Electronic Spectroscopy of Squaraine - American Chemical Society

Chapter 13

Electronic Spectroscopy of Squaraine Its Relationship with the Stabilization Mechanism of Squaraine Particles in Polymer Solutions Kock-Yee Law Downloaded by UNIV OF ARIZONA on November 29, 2012 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch013

Xerox Corporation, Webster Research Center, Webster, NY 14580

The fluorescence emission of bis(4dimethylaminophenyl)squaraine, 1, in CH Cl containing varying concentration of poly(vinyl formal) (PVF) has been studied. Three emission bands ( α , β and γ in the order of decreasing energy) are observed 2

in

2

CH Cl 2

solution and are

2

found to

be

the

emission from the excited state of 1, from the excited state of a solute-solvent complex and from a relaxed twisted excited state of the solute-solvent complex, respectively. Model compound studies show that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complexation process between 1 and the OH groups in PVF is also shown to occur. A model for the stabilization of particles of 1 in polymer solution is put forward where we propose that the s t a b i l i z a t i o n mechanism is a steric effect achieved by adsorption of PVF macromolecules onto particles of 1 via the formation of the PVF:1 complex. Bis(4-dimethylaminophenyl)squaraine, of

its

derivatives

photoconductive Although optical 670

class

absorption

nm,

solid

and

this €

m a

state

in

x~3x105 is

are

known

1 to

(J_),

semi-conductive of

compounds

the red i n cm-lM-1),

intense

and

properties. sharp

(X ax ~ 6 2 0 m

absorption

panchromatic

many

useful

exhibits

solution its

and

possess

in

from

0097-6156/87/0358-0148506.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

the the

13. L A W

Electronic Spectroscopy of Squaraine

visible

to

the

near-IR

characteristics xerographic

by

casting

various

appropriate in

quality squaraine the

(2)

the

the

dispersity but

also

S c i . ,

1

generally

was

in

and

superior

dispersing

because

poly(vinyl

on

1

Here property effect

studies, is

solution, the

Further

the

emission solvent

hydroxy

is

twisted strong

on

formal). process

particles

1 in

of

the

polymer

various K.Y.

poly

The acetal)

principle for

the

1. multiple

structure-

and

temperature

that

of

the

free

multiple

squaraine

solute-solvent relaxed

using

2

excited

as

a

is

complex-

1

and

the

chains

role

stabilization

shows

complexes

Analogous

important

state.

model

between

in

complex

solute-solvent

solution

of

(vinyl

From

effect show

J .

used.

on

2.

on

particles

macromolecular

The on

of

solvents

detected

the

in

poly(vinyl

molecules.

also

complexation

the

study

solvent

groups

poly(vinyl

of

a

forms

process

to

1

solubility

and

emission

effect

alcoholic

ation

able

the of

1

the

device.

(Law,

results

solvent

emission

squaraine

with

of

we a r e

from

of

when

poor

film

in

influence

were

general are

the

resulting

these

preliminary

relationships,

emission

that

against

emission

profound

butyral) of

technological

particles

studied

onto

squaraine

of

except

effect

solvents

we r e p o r t

fluorescence

and

is

ethereal

of

Flocculation

observed

of

affects

particles

been

press).

formal) polymers

of

has

squaraine

dispersions

thus

the

cell

fabricated

only

has

of

stability

solutions

Imaging

is

not

for

solar

generally

stability

solution

photoconductivity

polymer

organic

are

The

it

optical

attractive

applications,

devices

because

layer,

Recently,

and

these

These

very

squaraine-polymer

polymer

and

nm).

them

substrates.

significance

Downloaded by UNIV OF ARIZONA on November 29, 2012 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch013

In

(3).

photoconductive

particles

(400-1000 made

photoreceptor

applications based

have

149

of

of this

mechanism

of

discussed.

Experimental Squaraines and

1 and

described

by PVF,

hydroxy

content

Polymer

Products

Fisher

molecular

formal

and

sieves

synthesized

derivatives

Sprenger

formal),

from

were

2

Ν,Ν-dialkylaniline

and

Ziegenbein

content

6%,

was

Inc. were before

from

using

82%,

(4).

acetate

purchased

Solvents

acid

procedure Poly(vinyl

content

from

were

routinely use.

squaric the

12%,

Scientific

spectro

stored

Fluorescence

grade

over

3A

spectra


PHOTOPHYSICS OF POLYMERS

150 were

t a k e n

on

a

spectrofluorimeter differential Results

corrected

And

Figure

shows

was


spectrum. are

(b)

state

any

some

effects as

a

its

model

Results

in

Figure

bathochromic this

is

composition Figure the

2).

For

emission

with

identical

641 of

to

the and

result

and

in

emission

Figure

emission

XF

698

nm

emission according

to

structures, the the

(0,0)

also are

their

multiple

the

splitting

a

seen

in

assigned Stokes

α-band (0,1)

between

Figure the

shifts. due

and

the

transitions a-

and

a

3a βIf

to

of

the

between

the

α-band

is

state

of

and and

these the

two that

vibrational

would

β-band

2. at

y-band

be

respectively. the

an

band

assumes

fine

j3-band

spectral

small

one

is is

the

the and

α-band and

Because

excited

nm

to

the

in 3a),

spectrum

wavelength

that

the

(Figure

spectrum,

is

the

in

by

a

increases inset

relationship

660

XF

undergoes

(see

ether

of

solvents.

change

spectrum.

suggests

chosen

because

π*

dominated

image

multiple

was

2

excitation

monitoring

emission

and

is

The

at

the 2

of

spectra

Franck-Condon

shoulder are

Xmax

emission

3a

the

bands

2

on

organic

diethyl

absorption the

from

An

of

mirror

In the

with

in

nm. the

overlap

absorption

the

example,

(c)

excited

complex.

parameter

emission

spectrum

at

λγ

independent good

the

or

an

p o s s i b i l i t i e s ,

various

accompanied

of

of

emission

state or

investigation

that

the any

emission

excited

solvent)

solvent

that from

the

Squaraine

our

are

1.

of

temperature

show

as

than of

—702

y-band

showed

multiple

these

in

2

shift

the

solute-solvent

for

solubility

and

states

studied.

compound

they

the

(with

and

were

and

relaxed

of

solvent

bands

high

and

kind

660 and

structure

differentiate

of

emission

a

monitoring absorption

X F 646,

rather

1

for

exciplex

the the

experiments

fine

from

of

and

excitation

spectrum β-

aggregational

an

The

to

at

the

explanations

from

to

bands

from

vibronic

of

order

is

emission

emission

a

excitation

CH2CI2.

emission

Controlled

or

(a)

band;

44A

with

(DCSU-2).

independent

α-,

emission

Probable are

the

the

respectively.

1

unit

identical

emission in

as

impurities

be

was

Three

designated

in

1

to

and

observed

multiple

MPF

equipped

fluorescence

of

found

wavelengths nm

spectra

the

spectra

spectrum

was

Discussions

1

emission

P e r k i n - E l m e r

which

in


from From Figure

151



13. LAW

Figure emission

1.

Corrected

spectra

of

fluorescence

1 i n methylene

excitation

chloride

and

([1]—3x10-7

M).




152

Figure emission (b)

3·

Corrected

spectra

a t 77°K

(cone.

of

2

fluorescence in

diethyl

- 5 x 1 0 -7 M ) .

excitation

ether

(a)

at


and 298°K

13. LAW 3a,

the

(0,2)

γ-band

3a,

emission the

the

effect

taken

of

solvent

on

(Ham e f f e c t )

relative

emission

in

of

intensity

structure

of

be

vibrational the

in

(0,2)

Major observed

in

found

depart

in

to

2

to

around

nature,

in

the

nm

secondary

alcohols In

accompanying and

identical

significance

to

that

the

and

(rather

emission

between

dimethylaminobenzonitrile reported long to

by

Wang

wavelength the

spectra

interactions also

short

solvent that

sensitive

in

effect

steric

in

α-band

short of

assigned

the

sensitive, and

observed

suggests

alkyl in

that

solvent 2

forms

of

be

the

soluteSince

published)

squaraines around

the

molecules

solvents. to

pwas

DMABN

(Law, K . Y . ,

both

amines

between

that

the

of

state

he

work

The in

similar

steric

dependence

the

alcohols 2.

is

factors

amyl shift,

lies

where

emissions

in

the

and a l k y l

alcoholic

studies

is

nm

between

2

Very

We p r o p o s e in

the multiple to

this

Xmax

dependence)

2 and a l c o h o l i c

range.

complexes

structural show

between

steric

for

excited

which

emission

The s i m i l a r

electronic

recently

Xmax

tertiary

Figure

π*

(DMABN)

emission,

exciplex

amines.

is

(U))

e.g.

of

is

hindrance

—639.4

alcohols

the

2

relationship

Instead,

in

in

than

effects of

steric

spectra in

structure

hypsochromic

composition.

interactions

π*

correlation

seen

results

spectra

Xmax

intensity

The

emission

dependence

range

in

4).

nm

the well

absence

solvent

versus

the

as

best.

increases,

with

the

may

emission

alcohols,

the

of

y-band

The

—634.1

increase

(Figure

absorption

Xmax

and

conjunction

observed

steric

group

in

persistent

the

on

variation

vibronic

as

of

solvent

vary

at

in the

structure

solvents.

primary

alcohol. is

Xmax

wavelengths

in

an

the

affect

are

The

specific

alcoholic

hydroxy

642.6±0.2

is

the

to

of

the

50

solvents.

from

shorter

the

tentative, the

to

structure

and

over

against

fails

bands

fine

solvent

but in

from

known

(6-9).

the

in the

corresponding

effect

fine

the βand

alcoholic

Figure

the

pyrene

transition

comes

the

nm

a l l

examples

absorption

of

of

vibrational

and

α - ,

evidence

are

The

—675

laboratory

the

an assignment

assignment

shifts

of

of

be

shoulder

(5)

squaraine

of

such

the

our or

Notable

intensity

fluorescence

transition. to

inspection

Solvents

intensity

hydrocarbons.

benzene

makes

in band

transition.

relative

(0,3)

expected

Careful

emission

(0,2)

the

is

missing.

any

aromatic


is

be

which

spectra

identify

the

would

transition

Figure

to

153


are

the


not

N,N-

154



Figure

4.

Corrected

i n amyl a l c o h o l s

fluorescence

emission

spectra

([1]—3X10~7M) .


of 2

13. LAW


155

DIALKYLAMINO GROUP, OUR RESULTS SUGGEST THAT THE SITES OF COMPLEXATION IN ALCOHOLS ARE THE OH GROUP OF THE ALCOHOL AND THE FOUR-MEMBERED RING OF THE SQUARAINE (SCHEME I ) . Scheme

I


Η

AS THE STERIC HINDRANCE AROUND THE OH GROUP DECREASES, SOLUTE-SOLVENT COMPLEXATION INCREASES, RESULTING IN THE BATHOCHROMIC SHIFT OF XMAX AND AN INCREASE IN /3EMISSION INTENSITY. THE GENERAL SOLVENT EFFECT ON THE XMAX AND THE EMISSION COMPOSITION OF 2 (FIGURE 2) SUGGESTS THAT THE COMPLEXATION PROCESS IS VERY GENERAL AND COMPLEXATION BECOMES VERY PRONOUNCED IN SOLVENTS OF Π* > 0 . 6 5 . ACCORDINGLY, Α-BAND IS THE FRANCKCONDON EMISSION OF THE EXCITED STATE OF THE SOLUTE AND J3-BAND IS THE FRANCK-CONDON EMISSION OF THE EXCITED STATE OF THE SOLUTE-SOLVENT COMPLEX. ADDITIONAL EXPERIMENTAL EVIDENCE IN FAVOR OF THE SOLUTE-SOLVENT COMPLEX MODEL COMES FROM THE LOW TEMPERATURE ELECTRONIC SPECTRA OF 2 IN ETHER. SQUARAINE 2 EXHIBITS AN ABSORPTION AT XMAX 654 NM AND A SINGLE EMISSION AT X F 664 NM AT 77°K IN ETHEREAL MATRIX (FIGURE 3 B ) . THE OBSERVATION OF A BATHOCHROMIC SHIFT IN THE EMISSION SPECTRUM IS CERTAINLY AGAINST GENERAL EXPECTATION FROM VIBRATIONAL FINE STRUCTURES WHERE A HYPSOCHROMIC SHIFT OF X F SHOULD BE OBTAINED (JUL). THE ANOMALOUSLY LARGE SHIFT LEAD US TO CONCLUDE THAT THE ABSORPTION IS FROM THE SOLUTE-SOLVENT COMPLEX AND THAT ITS FORMATION IS PROBABLY A TEMPERATURE STABILIZATION EFFECT. SINCE THE EXCITATION SPECTRUM AT 77°K IS IDENTICAL TO THE ABSORPTION SPECTRUM AND SHOWS GOOD OVERLAP AND MIRROR IMAGE RELATIONSHIP WITH THE EMISSION SPECTRUM, THE SINGLE EMISSION AT XMAX 664


156 nm

PHOTOPHYSICS OF POLYMERS can

the

be

study

of

towards in of

assigned 2

ether, the

that is

solute.

from

Another emission

Since the

of

suggest

of

four

membered

the

C-C

the

y-emission

in

previous

results,

rotation

is

the

both

however,

complexing Addition

known

to

ternary

(e=4.7) hexane

in

red

as

of

and

isoemissive spectral

results is

of

is

at

we

bond

nm

positive

not

In

our

€

and

only the

is

also

order

chloroform

addition

of

n-

constant

as

the

Results shift

XF

at

the M.

the

a and

experiment

ether

The

point

red

that

process.

Xmax

the

an

the As

increases

performed

that

from

in

j3-

solvent

the

fact

medium,

keep

the

increase to

also

ether

to

—662

give

indeed

of our

C-C

and

polar

the

isosbestic

point

a and

with

complex.

Such

solvent

chloroform

an

a-

the

(e=1.9).

show

ring

complexing

shift

more

increases

[CHCI3]

Simultaneously,

emission

b

a

by

absence

the

should

by

namely

mixture

concentration 5a

2

complexation

n-hexane

the

a

but

dilemma,

system,

and

Figures

the

this

the

of

increase.

complexing

affect

phenyl The

of

should

XF

(e)

from

formed

complementary

of

are

constant

circumvent a

a

of

published)

is

that

of

quantum

emission

the

at

effects

77°K.

complexation,

dielectric

is

temperature

be

solute-solvent

solvents

the that

emission

squaraine.

addition

should

of

with

the

an

indicating

and

an ot-

y-emission

which

between

complicated

of

induces

in

Xmax

can

is

The

low

to

simply

the

β-emission

is,

is

state

solution

of

we

fluorescence

K.Y.

assignment

ethereal

result,

total

at

b,

state

the on

3b

correct,

concentration

the of

Figure

above

was

and

fluorescence (Law,

prohibited

the

in

results

of

nm

comparison

temperature

excited

absence

bond

— 660

by

study.

the

ring

insensitive λγ(β)

identical

y-emission

the

of

effect

complex.

the is

excited

rotation

an

on

the

twisted

If

the

multiple

that

3a

room

finding

squaraines

relaxed,

emission

from

recent

changes

number

at

effect

is

structural

(e.g.

Figures

deduction

yield

relatively

solute-solvent

solvent

emission

solvent

acetonitrile),

in

emission

spectrum

and

is

polarity

in

significant

nm.

Since

/^-emission

This

obtained

nm

the

an

Franck-Condon

XF

data

the

from

emission

—700

that

solvent

XF(J3)=665

emission


shows

spectral

the

complex.

increasing

conclude

in

to

solute-solvent

that state

an

These the of


J8the

13. LAW

157



(a)

600

Figure (a)

5.

Effect

absorption

spectra 0.28

M,

4.48

M).

of

2

i i i .

of

650 700 X(nm) chloroform

([2]—10-5

([2]~3x10-7 0.56

M,

M) M)

iv.

(b) in

750

concentration corrected

ether

1.12

M,

([CHCl3] = i v.

2.24

on

the

fluorescence M,

0 M, and


i i . v i .


158 solute-solvent excited complex

is

of

in

emission

2

to

the

(Ιβ)

Complexation

and

versus

however,

of

the

emission.

and β

further

bathochromic

increase

Absorption through

curves

their

observation solvation as

the

of of

shell

As acetal) in

the

polymers

are

organic

these found

to

have

no

experiments. are

the

latter

is

complexes model

shown with

the

studied. that

the

of

In

typical

a

PVF

effective exists

are

as

of

1

in

interaction

between squaraine

of

macromolecules of

1

and

groups

the for

representation

PVF

and

CH2CI2

with

the of

occurs in

onto

groups

in

the PVF

adsorption the

on

of

the to

majority the

the

OH

CH2CL2«

Due

the of

1

major

surfaces

solution.

As

p r o c e s s ,

a

PVF

of

particles

actually

anchoring

process.

adsorption

the

the

in

that

surfaces are

as

that 1

show

dispersion, Μ),

a is

6a-d

increases

implies

c o m p l e x a t i o n

adsorb OH

PVF

as

CH2CI2

—10-2 M .

particles

the

strong PVF

Figures

(:2.24

curves

events

as

overlapping

Xmax

at

concentration

of

in

the

isoemissive

complexation

noted

chloroform close

on

M the

Figure

and

attributed

two

the

of

of

solute-solvent

these

the

of

the

chloroform

1:n

localized

solvation

or

be

of

inset

observed

solute-solvent

a

of

highly

2 to

emission

either

the

solute-solvent

1

and

from

[CHCL3]J_2) ·

pigment

the

acid

of

of

organic

using

carboxylic

stabilizes

resulting

dispersion

stabilization

interactions

able

sterically

solution,

of

these

polymers

anchoring

effect

process,

which

p a r t i c l e s experiments

functional

in also

groups,



13. L A W


161

e.g., cyano g r o u p , c a n a l s o be u s e d a s an a n c h o r i n g group f o r t h e p o l y m e r i n t h e a d s o r p t i o n p r o c e s s and s t a b l e d i s p e r s i o n s c a n be p r e p a r e d a c c o r d i n g l y . For example, b e t t e r d i s p e r s i o n s t a b i l i t y i s o b t a i n e d from s t y r e n e - a c r y l o n i t r i l e c o p o l y m e r i n C H 2 C I 2 a s compared to p o l y s t y r e n e itself. In summary, the s t a b i l i z a t i o n mechanism of squaraine particles in organic solvents h a s been understood as a steric stabilization process by spectroscopic technique. The knowledge g a i n e d i n t h i s work has enabled us t o formulate other stable squaraine-polymer dispersions using polymers other than p o l y ( v i n y l a c e t a l s ) .

Literature Cited 1. For nomenclature, see West, R. Oxocarbon, Academic Press, New York, 1980, Chapter 10. 2. Wingard, R.E. IEEE Industry Applications, 1982, 1251. 3. Loutfy, R . O . ; Hsiao, C . K . ; Kazmaier, P.M. Photogr. S c i . Eng., 1983, 27,5. 4. Spenger, H.E.; Ziegenbein, W. Angew. Chem. Int. Ed. Engl., 1966, 5, 894. 5. Ham, J . S . J. Chem. Phys., 1953, 21, 756. 6. Nakajima, A. B u l l . Chem. Soc. J p n . , 1971, 44, 3272. 7. Nakajima, A. J. Mol. Spectroscopy, 1976, 61, 467. 8. Hara, Κ.; Ware, W.R. Chem. Phys., 1980, 51, 61. 9. Dong, D . C . ; Winnik, M.A. Can. J. Chem., 1984, 62, 2560. 10. Wang, Y. Chem. Phys. L e t t . , 1985, 116, 286. 11. Becker, R . S . Theory and I n t e r p r e t a t i o n of Fluorescence and Phosphorescence, John Wiley & Sons, 1969, p. 42. 12. P a r f i t t , G.D. Dispersion of Powders in Liquid, 2nd Edition, John Wiley and Sons, 1973. 13. Napper, D.H. Polymeric Stabilization of Colloidal Dispersion, Academic Press, New York, 1983. 14. Verwey, E.J.; de Boer, J . H . Rec. Trav. Chim., 1938, 57, 383. 15. van der Waarden, M. J . Colloid S c i . , 1950, 5, 317. 16. van der Waarden, M. J . Colloid S c i . , 1951, 6, 443. RECEIVED April 27, 1987


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