PERTURBATIVE EFFECTS ON THE LOWEST EXCITED STATES OF ANTHRACENE STUDIED BY TWO-PHOTON EXCITATION SPECTROSCOPY

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PERTURBATIVE EFFECTS ON THE LOWEST

EXCITED STATES OF ANTHRACENE STUDIED BY

TWO-PHOTON EXCITATION SPECTROSCOPY

G. Marconi, P. Salvi

To cite this version:

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PERTURBATIVE EFFECTS

ON

THE LOWEST EXCITED STATES OF ANTHRACENE STUDIED

BY

TWO-PHOTON EXCITATION SPECTROSCOPY

G.

Marconi and P.R. ~alvi'

I s t i t u t o

F.R.A.

E.-C.N.R.,

Bologna, I t a l y

+ ~ i ~ a r t i r n e n t o

d i Chimica, Laboratorio d i Spettroscopia MoZecolare, universitd-

d i Firenze, Firenze, I t a l y

Abstract- The two-photon e x c i t a t i o n spectrum o f anthracene i n t h e region o f t h e two lowest t r a n s i t i o n s has been measured both i n s o l u t i o n and i n Shpol s k i i matrix. The main features o f t h e spectrum are discussed i n terms o f pey t u r b a t i v e e f f e c t s such as t h e inductive e f f e c t and t h e v i b r o n i c interactions, on t h e p r o p e r t i e s o f t h e lowest s i n g l e t s t a t e s .

I

-

I n t r o d u c t i o n

I n t h e past years, some p r o p e r t i e s o f anthracene, l i k e t h e very high quantum y i e l d o f fluorescence, t h e photoconductivity and t h e p o s s i b i l i t y o f o b t a i n i n g molecular c r y s t a l s o f great p u r i t y , have a t t r a c t e d a number o f experimental and t h e o r e t i c a l studies i n t h e f i e l d o f sol

id

s t a t e physics /1/.0n t h e other hand some p r o p e r t i e s o f t h e i s o l a t e d molecule, such as t h e exact o r d e r i n g o f t h e low-lying e x c i t e d sta- t e s and t h e i r nature are not completely clear, due t o t h e superposition o f t h e two lowest e l e c t r o n i c bands i n t h e ordinary one-photon absorption spectra. The two-phc t o n e x c i t a t i o n spectroscopy has proven very h e l p f u l t o i n d i v i d u a t e hidden t r a n s i - t i o n s , t a k i n g advantage o f t h e d i f f e r e n t s e l e c t i o n r u l e s which allow f o r gcsg t r a n s i t ions instead o f t h e usual gt-w t r a n s i t ion r u l e s f o r one-photon processes

12,'.

I n t h i s note we present t h e r e s u l t s o f a two-photon study o f anthracene, w i t h spec t r a obtained both i n s o l u t i o n and i n a Shpolskii m a t r i x a t d i f f e r e n t concentrations.

I t

i s we

l

known t h a t a Shpo

l

sk

i i

m a t r i x (frozen normal alkane s o l u t i o n ) provides weakly i n t e r a c t i n g host c r y s t a l s f o r aromatic molecules, so t h a t t h e guest molecule can be thought as imbedded i n "cold d i l u t e d gases". Therefore these matrices allow f o r t h e study o f t h e molecular eigenstates as " i s o l a t e d " and w i t h t h e advantage o f b e t t e r spectral r e s o l u t i o n than i n normal s o l u t i o n s . The spectra so obtained, are

duate both t h e o r

i

-

g i n o f t h e second s i n g l e t s t a t e (1B2

)

o f anthracene and t h e source o f v i bron i c ac-

t i v i t y o f t h e various v i b r a t i o n a l moles.

I

-

Experimental

The two-photon exper

i

menta

l

apparatus has been described

i

n deta

i I

e l sewhere

/4/.

Solutions a t 10-3 and 10-5 o f anthracene i n n-heptane were q u i c k l y frozen i n a c r y 2 s t a t a t 10

K.

Solutions o f anthracene and 9-methy I-anthracene 10-I

M

i n

CHCl

were examined a t room temperature.

3 I I I

-

Results

I n f i g .

1

t h e two-photon e x c i t a t i o n spectrum o f anthracene and 9-methyl-anthracene i s reported whereas i n f i g . 2 t h e spectra o f anthracene dissolved i n a n-heptane

ma

_- t r i x a t 10

K

are reported f o r two d i f f e r e n t concentrations i n t h e second spectral region. As expected, t h e s o l u t i o n spectrum appears unresolved while t h e spectra i n m a t r i x are more d e t a i l e d and a1 low f o r a q u a l i t a t i v e a n a l y s i s o f t h e v i b r o n i c

con^

ponents. I n p a r t i c u l a r , t h e s o l u t i o n spectrum a t 10-3

M

c l e a r l y shows t h e presence o f a v i b r o n i c t r a n s i t ion i n t h e region 26221-28000 cm-1; t h e matr i x spectrum a t 10-5

M

shows severa

xb and a mors intense region above 28000 cnl-1, a t t r ibus l u l u '

two-photon wanenurn bar ~cm-') 21000 30000

dye wavelength ( A )

F i

g

.

1-Norma

I i

zed two-photon fluorescence exc

i

t a t ion spectra o f anthracene ( f u

l

_{l i}

ne) and 9-methy I-anthracene (dashed

I i

ne) i n

CHC

l

10-I

M

sol u t ion a t room tempera-

3

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low-lying singlets of anthracene, is also in agreement with previous MCD data which

locate the origin of the B

state at

28600

cm-1 in solution at room temperature

/6/.

2u

two-pho~on wavenurnber icm-')

Fig.

2 The two-photon excitation spectrum of anthracene in Shpolskii matrix in

the region of the second trans

it

i

on: lower trace

10-3

M;

upper trace

10-5 M.

IV

-

Discussion

In this section we discuss the results of-two-photon spectra in terms of basic con-

cepts, such as the inductive effects and the vi bronic perturbat

ions on the el ectro-

nic states.

a)

I nduct

i

ve effects

As pointed out by several authors

/ 7 , 5 / ,

the one- and mu It i-photon spectra of alter

_-

nant aromatic molecules can be rationalized taking into account the pseudosymnetry

of the lowest molecular eigenstates, jointed to the usual point group selection ru-

les for dipole transitions. In particular, the pseudoparity selection rules for the

two-photon absorption are

+cr

+

or

---.

ln the-case

yf

anthracene the+low-lyingtr~(*

singlet states are in the sequence 1 ~ - ,

2A7, 2BJu

/8/.

For

pseudo sypetry select

ion ru

Ies the &tl:~~l

v?$;n:~~?~n~B~&

Id

&

1A

--+

la-

9 2u

1A---+

18 and 1~---+

2A-, but the first resu

Its point group forbidden. On the

o t R r hana:

one el &tron krturbat

ions cause m

ix

i

ng of electronic states and possi

ble enhancement of transitions forbidden by pseudosymmetry or point group selection

rules are in order. In the case of anthracene, the lowest two states can acquire dl

pole transition from allowed final A

states through b

and b

vibrations. Other-

1 u

2u

w

i

se, enhancement of forb

i

dden transyt

ions can be produced by

1

ntroduc

i

ng su

i

tab

l

e

(5)

C7-444

JOURNAL

DE

PHYSIQUE

the on

I

y non-van

i

sh

i

ng ei ements are the one-center di agona

l

e

l ements, whereas for

states of the same pseudoparity the one-electron perturbation will be most effec-

tive for variations of the nearest neighbour resonance integrals. For anthracene,

therefore, we expect, that+the introduction of substituent will enhance the pseudo

parity forbidden 1A---+ 18 transition and that v

i

bron

i

c perturbations w

i

l l

be mc

1 u

re effective for ths 1A---+

18-

transit

ion. Moreover we expect that the inductive

effect

w

i

l l

be the larysst whe?

i

nduced by substitution on atom

9. These consider?

tions are confirmed by a qualitative analysis of the spectra of fig. 1, which show

i

ntens

i

ty enhancement for 9-methy

l -anthracene

i

n the region below 28000 cm-1 w

i

th

respect to anthracene. Thyefore we tend to attribute the lower portion of the

spectrum to the 1~---+

16 transition. This ass

ignement is also supported by the

1 u

two photon spectru8 of acr

I

d

i

ne in Shpo

l

sk

i

matr

i

x, wh

i

ch shows a weak enhance-

ment of the spectral region around 26.500 cm-1 with respect to anthracene

/5/.

Fig.

3 The trans

it ion density maps for anthracene (from ref.

5)

b) V

i

bron

i

c effects

As pointed out before, the two lowest transitions of anthracene, forbidden by point

group selection rules, become allowed through vibronic borrowing from

A

states.

In

spect

i

on of fig.

3 shows that the coup

l

i

ng should be most effective whe8 connecting

the 1A- state with the 16- state, and pseudoparity considerat

ions lead to the expec

2u

tat

i

on9that the+act

i

v

i

ty promoted by b

modes

i

n 1B-

w

i l l

be larger than that of

2u

b

modes in

18 .

To test these ideas we performed &ta

i

led MO calculations of the

v?~ron

i

ca

l l

y

i

n&ced

two-photon cross sect

i

ons, by us

i

ng a method already successful

l

y emploied to elucidate the two-photon vi bron ic spectrum of naphtalene

/3/

and pyre

ne

/9/.

In this scheme the two-photon intensity is calculated by:

where

So(

p

is the two-photon scattering tensor component a

long the

a,

directions,

defined as:

By expanding the components

4(Q)

and

p(0)

around the equ

i

I

i

br

i

um geometry, one ob-

ta ins a sum of three terms

:

the first

(A

term) couples the final state with the

i"

(6)

2 2 -1

Tab 1 e 1. I NDO/S ca

1 cu l ated Syy and Szz(e A eV

)

and i ntens i

t

i es, normal i

zed t o

l(1384) (see eqs. 1 and 2).

f o l low i ng framework /3/. The necessary cartes i an d i sp

l

acements o f t h e norma l modes

have been obtained by using t h e we

l

I -known valence force f i e l d o f Neto e t a I .

/lo/.

The r e s u l t s o f t h i s calculations are reported

i n

tab. 1 and show t h a t t h e t o t a l in-

t e n s i t y calculated f o r t h e b

modes i s larger than t h a t o f b

modes, as expected

from t h e pseudosymmetry sele?? ion r u l e s .

A

f u r t h e r analysis o p t h e resu I t s shows

t h a t the C term, which includes perturbation o f the ground state, i s p a r t i c u l a r l y

important as a source o f i n t e n s i t y f o r t h e b

modes, whereas t h e B term plays an

2u

important r o l e f o r the bl

modes. I n conclus~on

we f e e l t h a t t h e approach here p r e

sented, including a qua l I f a t ive analysis o f the symmetry select ion r u l e s f o r t h e

two-photon spectra, and d e t a i l e d vibronic coupling calculations

is

able t o give a

comprehensive p i c t u r e o f t h e nature o f t h e low-lying states o f anthracene, as revea

-

led by t h e two-photon e x c i t a t i o n spectroscopy.

References

/1/

E.A. S i l i n s h , "Organic molecular crystals", Springer series i n solid-state

science

16,

Spr i nger Ver

l

ag 1980.

/2/ L. Goodman and R.P. Rava, Acc. Chem. Res.

2 (1984) 250.

/3/ G. Marconi and G. Orlandi,

J. Chem. Soc. Faraday l l

28 (1982) 565 and r e f e r e p

ces therein.

/4/ E. Castel lucci, P. Foggi and P.R. Salvi, Chem. Phys.

6 J

(1981) 437.

/5/ P.R. Salvi, Chem. Phys. L e t t . 116 (1985) 472.

/6/ R.P. Steiner and J. Michl, J.

Am.

Chem. Soc. 100 (1978) 6861.

/7/ P.R. Cal l is, T.W. Scott and A.C.

A

l brecht, J .%em.

Phys.

78 (1983) 16.

/8/ D.M. Friedrich, R. Mathies and A.C. Albrecht,

J. Mol. Spectry.

2 (1974) 166.

/9/

G. Marconi, P.R. Salvi and

R. Quecquarini, Chem. Phys. L e t t .

107 (1984) 314.