Luminescence studies of 2,′2:′6,″2-terpyridine and its protonated species

JOURNAL OF

LUMINESCENCE ELSEVIER

Journal

of Luminescence

63 (1995) 143-148

Luminescence studies of 2,2’: 6’,2”-terpyridine and its protonated species Aindrila Sarkar, Sankar Chakravorti* Department

of Spectroscopy,

Received

14 March

Indian Association for the Cultivation of Science, Calcutta 700 032. India 1994; revised 20 May 1994, 18 July 1994; accepted

26 July 1994

Abstract Analysis of emission spectra of 2,2’: 6’,2”-terpyridine (TP) in glass matrices at 77 K reveals interesting photophysics sensitive to phenyl or pyridyl ring substitution in the 2,6-positions of pyridine. From the emission and absorption spectra in different solvents at 300 K, it is found that while the protonated form of TP deactivates via a radiative channel, its free base form prefers non-radiative processes. Formation of a covalent hydrate of TP in neutral aqueous solution is observed which becomes deprotonated in basic solution. Protonation in an acidic medium is limited to two sites. The excited state is more planar and basic than the ground state.

1. Introduction

In spite of much previous work, our knowledge of the dynamics of radiationless processes in large molecules is still incomplete. The reason for this may be, the lack of knowledge concerning the structures and vibrational spectral characteristics of different electronic manifolds of large molecules and the difficulty in measuring the decay properties of well-defined initial states of such molecules. While the benzene family is rather well-characterized, for large polyphenylic molecules containing N-heteroatom structures, our knowledge is far from complete. In a recent work [l] on 2,6diphenyl pyridine (DPP) it was shown that the bridging of the n-electronic clouds of the outer rings modulated the photophysics of the system dramatically. Among the substituted pyridines, the *Correspondingauthor.

polypyridinic compounds derived from the parent molecule 2,2’-bipyridine (BP) are widely used as ligands for transition metal complexes, which are interesting for their luminescence and redox properties. BP is very weakly fluorescent in inert solvents, whereas, its protonated adducts luminesce. The amount and type of emission depends on the pH of the solution. The very low fluorescence quantum yield of this molecule [2] (c#+N 5 x 10e4) is due to a very effective intersystem crossing to a local triplet state. The presence of relatively close lying II, n* and R, R* excited states of BP leads to vibronic coupling by out-of-plane vibrations with subsequent modification of the potential energy surface and mixed spectral and photophysical properties. This effect has long been recognized for azaaromatics and it is known as the ‘proximity effect’ [3]. These facts have given us an impetus to take up the detailed study of the photophysics of a 2,6-substituted polypyridinic compound,

0022-2313/95/%09.50 0 1995 - Elsevier Science B.V. All rights reserved SSDI 0022-23 13(94)00049-2

144

A. Sarkar, S. Chakravorti

I Journal of Luminescence

2,2’:6’,2”-terpyridine (TP), in relation to that of DPP and BP and to examine the change of the spectral behaviour of the protonated form of those molecules.

2. Experimental The compounds DPP and TP (Aldrich Chemical Co. Inc.) were sublimated several times under reduced pressure to obtain pure samples. Their melting points were checked before use. The solvents methylcyclohexane (MCH), ethanol methanol (MeOH) and sulphuric (EtOH), acid (E. Merck, Spectrophotometric grade) were used as supplied, but only after checking the purity fluorimetrically in the wavelength range of interest. The absorption spectra at room temperature were recorded with a Shimadzu absorption spectrophotometer model UV-21OlPC, and the fluorescence and phosphorescence spectra were obtained with a Perkin-Elmer MPF 44A Spectrofluorimeter. For emission measurements, the sample concentration was in every case maintained at - lo-’ M, in order to avoid aggregation problems presumably present as dimeric species for concentration > lop4 M. The phosphorescence lifetime was measured from the decay of phosphorescence intensity with time on the recorder. The quantum yields were measured from the corrected area under the emission curve. DPP in organic solvents and 4-Phenyl pyridine in water and in a 0.1 N H,S04 solution were used as fluorescence quantum yield standards [1,4]. To measure the phosphorescence quantum yield, benzonitrile was taken as a standard assuming the value of 0.43

ca The fluorescence lifetime was measured with a time-correlated single-photon counting fluorimeter (model 199, Edinburgh Instrument, UK) with a conventional L-format arrangement. The excitation source was a N,-filled nanosecond flash lamp with an instrumental response function of about 1.2 ns (full width at half maxima (FWHM) at 30 KHz repetition rate). The decay functions were deconvoluted with the Applied Photophysics Decon Programme.

63 (I 995) 143- 148

3. Results and discussion 3.1. Photophysics

of TP

At room temperature TP shows two well-defined absorption bands in nonpolar (MCH, CH) and polar (EtOH, MeOH) solvents like DPP in the UV region. In MCH two band systems appear at 35984cm-’ (277.8 nm) and 42463 cm- ’ (235.4 nm) with extinction coefficients 14.2 x lo3 and 15.4 x lo3 dm3 mol- ’ cm-‘, respectively. On going to a polar solvent EtOH, the long wavelength side band is shifted 346 cm-’ towards the red and the other band is unshifted. Comparing the position and extinction coefficients with other heteroatomic compounds [4,6], these two rc, 7c* transitions are assigned as ‘Bi (corresponding band in benzene is iLi, t 1Al,) and ‘L,, respectively. A transition to the n, 7c*state, which should be the lowest excited singlet in N-heterocyclic molecules, is expected to appear above 300 nm as a tail on the long wavelength band. However, it could not be detected in solution spectra of DPP and TP. Two shoulders on the ‘Lb band of TP around (301.9 nm) and 31949 cm-’ 33113cm-’ (312.9 nm) are observed in nonpolar solvents. Unfortunately, it is impossible at this stage to say definitely whether the 31949 cm-’ is the n, rt* transition since it is generally submerged under the ‘Lb band of heterocyclics in polar solvents or the &0 of that transition. But each of these shoulders has too high an extinction coefficient to be an n, rc* transition [7,8]. Emission and fluorescence excitation spectra of TP in polar and apolar solvents were obtained both at room temperature and at 77 K. The fluorescence and phosphorescence spectra in ethanolic glass at 77 K is well-structured (Fig. 1) and the vibronic assignment is shown in Table 1. The 0,O bands in fluorescence and phosphorescence have been assigned to 31397 cm-’ (318.4 nm) and 22962 cm-’ (435.5 nm), respectively. The vibronic assignment shows that the vibrations involved are mainly the progressions and combinations of totally symmetric vibrations. From the nature of the emission spectra it seems that TP is planar in both the excited singlet and triplet states. Room temperature (300 K) emission spectra in different solvents show

A. Sarkar, S. Chakravorti I Journal of Luminescence

from the @J& ratio (3.89). A comparison of the c#+,/& ratios of 2,6-lutidine (LTD) [l], BP, TP, DPP (Table 2) and 2-phenyl pyridine (PP) [9] indicates the presence of interesting photophysics due to the substitution of different groups in the 2,6positions of pyridine. In PP the fluorescence quantum yield is small (0.09) but the phosphorescence yield is even smaller (0.01) which is indicative of highly efficient nonradiative channels. Addition of one phenyl ring in PP, i.e. DPP, causes a substantial increase in the fluorescence yield (0.62) with a simultaneous closing of the nonradiative channels. We find for BP, a very high @,,/& ratio (340) because of negligible fluorescence and the presence of another pyridine ring on BP, i.e. TP, completely alters the photophysics. The analysis of the rate parameters (Table 2) of TP and BP reveals that the radiative fluorescence rate constant (kp) decreases with the simultaneous increase in klsc. The internal conversion rate k, of BP also seems to be very high along with klsc (Table 2) compared to that of TP. While the value of ki, the radiative phosphorescence rate remains unchanged, an increase in the nonradiative triplet decay rate kNR is discernible,

TP fl

Wavelength Fig. 1. Emission (-) at 77 K.

spectra

(nml

of TP in EtOH

(----)

and in MCH

a red shift as the solvent is changed from one of nonpolar to polar character. A similar red shift is observed in fluorescence spectra at room temperature when compared to that obtained at low temperature (77 K). The phosphorescence spectrum in EtOH is more structured and intense compared to the fluorescence spectrum at 77 K, which is evident Table 1 Vibronic

analysis

of fluorescence

and phosphorescence

spectra

145

63 (1995) 143-148

of TP in EtOH

Fluorescence

glass at 77 K

Phosphorescence

Bands (cm-‘)

Av (cm-‘)

Assignment

31397 31104 30959 30487 30 303 30084 29958 29481

0 293 438 910 1094 1313 1439 1916

0, 0 O-293 (vl) O-438 (vz) O-910 (Yj) op 1094 (v‘$) o-1313 (vs) o- 1439 (vg) o-2 x !J-Vr,

Observed fundamental in Raman or IR [18] (cm-‘)

285 420 920 1105 1324 1443

AV

Bands (cm ‘)

(cm ‘1

22 962 22 296 21978 21857 21551 21367 21322 21150 20 964 20929 20 682 20 555 20449 20 141 19821 19 157

0 666 984 1105 1411 1595 1640 1812 1998 2033 2280 2407 2513 2821 3141 3805

Assignment

Observed fundamental in Raman or IR [18] (cm- ‘)

08 O-666 (V,) O-984 (vs) o-1 105 (Vq) o-141 1 (Yg) o-1595 (1’10) oGv,~v* o&v,+, o-2 x v* o-v,+, OGV,~V, 0 0-V*-V, o-v,-Vg o-2 x Vg 0-2x v*-v‘$ opvsp2 x vg

658 996 1105 1413 I596

146

A. Sarkar, S. Chakravorti 1 Journal of Luminescence 63 (1995) 143- I48

Table 2 Emission Name of the sample

TP BP DPP’ MTP’ a Ref. b Ref. ’ Ref. ’ 7p =

data and excited state rate parameters Fluorescence ~0.0

Phosphorescence

q$

of TP and some other related compounds 4,

or

rp

k,O

k, E O*

(nd

(s)

(lo-s-‘)

:s$

k,, (s-l)

knc (lo-as-‘)

k, (lO-*s-l)

k; (SC’)

kISC (lo-ss-1)

2.1 0.05” 1.8 28.5

2.1 0.9 3.2 5.6

0.33 0.10 3.49 0.03

0.18 0.19 0.03 0.02

0.29 0.92 0.28 0.16

3.34 199.9 2.14 0.35

2.06 165.9 1.91 0.31

0.47 1.18 0.3 1 0.18

1.28 34.0 0.23 0.04

%.o

(cm-‘)

(cm-‘)

31397 27 397 30911 30 959

22 962 23 529 22 573 22 896

[2]. 1191 [l]. l/(k,o + k,,), kNR = (l/r,

= k,,, = k,O&$P,> k. = kWl$r

0.09 5 x 10-4s 0.62 0.08

- l/r;),

k,,

0.35 0.17b 0.04 0.11

in EtOH at 77 K

= k,o(l - $~r)/& k; = &Al

kNRE 0’

- #r)r,

- 1) - k,sc.

which may be the reason for the low phosphorescence quantum yield in spite of the high intersystem crossing rate. The fluorescence lifetime (rr) of TP was measured to be approximately 2.7 ) 0.13 ns. Regarding the positions of singlet and triplet energy levels and the lifetimes, TP seems to be more similar in nature to other molecules of CZVsymmetry, DPP and meta-terphenyl (MTP) than those of BP (Table 2). This observation is evident, because the electronic states are extensively affected upon pyridyl substitution to BP. In MCH a weak fluorescence whose band origin is at 30 864 cm - ’ (323.9 nm) should also be observed for TP, but no phosphorescence could be detected (Fig. l), which may be attributed to a solvent-dependent intersystem crossing (T, -+ S,) process [lO,ll]. 3.2. Spectra of protonated DPP and TP While the solubility of DPP in pure water is very low, it can be solubilized in the presence of sulphuric acid. The absorption spectra of DPP in neutral aqueous solution is almost the same as that obtained in nonpolar solvents. Unprotonated DPP shows two absorption bands around 33 400 cm- ’ (299.3 nm) and 41494cm-’ (240.9 nm), but in 7 x lop4 N H,SO, solution, the band around 33400 cm-’ shifts to 31221 cm-’ (320.2 nm) and the 41494 cm- ’ band becomes resolved into two vibrational bands. This character of the spectrum is maintained upto [H,SO,] = 9.7 N, but the inten-

sity of the bands increases with the concentration of the H+ ion. DPP in sulphuric acid solution fluoresces at 25 316 cm-’ (394.9 nm) and its intensity increases with increasing acid concentration but at very high concentration the fluorescence is quenched. In alkaline solution (0.1 N NaOH) DPP absorbs at the same energy as that of the neutral molecule and re-emit at around 29000 cm- ’ (344.7 nm). Therefore, the spectra in acidic solution is identified as that of singly protonated DPP. We point out some interesting features, noted below, of the absorption and emission spectra of TP at room temperature (300 K) in different solvents. The absorption spectra of TP in EtOH, MCH and aqueous NaOH solution are similar. An extra band system with two peaks at 31437 (1 - 318.0 nm, E - 7.2 x 103) and 30003 cm-’ (333.2 nm) and a broad shoulder around 33 898 cm- ’ (294.9 nm) in the absorption spectrum of TP in neutral (de-ionized) aqueous solution have been observed (Fig. 2) compared to that in EtOH or MCH solutions. In mild acidic solution ( - 7 x lop4 N H,SO,) the shoulder vanishes and in 0.1 N H,S04 solution, a much intensified (a - 13.4 x 103) band appears in approximately the same position (Fig. 2) mentioned above. Like absorption spectra, fluorescence spectra of TP in EtOH, MCH and in aqueous NaOH solution seem to be similar. In fluorescence spectrum of TP in neutral aqueous solution two sharp peaks are observed at 29 674 cm-’ (336.9 nm) and 28 571 cm-’ (349.9 nm) (Fig. 3) which are red shifted by about

A. Sarkar, S. Chakravorti

0.8

/ Journal of Luminescence

Table 3 Fluorescence

’ A.

141

63 (1995) 143%148

quantum

yield of DPP and TP in different

media

at 300 K Molecule

EtOH

MCH

& in 0.1 N NaOH

TP DPP

0

225

250

275 Wavelength

Fig. 2. Absorption spectra 0.1 N NaOH (----), Hz0 300 K.

325

350

(nm)

of TP in EtOH (-..-), MCH (-,-), (--) and 0.1 N H2S04 (---) at

Wavelength Fig. 3. Fluorescence spectra (~~~~~), 0.1 N NaOH (-.-), (--) at 300 K.

300

(nm)

of TP in EtOH (-..-), MCH H,O (---) and 0.1 N H2S04

1286 cm-’ compared to that in EtOH or MCH solutions. The 0, 0 band in moderately acidic (0.1 N H,SO,) solution shows a larger shift ( - 2058 cm- ‘) in the red region and is also more

0.09 0.62

0.04 0.5

0.11

Water

0.21 Low solubility

0.1 N HzS04 0.61 0.9

intense. The intensity of the band was found to increase with HzS04 concentration upto 4.7 N. The different quantum yield data of DPP and TP in different media are tabulated in Table 3. It can be seen that in both molecules, shorter wavelength emission occurs in the basic medium indicating the increased basicity or proton affinity of the molecules in the excited state, which ensures migration of charge towards the nitrogen atom [6,12]. ApK, values of DPP and TP were estimated to be atleast 4 and 5.2, respectively. The absorption of TP in nonaqueous solvents has been rightly [ 133 assigned to the free base form and the repulsion between the N-lonepair electrons may cause TP to resemble transcoplanar MTP resulting in a splitting of the absorption band. The absorption spectrum of TP in acidic MeOH, assigned to the protonated form, is nearly identical in nature with that of aqueous acidic solution. The nature of the absorption band changes completely in alkaline aqueous solution and has a resemblance with that in polar or nonpolar solution with some red shift. The foregoing observations lead us to attribute the absorption bands of TP in neutral aqueous solution to a new species ~ a covalent hydrate [13,14] formed by the addition of Hz0 in TP in accordance with TP + H,O=TP.H,O. Studies of the formation of covalent hydrates of similar aromatic N-heterocyclics have revealed that [ 151 more than one H,O molecule may be added to the same ring and the added H20 may migrate to different sites of the ring. Ultimately all the possible hydrated species, including protonated forms, are in equilibrium with the anhydrous forms.

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A. Sarkar, S. Chakravorti

/ Journal of Luminescence

As NaOH is added to the aqueous solution, the absorption spectrum is changed (Fig. 2) and the new equilibrium is TP.H20

+ OH- =TP*OH-

+ H20,

where TP. OH- is the deprotonated form of the hydrated species. The exact nature of the protonated species of TP is unknown but the molar extinction coefficients of the absorption bands indicate that the doubly protonated species, TPH: + , dominates in a 0.1 N H,SO, solution [16,17]. The singly protonated TP form occurs at a much lower acid concentrations [ 171 but it could not be detected in aqueous acidic solution since different species are also present in the aqueous medium. The 28 902 cm- ’ (345.9 nm) emission observed from moderately acidic solution of TP is assigned as fluorescence from the Si state of TPH: + . From Table 3 it is evident that TP exists as a different species in neutral water as its fluorescence quantum yield is quite high with respect to that in polar solvents and quite low with respect to that in 0.1 N acidic solutions. Therefore, we assign the 29 674 cm- ’ emission as arising from the Si state of the covalent hydrate TP* H20. The $r of TP in 0.1 N NaOH solution is very close to that in polar solvents (Table 3) and the emission spectra of TP in alkaline aqueous solutions are virtually the same as that in neat nonaqueous solvents (Fig. 3). Therefore, it is assumed that TP. OH- is transformed to TP according to the equilibrium TP.OH-=TP

+ OH-.

We could not detect any triply protonated species of TP except at very high concentrations of acid. Previously it was established [ 173 that TP is a diacidic base and its composite pK, value is 7.0 f 0.1. It is expected that the addition of a third proton would be difficult because then its pK, would be negative due to the increasing positive charge of the base. If there is resonance across the internuclei bonds giving a partial double bond character, the molecule is practically planar. In all three possible conformations, the addition of a third proton would be greatly hindered.

63 (1995) 143- I48

From the above discussions it is expected that the pyridyl rings of TP are almost coplanar in both its ground and lowest excited states and that the nonradiative deactivation channel of S1 is highly reduced when TP is transformed into its protonated form. On the other hand, TP in MCH in its free base form is deactivated from its lowest singlet state mainly through nonradiative pathways.

Acknowledgement The authors thank Dr. R. Dutta of Physical Chemistry Department for lifetime measurement.

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