The effect of internal rotation on absorption and fluorescence of dye molecules

411

Journal o/Molecular Structure, 219 (1990) 411-416 Elsevier

Science

Publishers

THE

EFFECT

OYE

MOLECULES

G. 1

OF

Sektion

INTERNAL

P.

HAIJCKEI,

B.V., Amsterdam

-

Printed

ROTATION

CZERNEYI,

ON

H.-O.

in The Netherlands

ABSORPTION

ILGEl,

Chemie,

Friedrich-Schiller-Universitat,

Chemie,

Technische

0.

AND

STEEN1

FLUORESCENCE

and

H.

OF

HARTMANN

ODR-6900

Jena

(GDR) ‘Sektion Merseburg,

DDR-4200

Hochschule

Merseburg

“Carl

Schorlemmer”,

Leuna-

(GOR)

SUMMARY

Internal rotation around an essential single bond within a chromophoric system very often leads to a hypsochromic shift of the longest-wavelength absorption band, associated with a decrease in absorption coefficient and an increase of Stokes shift. In the case that a non-fluorescent nr* state lies near to a fluorescent CT this hypsochromic shift can cause a dramatic drop of the state, fluorescence quantum yield and/or a strong dependence of it on solvent polarity.

INTROOUCTION Molecules

another

where,

ring

different

conformers

as

tion

barrier

tate

around

these

absorption studied

escence

of

internal

In bridging

the

following

tion

and

small creased

of

typical

shift,

tendency the

observation

0022-2860/90/$03.50

the

to

the

was

features band,

fluorescence

form

aggregates, these

0 1990 Elsevier

features

expected and

roto

fluor-

where

that

for

of this

fragthe

case

the

molecular

fragments

3):

structured

absorp-

shift

of

absorption quantum

excimers can

capable

molecules

shown

(ref.

molar

activa-

molecular

bathochromic large

high of

of

of

occur

can

freely

potentially

respective

co-planariration

the

less

is

with

bond,

molecule.

hand, the

connected on

or

absorption

molecules

other

bridging

spectra,

absorption

Stokes

of

dye

more

rotation

comparing

publications

to

fluorescence

wavelength

on by

leads

of

is

single

Depending can

internal

by

ring

a formal

l-2).

fluorescence

and,

a number

that

an

types

prevented

by

(ref.

effect

different

aromatic

fragments

Such

and this

rotation is

merits.

Thus,

bond.

an system

molecular

this

We have

process

example,

any T-electron

exist

affect

for

or

be

Science Publishers B.V.

the

longest-

coefficient,

yield

and

and

exciplexes.

used

to

an

in-

examine

412 whether

absorption

geometry

of

EXPERIMENTAL The

and

the

originate

regardless

of

from

being

a co-planar

bridged

or

not.

DETAILS

synthesis

techniques

fluorescence

molecule

of

have

the

been

investigated

described

compounds

earlier

or

and

will

be

experimental published

(ref.

4-5).

RESULTS

AND DISCUSSION

Fig. (2)

1 shows

the

benzopyrylium

structured can

dye.

and

be

comparison

clearly

seen.

This

coefficient

for

In

MO model

a simple

tion

that

results This

arise

an

elevation

Therefore, in

the

the

if excited

(see

Fig.

the

band

accompanied

the

higher

the

order an

by

increase

be

of

larger in

the of

ground

state

transition

a more species

absorption

1). by

the

-bond

assump-

character

respective

thez--bond

the

of

a larger

some

a bridged

bridged

explained

the

and

feature the

Tabl.

possessing energy

(_t)

of

(see can

a bond of

the-rr-bond state,

observed

effects

around

is

generally

compound

these

elevation

a non-bridged

absorption is

bridged

twisting in

The

red-shifted

of

state. order is

is.

smaller

energy

than will

lb).

4 A

1. (a) Absorption spectrum of compound Fig. methane. (b) Energy level diagram illustrating ternal rotation on transition energy (solid dotted curve: bridged chromophore).

l_ and 2’2 in dichlorothe effect of incurve: non-bridged,

413 The

fluorescence

ly,

however,

is

lower

bridged

by one

spectra the

of

both

fluorescence

a factor (see

of

Tabl.

species

quantum

more

than

do

not

yield

three

if

of

differ the

significant-

non-bridged with

compared

one

the

1).

TABLE 1 Position shoulder); escence discussed benzene, hexane).

of

the longest-wavelength absorption band, A(a), (sh: molar absorption coefficient,E ; maximum of the fluorband, of the 3\(f), and fluorescence quantum yield, qi(f), compounds in different solvents (A: acetonitrile, 6: CH: cyclohexane, 0: dichloromethane, MCH: methylcyclo-

Solv.

Comp.

X(a)

(nm)

E. (lmol-lcm-1)

X(f)

(nm)

@f)

1

0

544sh579

23000

622

0.024

2

0

553sh593

28700

623

3

MCH 0 A

327 334 329

13000

-----

0.087 -_-_ -------

MCH 8 0

355 360 360

20800

417 422 418

0.62 0.68 0.74

CH 6

338 344

16400

--_

CH B II

343 360 360

38800

381 388 390

0.027 0.13 0.34

415 417 414

0.63 0.68 0.69

397 400 437

0.029 0.21 0.48

29300 28600

450 473 489

0.58 0.86 0.84

98500

479 508 532

0.62 0.70 0.65

absorption

spectrum

4

5 6

7

MCH B Cl

363sh376 382 383

8

MCH 6 A

347 355 354

CH !3 0

445 452 448

MCH B II

474 489 498

9

10

Fig.

2a

compound has

the

shows 1. same

the It

36000 31900

temperature

is

quite

obvious

effect

as

bridging.

effect that

of

the

decreasing

the

temperature

of

414

-

t

lGOK

lb)

A

\ 1

30 c-

24

Fig. pound

2.

Such

effects

16

Temperature dependence in butyronitrile. (b)

L

could Fig.

20

vllo3crn-’

of

bridging

be verified 3a compares

for the

the

phenyl

substituted

is

large

the

2

spectra.

in

group

variety

of

of of

dioxaborine.

bathochromic

t

(a)

benropyrylium

salts

(ref.

6).

a non-bridged The most

shift

com-

methyltetrahydrofuran

compounds

spectrum

28

VjlOJcm~’

absorption

phenyl

absorption

fect

here

of

/

30

3; +---

Compound

a large

a bridged

,

of

the

(3)

striking

bridged

and ef-

species.

i

A

A

xl

-

fig.

3.

P /10~cm“ Absorption

Although

some

bridging

effect

dyes respect

25

bridging to

spectra

doubt

is

alone, yields

fluorescence

in

one

of

order can

dioxaborines

whether assume

a bathochromic of

the

above

in

this that

shift. examples

benzene.

shift also The

is

for

due

this

consequences

are

quite

to

the

type

of

with dramatic.

415 Thus

the

yield

in

whereas

species

a broad

range

the

escence can

bridged

be

lying

non-bridged

in

these

seen

in

in

Decreasing

leads

that

internal

Turning the

bridged

tional

strong

pared

with

practically

planar

ground

pound

probably

due of

to

the

the

electron

a co-planar variety

which (Tabl.

8.

can

This

from

of (Tabl.

1).

A similar

a co-planar

7-diethylamino-coumaryl

polarity for

out

and

conformation

non-bridged

to

are

3)

not

bi-

hindrance Like yield

in

to

(6)

polar found

for

excited

Sl-state

a nearby

np*-state

lb). where

and

one

a more is

from Fig

a

cyclohexane

spectrum

the and

struca small

fluorescence

5,6-benzocoumaryl dioxaborine

the

non-bridged in

a co-planar

absorption

ab-

and

possible.

molecules

the

distortion

steric

going

fluorescence

both

character

a structureless

(Fig.

the

also

a

com-

bond That

yield

downwards (see

substituted

a co-

behaviour

of

non-bridged that

from

of

quantum

also on

terms

removes

absorption

point

now

fluorescence in

which

solvent

of

But

increases

interpreted

examples

is

quantum

considerably

be

shift

1).

state

is

occur

fluorescence

fluorescence

increasing

shape

ground a high

due

com-

process

to

substituent.

where

if

presence

to

addi-

structure absorption

the

2

compared the

compound

the

leads

if

reveals

double

compound

8,

the

reveals

of ‘JOG-* nature

Instructive Stokes

dioxaborine

solvents

solvent

tured

indeed by

a measurable

(0.027),

methoxy

demonstrated

z

for

(6_) and

strength

seems

increased

conformation

geometry of

shows

with

donating

is

_$ also

the

a

quenching.

also

bridged

pronounced

to

reveals

vibrational now

also

from

and

assumption

non-bridged

It

species, more

conformation

substituted

compound

The

the the

that

non-bridged

results

band

phenyl

being

state

co-planar

sorption

of

indicating

and

3).

7).

dioxaborine

the

donor

(Fig.

finding

spectrum

the

a low-

(ref.

fluorescence

both

but

the

dioxaborine

increased

one,

conformation.

by

supporting for

effect

to

observed

absorption

the

non-bridged

bridged

both,

co-planar

to

non-fluorescence

phenyl

phenyl

l), fluor-

crossing

is

for

quantum (Tabl.

observable

supported

thus

compounds

identical,

this

non-bridged

Zb),

shift

due

any

effective

responsible

bathochromic the

the

Fig. is

(7-1

show

further

substituted

unsubstituted

polarity

for an

structured

bathochromic

species the

of

a more

methoxy

fluorescence

phosphorescence

(see

the

is

a high different not

reason that

This

rotation

to

expected

with

in

to

does

The

temperature

fluorescence

of

(2)

assumption solution

the

solution strong

one

occurs.

a glassy

exhibits

solvents

solvents. the

nV*-state

that

(ft) of

(12)

occur (2)

(Fig.

and 4).

the

416

Fig. 4. coumaryl

These

examples

acceptor be

Fluorescence substituted

and fluorescence dioxaborines in

demonstrate

relationship

the

that same

by

excitation cyclohexane.

means

effect

as

spectra

of

an

by

means

appropiate of

of

donor-

bridging

can

achieved.

REFERENCES Sheck, N.E. Kovalenko and M.V. Alfimov, Conformeric Yu.B. fluorescence and phosphorescence of aromatic molecules with a possibility of rotation of chromophoric fragments around sinole chemical bonds, J. Luminescence, 15 (1977) 157 and ref. cited therein. J. Donatsch and U.P. Wild. The influence of K. Rarinaaui. molecular geometry on the fluorescence spectra of biphenyl and the polyphenyls, Chem. Phys. Lett., 34 (1975) 285. I.B. Berlman, On empirical correlation between nuclear conformation and certain fluorescence and absorption characteristics of aromatic compounds, J. Phys. Chem., 74 (1970) 3085. G. Haucke and H. Hartmann, On the SynP. Czerney, C. Igney, thesis and spectroscopic characterization of bridged 2,2-difluoro-1,3,2-dioxaborines, Z. Chem., 28 (1988) 23. Photophysical chemistry of indigoid G. Haucke and R. Paetzold, Nova Acta Leopoldina, Suppl. 11 (1978). dyes, G. Haucke, P. Czerney, C. Igney and H. Hartmann, Absorption and fluorescence behaviour of benzopyrylium compounds, Ber. Bunsenges. Phys. Chem., 93 (1989) 805.

H.-D. Ilge, Photochemistry Oissertation

Conributions of aryl 8, Jena,

on Spectroscopy, Photophysics fulgides and 1,3_diketoborates, 1987.

and