Excimer emission in 9-aminoacridine hydrochloride

Volume 80, number 3

EXCIMER

CHEMICAL

EMISSLON XN 9-AMINOACRIDINE

lSJune1981

PHYSICS LE-FIERS

HYDROCHLORIDE

P. GANGOLA, N-8. JOSH1 e and D.D. PANT Department

of Physics, DS_B. College, Kumaun

University, Narni Tal, In&k

Received 6 February 1981; in t-ma1form 23 March 1981

Ewrmcr emission In concentrated aqueous solutions of 9-aminoacridine hydrochloride has been observed at 555 nm. A study of monomer, dimer and escimer emission, absorption and excitation spectra shows that the excited dimer emission is transformed into elcimer emissron at higher temperatures. Unusuaiiy huge hypochromism is observed on dimer formation.

Excimer emission in several fhrid solutions of aromatic hydrocarbons occurs due to the process M+M*+E*-+M+Mtizv, where M denotes the ground-state monomer, M* the excited monomer and E” the excimer which emits at considerably lower frequency than the monomer. Exciton and charge-transfer interactions are generally accepted to be the cause of the stabilization of extimers [ 1,2]. Besides the excimer formation by dssociation of an excited and unexcited monomer, a few cases of excimer emission due to dissociation of dimers have also been studied [3--51. We report here the excimer emission in I)-ammoacridine hydrochloride aqueous soiutions which appears to be due to the excitation of the ground-state dimer followed by a nonradiative transition to the excimer state, Baminoacridme hydrochloride monohydrate (Fluka AG) was purified by repeated crystalhzation. No impurity was detected by absorption spectra nor thin-layer chromatography. AnalaR grade glycerol was further purified by vacuum distillation and water was triply distdled. Absorption, excitation and emission specrra were recorded with a Spex Fluorolog model 1902, employing two grating double monochromators, a photon counting device and xenon arc for excitation. Absorption measurements were carried out in glass celIs of *

Present address: Department of Biophysics, Michigan State

University, East Iauskrg, Mxhigan 48823,

418

USA.

10-0.3 mm path lengths, fabricated in this laboratory. The cells were calibrated by measuring the optical density of suitable solution in a standard cell and the fabricated cell.

---? 3i/ E

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z=6

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1 ‘t

I

72 I

-

%

I

*2!2

Ei

6K s-3

---

-2

0

-L log

c

H x W

WAVELENGTH

1N

nm

Fig. 1. Concentration dependence of absorption spectra of 9aminoacridine hydrochIoride in water at 288 K: (1) 1 X low5 M; (2) 1 x lo4 M, (3) 1 X 10m3 M; (4) 1 X 10d2 M; and (5) 5 x 10M2 M. Inset: plot of E at 428 nm versus log concentratiOil.

0 009-2614/81/0000-0000/S

02.50

0 North-Holland

Publishing Company

Volume 80, number 3

15 June 1981

CHEMICAL PHYSICS LETTERS

The long-wavelength absorption spectra for various concentrations of 9-aminoacridine hydrochloride are given in fig. 1. Although there is an unusually large hypochromism due to aggregate formation, the redshift is very small (275 cm-l). Following Hosoya et al. [6] the plot of e versus log C is shown in the inset of fig. 1, where E is the extinction coefficient and C the concentration. The curve tallies with a similar plot for benzoic acid reported by these authors. We conclude that in spite of the probability of higher aggregate formation, only monomers and dimers are present at room temperature even in the most concentrated solution. The two dotted horizontal lines in the inset are the asymptotes for the curve and represent the values of extinction coefficients eD and ebl for dimer and monomer respectively. Log C at E = $(eD + eM) gives the pK value. The equihbrium constant K for the monomer-dimer equilibrium in water at 288 K isthus found to be 1 X 103 mol-l_ Fig. 2 shows the emission spectra at 288 K for vari-

ous concentrations

on excitation

with 400 nm radiaspectrum with at 460 nm. All the spectra are normalized with &na?c constant intensity at 460 run. The broad band with tion. The monomer has a structured

&,,,

at 555 mn has a large Stokes shift and has the

characteristics of excimer emission. In viscous solutions its intensity is decreased and at low temperatures it vanishes. On excitation of the 5 X 10-z M solution with 450 mu, only the excimer emission is obtained to the total exclusion of the monomer emission. The excitation spectrum monitored at the monomer peak resembles the monomer absorption spectrum for low concentration while the excitation spectrum monitored at the excimer peak resembles the dimer absorption spectrum, i.e. red-shifted by 275 cm-l (fig. 3). This leaves no doubt as to the origin of the excimer emission_ The excimer emission is thus due to radiationless crossing from the excited dimer to the excimer state stabilized by relaxation and charge-transfer interaction. It is difficult to assign the 555 nrn band

to dimer emission not only on account of the large

,

WAVELENGTH

.’

IN

nm

Fig. 2. Concentration dependence of fluorescence spectra of 9-arninoacridinehydrochloride in water at 288 K excited at 400 NIB:(1) 1 X 1O-5 M; (2) 1 X lo-* M; (3) 2 X lo-* M; (4) 3 x lo-* M: (5)4 X lo-* M; and (6) 5 X lo-* M. Inset: plot of [C] /I versus (l/r) y 2.

WAVELENGTH

IN nm

;. PAbsorptionand excitation spectra of 5 X 10S2 M 9aminoacridine hydrochloride in water at 288 KE (1) absorp tionspectrum; (2) excitationspec~monitoredat540 run; (3) excitation spectrum monitored at 460 nm; and (4) absorption spectrum of monomer (1 X lo-’

I@.

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15 June 1981

CHEMICAL PHYSICS LETTERS

Volume 80, number 3

Stokes shift but also because we have obtained direct evidence for dimer or aggregate emission at low temperatures. Additional evidence for excimer emission due to dissociation of dimers is obtained by separately calculating the equilibrium constant from the data of excimer intensity using the expression [7] K=

[D]/([Cl

-

60-

W1)2,

where [C] IS the total concentration and [D] the concentration of the dimer. Assuming the excimer intensity I to be proportional to the dimer concentration [D] the following equation is obtained: [Cl/l=

214 + (1/@3I/‘(l/r)‘/2

2

where y is a constant including several experimental factors. The plot of [Cl/I versus (1/1)lj2 as shown in the inset of fig. 2 is a straight line confirming our assumption I = [D]. The equihbrium constant K = 1.05X lo3 mol-I, calculated from the slope and intercept of the straight line, is in good agreement wrth the value obtained from absorption measurements. We did not succeed in the softenmg glass technique of Ferguson [4] to ensure the presence of only dimers at low temperatures as, in spite of several trials, the results remained the same. A 5 X 10e2 M solutron in a glycerol : water mixture at 80 K shows an absorption spectrum similar in structure to the room-temperature dimer spectrum except for a further red-shift of 270 cm-l; whereas the monomer spectrum does not shift with temperature. We assume that even at low temperatures only monomers and dimers exist in concentrated solution. Any small change in shift may be

due to geometrical or relaxation (in emission) changes. The concentration variation of emission spectra at 80 K proves that only two types of species are present.

450

650

550 WAVELENGTH

IN

nm

Fig. 4. Concentration dependence of fluorescence spectra of 9aminoacridine hydrochloride in gIyceroI : water at 80 K excited at 400 nm. (1) 1 X 10m5 M; (2) 1 X low3 M, (3) 5 X low3 M; (4) 1 X lo-* M, and (5) 5 X lo-* hl.(6) 5 X lo-* M excited at 450 nm.

Frg. 4 shows the 80 K emission spectra of 1 X 10m55 X 10-Z M solutions. It will be observed that broadening of the bands appears at -5 X 10-S M and the broad band shifts to 480 nm for a concentration of 5 X IO-2 M. But actually all the spectra in this range are due to the superposition of the monomer spectrum with structured peaks and a structureless peak at 480 nm of the dimer spectrum. For example, the 1 X 10M2 at 470 nm, on excitation with M spectrum with h,, 450 nm, exhibits only the broad emission peak at 480 nm which also is the wavelength for 5 X 10B2 M

Table 1 Absorption, fluorescence and phosphorescence maxima (in run) for aqueous solutions of 9-aminoacridine hydrochloride

Itl0Il0Iller

d’krrer excirrer

420

Phosphorescence

FIrrorescJnce

Absorption 288 K

80K

288 K

80K

80K

424,402, 384.363 429,405, 387,365 -

424,402, 384,363 435,410, 390,372 -

435,460, 488,520 -

432,455, 483,520 480

560

555

-

-

570

Volume 80, number 3

CHEMICAL PHYSICS LETTERS

emission (fig. 4). The dimer spectrum thus has its peak at 480 run irrespective of an apparent shift with concentration. The temperature variation of emission from the 5 X 10e2 M solution was done on excitation by 400 and 450 M-I. Whereas in the former case, with increase in temperature, dimer (Amax = 480 nm), excimer (%K3X = 550 nm) and structured monomer bands = 460 nm) appear, with 450 nm excitation only (x th?%ner emission is seen at low temperatures, which changes to excimer emission on increasing the temperature. The potential-energy diagram to explain these observations is of a formal type [S] and the processes involved in excimer formation are therefore not reproduced here. On dimer formation an enhanced phosphorescence band appears at 570 nm and shifts relative to the weak

15 June 1981

monomer phosphorescence at 560 nm. The relevant spectral data are given in table 1.

References [l] T. Azumi, A.T. Armstrong and S.P. Md=ly~, J. Chem. Phys. 41 (1964) 3839. f2] 3-B. Birks, Progr, Reaction Kinetics 5 (1970) 181. [3] J. Tanaka, Bull Chem. Sot. Japan 36 (1963) 1237. [4] J. Ferguson, J. Chem. Phys. 44 (1966) 2677. [S] N. Mataga and T. Kubota, Molecular interactions and electronic spectra (Dekker, New York, 1970) p. 424. [6] H. Hosoya, 1. Tanaka and S. Nagakura, J. MoL Spectry. 8 (1962) 257. [7] K. Inoue and M. Itoh, BulL Chem. Sot. Japan 52 (1979) 45.

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