1

Excited States of Aggregate FilmsExcited States of Aggregate Films

@ MITFebruary 21, 2002 – Organic Optoelectronics - Lecture 6

Anouncement: the course website has moved to hackman.mit.edu/6977Handout: Directions for Laboratory #1

Acknowledgment:

figures in slides 9,10 are from Organic Chemistry by G.M. Loudo,figure in slide 11 is from Solid State Physics by N.W. Ashcroft and N.D. Merminfigures in slides 12-22 are from Electronic Processes in Organic Crystals and Polymers

by M. Pope and C.E. Swenberg

•Van der Waals bonding•Hydrogen Bonding

•Dimers•Excimers

•Generation of excitons

•Wannier exciton•Charge-transfer exciton•Frenkel exciton

T1S1

S0

FL

UO

RE

SC

EN

CE

PH

OS

PH

OR

ES

CE

NC

E

ENERGY TRANSFER

FÖRSTER, DEXTERor RADIATIVE

INT

ER

NA

LC

ON

VE

RS

ION

AB

SO

RP

TIO

N

10 ps

1-10 ns

>100 ns

Ener

gy

density of availableS and T states on

surrounding molecules

Electronic Processes in Molecules

JABLONSKI DIAGRAM

INTERSYSTEMCROSSING

S: spin=0 (singlet) statesT: spin=1 (triplet) states

2

• symmetry conserved

fast process ~10-9s

• triplet to ground state transition is not permitted

slow process ~ 1s

Fluorescence

E

ground state(singlet)

singletexcitedstate

tripletexcitedstate

S1 T1

S0

FLUORESCENCEFLUORESCENCE

singlet excitonsinglet exciton

E

Phosphorescence

PHOSPHORESCENCEPHOSPHORESCENCE

triplet excitontriplet exciton

S1 T1

S0

Why do we care about singlets and triplets?• only singlets contribute to fluorescence• triplets contribute to phosphorescence

(low efficiency process)

Generation of Excitons

Photo generation Electrical generation

if molecule absorbs a photon,symmetry of molecule is

unchanged

⇒ only singlets

if electrons and holesrecombine to form an exciton,

their spins are uncorrelated

⇒ singlets and triplets

3

PTCDA Solution (~ 2µM in DMSO)

1.5 2.0 2.5 3.0 3.5

Energy [eV]

AbsorptionFluorescence

S1

[0-3

]

S1

[0-2

]

S1

[0-1

]

S1

[0-0

]

CT

[0

-ST

]

CT

[0

-F]

S1

[0-1

]

S1

[0-0

]

Solution Absorption

1.8 2.0 2.2 2.4 2.6 2.8 3.00

2

4

6

..

.

2 µM

Abso

rptio

n [a

.u.]

Energy [eV]

PTCDA in DMSO

AGGREGATEAGGREGATEStateState AGGREGATE

STATE ABSORPTION

increases with PTCDA solution concentration

1.6 µM

0.25 µM

4

Crystalline Organic Films

PTCDAPTCDA

x

y

z

11.96 Å

17.34 Å

3.21Å

substrate

GOOD CARRIER MOBILITYIN THE STACKING DIRECTION

µ = 0.1 cm2/Vs – stacking directionµ = 10-5 cm2/Vs – in-plane direction

CHARGED CARRIER MOBILITY INCREASES WITH INCREASED

π−πORBITAL OVERLAP

Highest mobilities obtained onsingle crystal

pentacene µ = 10 5 cm2/Vs at 10Ktetracene µ = 10 4 cm2/Vs at 10K

(Schön, et al., Science 2000).

Organic Molecules are typically held together byvan der Waals (Fluctuating Dipole) forces

Consider Non-Polar molecules:

Although the average charge distribution in a non-polar molecule may be spatially symmetric, due to molecular vibrations at any instant there may be a net dipole moment (whose time-averaged value must vanish).

If the instant dipole moment of molecule 1 is p1 then the associated electric field is E = p1/r3. This will induce a dipole moment in molecule 2 proportional to the filed: p2 = αE = α p1/r3, where α is the polarizability of the molecule.

Then energy of dipole interaction = p2p1/r3 = α p12/r6

(p12 does not average to zero)

5

A stop-frame cartoon of the formation of induced

dipoles as two hypothetical molecules collide and rebound.

Notice that the distortion of electron clouds is not

permanent but varies with time. The electronic

distortion of one species is induced by the proximity

to the other.

t1:

t3:

t2:

t4:

t5:

Induced Dipole Formation

Van der Waals Attraction

Energy curves for the approach of two spheres of radius r.

SOFT SPHERES ATOMS / MOLECULES

6

Hydrogen Bonding

Hydrogen is unique in three way:

1. Radius of the ion core of hydrogen (bare proton) is 10-13

cm, a factor of 100,000 smaller than any other ion core

2. Hydrogen is one electron shy of the stable He configuration which is unique in that it has two electrons in its outer most shell

3. First ionization potential of hydrogen is unusually high (H, 13.59 eV; Li, 5.39 eV; Na, 5.14 eV; K, 4.34 eV; …)

The crystal structure of one of the many phases of ice. The large circles are oxygen ions and the

small are protons

PHYSICAL DIMER CHEMICAL DIMER

Exciton band splitting and energy shift for a physical dimer

Dimers

7

Absorption Spectrum of a Monomer and a Corresponding Dimer

Spectral Properties of Symmetric Sandwich Dimers

1. All dimer maxima lie at a lower energy than the corresponding maxima of the monomer spectrum

2. The Frank-Condon maximum of the dimer is represented by the 0 -> 1 transition, as compared to the 0 -> 0 transition in the monomer. (There is typically a greater nuclear displacement in the dimer.)

3. The dimer spectrum is broader and less well defined as compared to the monomer spectrum due to the presence of new vibrational modes associated with the dimer.

4. There is a decrease in dimer luminescence intensity as compared to the luminescence of monomers

8

Exciton Splitting in Dimers

a

b

c

Crystal structure of anthracene

Each unit cell is monoclinic, i.e. two of the axes (a,b) are at the

right angles to each other , but the third (c) is at an angle other than the right angle to the other two

9

Davydov Splitting

A given molecular energy level may split into as many components as there are inequivalent molecules per unit cell. This splitting is in addition to the level splitting produced by the interaction energy between two adjacent identical molecules. For triplet state the splitting is ~ 10 cm-1, for singlet states splitting is few 100 to several 1000 cm-1.

Splitting of the 2N-fold degenerate level for a

crystal with two molecules per unit

cellinto two distinct Davydov bands.

a and b denote the corresponding

polarizations associated with each band. WD is defined as the Davydov

splitting energy.

10

Davydov Splitting

… in anthracene

This is the delayed fluorescence excitation

spectrum corresponding to the T1 -> S1 (0-0)

transition.

Excimers

A class of compounds that exhibits the optical absorption characteristic of a monomer, but their luminescence is characteristic of a physical dimer.

In excited state - dimer existsGround State - is dissociative

Examples: Hg, He, Xe form excimersSo does pyrene, …

1000

cm

-1

Intermolecular Distance

11

1 x 10-2 M8 x 10-3 M5 x10-3 M3 x 10-3 M1 x 10-3 M

1 x 10-4 M

Fluorescence Spectra of Pyrene Solutions

EXCIMERPEAK

MONOMERPEAK

12

PTCDA monolayer on HOPG(STM scan)

Organic Semiconducting MaterialsVan der Waals-BONDED

ORGANIC CRYSTALS (and amorphous films)

HOMO of3,4,9,10- perylene tetracarboxylic dianhydride

Solution Absorption

1.8 2.0 2.2 2.4 2.6 2.8 3.00

2

4

6

..

.

2 µM

Abso

rptio

n [a

.u.]

Energy [eV]

PTCDA in DMSO

AGGREGATEAGGREGATEStateState AGGREGATE

STATE ABSORPTION

increases with PTCDA solution concentration

1.6 µM

0.25 µM

13

1.8 2.0 2.2 2.4

0

2

4

6

.

.

.

0.25 µM

2 µMFl

uore

scen

ce [a

.u.]

Energy [eV]

0 1 20

1

2

3

4

I ∝ [k]0.65In

tegr

ated

Flu

ores

cenc

eIn

tens

ity [a

.u.]

Solution Concentration[k] [µ M]

Solution Luminescence

10.01

0.1

1

20.5Nominal Solution Concentration [k]

Monomer and Aggregate Concentration in Solution

AGGREGATE

[ka] ∝ [k] 1.75

MONOMER

[km] ∝ [k] 0.65

[µM]

MonomerConcentration

[km]

AggregateConcentration

[ka]

[µM]

rate of riser = 1.75

Monomer and Aggregate

concentrationsare derived from

integrated solutionfluorescence efficiency by

assuming thatthe 2.3 eV solution

fluorescence is dueto monomers and

that the most dilutesolution primarily

contains monomers.

14

1

0.01

0.1

1

0.5 42

r = 0.53 ± 0.02

r = 1.75 ± 0.02

r = 1.00 ± 0.02

r = 0.73 ± 0.02S1 [0-0]: 2.39 eV

S1 [0-2]: 2.74 eV

CT [0-ST]: 2.23 eV

S1 [0-1]: 2.55 eVR

elat

ive

Osc

illato

r Stre

ngth

PTCDA Concentration in DMSO [ µM]

2.0 2.4 2.8

0

2

4

6Measurement

Fit

[k] = 0.8 µM

Abso

rptio

n [a

.u.]

Energy [eV]

Absorption of Vibronic Transitions – Change with Solution Concentration

PTCDA Thin Film

1.5 2.0 2.5 3.0 3.5

Fluorescence

Absorption

CT

[S

T-1

]

CT

[S

T-2

]

CT

[S

T-3

]

S1

[0-3

]S1

[0-2

]

S1

[0-1

]

CT

[0

-ST

]

S1

[0-0

]

CT

[0

-F]

Energy [eV]

15

PTCDA Solution(~ 2µM in DMSO)

1.5 2.0 2.5 3.0 3.5

Energy [eV]

AbsorptionFluorescence

PTCDA Thin Film

0.6 eV

Solution and Thin Film Fluorescence

1.8 2.0 2.2 2.4

0.0

0.2

0.4

0.6

0.8

1.0

2.0 µM Solution

0.25 µMThin Film

(E + 0.60 eV)Fluo

resc

ence

[a.u

.]

Energy [eV]

0 20 40 60

101

102

103

Solutionτ = 4.0 ± 0.5 ns

Thin Filmτ = 10.8 ± 0.5 ns

Nor

mal

ized

Cou

nts

Time [ns]

FLUORESCENCE LIFETIME* Thin film fluorescence is red-shifted by 0.60 eV from solution fluorescence

* Minimal fluorescence broadening due to aggregation

* Fluorescence lifetime is longer in thin films

16

Thin Film Excitation Fluorescence

2.0 2.4 2.8 3.2 3.6

0

10

20

30

650 nm PTCDAThin Film

Inte

grat

ed F

luor

esce

nce

[a.u

.]

Excitation Energy [eV]

CT

[0-S

T]

CT

[0-F

]

S 1[0

-0]

S 1[0

-1]

S 1[0

-2]

S 1[0

-3]

* Fluorescence energy and shape is not affected by the change in excitation energy

* Fluorescence efficiency increases when exciting directly into CT state

MOLECULAR PICTURE

GROUND STATE FRENKEL EXCITON

SEMICONDUCTOR PICTURE

GROUND STATE WANNIER EXCITON

CONDUCTIONBAND

VALENCEBAND

S1

S0

Wannier exciton(typical of inorganic

semiconductors)

Frenkel exciton(typical of organic

materials)

binding energy ~10meVradius ~100Å

binding energy ~1eVradius ~10Å

treat excitons as chargeless

particlescapable of diffusion,

also view them as

excited states of the

molecule

Charge Transfer (CT) Exciton

(typical of organicmaterials)

Excitons (bound

electron-hole pairs)