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ADVANCES INPHOTOCHEMISTRY

Volume 29

Editors

DOUGLAS C. NECKERSCenter for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

WILLIAM S. JENKSDepartment of Chemistry, Iowa State University, Ames, Iowa

THOMAS WOLFFTechnische Universität Dresden, Institut für Physikalische Chimie und

Elektrochimie, Dresden, Germany

WILEY-INTERSCIENCE

A JOHN WILEY & SONS, INC., PUBLICATION

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Volume 29


Volume 29

Editors

DOUGLAS C. NECKERSCenter for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

WILLIAM S. JENKSDepartment of Chemistry, Iowa State University, Ames, Iowa

THOMAS WOLFFTechnische Universität Dresden, Institut für Physikalische Chimie und

Elektrochimie, Dresden, Germany

WILEY-INTERSCIENCE

A JOHN WILEY & SONS, INC., PUBLICATION

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CONTRIBUTORS

F. C. De Schryver

KULeuven Department of Chemistry

Celestijnenelaan 200F

Heverlee B-3001

Belgium

Mamoru Fujitsuka

The Institute of Scientific and

Industrial Research

Osaka University

Mihogaoka 8-1

Ibaraki, Osaka 567-0047

Japan

J. Hofkens



Heverlee B-3001

Belgium

M. Lor



Heverlee B-3001

Belgium

Tetsuro Majima

The Institute of Scientific and

Industrial Research

Osaka University

Mihogaoka 8-1

Ibaraki, Osaka 567-0047

Japan

G. Schweitzer



Heverlee B-3001

Belgium

Bernd Strehmel

Kodak Polychrome Graphics

Research and Development Division

An der Bahn 80

D-37520 Osterode

Germany

Veronika StrehmelUniversity of Potsdam

Applied Polymer Chemistry

Karl-Liebknecht Str. 24/25

D-14476 Golm

Germany

v

M. van der Auweraer

KULeuven Department of

Chemistry


Heverlee B-3001

Belgium

vi CONTRIBUTORS

PREFACE

Volume 1 of Advances in Photochemistry appeared in 1963. The stated purpose

of the series was to explore the frontiers of photochemistry through the eyes of

experts and pioneers. As editors we have solicited articles from scientists who

have strong identifications with the work presented, and strong points of view.

Photochemistry has expanded enormously since those first days. A serious

percentage of the papers in any single volume of the Journal of the American

Chemical Society, for instance, can rightly fall in its purview. The emergence

of the laser and the evolution of theoretical methods strongly influenced research.

With new computational methodology almost no intermediate lives too short a

time to be detected and its dynamics characterized. The fundamental objective

of our field, elucidation of the history of a molecule that absorbs radiation, is

now within reach in even the most complicated cases. We hope that the series

continues to reflect the frontiers of photochemistry as it evolves into the future.

We report, sadly, that one of the founding Editors, George S. Hammond,

passed away in Portland, Oregon on October 5, 2005. George will be sorely

missed.

With the publication of this volume, Douglas Neckers will be leaving the post

of Senior Editor. He has served, first as Associate Editor and more recently as

Editor, for nearly half of the volumes in the Advances series. He wishes to express

his appreciation for all of the cooperation he has received from everyone involved

in the series. He will remain as a consultant, and Pavel Anzenbacher will take

over as Editor beginning with Volume 30.

Douglas C. NeckersThomas WolffWilliam S. Jenks

Bowling Green, Ohio, USA

Dresden, Germany

Ames, Iowa, USA

vii

CONTENTS

Ensemble Photophysics of Rigid Polyphenylene Based Dendritic

Structures 1

M. LOR, G. SCHWEITZER, M. VAN DER AUWERAER, J. HOFKENS,

AND F. C. DE SCHRYVER

Photochemistry of Short-Lived Species Using Multibeam Irradiation 53

MAMORU FUJITSUKA AND TETSURO MAJIMA

Two-Photon Physical, Organic, and Polymer Chemistry: Theory,

Techniques, Chromophore Design, and Applications 111

BERND STREHMEL AND VERONIKA STREHMEL

Index 355

Cumulative Index Volumes 1–29 379

ix

ENSEMBLE PHOTOPHYSICSOF RIGID POLYPHENYLENE

BASED DENDRITIC STRUCTURES

M. Lor, G. Schweitzer, M. van der Auweraer, J. Hofkens,and F. C. De Schryver

KULeuven Department of Chemistry, Celestijnenelaan 200F,

Heverlee B-3001, Belgium

CONTENTS

I. Introduction

II. Electronic Excitation Transfer

III. Stationary Measurements

IV. Single-Photon Timing Measurements

A. Time-Resolved Fluorescence Measurements Performed Under Magic

Angle Polarization Condition

1. Para-substituted Carbon Core Dendrimers

2. Meta-substituted First Generation Carbon Core Dendrimers

B. Time-Resolved Fluorescence Polarization Measurements

1. Meta-substituted First Generation Carbon Core Dendrimers

2. Para-substituted First Generation Carbon Core Dendrimers

V. Femtosecond Fluorescence Upconversion Measurements

VI. Femtosecond Transient Absorption Measurements

A. p-C1P1 and m-C1P1B. p-C1P3 and m-C1P3

Advances in Photochemistry, Volume 29 Edited by Douglas C. Neckers, William S. Jenks,and Thomas Wolff # 2007 John Wiley & Sons, Inc.

1

C. m-C1P3D. p-C2P1 and p-C2P4

VII. Conclusions

Acknowledgments

References

I. INTRODUCTION

Excited state processes in multichromophoric systems have attracted attention

for a long time [1], since these processes are of great importance in biological

and material science. Indeed, in the light harvesting system as well as in poly-

conjugated polymers, multiple chromophores are present and the efficiency

of the system in the energy cascade to the reaction center or in the efficiency of

the charge generation is influenced by excitation and electron transport along the

multichromophoric system. Because of the controllable incorporation of various

functional groups in different parts of their structure, dendrimers have attracted

much attention recently as model systems for the study of photoinduced intra-

molecular energy and electron transfer. Dendrimers can act as scaffolds that

tether the donor and acceptor chromophores [2], providing versatility such

that additional features can easily be introduced by simply changing the various

components of the dendrimer.

Alternatively, the dendrimer backbone itself can concurrently be used as the

energy donor or acceptor. Several types of chromophoric dendrimer backbones

such as poly(phenylacetylene) [3], poly(phenylene) [4], and poly(benzylether)

[5] have been used as light absorbers, and the energy was efficiently transferred

to the core acceptor. While most of these systems have high energy transfer effi-

ciencies, they still suffer from a weak fluorescence or a low fluorescence quan-

tum yield. However, polyphenylene dendrimers composed of tens or hundreds of

out-of-plane twisted phenyl units can be used as chromophoric backbones [6]

carrying highly luminescent dyes at the periphery.

The earliest work on intramolecular energy transfer in dendritic macro-

molecules originates from Moore and co-workers [7], who synthesized den-

dritic structures based on phenylacetylene units with perylene in the center.

The excitation of the phenylacetylene units at the rim at a wavelength of

310 nm leads to fluorescence emitted by the center perylene unit, indicating

intramolecular excitation energy transfer. A significant increase in the rate

of excitation energy transfer was achieved by modifying the dendrimer

skeleton. This was done in such a way that additional phenylacetylene units

with lower excited state energy and larger conjugation length toward the core

were introduced near the perylene unit. Recently, Bardeen and co-workers

2 ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

reported the role of Förster, Dexter, and charge transfer interactions in pheny-

lacetylene dendrimers [8].

They demonstrated by steady state spectroscopy, picosecond time-resolved

emission and anisotropy measurements, and ab initio calculations that while

the subunits of polyacetylene dendrimers (Fig. 1.1) are weakly coupled in their

equilibrium ground state geometry, they can become strongly coupled in the

excited state. This geometry-dependent electronic coupling will affect the

modeling of energy transfer in these molecules. They found that the variation

of the electronic coupling V with molecular geometry is due to the through-

bond or charge transfer type of interaction rather than due to variation of

the more familiar dipole–dipole and Dexter terms. These dendritic struc-

tures are rigid systems in which the branches are also the absorbers and the

Bardeen study underlines the complexity of these systems in terms of excitation

transfer.

Most of the dendritic molecules investigated for excitation transfer between

chromophores attached at the periphery belong to a class in which the arms are

rather flexible. This of course leads to data related to excitation transfer, which

are averaged over all the possible branch conformations leading to a distribution

in distances between donor and acceptor.

Balzani et al. [9] reported metal-containing dendrimers, where the core

and branching unit are built up from ruthenium complexes of a polypyridine

R

R¢

R¢

RR

R

RR

1-H:

1-TMS:

R = 3,5-di-t-butylphenyl, R¢ = t-butyl1-Ph:

R = H2-H:

R = Si(CH3)32-TMS:

R = 3,5-di-t-butylphenyl

R = H

R = Si(CH3)3R = 3,5-di-t-butylphenyl2-Ph:

3-H:

3-TMS:

3-Ph:

R = H, R¢ = HR = Si(CH3)3, R¢ = H

Figure 1.1. Building blocks of phenylacetylene dendrimers studied by Bardeen and

co-workers [8].

INTRODUCTION 3

ligand serving as core and branching units. By varying the ligands and metals

used, different directional excitation energy transfer processes were observed,

either from the center to the rim or from the rim to the core [10]. The molecular

structure of such a dendrimer with a ruthenium complex in the center is depicted

in Figure 1.2.

Recently, Balzani and co-workers published results on dendrimers consist-

ing of a benzophenone core and branches containing four and eight naphtha-

lene units (Fig. 1.3) [11]. In both dendrimers, excitation of the peripheral

naphthalene units is followed by fast singlet–singlet energy transfer to the

benzophenone core; but on a longer time scale a back energy transfer takes

place from the triplet state of the benzophenone core to the triplet state of the

N N

O

OO

O

O

O

O O OO

O

O

N

N

O

O

O

OO

O

O

O

O

O

OO

N

N

O

O

O

O

O O

O

O

O

O

O

O

Ru2+

Figure 1.2. Molecular structure of a metal-containing dendrimer investigated by Balzani

and co-workers [10].


peripheral naphthalene units. Selective excitation of the benzophenone unit is

followed by intersystem crossing and triplet–triplet energy transfer to the

peripheral naphthalene units, which could be observed by nanosecond tran-

sient absorption.

Using a similar type of branch, developed by the Fréchet group, they have

published extensively on chromophore labeled dendrimers [12]. The dendri-

mers possessing coumarin-2 dyes at the periphery, and either coumarin-343

(Fig. 1.4) or a heptathiophene dye at the core, were studied by time-resolved

fluorescence and transient absorption spectroscopy. It was revealed that

upon excitation of the rim chromophores almost no direct fluorescence

occurred from these initially excited chromophores. Instead, only the center

O

O O

OO O

O

O

O O

OO O

O

O

O

OO

OOO

O

(a)

(b)

Figure 1.3. Molecular structures of dendrimers with 4 (a) and 8 (b) peripheral

naphthalene units and a benzophenone core investigated by Balzani and co-workers

[11].

INTRODUCTION 5

chromophore showed emission; thus proving efficient excitation energy

transfer within this dendrimer. The efficiency of the excitation energy transfer

decreased by increasing the generation number from 3 to 4. This comes from

the fact that increasing the generation number increases the average distance

between the chromophores and thus the overall efficiency of excitation energy

transfer decreases.

Recently, Fréchet and co-workers reported intramolecular energy trans-

fer in dendritic systems containing one or more two-photon absorbing chro-

mophores at the periphery, which act as energy donors, and a Nile Red

chromophore at the core that acts as energy acceptor as well as fluorescence

emitter [13]. The two-photon energy absorbed by the chromophores at the

periphery was transfered to the core, where the core’s emission was strongly

enhanced. The emission from the core chromophore in these dendritic sys-

tems was significantly greater than the emission from the core itself when

the core was not connected to the donor chromophores. This increased emis-

sion arises from the much larger two-photon absorbing cross section of

the donor chromophores compared to the core acceptor at the excitation

wavelength.

N

NO

O

O

O

O

O

O

O

N

O

O

N

N

O

O

O

OO N

N

O

O

O

O

OO

N

N

O

O

O

O

O

Figure 1.4. Molecular structure of a third generation dendrimer with coumarin-343 at the

center investigated by Fréchet and co-workers [12].


Meijer and co-workers investigated the dynamics of excitation energy trans-

fer for a series of spherical porphyrin arrays based on different generations of

poly(propylene-imine) dendrimers using time-resolved fluorescence anisotropy

measurements in a glass environment [14]. They demonstrated that the multipor-

phyrin functionalized dendrimers were able to absorb light and efficiently distri-

bute the excitation energy by hopping over the chromophore arrays with

minimal loss during the energy migration process.

Goodson and co-workers investigated excitation energy transfer proces-

ses in nitrogen cored distyrylbenzene and triarylamine dendrimer systems

(Fig. 1.5) by photon echo and polarized fluorescence upconversion spectro-

scopy. Observed components of less than 1 ps were attributed to a coherent

energy transport mechanism. The contributions from his group were recently

summarized [15].

De Cola and co-workers recently published [16] a study of the photophysical

properties of a molecular system consisting of a bay-functionalized perylene

N

Figure 1.5. Molecular structure of a second generation triaryl dendrimer investigated

by Goodson and co-workers [15].

INTRODUCTION 7

bisimide, containing four appended pyrene and two coordinating pyridine units

(Fig. 1.6) using steady state, time-resolved emission and femtosecond transient

absorption spectroscopy.

Analysis of the data showed the presence of a fast intramolecular photoinduced

energy transfer process from pyrene*–perylene to pyrene–perylene* (ken �6:2� 109 s�1) with a high yield (>90%), followed by efficient intramolecularelectron transfer from pyrene–perylene* to pyrene.þ–perylene.� (70%, ket �6:6� 109 s�1). Both processes occur from the pyrene unit to the perylenemoiety. The Förster distance was calculated to be 3.4 nm and the corresponding

donor–acceptor distance was calculated from the energy transfer rate as 0.9 nm.

No indications for energy hopping between different pyrene moieties were

observed.

Similarly, a number of terrylenediimide core dendrimers with semiflexible

arms were investigated by our research group at the ensemble [17] and at the

single molecule level [18]. Different generations of a polyphenyl dendrimer

containing a terrylenediimide core with peryleneimide chromophores at

NN

O

O

O

O

NN

OO

OO

OO

OO

OO

OO

OO

Figure 1.6. Molecular structure of a bichromophoric pyrene–perylene bisimide system

investigated by De Cola and co-workers [16].


the periphery (first generation depicted in Fig. 1.7) have been studied

with respect to intramolecular energy transfer processes. Excitation of the

peryleneimide at 480 nm resulted in fluorescence of the terrylenediimide

chromophore at �700 nm with an almost complete disappearance of the fluo-rescence of the peryleneimide chromophore at �550 nm, indicating a veryefficient energy transfer process between the peryleneimide and terrylenedii-

mide chromophore.

Single molecule data measured at room temperature indicated that a distri

bution of excitation transfer rate constants could be observed [18], while Basché

and co-workers [19] showed, studying linewidths at low temperature, that the

observed rates are larger than expected from classical Förster excitation transfer

theory and suggested that in these systems through-bond interaction might play

a role.

Similarly, phenylacetylene based dendrimers [7, 8] and those investigated

by Goodson and co-workers [15] show substantial coupling between the

branches while all others discussed above, due to flexibility of the connecting

arms, have an undefined three-dimensional structure and hence variable donor–

acceptor distances.

N OO

N OO

OO

OO

N

O

O

N

O

O

N

O

O

N

O

O

Figure 1.7. First generation polyphenylene dendrimer with terrylene as a luminescent

core.

INTRODUCTION 9

In the present contribution we want to focus on rigid dendritic structures

in which the coupling between the chromophores is weak and in which the

distance between the chromophores involved is fixed in space. To achieve this

goal together with the Müllen group (MPI Mainz), a series of molecules was

developed based on the general structure in Figure 1.8.

Besides these first generation dendrimers, second generation dendritic struc-

tures p-C2Pn were also investigated (p-C2P1, p-C2P2, p-C2P3, p-C2P4) (see

Fig. 1.9).

II. ELECTRONIC EXCITATION TRANSFER

One of the basic mechanisms in multichromophoric systems, electronic excita-

tion transfer has been in the past and still is in many studies largely described

using Förster theory. As stated by Förster [20], this model is developed for the

weak coupling limit as it is based on an equilibrium Fermi Golden Rule

CR2

R2

R3R3

R1

R1

N

O

O

C

NO

O

N

O

O

R1 R2 R3 R1 R2 R3

p-C1P1 H H Hp-C1P3 PI PI Hp-C1P4 PI PI PI

m-C1P1 H H Hm-C1P2 PI H Hm-C1P3 PI PI Hm-C1P4 PI PI PI

p -C1Px m-C1Px

PI

Figure 1.8. Molecular structures of p-C1Pxðx ¼ 1; 2; 4Þ, para-substituted first generationdendrimers, and m-C1Pxðx ¼ 1; 2; 3; 4Þ, meta-substituted first generation dendrimers; PI,peryleneimide chromophore.


approach and the derived Förster equation is valid provided a number of condi-

tions are fullfilled as recently discussed by Scholes [21]: ‘‘(a) A dipole–dipole

(or convergent multipole–multipole) approximation for the electronic coupling

can be employed appropriately for the donor–acceptor interaction. (b) Neither

the donor fluorescence lifetime, emission line shape, acceptor absorption line

shape, nor oscillator strength is perturbed because of interactions among donors

or acceptors, respectively. (c) Static disorder (inhomogeneous line broadening)

is absent in the donor and acceptor line shapes. (d) the energy transfer

dynamics are incoherent.’’

Different complicating factors led to the development of a more generalized

approach [22, 23] in which the Coulomb interaction is now considered in terms

of local interactions between donor and acceptor transition densities. This is

CN

O

O

NO O

NO O

N

O

O

Figure 1.9. Molecular structure of p-C2P4.

ELECTRONIC EXCITATION TRANSFER 11

particularly important when the donor–acceptor ‘‘chromophores’’ are large com-

pared to their center-to-center separation.

To verify if the above-mentioned boundary conditions are valid for the mole-

cular structures reported in Figure 1.8, electronic coupling constants were cal-

culated. Doing this, one needs to take into account that, as a result of the

asymmetric building blocks used in the Diels–Alder cycloaddition in the course

of the reaction, the attachment of the chromophores leads to structural isomers.

Therefore, if multiple chromophores are present, small differences can occur in

the efficiencies of photophysical properties among different isomers. An exam-

ple of possible structural isomers (2D picture) and one example of a 3D isomer

of p-C1P2 (2A2B) are given in Figure 1.10. As depicted in Figure 1.10b, there

are four attachment places for the chromophores and this normally results in four

possible isomers for p-C1P2. However, there is an asymmetry in the four poly-

phenyl branches resulting in two possible ways in which the two chromophores

can be attached. The arrows indicate the possible substitution patterns of the

chromophores in the different structural isomers. The positions where a chromo-

phore can be attached are A2, A3, B2, B3, C2, C3, D2, and D3, where A, B, C,

and D represent the different branches and 2 and 3 the second or third phenyl

group within each branch where a chromophore can be attached.

For p-C1P1, however, the two different structural isomers that can be formed

will show similar photophysical behavior. Also, for p-C1P4 there are a number

of possible structural isomers as can be seen in Figure 1.11a, b. These two

(a)

(b)

(c)

Figure 1.10. (a) Chemical structure of p-C1P1. (b) Two-dimensional (2D) representation

of where chromophores can be attached to the dendrimer and (c) three-dimensional (3D)

representation of isomer 2A2B of p-C1P2. The arrows indicate the possible substitution

patterns.


minimized structures were obtained using a molecular mechanics optimization

method (Merck molecular force field) present in SPARTAN1.

Geometry optimization of p-C1P4A shows that the center-to-center distance

between the chromophores is on average 3.17 nm. For two structural isomers of

this compound (p-C1P4A and isomer p-C1P4B, respectively), the difference in

interaction between the chromophores in each isomer was calculated. Calcula-

tions of the electronic transitions of the two depicted structural isomers of

p-C1P4 were done by using the CEO-INDO/S procedure [24]. Besides revealing

the energy of the electronic transitions, this method allows for the calculation of

the electronic coupling constants between the transition dipole moments of the

chromophores. All reported values apply to a molecule in vacuum at 0 K. CEO

calculations were performed on two isomers (p-C1P4A and p-C1P4B) of p-C1P4,

Figure 1.11. (a, b) Two-dimensional representation of the two structural isomers

p-C1P4A and p-C1P4B of p-C1P4. ða0; b0Þ Three-dimensional representation of the twostructural isomers of p-C1P4.

ELECTRONIC EXCITATION TRANSFER 13

obtained by energy minimization (see Fig. 1.11a and 1.11b). The results of

the CEO calculations on both isomers show an average value for this coupling

of the chromophores of p-C1P4A to be 22.6 cm�1. In p-C1P4B, the average

distance between the chromophores is 3.3 nm except for pair 1–4, where the

distance is only 1.7 nm. The average value for the coupling constants is

21.22 cm�1, except for pair 1–4 for which a value of 62.6 cm�1 is obtained.However, one needs to take into account that all the calculations are done

assuming a temperature of 0 K, and hence at room temperature these couplings

will be minimal. Furthermore, in collaboration with Beljonne and co-workers,

transition densities were calculated [25] for excitation transfer between two per-

yleneimide chromophores coupled by a fluorene trimer (separation 3.4 nm) and

found to be in line with the Förster approximations.

III. STATIONARY MEASUREMENTS

The steady state absorption and fluorescence spectra of all first generation den-

drimers in toluene are depicted in Figure 1.12. Within experimental error, the

former ones are identical for all compounds. In the emission spectra, however,

Figure 1.12. Steady state absorption and emission spectra of the first generation

dendrimers in toluene: p-C1P1, p-C1P3, (solid lines,—), m-C1P1 (short dashes, - - -), and

m-C1P3 (long dashes, – – – ).


a small shift and broadening of the meta-substituted compounds spectra rela-

tive to the ones of the para-substituted compounds can be seen. Moreover, a

change in the intensity ratio between the two vibronic maxima is also visible.

For the meta compounds, the vibronic maximum at 595 nm is relatively more

pronounced as compared to the one for the para compounds. The para coupling

allows for a better conjugation of the p-electrons of the peryleneimide over thearomatic phenyl ring of the branch. As this effect is more important in the

excited state than in the ground state, it will alter the perpendicular orientation

of the neighboring phenyls in the excited state compared to the ground state. The

width of the fluorescence band at half maximum (FWHM) increases slightly

with the number of chromophores from 2680 cm�1 for m-C1P1 to 2750 cm�1

for m-C1P4.

The fluorescence spectra of the first generation para-substituted dendrimers

p-C1Px (x ¼ 1--4) are independent on the number of PI chromophores. Similarly,the absorption and emission spectra of the second generation rigid dendrimers

(p-C2P1, p-C2P2, p-C2P3, p-C2P4) were found to be independent of the number

of chromophores present in the dendrimers. The fluorescence quantum

yield (�f ) is calculated to be 0.98� 0.05 and is identical within experimentalerror for all compounds. The similarity of the fluorescence properties of all

the para-substituted dendrimers in terms of spectral shape, fluorescence maxima,

and fluorescence quantum yield suggests that the emission occurs from the same

state in all the dendrimers. Triplet formation is very inefficient in these chromo-

phores: the rate constant of intersystem crossing could be measured using single

molecule spectroscopy and was found to be equal to 7� 103 s�1 [25, 29].

IV. SINGLE-PHOTON TIMING MEASUREMENTS

A. Time-Resolved Fluorescence Measurements PerformedUnder Magic Angle Polarization Condition

In order to examine the properties of the fluorescent states for the dendrimers

more closely, fluorescence decay times for all first generation dendrimers were

determined in toluene by single-photon timing detecting the emission under

magic angle condition.

1. Para-substituted Carbon Core Dendrimers Table 1.1 shows that thelifetimes of p-C1P1, p-C1P3, and p-C1P4 are identical with the fluorescence

decay measured for an adequate model containing a peryleneimide chromo-

phore. A representative plot of the fluorescence decay of the first generation

para-substituted dendrimers is given in Figure 1.13 for p-C1P4.

SINGLE-PHOTON TIMING MEASUREMENTS 15

The corresponding decay parameters are collected in Table 1.1.

Similarly, the decays of the second generation dendrimers were measured

and all decays could be fitted globally by a single exponential with a time

constant of 4.2 ns (Table 1.2).

TABLE 1.1 Fit Parameters of the Fluorescence Magic Angle and Anisotropy

Decays Measured for p-C1Px (x ¼ 1; 3; 4) in Toluene with kexc ¼ 488 nm andkflu ¼ 600 nm and Average Peryleneimide–Peryleneimide Distances (dDA)Compound t (ns) r0 y1 (ns) y2 (ps) b1 b2 b2/r0 (%) dDA (nm)

p-C1P1 4.2 0.34 1.4 — 0.34 — — —

p-C1P3 4.2 0.31 1.6 70 0.09 0.33 71 2.7

p-C1P4 4.2 0.34 2.0 50 0.07 0.37 79 2.7

Figure 1.13. Time-resolved fluorescence decays of p-C1P4 with fits at 600 nm and

700 nm detection wavelengths. The upper panel shows the weighted distribution of

residuals (Ri) and the lower panel represents the autocorrelation (ac) function for the

decays. Inset reports on a shorter time scale.


2. Meta-substituted First Generation Carbon Core Dendrimers The cor-responding decay parameters are collected in Table 1.3. The fluorescence inten-

sity of the dendrimer having only one chromophore (m-C1P1) decays single

exponentially with a decay time of 4.25� 0.05 ns. However, as the number ofchromophores is increased in the dendrimer, a small contribution of an addi-

tional long decay component of 7.4� 0.6 ns is found essential to fit the experi-mental data. It has to be noted, however, that the amplitude of this long decay

component is very small in m-C1P2 and m-C1P3. Thus, in order to minimize the

error in the fit procedure, an additional component with a fixed decay time of

7.4 ns, as obtained for m-C1P4, was introduced in the analysis of the fluores-

cence decays of m-C1P2 and m-C1P3 to allow a better comparison of the corre-

sponding amplitudes. It was furthermore observed that the relative amplitude

of the longer decay time is larger at the red edge of the fluorescence spectrum

for all multichromophoric dendrimers as shown in Table 1.3 by the comparison

of results obtained at 600 nm and 725 nm emission.

From the small difference in the spectral width (vide supra), the assumption

of an excited state excimer-like (or dimer) chromophore–chromophore interac-

tion is possible but not conclusive. Better insight into the extent of excimer-like

emission is obtained from the fluorescence decays, where only for the multi-

chromophoric dendrimers is a long decay component of 7.4 ns observed along

TABLE 1.2 Fit Parameters of the Fluorescence Magic Angle and Anisotropy

Decays Measured for p-C2Px (x ¼ 1; 2; 3; 4) in Toluene with kexc ¼ 488 nm andkflu ¼ 600 nm and Average Peryleneimide–Peryleneimide Distances (dDA)Compound t (ns) r0 y1 (ns) y2 (ps) b1 b2 b2/r0 (%) dDA (nm)

p-C2P1 4.2 0.32 2.7 — 0.32 — — —

p-C2P2 4.2 0.36 3.1 410 0.18 0.18 50 3.6

p-C2P3 4.2 0.36 2.7 310 0.14 0.22 61 3.7

p-C2P4 4.2 0.35 3.0 280 0.08 0.27 77 3.8

TABLE 1.3 Fluorescence Decay Times (si) and Associated Relative Amplitudes (ai)

for m-C1Px (x¼ 1–4) Measured in Toluene at Room Temperature Usingkexc ¼ 488 nmCompound t1 (ns) t2 (ns) a1-600 (%) a2-600 (%) a1-725 (%) a2-725 (%)

m-C1P1 4.25 0.32 100.0 0.0 — —

m-C1P2 4.25 0.36 99.2 0.8 98.7 1.3

m-C1P3 4.25 0.36 98.1 1.9 96.2 3.8

m-C1P4 4.25 0.35 96.0 4.0 93.7 6.3

SINGLE-PHOTON TIMING MEASUREMENTS 17

with the typical peryleneimide fluorescence decay time of 4.25 ns as obtained

for the monochromophoric model compound m-C1P1. The attribution of this

long time constant can be made to an ‘‘excimer-like’’ species as the decay

time is similar to that reported for the higher generation dendrimers having a

flexible biphenyl core [26].

Further evidence for this assignment can be derived from the dependence of the

amplitude a2 connected with the 7.4 ns component on the number of chromo-phores and the dependence on the emission wavelength (lflu), respectively. Asreported in Table 1.3, this amplitude is 0.8% for m-C1P2 and increases to 4%

for m-C1P4 at lflu ¼ 600 nm. This is reasonable as the probability of formationof the ‘‘excimer-like’’ entity increases as the number of chromophores in the den-

drimer increases. By detecting at lflu ¼ 725 nm, a2 increases to 1.3% for m-C1P2and to 6.3% for m-C1P4. The larger contribution of that component at longer emis-

sion wavelengths is also consistent with a red-shifted fluorescence from ‘‘excimer-

like’’ entities. This suggests that a fraction of the molecules have a substitution

pattern in which two of the PI chromophores are relatively close in space.

No such long decay component of 7.4 ns is observed for para-substituted

dendritic structures p-CnPn. The absence of the long decay component is there-

fore due to the different position of substitution leading to a better spatial sepa-

ration of the individual chromophores. This is also supported by a comparison of

the molecular structures of the para- and meta-substituted dendrimers obtained

from molecular modeling, since the average center-to-center distance among

the chromophores is 2.9 nm for the para series but only 2.6 nm for the meta

series in the first generation series.

B. Time-Resolved Fluorescence Polarization Measurements

From time-resolved fluorescence depolarization measurements, the anisotropy

decay times (�) and the associated anisotropy (b) have been determined forall first generation dendrimers using Eq.(1):

rðtÞ ¼X

bi expð�t=�iÞ with r0 ¼Xi

bi ð1Þ

The sum of all bi is called the limiting anisotropy r0.

1. Meta-substituted First Generation Carbon Core Dendrimers For themonochromophoric meta-substituted dendrimer (m-C1P1), a monoexponential

fit of the anisotropy decay function is sufficient, which gives a relaxation time

of �1 ¼ 950� 30 ps with b1 ¼ r0 ¼ 0:38 (Table 1.4). However, the anisotropydecay functions for the meta-substituted dendrimers having more than one

chromophore (m-C1P2 to m-C1P4) can only be fitted with two exponential decay


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