Fluorescence and competing events on the picosecond time scale

221

JoumalofMoleculnrStructure,47(1978)221-235 0 ElsevierScientZic PublishingCompany,Amsterdam-PrintedinThe

FLUORESCENCE

AND COMPETING

G. W. ROBINSON*, * Department

J. M. MORRIS,

of Chemistry,

Department of Physical (Australia) 3052 f

EVENTS

The Royal

ON TBB PICOSECOND

R. J. ROBBINS

Texas

Tech.

Chemistry,

Institution,

TIME

SCALE

and G. R. FLEHtNG+

University,

University

21 Albemarle

Netherlands

Lubbock,

Texas

of Melbourne,

St., London

79409

Parkville,

(England)

WlX

Victoria

4BS

ABSTRACT &I experimental is described proposed. between

system

A special radiative

isation

reaction,

changes

might

experimental

These

have with

extends

reaction,

an intrinsic

probe,

studies

are interpreted

using

are also useful

decays,

temporal

domain

picosecond

for assessment

dependent

chemical

reaction

of such studies probe

is described. of whole

The

these

rates.

molecules

is being

fluorescence

of the latter

both

extrinsic

two molecules

in terms of inhomogeneities

Initial

on surfaces

or in

emission

to small molecules of photoion-

molecules

such as

sulphonate)

probes.

Mixed

sites

and

solvent

show nonexponential of solute

dependent

the existing

assessed probe

and AN.9 (1,8 anilinonaphthalene

sulphonate),

depolar-

structural

The picosecond

macromolecules

importance

in biological

is described.

of solvent

using

are

diffusion

fluorescence

and the relationship

of fluorescence

spectroscopy area

and competition

a rotational

molecules

of rotation

path

(2,6 toluylnaphthalene

which

in this

chemical for study

segments.

as a nonradiative

(water/ethanol)

processes

or macromolecules

the study

fluorescence

in this research

on nonexponential

the feasibility

Filially, the study membranes

tryptophan, TNS

solvent

show

or small macromolecular isation

methods

of flexible

results

technology. biological technique

controlled

is proposed

structure

of experiments

is placed

and nonradiative

methods.

molecular

classes

emphasis

A new type of diffusion controlled

for the study of subnanosecond

and some general

decays

in the mixed

solvent.

INTRODUCTION This molecules

paper

picosecodds. first

will

following There

is to measure

describe

some recent

excitation

work

on the emission

by light pulses

are two primary

reasons

emission

directly

decays

having

for carrying in those

spectroscopy

temporal

widths

out such studies.

cases where

of

of a few The

nonradiative

events

have

reduced

iments

the nature

variability

with

investigations

of various system

into

rapid

the subnanosecond

radiationless

parameters

expected

by quantum

on this

t&e

interference

the many

scale.

effects,

regime.

transition

can be studied.

is to seek out and study

theoretically caused

the lifetimes

These

processes

The second

types

In such experand their

reason

for these

of nonexponential

nonexponential or

or by the coherent

decays

decays

can be

incoherent

super-

position of simultaneous temporal events. Currently the best method for carrying single

shot experiments,

selected

pulse

of emission display with

the direct

tuned

during

mode.

of temporal

emission

pulses

bank

the current

length

region,

second

harmonic.

technology

has removed

many

the advent

output

undoubtedly

pulsed,

frequency

in a signal

allowed

beyond

the frontiers

the 10

the recent

of detection

of the limitations

will

detector

pushed

Furthermore,

whose

(OMA) interfaced

methods

has already

Detection

laser.

repetitively

similar

use of

single

camera,

analyser

These

to be routinely

and

streak

power,

or some

set time regime.

for excitation,

of a Nd+3/glass

multaannel

by lower

camera

spectroscopy

- lo-l2

train

is through

of an amplified

an ultrafast

of a computer.

few years

a streak

Bowever,

the.1O-11

laser

the next

with

studies

harmonics

to an optical

memory

used with

lasers,

averaging

access

pulse

possible

coupled

out these

var(ous

the mode-locked is made

is optically

be replaced

into

from

events

utilizing

use o,8u~::a~~~~t

sensitivity

in this wave-

in the use of the Nd+3

inherent

EXPEEIMENTAL The experimental has already system

been

at Texas

of the authors

(a)

Laser The

described Tech

Oscillator

laser

heads

the amplifier allows

head

firmly

in some detail

That

we&

equipment

head

is built

accommodates radial

the thermal

be described

by Apollo.

a 15.9 mm diam. pressure

screws shock

The

are

caused

rod compression.

rod,

to be applied

by the authors

[1,2,3].

A similar

of completion

by one

here.

firing

of the oscillator

transverse

beam

deflection

are identical

Because

rod, minor

design

pressure

rise

from beam walk-off

rod, while

rod by an seats

the rod disturb

effects

- Brewster

displacements

except

of the Apollo

to lensing

of the Brewster

longitudinal

and misalignment

The

to the laser

laser

does not appreciably

it is not so tight as to give

figuration

two heads

This

tightened.

of laser

yet

(slight)

stages

to accorrrmodate a 12.7 mm diam.

the rod alignment; by

will

of Melbourne

in the literature

is now in the final

manufactured

the retaining

so that

at the University

and Amplifier

a uniform

"0''-ring when

built

University

(GWB).

that the oscillator

heads

apparatus

end con-

can lead

along

concave

to

223 surfaces

in the laser

A new cavity

cavity.

arrangement

flat end configuration, antireflectance

the line-up

and will

reduce

reliability

This

the number

to silicate

EV2) with

glass

used

in the laboratories

have

proved

Both

These

because

[2].

hold

condition

to about voltage

and an output

+1 volt

shot-to-shot.

to 2 min maximum

timing

circuit

pulses

per pulse

adequate

for most

techniques.

supply.

experiments

or by using

the first

power

further

Tech

glass

laser

glass is superior

and higher LEG-5

rods,

for the past

synchronously

amplifier have been

two years

from a common

oscillator-amplifier

Each

and

the 35uf capacitor

with

50% transmission

voltage

modern

mode,

firing

2nd,

amplification

internal of about

are usable. and

this

The

can be

This

3rd or 4th harmonic if needed

A

to within

a train

15 millijoules.

can be obtained

is charged

20 set minimum

produces

1 millijoule,

thres-

is used,

by an adjustable

ten or so of which

to about

The

is reproducible

accurately

laser

Apollo

timing.

when

is around

using

more

Better

to be achieved,

power Boya

is in a "rep-rate"

by amplification

Of course,

the amplifier

fired

reflector

the oscillator

increased

and Texas

as determined

by this means, from

pumping

is achieved

Firing

in the power

by one.

The phosphate

that the firing

period,

cavity

configuration

are both Nd+3/phosphate

equivalent

optimizes

cavity

[1] insures

period

conveniently

delay

the other

- Brewster

components

a flat-

respect.

are

for the oscillator

4.5kV

40 intense

in every

and amplifier

comparator

energy

at Melbourne

A variable

supply.

threshold

and

in the laser

are expected

at 1055 nm.

of lower

essentially

reflectance

coaxially

rates

rod materials

output

rod with

the Brester

optPca1

repetition

or the essentially

to be reliable

oscillator

power

about

rods,

with

of adjustable

firLng

gain

be used

attendant

and amplifier

(Owens - Illinois

an oscillator for maximum

rod will

problems

and higher

The oscillator

employ

one end coated

coated.

avoiding

will

power

is

generation

by double-passing

stages.

)(b Pulse

selection

gap initiated with

by focussed

the polarization

of intensity by means pulse

of a "selected" spark

impedence amplitudes.

of the laser

density

that have

(approx.

pulse,

onto

with

A coaxial

output

intsnsity.

cell

the 50

manner

reflected

to select

The spark

Q

attenuator

crossed The level

early

10-20X in the

the half-wave

the polarisation

by the crossed

as evidenced

L3,5] in the spark

of a spark

within

out pulses

rotating

by Interactive

cables,

adjusted

gap switches

thereby

it to be transmitted supplied

to the table).

be critically

cell,

[4] by means

from a polarizer

(horizontal

in order

a Pockels

causing

gap and pockels

pulses

light must

filters

highest

6.6kV)

matching

out in the usual

fundamental

of this initiating

of neutral

train

voltage

The

can be carried

Radiation

polarizer.

Inc. have

good

from the low reflection gap input

cable provides

a

224 rectangular

trigger

synchronously like a voltage wave

voltage

our spark allow

divider

to the spark

An improved

on

consists

volt

1OA pulse trigger

(c)

selected

train.

It is amplified, combinations

crystals)

Interactive erature

phosphate

about (RDP).

l% respectively.

satisa

circuit

pockels

cell

into a ?in camera

driver

with

The 20-100

diode.

and rather

perpendicular

or

University

long delay

control

(for the angle

The present

compared

angle

of the phase

of:

arsenate

the fundamental matching

obtained

doubler, about

from

optimum dihydrogen

type I ammonium

35OC.

The photon

are approximately

15X,

the frequency

that the temporal

that of the 2nd harmonic

as

temp-

2nd harmonic,

type I rubidium

condition

It is felt

were

(CPA) crystal,

tuned

temperature

by

crystals

(for the temperature

crystal)

consists

temperature

[2].

with

tuned

or quadrupled

generating

units

tuned mixer,

optimum

with

is compressed

system

dihydrogen

4th harmonic,

tripled,

The harmonic

and temperature

mount

compared

is doubled,

generators.

to that of the pulse

2%,

bandwidth

pulse

width

161.

Camera

streak

camera

[7] is a model

transmitting

transmitting

CaF2 achromat

with

the photocathode

gap,

entrance

substrate. f/2 aperture

675 II

(Hadlands

and an S20

The system focusses

Photonics)

(U.V.) photocathode

is sensitive

to about

equipped

with

an

set on an ultra200 run. A quartz

the slit image

of the streak

triggered

the coaxial

camera

onto

surface.

In the "streak the spark

pulses

a krytron

is polarized

(ADP) crystal,

Because

compressed

ultraviolet

by laser

3rd harmonic,

crysal;

of the 4th harmonic

violet

in

does not

gap is required,

at Melbourne

Inc. PC-6

then its frequency

34OC;

efficiencies

The

using

tested

for the 675 II streak

type I cesium

conversion

Streak

device,

being

pulse

Inc.

doubler,

phosphate

(d)

in the spark

of switching).

and gimbal

dihydrogen

is also

breakdown

camera,

gap at 6.6kV without

in this mode

housings

Radiation

tuned

temperature

the half-

A not altogether

the spark

insulation

Associates

of harmonic

as the crystal

tuned

high voltage

problem

fundamental

various well

double

for acts

Generation

Harmonic The

circuit

of the streak

to be employed.

switching

triggered

a minor

risetime

cell.

of a Pulsar

requirement

fXmes may present

triggering

the one presently

generator

pulse

and sins

to apply

Unfortunately,

1OkV

the use of an electronic

(that system

necessary

of the high voltage

li.ke

amplitude

The pulse selecting

is to operate

the pockels

device

model

than about

design

a commercial

volt

camera.

to spurious

to this problem

50 Q termination

25-50

gap input.

rise

greater

solution

or better

of about the streak

and it is therefore

gap, giting

voltages

factory

pulse

triggering

mode" , a voltage

is applied

ramp,

so that the middle

&near)

from

range

attenuator

of photoelectron

of de-

-

225 flection

is centered

"focussing reins

mode"

fixed

the voltage nonlinear

near

An EMI

3-stage

intensifier

methods, corrected

these and,

Another

Kowa

mode

or cable

of operation delays

ramp when

may be

operating

in

gain,

the 1205D

has been

flat

intensifier

field

the 1205D stage

intensifier,

itself

head

in this respect,

head,

of

light

flux from charge

of the streak

The OMA

or

to a minor

as it was

Another

Even

under

running

that an image

signals

needed

to obtain

usable

tube photocathode apparent.

in both without

signals

the the

so greatly

that considerable

In order

intensities

in an

this fault.

believed though

large

conditions.

the1205D

be detected

was

with

are apparently

humid

routinely

low input

of that

problem

an incredibly

for correcting

originally

amount

in Melbourne,

A few tubes

particularly

detector

end of the detector.

compresses

volume.

consists

(1205D)

could

the streak effects

rise

in the front

altogether.

camera,

moni-

them altogether

target

is adequate.

method

it was

intensity

curved (time)

be partly

to avoid

gives

In any case,

the camera

highly

horizontal

to the OMA.

be found,

breakdown

corrected.

rather

coupled

for photographic

intensified

into a small

be avoided

due to space

head

is a stop-gap

modes"

the electron

intensifier silicon

detector

is because

the slit image.

intensifier

undoubtedly

best

of the tube, which

detector

could

could

seemed

chosen.

it will

corona

components

the excess

time resolution

1205D

1205B

from

to the design

and "streak

Corp.

and perhaps

of dry nitrogen

By using

the image

The

problems

it seemed

(Kowa).

but

This

along

and a nonlinear

fewer

intensifier

Research

is now hopefully

atmosphere

defocussing

field

than the majority

The problem

distortion

format

the OMA,

of high voltage

much worse

causes

This

lens

in the intensifier

tube.

using

noise

intensity occurs

ccupled

camera,

analyser.

plate

even when

console.

aperture

the 675 II - a channel

pose

distortion

traced

integrates distortion

of the streak

couples

is optically

the streak

multichannel

would

flat

lens

the lower

increased

with

cathode

with

effects

Applied

and a 1205A

pin cushion

display,

in the data analysis,

the low noise,

of a Princeton

the optical

to the streak

While

bialkali

supplied

and pin cushion

at the edges

toring

"focus"

using

supplied

mostly

number

the voltage

mode",

so that a somewhat

latter

tube by an ultrahigh

normally

a photographic

optically

This was

This

so that optical

with

of the slit

and in the "synch

position,

screen.

with

of the streak

of noise

normally

baseline.

head

image

phosphor

unlike

slit images

with

screen,

In the

and OMA

the intensifier

A fair amount

fibre

the phosphor

tube.

so that the image

the focus

in the time domain

for applications

system

from

for synchronisation

Intensifier

is not

the OMA,

of the phosphor

of the streak

mode".

to the output

better

screen

is applied

to rise

half

pulses

orsubtracted

Image

This

covers

phosphor

voltage

the center

commences

streak

the "streak

(e)

a constant

ramp

is for locating added

on the output

to preserve be used

the

[7,8].

The

final

input

signal-to-noise

signals

only about range over

to avoid

15 t 5:1,

tion effects

reducing

components

axis

For

table.

collection

to the laser

optical

path

It is most

this reason,

optics

solidly

divergence.

Most

all reflecting wavelengths.

delay

unnecessary path

as much avoids

the use of an added

allows

camera

for slit

or by image

the streak EL300

to

in a two-dimensional

for correcting

of the streak

data

camera

are displayed

while

over

optical

the rather

prevents

30

the loss

In other words,

it

stage. access

The caused

memory

aperture

a holographically

of a PDP

so that the

2-D configuration streak

at high

It is also

in the 1205D.

to a high

beam

coated,

as it accumulates

curvature

(>l m

are approximately

intensity.

be buffered

the length

lenses

to avoid

of intensity

the direct

distortions

with

along

pulse

the and also

to minimize

to these problems

with

raises

at the appropriate

that there

configuration.

coupled

monochromator

path

attached

board

length

to A/10 or h/20

amplification

image

focal

reflectance

and loss

attention

device

on

and fluore-

board

are antireflectance

the fact

as 90% of the original

This

OMA can be used

data

divergence

bread

on the bread

Long

camera

generators,

optical

the optical

and polished

path,

harmonic

20ns.

for maximum

of the OMA is interfaced

computer.

length

satura-

the optical

the streak

beam-steering

elements

Considering

in the light

effectively

The output

beam

(-6m).

of perhaps

using

of the system

18cm above

to mount

mounted

along

optical

are ground

distortions,

surfaces

of about

are coated

Elements

optical

positions

transmitting

long optical

J-Y model

the gain

dynamic

and OMA - intensity

is about

on an elevated

A rigid

optical

elements

camera

convenient

of the components

f-L) can be used at various

when

low

on the OMA is

the attainable

spreading

the amplifier,

are mounted

table.

to the height

a required

important

by

intensifier

of the 675 II streak

the laser

IIT34

tube,

sufficiently

and as read

of magnitude

It is hoped

- streak

output.

coated

distortions

with

can be minimized.

The optical

provides

an intensifier

by an order

system.

axis of the laser

scence

without

the space-charge

of the detection the three

ratio

monochromator,

ruled

of the streak

grating,

camera

is

speeds important e.g. a

where

entrance

waveslit.

SOME RESULTS

In the past

the presence

of nonradiative

in the subnanosecond

region

fluorescence

quantum

yield

nonradiative

decay

cause

of lifetime

radiative second

rate

emission

he measured

rates,

could data,

it was

shortening-

changes

2nd

technique

only be assessed Since never

often sometimes allows

in this interesting

processes

yields possible

some

through

depend

complicated shapes

time domain,

of

emission

the evaluation

upon both

to determine

combination

the exact

[9] causing

both

radiative

decays of and

the quantitative radiative

by nonexponentiality, of fluorescence

decay

and nonThe picocurves

to

227 (a)

Nonexponential Later

fically.

certain

Nonexponentiality

superposition events,

and quantum

species

weighted

species.

Less

sensitive. solid

These

molecule, [lo] where Examples where

the microenvironment, virtue

of the mixed

time scales

sensitivity

Coherent

may occur,

These

system

of molecules

arises

in time dependent

reorientation Of course,

which

power

excitation

(b)

found

resides

lifetime

in a solid

is

or on a

membrane

or a macro-

in a fluid

solution,

the emission in mixed

molecule

decay

solvent

time. systems,

to some property

this solvent

property,

highly

decays

of

by

nonuniform

on the

from

energy.

emission

[12].

curves

followed

a

example rotational

In fact,

there

by an exponential

effects

of dye molehigh

peak

is a strong

on emission

or the excited

decay

theoretically

of the dye

dyes,

angle,

directly

optical

solutions

absorbing

The emission and

where

from nonlinear

on viewing

is observed

experimentally

or within

A simple

[l,ll],

in dilute

for strongly

by absorption.

of dependent

the same molecule,

can also arise

decay

decay,

the presence

as a sum of

decay.

observation

the emission

to a weighted

can be analysed

depolarisation

Stimulated

lifetime

A type of coherence

arise

fluorescence

particularly

Interference

the decay

excitation

then a steep

interference

does not lead

can exchange

fluorescence

Quantum

cases

fluorescence

is monitored

events

within

of the fluorescence

has been

Quantum

sites

is spatially

expon-

species

its fluorescence

to arise

for example,

and "head-on"

and on whether

the normal

in some

with"

powers.

can be important,

time,

of temporal

nonexponential

cules

rise

than

and where

superpositions

"competes

at highlaserpeak

stances

If each

decay.

if the emitting

of the emitting

of the solvent,

although

events.

population

likely

two separate

of the two emitting

microenvironments

such as polarity,

exponentials.

length,

case when

on a biological

is longer

case are more

superposition

of decays,

dependence

of such

in question.

A coherent average

speci-

the incoherent

a sum of the two separate

to which

sites

of change

nature

arise

superposition

a temporal

may be different

even be different

is a great

will

of causes:

concentrations

of this case arise

microenvironments

of the latter

there

is simply

microenvironments

the time scale

trivial

undergoes

to the relative

they may be different

or they may

kinds

the coherent

in the most

independently

examples

different

surface,

events,

then the result

according

to three

decays

effects.

arises

and each

trivial

in two or more

temporal

interference

is exponential,

of nonexponential

can be traced

superposition

are excited

entials,

examples

of simultaneous

Incoherent

decay

Decays

in the paper

under

to have

such

wave-

state

circum-

a resolvable

tail decay which

matches

[K!].

and Dephasing effects

can result

[9,13,14] have been discussed from

the suddenness

in the literature.

of excitation

because

all the

phase

factors

equal.

in the prepared

Following

state

such

are caused

ionary

ensemble

uncertainty

the prepared

states

trum is a uniform

continuum,

Lorentzian

shaped

frequency

coherency

can also

providing

there

excited

These temporal

limited

coherence,

of effects

require

coherence

loss

mixed

and

frequencies

interference

tion of quantum would

confirm

studying fields

(c)

theoretical

and

their

Fluorescence The

dephasing

chromophore solution, Even

shape

pendent

states

difficult.

states

excited

(S 10 psec)

giving

the initial to prepare

of random

time.

In all cases,

a smear

of

molecular

levels

the quest

for this

with

with type

a low temperature

a tunable,

cavity

for this type of study, The observa-

time dependent

and could prwide

spectroscopy

a further

of the beats

wavelength

than

to a transform

in this direction.

Perturbation

spaced

to create

Probably

candidate

in picosecond

on excitation

produced

the large

sufficient

would

vapour

angular

necessary

pressure. experiment

depolarisation

dipole

method

of

by various

be of great

membranes

though

interest.

of molecules

from

in

or macromolecules.

rotational

if the molecular

be unnecessary,

can be calculated

in a

this has not been

for nanosecond

Naturally, would

orientations

displacements

in biological

can be studied,

molecules

the gas phase

fluorescence

transition

of following

or incorporated

in the gas phase

are known,

finely

close

in order

has made

been made

expectations

a method

on surfaces,

do not have

better

of matching

pulses

even more

effects

of initially

the past because

since

Depolarisation

offers

molecules

levels

the observational

of initial

in molecules.

dependence

of the molecular

requires

range

[19] is the best

interference

fine structure

somewhat

and also

the difficulty

have

a

spectrum

because

efforts

having

type of

This

but suffers

energy

spec-

in the temporal

time resolution,

ensemble

be produced

frequency

by a pulse

cases.

with

as Lorentzian,

whose

is easier

defined

but so far no extensive

will

spectrum

to observe

frequency

of picosecond

dye laser

decays

c> 10 psec)

effect

with well

Ar+ pumped

in accordance

[17,18].

difficult

etc during

of a large

Ao of the prepared

of the stat-

structure

collisions

characteristics

crystal

dumped

pulse

temporal

not as good

from

similar

spectrum

a time resolution

spaced

width

excited

[16], e.g. beats,

been

the appropriate

population

of quantum

or other

in the prepared

parts

by a pulse

continuum

are virtually

-iot e

Nonexponential

excited

so far to this problem

Coarsely

the available

have

of the

the frequency

in the frequency

requires

over

and would

temporal

spectrum,

changes

in the frequency

line

to structure

states

is the dephasing

[l5].

a molecular

is structure

applied pulse

thermal

lead

structure

has been

molecular

or in the exciting

types

greater

are weighted

shaped

temporal

dephasing

the faster

relation

i.e. a Lorentzian

being

The

states,

of stationary

preparation,

mechanical

functions.

of stationary

the time-energy unless

a coherent

by quantum

state wave

superposition

done

in

times

weight

and

as the time de-

the moments

of inertia

229 and

the temperature. Combined

ment

with

energy

of aecompsnying

angular

dependence

required scence

for the photochemical

from

out"

between

the molecule may arise polar

parts

take on different

affording

of the molecule

the greatest

such effects,

i.e. if the chemically

in the above

depolarisation dependent

on solvent

with

the present

of experiment

Picosecond

shape

will

Solvent

should

changes,

and,

reaction

has already

be described

combined

molecule

environment.

reactive

with

other

such

been demonstrated

may

The same

chemical

part

of

reactivities

site were shape

to provide

That

diffusion.

of interaction

of molecular

be able

rates.

later

fluorecontrolled

the hydrophilic

sensitive

The determination

studies

dependent

technology

which

Picosecond

example.

hiding

is

depending

A polar

amount

and its solvent solvent,

shapes

[Zl].

layer.

solvent

the

by means

information studies,should

studies

are possible

1221 by another

type

in the paper.

Depolarisation spectroscopy

to be extended

in media

allows

11,233

than any earlier

100-1000

solvent

picosecond

to that of rotational

a hydrophobic

light

faster

molecules

the

orientation

of a diffusion

within

shed

studies

diffusion

the measureabout

mutual

Thus,

the concept

up" in a non-polar

of fluorescence about

'What

to proceed?"

extend

studies,

information

"bunch

from just

group

yield

in whzfch they are dissolved

in a polar

the polar may

reaction could

that flexible

reaction

would

or the reaction.

of translation

the type of solvent

molecule

(d)

studies

the realm

It is suspected

"stretch

or photochemical

displacements

of the transfer

depolarisation

reaction

upon

transfer

angular

time dependent

into a temporal

experiments.

of low viscosity

fluorescence

region

Molecules

three

having

can be routinely

depolarisation

orders

molecular

studied

of magnitude weights

between

by the picosecond

technique. The obvious equations

extent

The derivation

of the solute

extreme

molecule

caught position

tensor

more

appealing

axis.

The only

vector

is the case where

model

source

displaced

concerns

solute

equations

of the solvent since

the classical

there

(241.

extreme

exactly

and rotates

friction

To Interposition.

matching

the solvent with

are no tangential

the sticks

it. components

of the solute.

predicts

or a spheroid

as the molecule

motion

the solute?

In other words

at the surface

of rotational

with

of the solvent

it correctly

molecule,

the relat+ve

take a rather

solute

hydrodynamic

as it rotates

to and rotate

at its surface.

in the case of a spherical

of fluid being

with

stick

up by the rotating

stress

symmetry

question

on the velocity

of the normal

because

A major

molecules

an intuitively friction

is to check

used Stokes-Einstein

depends

to or is othetise Another

studies

solvent molecules compared

do the solvent

the often

estingly,

velocity

for these

for small molecules.

of neighboring what

reason

This

zero rotational

rotating

about

its

in this limit arises

rotates

(a "molecular

paddle

is

230 wheel

effect") .

It has been

another

either

to zero

[24] or as the roughness

The

above

as the ratio

showo

extreme

of these

of minimum

cases

boundary.conditions

relate

relaxation

nonpolar

For

much

solvent.

better

agreement

The experiments been based

on lineshape

large molecules, roughness,

not

the limit must

congruous

cases where solute

such

carried

with

important

either

must

of the above

molecules

exist.

to the "slipping"

of interaction

with

as the temperature

Another

"limit"

solute-solvent

binding.

of solvent

molecular

weight,

conditions bulk

exists

should

solvent.

are now

Here,

molecules

it is not clear whether then be used

in the molecular

course,

intermediate

weight

cases

exists.

is sufficiently and size

between

"sticking"

of

which

are

slip past

the

which

of a special over

kind

to such boundary then be

[28].

(compared

with

ICC)

it is obvious

that

or "slipping" rotating

long range,

determination

For

Intermediate

determination

solvent

the

quite well.

~1000)

cases might

While

"sticking"

for the bound

If theinteraction

solvent

switch

strong

inthe

that

degree

to apply

at Melbourne

is infinitely

be included

a high

(MW

because

Intermediate

an ambiguity

must

seems

in a

picture.

of small molecules,

might

place

(BBOT)

11 showing

have

result.

recently

molecule

conditions.

conditions

taking

there

lead to

fluorescence

to and partially

classes

surroundings

is raised.

when

stick

certain

boundary

solvent

Such experiments

observed.

a layer

their

two boundary

until

theoretical

nature

conditions

in a

Stokes-Einstein

of small molecules

partially

In fact,

conditions

have

nonpolar

by their

boundary

classes

do not conform

conditions

which

values

molecules

picosecond

the correct

Application

theoretical

27 and Table

[Ref.

give

and "slipping"

medium.

boundary

by the way,

small,

out

indeed

of "sticking"

be many

the solvent

surface

on a relatively

as proteins,

"sticking"

nonpolar

A direct

tends

1251.

than the classical

1261.

into one

of the spheroid

different

smooth,

the "slipping"

measurements

conditions

known

merge

in a viscous

of small,

experiment

has now been

boundary

semiaxes increases

can give widely

times

to this question,

experiment

medium

"slipping"

There

with

to maximum

of a spheriod

systems

pertaining

depolarisation nonpolar

such

two limits

to the now well

conditions

for the rotational

the above

of the molecule

for rotation

two boundary

that

of volume boundary

against

the unbound

a second

layer

may be appropriate.

and "slipping"

and

are again

of Of

possible

throughout. The

case of attached

See also Table aqueous

1.

The

and alcoholic

solvent

fluorescein media

molecule.

In other words

"sticking"

boundary

"slipping"

pathological

of the negatively

charged

been

observed

derivatives,

as if their volume

they rotate

conditions

and "sticking"

this seemingly

has already

much

which were

slower

limits.

condition regions

are diai&ons,

about

double

the region

the interaction

is strong

of the solute.

hydrogen

in

that of the free

bounded

by the by

responsible

bonding

(1).

rotate

even than predicted

and thus lie outside We believe

in our laboratory

for

in the vicinity

231

When

the solute

cause

positively

solute.

dental

[see Table

conditions

between

defined

effects,

lead

of the solute,

relevant. solvent

Weaker to deviate

caused

from

- solvent

that of the solute

molecule,

a high

we conclude

coefficients

degree

that

only

in Table

seems

between 1 and

that becomes field

experimental

of the

rise

of "roughness"

intermolecular for a small rotational

using

diffusion

"sticking

boundary

is fortuitous.

1

Rotational

Correlation

-Molecule

Times

(psec)

r(measured)

Bengal

Rhodomine

in Ethanol

68090

6G

r(slip)

~(stick)

220

120

250'30

220

120

DODCI

160=30

160

80

BBOT

210r30

460

260

(e)

to

and "slipping".

the effect

the theory

as mentioned

thus giving

- solvent

purely

an integral

molecule

improbable

is

compared

Rather

"sticking"

through

part of the solute

of roughness

the agreement

for the molecules

between

our

and solvent.

the velocity

at its surface,

conditions

can thus be achieved

by the repulsive

Since

conditions

boundary

cause

from

through

to be considered

of the enlarged

interactions

boundary

conditions

interaction

conditions.

in that case has

by an acci-

stems

the solute

attractive

boundary

to apply

and intermediate

physically

between

- solvent

by the

in a number

caused

conclusion

less

would

seems

of "sticking " boundary be achieved

and it is the rotation

limit

primarily

potential.

Rose

solvent

solute

This

limit observed

likely

attachment

interactions

solute

to the "sticking"

sort of intermediate

The "sticking"

the limit

it can never

strong

the neighboring

earlier,

though

which

to be repelled

the "sticking"

solvent

is evidently

effects

this has now been

to us that it is very

i.e. attractive

a very

kT does not

of the solvent

','slipping" and "sticking".

that even

bonding

electrostatic

enough,

though

- partial

mathematically,

In particular,

Table

l] it seems

the hydrogen

purely

atoms

Even

of effects

feeling

sticking

some

from

hydrogen

[22,23,29].

balancing

present

part

charged

exactly

of cases

with

charged,

be expected

In these cases,interestinglY

almost

well

is positively

This would

strong.

Biological

Probe

Molecules

A part of our current earlier extrinsic spectral

in the paper probes

work

that illustrates

deals with

ANS and TNS.

characteristics

the intrinsic The important

and fluorescence

a number probe property

quantum

of concepts

molecule

tryptophan

of probes

yields

discussed and

is that their

are sensitive

to the

microenvironment in which they reside.

This sensitivity arises because solute -

solvent interactions In the excited electronic state are different from what they are in the ground state, and the differences vary with solvent properties such as polarity and hydrogen bonding strength.

Spectral shifts therefore may be

highly dependent on these properties of the microenvironment, as are nonradiative processes, such as ionisation and other fast spin allowed or spin forbidden nonradiative transitions, which compete with light emission, reducing lifetimes and yields. A simple example of a probe is the dye rose bengaf (abs.max. = 550 nm) where the fluorescence lifetime drops from 820 psec in ethanol to 95 psec in water [30]. Because of a decrease in the energy level spacing between the lowest singlet and triplet electronic states, a fast nonradiative transition from the singlet to the triplet can occur more easily in aqueous solution.

The energy level shifts

arise from relative hydrogen bonding strengths in the two solvents. An even more striking example of the lifetime shortening effect occurs in ANS and INS, which can conveniently be excited by the third harmonic of Nd+3/glass

1311.

In INS, for example, the lifetime in ethanol is abnut 9 nsec while in

water solution it drops all the way down to about 60 psec, a change of l!?O-fold, In AM

the difference is almost as striking.

Accompanying these lifetime

changes are emission spectral shZfts of about one-third of an electron-volt. Because of this great sensitivity to the solvent microenvironment, mixed water/ ethanol solvents show highly nonexponential decays caused by solvent inhomogeneities throughout such solutions. In the case of ANS and TNS the cause of ,the lifetime shortening is primerfly photoionisation

[32], which is much more probable in water than alcohol, though

other effects play a smell role as well.

One-photon ionisation is fairly common

in aqueous solut%ons of organic molecules containing electron donating groups 133 lPhotoionisation is very temperature dependent, a hebaviour which acts as a diagnostic method for distzinguishing this from other nonradiative processes, and of course the solvated electron has an absorption spectrum of its own which can be studied temporally. The huge shifts of the emission spectrum of ANS and TNS inaqueous solution are very likely caused by the great increase in polarity and solvent binding in the excited state.

The geometrical rearrangement of solvent molecules must be very

great following excitation, but in nonviscous solvents it is accomplished rapidly compared with the short excited state lifetimes.

Emission takes place primarily

from the relaxed conffguration in nonviscous solvents but from an unrelaxed configuration in viscous solvents [34j.

We are now studying picosecond fluore-

scence depolarisation of these molecules to try to determine the extent of this solvent binding.

Because of the shortness of the time-scale in these experiments,

the pjspaecond technjlque is ideal, rather subtle molecular shape changes 1221 and

233

solvent attachment [l] having already been detected by

this

method.

Another interesting molecule is tryptophan, which can be excited by the fourth harmonic of Ndf.3/glass. Tryptophan is indole with an amino propionic acid It is an important a&o

attached.

acid and a good intrinsic fluorescence probe Our recent picosecond work [35] has revealed

for biological structural studies.

nonexponential decays, which can roughly be expressed as sums of two exponentials, whose

rate

study

1351 of a simple

ential

constants

both

increase

indole

markedly

derivative

with

A picosecond

temperature.

<3-methyl

indole)

shows

a single

expon-

decay matching the long lived decay of tryptophan, whzile 3-methyl indole

in concentrated glycine solution gives two exponentials similar to tryptophan. The simplest way of interpreting these results is to assume tbat photoionlsation (pli4-9) occuring at the indole njttrogen can be enhanced in tryptophan by the intramolecular electron scavenging ability of the amino group and in 3-methyl indole/glyctie through intermolecular scavenging by glycine.

The nonexponentia-

lity arises in both cases because of an incoherent superposition of:

(1) slow

photoionisation tithout the help of a scavenger; and C2) rapid photoionisation when the scavenger is ideally located.

that

The experiments suggest

exchange

between the "two kinds of molecules" is slow on the time scale of the fluorescence - in the 3-methyl

indole/glycine

case because

translational

is

diffusion

not fast enough and in the tryptophan case because "trans-molecular" solvent bonding prevents rapid rotations of the amino propionic acid about its own bonds.

REFRRRNCES 1.

G. R. Fleming, J. M. Morris and G. W. Robinson, "Direct Observation of Rotational Diffusion by Picosecond Spectroscopy", Chem. Phys., 17 (1976) 91.

2,

G. R. Fleming, I. R. Barrowfield, A. E. W. KrUght, J. M. Morris, R. J. Robbins and G. W. Robinson, "Properties of Single Picosecond Pulses Phosphate

3.

Glass",

G. R. Fleming,

Opt.

Comm.,

S, M. Morris

20 (1977)

from Neodymium:

36.

and G. W. Robinson, "Picosecond Fluorescence

Spectroscopy with a Streak Camera", Aust. J. Chem., 30 (1977) in press. 4.

D. von der Linde,

0. Bernecker

for the Selection

of Single

and A. Laubereau,

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Opt.

Comm.,

Shutter

2 (1970)

215-218. 5.

J. M. Ley, T. MI, Christmas and C. G. Wildey, Light

6.

Switch",

G. R. Fleming,

Proc.

Opt.

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R. Eadland, cl.975).

Electr.

I. R. Harrowfield,

and G. W. Robinson,

7.

Inst.

"A Nonlinear

Engr,,

117

%olid&tate (1970)

A. E. W. Knight, Optical

Method

Subnanosecond

1057-1062.

J. M. Morris,

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R. J. Robbins Conpression".

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N.

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editor,

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ILL.

T. Tao,

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Electron

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in Single-

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28

Phys.,

Naphthalene

14.

M. S. Slutsky

15.

G. W. Robinson

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Phys.

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301-303. Rotational

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609-632.

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in

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6 (1974)

with

34-53.

Collisionless

272-283.

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Can. J. Phys.,

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in Qibronically

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53 (1975)

l-34.

61-70.

Singlet",

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Rev. A, I5 (1977)

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'Time

3 (1960)

and Brownian

and Stimulated

23 (1977)

Second

from

1 (1974)

R. 3. Robbins

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Resonances:

Phys.,

Biopolymers,

J. M. Morris,

Studies Chem.

Mol.

New York

Depolarization

of Macromolecules",

Spectroscopic

Transitions'

Press,

in Liquids",

A. E. W. Knight,

Dye Molecules",

Tangled

Sites

Radiationless

Academic

Fluorescence

Coefficients

G. R. Fleming, "Picosecond

16.

Resolution

999-1010.

10.

13.

Photography",

G. W. Robinson,

G. W. Robinson,

12.

and M. El. Key, "Time

Abrned, B. C. Gale

"The Level

Perturbed

Shift

Spectra",

Operator Mol.

and its Effect

Phys.,

29 (1975)

613-622. 17.

C. A. Langhoff,

18.

W. Rhodes

"On

Molecular

Excitation

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19.

Light",

C. Q. Shank Locked

27

21.

Chem.

(1975)

23.

Appl.

Chem,

"Subpicosecond

Spectroscopy

"Nanosecond

Lett.,

Transitions:

Phys.,

22 (1977)

Kilowatt

24 (1974)

Pulses 373-375;

and Subpicosecond

Fluorescence

of the

Pulse

Selective 95-103.

from a ModeE. P. Ippen Compression,

and ibid

D. Chandler

and L. R. Pratt,

Phys.,

G. R. Fleming,

Chem.

Lett.,

T. 3. Chuang Orientational

Mechanics

of Chemical

of Nonrigid

Molecules

in Condensed

J. M. Morris, R. J. Robbins

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368-370.

Phases"

Dye DODCI

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and Its Photoisomer",

l-7.

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65 (1976)

"Rotationa

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498-578.

26 (1972)

(1971)

Pulses",

The Role

357-366.

of Radiationless

Phys.

Enzymology,

11

Fluorescence.

488-490.

J. Yguerabide,

J. Chem.

20 (1977)

by Laser

"Dynamic

Resonance

Theory

and E. P. Ippen,

and Intramolecular

22.

Phys.,

the Dynamic

CW Dye Laser",

C. Q. Shank,

20.

"Nonexponential

Using

'Studies

Picosecond

of Effects Light

of Hydrogen

Pulses",

Chem.

Bonding

Phys.

on

Lett.,

235

24.

C. M. Hu and R. Zwanzig, with

25.

the Slipping

S. Richardson,

"Rotational

Boundary

"On

Friction

Condition",

the No-Slip

Coefficients

.I. Chem.

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Phys.,

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for Spheroids

60 (1974)

J. Fluid

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59

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D. R. Bauer,

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Experimental

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96:22

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6840-6843. 27.

G. R. Fleming,

A. E, W. Knight,

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Conditions

Boundary

scence

Depolarization

28.

G. R. Fleming,

29.

K. B. Eisenthal,

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31.

G. R. Fleming,

Sot.,

Chem.

Res.,

8 (1975)

"Picosecond

99:22

(1977)

Time

Phys.

R. J. Robbins,

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Fluore-

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399-402

and G. W. Robinson,

work

G. Porter,

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Decay

33.

L. I. Grossweiner

34.

R. P. DeToma,

R. .I, S. Morrison

Studies

G. R. Fleming, Studies

Sulphonate

R. J, Robbins

Pathways

of Zanthene

Picosecond

and

Dyes",

J. M. Morris,

J. Am.

A. E. W. Knight

of the Fluorescence (ANS)",

Israel

Probe

J. Chem.,

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in the Fluorescence

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(ANS)",

and H. I. Joschek, Compounds",

J. H. Easter with

with

Submit-

in preparation.

from Aromatic

Probes

Processes

118-124.

J. M. Morris,

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ted; and other

Anilinonaphthalene

and Physical

Fluorescence

1,8 Anilinonaphthalene

Electrons

Chem.

and G. W. Robinson

4306-4311.

G. R. Fleming,

cence

of Chemical

A. E. W. Knight,

G. W. Robinson,

Singlet

Rotation:

of BBOT",

J. M. Morris , R. J. Robbins

"Studies

and R. J. S. Morrison,

32.

R. J. Robbins

work.

Accts.

Molecule

for Molecular

Studies

F. Howie,

G. W. Robinson, Chem.

3. M. Morris,

Molecule

1,8

to be published.Chem.Phys.LettinPress "Optical

Advan.

and L. Brand,

the Solvent

Probe

"Excited

Generation

Chem.

"Dynamic

Environment",

Ser.,

of Hydrated 50

(1965)

Interactions

J. Am. Chem.

Sot.,

279-288.

of Fluores98:22

(1976)

5001-5007. 35.

G. R. Fleming,

J. M. Morris,

and G. J. Woolfe, Tryptophan

using

R. J. Robbins,

"Observation Picosecond

G. W. Robinson,

of Nonexponential

Spectroscopy",

Fluorescence

to be published.

P. J. Thistlethwaite Decay

of