High resolution excitation spectroscopy on triplet excitons in organic molecular crystals comparison of naphthalene and anthracene

JoUmalofMok?cularSt7%cfu???~ 45(1978)455-464 0 E~vierScientificPubiishingCompany, Amsterdam- PrintedinTheNetherlancis

High

resolution

on triplet

excitation

excitons

Comparison

spectroscopy

in organic

molecular

of naphthalene

H.

Port

and

Physikalfsches

and

crystals

anthracene

D. Rund

Institut,

Teil

3,

Stuttgart

Universitzt D 7000

Stuttgart-80,

exciton

absorption

W-Germany

Abstract Triplet thracene cence the

have

and

been

delayed

temperature

dependence similar non

The

directions

to the red

is explained the

crystals.

In the

can

1.

in the

solvent

upper

indicating

between

not be explafned

and

phosphoresin

thermal

shift

with

Davydov

300 K. The

comparable and

to the

the

so

an-

spectroscopy

however,

helium

naphthalene 0.0

2 K and

positions

with

and

temperature line shape and line width is very

for naphthalene

affecting ture

line

applying

excitation

between

crystals

of naphthalene

indirectly

fluorescence

of the exciton

coupling.

This

measured

range

in both

different

spectra

are room

blue

opposite

phoin

temperature, anthracene.

of the

sign

below

component

shifted

for

expansion

spectra

exciton

crystal

in these

30 K a substruc-

is observed,

which

far.

Introduction

Preliminary experiments on triplet excitation spectra of anthracene between 2 K and 300 K have been reported recently cll. Temperature effects on the triplet excitation line shapes, line

455

456 widths

and

line

result

was

the

for

the

a- and

has

been

changes

both

of

have

static

and

excitation

energies.

The

study

of

exciton

ded

to pure

clinic

with

interest, two

as

for

ture

has

case

of naphthalene

triplet Shift

anthracene

been

to

In

report

this

tronic

2.

the 0.0

the

results

in direct discussion

of

of

the

CSJ,

spectral

naphthalene

other

in the

in the

no measurable

3 in contra-

experiments

excitation of

the

struc-

hand

observed

the

c13.

spectra

anthracene

region

triplet

is

band

emissionC6

of

the

exten-

cell)

whereas

naphthalene

of

c2 3 (mono-

excitation

to those

lowest

with

unit

been

to

been

exciton

On

has

recent

the

the the

the

anthracene

the

contrast on

transition

c4I.

spectra

the

in

now

structure

in

red-shift

for

blue-shift

has

a similar

previously

transi-

concen-

pure

elec-

state.

Experimental

The

experiments

scribed For

in t11,

naphthalene

tive

triplet

were

performed

with

a spectral

the

fluorescence,

light

by

Schott

7 - 37,

GG

using

the

Coumarin

monitored

were 495

CW dye

resolution

47 was by

separated and

laser

of better used,

set-up than

and

the

phosphorescence from

choppers

one

or by

or

another Corning

de-

0.1

cm

crystals

ab cleavage

have

been

plane.

perpendicular

3.1

Excitation

The

general

spectra

grown The to

the

at helium

characteristics

of

by

the

samples

Bridgman were

method

about

1 mm

rela-

and

laser

filter

and thick

cut and

ab-plane.

temperature the

excitation

spectra

-1

de-

respectively.

in

irradiated

dye

was

which

filter

Single the

laser

absorption

layed cs

Also

its

a superposition

comparison

molecules

a large

obtained

the

presented

trating

crystal

phosphorescence

been

diction

are

The

calculated

exciton has

crystals.

C3 1.

in

contributions

behaviour

the

striking

of the 0.0

as

dynamical

most

differing

components

line

inequivalent

same

The

lines

since

naphthalene

is of special

observed. the

tentatively

molecular

triplet

of

b-polarized

explained

the

been

blue-shift

This

amcunt tion.

positions large

at helium

.

457 temperature

are

The

consist

spectra

very

phonon

satellites

of

spectra

the

EiXiS,

two

to higher are

lines

and

intense

zerophonon

lines

energies. The

main

0.0

phonon

inverted

between

Fig.

1:

km-‘3

Polarized 0.0

region

The

relative

is ten bands

The band

times are

phonon

of naphthalene

maxima

lines.

that

of

observed

lines

numbers

intensity

as

from

in the

the

and

energy

in

indicated

the

the

by

la50

of

zerophonon

ucmo

u%ol5mo6(60 -

and

anthracene

na.

II b of The

(right). anthracene

phonon

side-

than

the

the

X40,

x400,

of

side-

and

sensitivity

energetic

center

the

K;

spectrum

spectrum

differ

behaviour

uem

u7so

4.2

(left)

at higher

measure

at

into

in relative

ENERGY km3

spectra

regions

b crystal

drastically

-

excitation

weak

naphthalene

lower

no ENERGY

0.0

observed

sidebands

the

with

splits

polarization

In naphthalene

lines:

are

anthracene.

a and

which

changes

and

1 the

transition

difference

the

fig.

parallel

components)

lines

is

In

excitation

Further

The

however,

zerophonon

naphthalene

(Davydov

zerophonon

substructure.

in both

given.

intensities. of

anthracene, the

sharp

at polarized

aerophonon

intensity band

of

respectively,

in relative

similar

factors distance

gravity

of

of the

some

zerox600. side-

zerophonon

21212 cm-1

212025cm-l

n

1cm-’

a-camp.

l&7595 cm-’

14738 cm -1

ENERGY Fig.

2:

Zerophonon energy

line To

is mainly

emphasize

of an

agreement

the

elements molecules

A between

9.5

The

zerophonon

scale. width one

with

the

the

lower

in naphthalene

maximum

in

the

and

order

and

are

for

K.

The

naphthalene

0.0

parallel

components

b-components.

L7,81 in

and

the

of

This

the

and

was

21.5

by

cm-'

in

calcuinequi-

energetic

differen-

the

measured

reis

interaction

translationally The

magnitude to about

in anthracene

in

f7,8,10,113. given

in

differently

component

is

fig-

2 in

larger

polarized, smaller

than

the the

energy line upper

anthracene as well (full width at half -1 cm ). In both cases the lines are

of 0.2

a.

are

components theoretical

sign

is determined

to about

are

4,2

anthracene

Davydov

lattice.

lines

alone lines

the

the

A = 8 JAB) and

energy

in

neighbouring

literature

lines

Although of

the

aof

a change

zerophonon

in naphthalene

agreement

as

B in the

splitting

traces,

but

b,

experiments

between

A and

the

(Davydov

cn?

JAD

at

(below).

polarization

following

to

excitation

in all

parallel

previous

valent

same

anthracene

polarization

It is due

matrix

of JAB

is the for

in the

with

in polarized

different

inverted

lations[93.

ce

and

polarized

distinguished sult

lines

scale

(above)

-

459

inhomogeneously depending cene

on

also

thalene

exhibits

instead

comparable tail.

changes

on

probably

line

and does

crystal

than had

with

further not

it

naphthalene

those been

is more

or

Both

crystals reports

experimentally

on

between

two

the

different less much

C8,12],

width

in anthraIn naph-

lines

high

energy due

b-component smaller

which

of

samples:

resolved

a- and

show

line

shape.

with

substructure

the

component

line

appears

width.

in previous

shape,

energy

symmetrical

change

line

line

higher

structure

quality

inhomogeneous

investigated

widths

one

Gaussian The

a multiplet

structure

in

with

quality.

intensity

The

depending all

broadened crystal

to in

line

therefore

limited.

(a)

ENERGY -

(b)

I 21100

Fig.

3:

2l300

212cm ENERGY

Temperature

km-‘1

-

-

dependence

of

the

zerophonon

components

naphthalene (a) a- and

b-component

(b) b-component

between

between

7 K and

40 K and

300

K

40 K

in

460 3.2 Temperature

dependence

The temperature

dependent

is very

to that of anthracene,

similar

line shape

behaviour which

of naphthalene

has been

characte-

rized already in c 13. Symmetrical line broadening of both zerophonon components could be followed up to about 100 K. At higher temperatures

the phonon

sideband

contributions

are increasingly

superimposed at first only with Stokes contributions at the high energy side of the zerophonon line, appearing already at 4.2 K

(fig. l), later on with

additional

tions

at the low energy

Examples

side.

contribu-

the low and high

I

t

7-

tD

yg 6z sE g

Anti-Stokes for both

4-

3u. $ 2l20

40

60

80

loo

60

80

100

TCKI7-

20

40 TCKI

Fig.

4:

Temperature

-

dependence

of the line width

r of the zero-

phonon a- and b-components of naphthalene (above) and anthracene (below). The solid lines merely connect experimental points.

461

0

a-camp.

1 __-A

A cizf+Gq

L

I 40

.II

I

.

120

I

I

200

I

280

TCKI-

Fig.

5:

Shift

and

a-

The

zero

for

naphthalene

cene

(DTGHT).

in

lines

bands

at

any

room

The

zerophonon

low

temperature

more

the

intense

width

Fig.

for

a further there

to

results

K

100

is

energy

is

to

evaluation

at

21202.5

in is

line

width

at half

plotted

taken,

above

to

the

than

anthra-

fig.

3. The

smeared

out

structureless

width

(full

maximum) fig.

width

in

the

4. For

the

line

width

of

40 K its

total

line

different

component

-1

temperature

in

40 K the

cm

for

The

a total -1 .

are

the

temperature.

cm-’

given

of

of

in the high

below

smaller are

14738

component K

I' (half

both that

polarization,

for

of

the

anthracene

the

upper

very

similar

and

lead

to

temperature

effect

not

mentioned

and

one.

a line

The width

K.

a monotonic

temperature,

a-component

cm

correlated

dependencies -1 at 100

increasing the

up

lower

at

disappears.

exhibit 200

positions

taken

are

300

naphthalene

6 cm

3 shows

and

structure

widths

not

the

to

roughly

range of

1

is

zerophonon up

is remaining

quantitative the

of

scale

T,=IFT

maxima

a function

naphthalene

upper

line

Qualitatively with

of

subcomponent

of

temperature about

(

the

as

energy

temperature

Obviously

naphthalene

of

the

sideband

component

width.

of

Equally

maximum)

(A) of

b-components

the

40 K.

at

total

upper

and

behaviour

substructure

at half

distance

0.0

temperature

already

TCKI-

for

shift the

the red, but is

of anthracene.

given

in

Plotted

of

the

b-component at

to

different

fig.

5 in

line

positions the

amounts.

comparison

are the positions

above.

blue

and

The with

of the line

462 maxima

as

a function

the

zerophonon

0.0

transition,

naphthalene the

plot

the

lines.

for

energetic of

the

the

anthracene

the They

distance is

A,

total

shift

of

helium

and

room

for

Up

the

to

the

the

center

upon

gravity

of

nearly

a-

in

a-

of

and

their

The

amount

naphthalene

amount

and

equal

of

the

b-component

for

for in

of

the

line.

both the

to

plot

blue-shift

in dashed

a-component

is

the

positive

dependence

temperature

of

in

general

refers

anthracene

and

is different

5 drawn

room

temperature

negative

a temperature

fig.

scale or

red-shift,

effect to

in

larger

anthracene.

are

reflecting

leading

energy

naphthalene

general

temperature

thus

shift

in

of

respectively.

The

The

temperature.

reflecting

b-components

and

of

a-component

both

between

crystals.

4. Discussion The

present

thalene and

and

phonon

This

triplet

The

at

and

width

lene

coupling

processes

The

the

lower

optical

exciton

band

scattering and both

explains

up

than

the

The

exciton

of

the

discussion

naphlines

behaviour

proposed

t53

to

zerophonon

to

similar

coupling

in anthracene,

triplet

evaluation

zerophonon

shape

on

in naphthalene

100 K and

to

between

in both the

basis

in naphthalene.

leading

temperatures.

smaller

of

the

anthrabroade-

total

line

shapes

in

naphtha-

strength

a result

bandwidth

coupling

and line

in

re-

naphthalene

strength

phonon

probably at

sideband

c133.

different structure

elsewhereC141.

symmetrical the

same,

higher

and

components

the

amounts

given

supports dov

are

at higher

temperatures be

been

phonon

A quantitative

will

already

of

line

spectra

equal

to

the

emission

is somewhat

lated

had

correspondence

superposition

determines

effect

apparently

ning

The

the

exciton

exciton

cene

reveal

anthracene. sidebands

crystals. of

experiments

zerophonon

line

identification in

the

low

accessible c41.

This

probability the

i-n naphthalene

of

broadening the

and

line

is

to

width

in anthracene.

of

the the

with

In both

lying

to a reduction

30 K and

lines

range.

k = 0 level

as compared

smaller

zerophonon

temperature

leads

between

at

of

the

the

upper lower

the

bottom

K

Davy-

crystals

exciton

k = 0

100

the of

the

phonon

levelC1,151

zerophonon

line

463 The main differences and anthracene

in the excitation

are the reversed

spectra

polarization

of naphthalene

Of upper

and lower

component and the direction of the line shift to lower or higher energy, respectively. The change in polarization is understood as a consequence of the different signs in the ex-

zerophonon

change

integral

JAB

(see section

3.1). The

line shift

in anthra-

cene has been attributed cl 3 to the thermal expansion of the crystal lattice leading to both the blue shift of the center of gravity

of the two zerophonon

components

and the temperature

dependence of their energetic distance A. Since naphthalene has the same crystal structure, similar effects due to the thermal expansion were expected, This similarity is evident in fig. 5. The opposite

direction

of the shift

concerning

only

the A con-

tribution is due to the opposite sign in the exchange integrals. The relation A I= 8 aAB in the present experiments should be valid at least up to about 100 K, (the low temperature region defined above and indicated in figs. 5). In order to describe the total red-shift in fig. 5a it has to be assumed that also the second contribution opposite

stemming

from a change

sign in naphthalene

in the solvent

as compared

shift

has the

to anthracene.

The discussion so far revealed a complete correspondence between the results of naphthalene and anthracene. The exception is the substructure in the upper zerophonon component of naphthalene, which is not understood at present. Very probably this substructure is not due to impurities or minor made from different starting materials and the structure terated

naphthafene

is found

also

crystals

crystal quality. Crystals did not show any difference

in the excitation

f14j

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12 I

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