Singlet excited monomer and excimer absorptions of pyrene

Volume

10, number

4

SINGLET

CHEMICAL

EXCITED

MONOMER

PHYSICS

AND EXCIMER

M.F.M..POST, J. LANGELAAR Laboratory

LETTERS

15 August

ABSORPTIONS

1971

OF PYRENE

and J.D.W. VAN VOORST

for Physical Chemistry, University ofAmsterdam, Amsterdam, The Netherlands Received

7 June

1971

The absorption and polarization spectra from the first excited singlet state of pyrene and the absorption spectrum of the pyrene excimer are presented. These results were obtained by laser flash spectroscopy using a frequency doul bled ruby laser pulse of 6 nanoseconds halfwidth. A qualitative preliminary assignment of the bands is given.

1. Introduction The experimental evidence for the formation of singlet excimers upon excitation of aromatic molecules in a moderately concentrated solution is the appearance of a broad structureless fluorescence at the low energy side of the monomer fluorescence [I] . Many theoretical calculations, which were concerned with the excimer formation, could only be correlated with experiment by this shift of the fluorescence, which is made up of a stabilization in the excited singlet state and a repulsive term in the ground state [2]. Recently Ottolenghi and Goldschmidt [3] increased the available information by finding a weak absorption band which they could attribute to an absorption from the excimer state. Because the available calculations (e.g. that of ;ef. [2]) are restricted to the lower extimer states, it is hard to find an accurate assignment for this single band. Therefore we have followed another approach. A rough inspectidn of the MO description of the transitions between the singlet states of the monomer and those of the excimer of pyrene shows that the Sg +-ST @,+Bu or BlgCB3J transitions in the monomer, are more or less complementary to the excimer transitions, originating from Au -3g or BluCB3g the $+$ (Blg+Blu or A,+-B;,) transitions in the. monomer description. Thus, apart from the obvious need of obtaining an extension of the excimer absorption in the near IR and 468

visible region, our first task was to find the location, absolute polarization and transition probability of the Si +-ST transitions of the monomer and to compare this with results of MO calculations. Then, assuming that the shifts of the levels due to the additional interaction in the excimer, will be within rCasonable limits, we may obtain the possible assignment of the measured excimer transitions by comparing these transitions with the calculated SE-+Sl transitions and their relative transition probabilities in the monomer description, 2. Results The experimental method used to measure the absorption from the excited state has been described previously [4]. The method we followed here for the interpretation of the oscillograms will first be discussed. The transmittance of the light of the spectroflash as a function of time at a fixed wavelength (lower curve in figs. la and 2a, curve A in figs. lb and 2b) after laser excitation consists of contributions due to: (i) absorption from the lowest excited singlet state of the monomer; (ii) absorption from the lowest triplet state of the monomer; (iii) absorption from the lowest singlet excimer state; (iv) monomer fluorescence; (v) excimer fluorescence.

Volume

10, number

4

CHEMICAL

PHYSICS

15 August

LETTERS

20 500 cm-’

23 500 crd

b

b Fig. 1. (a) Oscilloscope traces of fluorescence (upper trace) and combined absorption (lower trace) at 20500 cm-l of 1.7 X 10p3M pyrene in cyclohexane. (b) Constructed decay curves from fig. la, cf. text for explanation.

The fluorescence was measured separately (upper curve in figs. la and 2a, curve B in figs. lb and 2b), hence curve C describing the change in the absorption as a function of time can be constructed. The amount of monomer absorption D”(t) contributing to curve C follows as a function of time from t: D”(t)

= [DS(0)-DT(m)]

1971

e&”

tDT(m)

.

OS(O), which denotes the optical density of the singlet

state of the monomer directly after laser excitation, is obtained from the value of the transmittance after the initial sharp rise in curve C. At this point, which is at 5 nsec after the maximum of the laser pulse, where the laser intensity has reached less than 5% of its maximum value, the concentrations of triplet monomers and singlet excimers are too low to give a contribution to the optical density. DT(m) denotes the optical density of the triplet after a time long as compared to the decay

Fig.

2. Same as fig. 1 at 23500

cm-l.

time of the singlet species. Because of the low rate of the triplet decay the value of DT(w) can be obtained from the height of the horizontal part in curve C. The actual lifetime 7 of the singlet monomer is obtained from the exponential fluorescence decay of the monomer. With these data the decay of the absorption of the monomer as a function of time can be constructed (curve D). The absorption of the excimer (curve E) t The optical density of the first excited function of time after laser excitation

DS(t)

= OS(O)

where state.

r denotes The triplet

the actual formation

LIT(t)

= LIT(-)

[ 1 -e-$

state as a by:

ect/7,

The triplet decay lifetime compared

DT(t) +DS(t)

singlet is given

-D”(t)

lifetime of the first excited can be described by:

is ignored because with 7. Hence = OS(O)

of its relative

eCtiT+DT(-)

= [/IS(O)-LIT(-)]

singlet

long actual

[l -eet17] eCt/T+DT(-).

469

Volume

10, number

CHEMICAL PHYSICSLETTERS

4

1.5August 1971

then follows from the difference of curves C and D. In the formula given above it is assumed that the decay of the monomer singlet is pureiy exponential. This assumption is justified by the observed exponential fluorescence decay. Moreover it cti be shown by using the formula by Birks for the monomer singlet decay [5] that under our experimental conditions (i.e. at room temperature and in concentrations of 1.7 X 10W3M pyrene in cyclohexane and 6.0 X lop3111 in ethanol) the decay can be approximated by a single exponential. The singlet absorption of the excimer and the excited monomer of pyrene in cyclohexane using the analysis as described before are given in fig. 3. Additionally, the absorption arid polarization spectra from the first excited singlet of the monomer as observed in a solid solution of polymethylmetacrylafe (PMM), are given in fig. 4.

-0 05 L

I1 10

Fig. 4. Sz -ST 10W2M pyrene

x 103cm

3

'1'

10

I'#

" 15

"

"

20

"

"1

-’ w

25

Fig. 3. Excited singlet state absorption spectra of 1.7X lop3111 pyrene in cyclohexane. Monomer above; excimer below. The curves are constructed by 45 point by point measurements. . The error is small (10%) between 16 000 and 25 000 cm-l, but may be considerable (up to 50%) in the near infrared region because of the slow response of the S-l photomultiplier.

470

x103cm-' I

I

absorption in PMM.

I

I 15

I1

1

and polarization

I

I 20

II

11

I 25

spectrum

of

The accuracy of the analysis is supported by the following facts: (i) the constructed excimer absorption curve E exhibits a maximum at a certain time after laser excitation. This time (t-max) is independent of the wavelength where the absorption has been detected, and is equal to t-max occurring in the excimer fluorescence; (ii) the shape of the SE + ST absorption spectrum of the monomer as obtained by this analysis (fig. 3) is in close agreement with the Si +S; spectrum measured in PMM (fig. 4). The value for the extinction coefficients for the monomer in PMM as given in fig. 4 were calculated from the measured ratio of singlet and triplet monomer absorption, the e-values of the triplet-triplet absorption [6] and the triplet formation efficiency &I, which is taken to be 0.27 [7]. The extinction coefficients of the Sl*,+- ST absorption in cyclohexane were obtained from the e-values

Volume

10, number

4

CHEMICAL

PHYSICS

of the singlet-singlet spectrum in PMM, after performing a minor correction due to a slightly different bandshape in cyclohexane as compared to PMM. This correction is easily made if one assumes that the oscillator strength of the various transitions is independent of the solvent. The value for the extinction coefficients of the extimer absorptions follow from the equation E=EM~M

--DE

cM c,

where DE/D, denotes the ratio of the excimer absorption at 1 = t,, and the absorption of the excited monomer at t = 0. The concentration ratio ) can be calculated using the methcM(,=O)/cE(,=r od giyen by ‘Birk% al. [ 5,8] .

3. Discussion It has been shown [2,9] that the fluorescence state of the excimer of pyrene contains a large contribution and experimental

Table 1 results on pyrene

symmetry of state b)

rl

calculated energy C)

sn

1

Biil

2

Gl

3

“ig

4 5

Big A;

6

calculated EC)

singlet

excited

*

s; ‘Sl wavefunction of state S, a)

1971

of the second instead of the first excited singlet state of the monomer. Therefore the absorption spectrum from the first excited singlet state-e&the monomer and consisting of transitions of the type Ap +-BFU and B,,+B,, is expected to be different from the absorption spectrum of the excimer (see figs. 3 and 4). Apart from contributions due to charge resonance states, the absorption spectrum of the excimer will derive its transition probabilities in terms of monomer states mainly from the A,+B;, and BI~+-B$~ transitions, i.e. Si +Sg. In order to obtain an idea about which monomer states will be mainly responsible for the transitions in the excimer we use the following procedure. Firstly we had to obtain a rough assignment for the transitions from the first excited state of the monomer by comparing the location and polarization of the experimentally observed absorption band with an SCF calculation extended with configuration interaction up to first order. The position of the excited levels with respect to the ground state are given in table 1 column 4. Although a more extended calculation certainly

’

Calculated

15 August

LETTERS

*

sz;+ experimental

f

states

EC)

experimental

calculated

f

EC)

EC)

f

d)

fe)

32.1 32.6 35.8

3.7

0.001

41.5

9.4

0

46.4

14.3

0.02

A;

47.5

15.4

0

7

A;

48.0

16.1

0

8 9

Big

50.5

18.4

0

52.3

20.2

0

53.5

21.4

54.0

21.9

56.0

23.9

57.2

25.1

3.2

0

10.9

0.018

8.9

13.0

0.005

13.8

0

0.17

14.9

0

=12

15.4 17.9

0.63 1 0.28

z:;

19.7

0

> 16.0

0.035

0.31

19.5

0.052

20.9

0

1.08

21.2

0.11

21.4

0

0.22

23.4

0

0.62

24.6

0

ai The numbers denote the bonding and antibonding SCF orbitals. b) The x and y axes are taken as the short and long axes in the molecular c) In 103 cm-l. d) Experimental excimer transitions. e) Mean values

=

9 0.13

1

=23

>

0.11 0.05

plane respectively. for the solvents.

471

Volume

10, number

4

CHEMICAL

PHYSICS

will change the location of .the calculated levels no drastic changes in the general trend, however, can be expected. With this in mind a preliminary assignment is given for the Si +Si transitions in column 7 of table 1. From the results of the calculations mentioned above the transition probabilities for the monomer transitions, originating from the second excited singlet were calculated. As we mentioned before, in a first approximation these transitions will be mainly involved in the transitions of the excimer. As follows from table 1 it is calculated that in first order only the transitions S, to S,, S7 and S, have some transition probability. However, it can safely be predicted that by a configuration interaction up to second order S9 will steal some oscillator strength from S4 and S, and that S6 will obtain some transition probability by mixing with S7 In this way a tentative assignment as given in column 11 can be given. Other calculated transitions with a significant oscillator strength are at such high energies (> 35000 cm-l) that they are not considered. Although in the excimer certainly charge resonance structures will be involved, and the amount of stabilization upon excimer formation is unknown, in a first approximation we may conclude that the excimer states involved in the transitions shown in fig. 3 (below), will originate to a reasonable extent from the Ag+-Bi, (S;+S;, ST;+-S;) and the Bl,+-Bl, (S, c-s;, s*, +s;, s; +Sz) monomer transitions.

LETTERS

1.5 August

1971

Further experimental and theoretical investigations to extend the work reported here are in progress. Acknowledgement The authors are grateful to Drs. C. Tetreau and S. de Bruijn for making their calculated results available to us, and to D. Bebelaar for his technical assistance.

References 111 E.

Doller and Th. Forster, 2. Physik. Chem. 34 (1962) 132. J. Chem. Phys. 41 (1964) 121T. Azumi and S.P. McGlynn, 3839; N.J. Murrel and J. Tanaka, Mol. Phys. 7 (1964) 363. and M. Ottolenghi, J. Phys. Chem. 74 131 C.R. Goldschmidt (1970) 2041. C.J. Werkhoven, D. Bebelaar, J. Langelaar [41 D. Lavalette, and J.D.W. van Voorst, Chem. Phys. Letters 9 (1971) 230. Proc. Roy. Sot. [51 J.B. Birks, D.J. Dyson and I.H. Munro, A275 (1963) 575. Beek, H. ten Brink and [61 J. Langelaar, J. Wegdam-van J.D.W. van Voorst, Chem. Phys. Letters 7 (1970) 368. [7] C.A. Parker, Photoluminescence of solutions (Elsevier, Amsterdam, 1968) p. 310. [8] J.B. Birks, M.D. Lumb and I.H. Munro, Proc. Roy. Sot. A280 (1964) 289. [9] E. Sackmann and D. Rehm, Chem. Phys. Letters 4 (1970) 537.