys. Chem. 64 (1960) 762. W.H. Melhuish, J. Phys. Chem. 65 (1961) 229. H.C. Borresen, Acta. Chim. 19 (1965) 2089Scand. 19 (1965) 2089. J. Drobnik, E. Yearghers, J. Mol. Spec. 19 (1966) 454. R.F. Chen, Anal. Biochem. 19 (1967) 374. A.N. Fletcher, J. Phys. Chem. 72 (1968) 2742. P. Gangola, N.B. Joshi, D.D. Pant, Chem. Phys. Lett. 51 (1977) 144. D.V. O’Connor, S.R. Meech, D. Phillips, Chem. Phys. Lett. 88 (1982) 22. (i) D. Pant, U.C. Tripathi, G.C. Joshi, H.B. Tripathi, D.D. Pant, J. Photochem. Photobiol. A 51 (1990) 313; (ii) D. Pant, H.B. Tripathi, D.D. Pant, J. Lumin. 50 (1991) 249. W. Solomon, Chemistry of the alkaloids, Van Nostrand, 1970 301. R. Wijnsma, R. Verpoorte, F. Constabel, I.K. Vasil (Eds.), Cell Culture and Somatic Cell Genetics of Plants, 5, Academic Press, San Diego, CA, 1988, p. 335. D. Pant, H.B. Tripathi, D.D. Pant, J. Lumin. 51 (1992) 223. S. Pant, D. Pant, H.B. Tripathi, J. Photochem. Photobiol. A: Chem. 75 (1993) 137. S. Pant, H.B. Tripathi, D. Pant, J. Photochem. Photobiol. A: Chem. 85 (1995) 33. T. Burgi, A. Baiker, J. Am. Chem. Soc. 120 (1998) 12920. H.C. Joshi, A. Upadhyay, H. Mishra, H.B. Tripathi, D.D. Pant, J. Photochem. Photobiol. A: Chem. 122 (1999) 185. H. Caner, P.U. Biedermann, I. Agranat, Chirality 15 (2003) 637. Y. Liu, H.Y. Zhang, Z. Fan, D.X. Guan, J. Bioorg. Chem. 31 (2003) 11. Y. Liu, Y.W. Yang, H.Y. Zhang, B.W. Hu, F. Ding, C.J. Li, Chem. Biodiv. 1 (2004) 481. H. Mishra, D. Pant, T.C. Pant, H.B. Tripathi, J. Photochem. Photobiol. A: Chem. 177 (2006) 197. R.A. Olsen, D. Borchardt, L. Mink, A. Agarwal, L.J. Mueller, F. Zaera, J. Am. Chem. Soc. 128 (2006) 15594. N.K. Joshi, R. Rautela, S. Pant, H. Mishra, J. Lumin. 130 (2010) 1994. G.D.H. Dijkstra, R.M. Kellogg, H. Wynberg, J.S. Svendsen, I. Marko, K.B. Sharpless, J. Org. Chem. 55 (1990) 6121. G. Hamza, T. Schubert, I.Papai Soos, J. Am. Chem. Soc. 128 (2006) 13151. J.L. Margitfalvi, E. Tfirst, J. Mol. Catal. A 139 (1999) 81. T.H.A. Silva, A.B. Oliveira, W.B.De Almeida, Bioorg. Med. Chem. 5 (1997) 353. T. Burgi, A. Vargas, A. Baiker, J. Chem. Soc., Perkin Trans 2 (2002) 1596. D. Ferri, T. Burgi, A. Baiker, J. Chem. Soc., Perkin Trans 2 (2002) 437. D. Ferri, T. Burgi, A. Baiker, J. Chem. Soc., Perkin Trans 2 (1999) 1305. J.L. Margitfalvi, E. Talas, F. Zsila, S. Kristyan, Tetrahedron: Asymmetry 18 (2007) 750. S.R. Calvo, R.J. LeBlanc, C.T. Williams, P.B. Balbuena, Surf. Sci. 563 (2004) 57. Y. Hiraya, K. Achiba, E.C.Lim Kimura, J. Chem. Phys. 81 (1984) 3345. M.F. Anton, M. Nicol, J. Lumin. 18/19 (1979) 131. M.S. Mehata, H.C. Joshi, H.B. Tripathi, Spectrochim. Acta Part A 59 (2003) 559. A.P. Demchenko, J. Lumin. 17 (2002) 19. K.A. Al-Hassan, T. Azumi, Chem. Phys. Lett. 163 (1989) 129. K.A. Al-Hassan, W. Rettig, Chem. Phys. Lett. 126 (1986) 273.