Photophysics and Photochemistry of Molecular Recognition and Sensing with Metal-Directed Macrocyclic Systems - Molecular Self-Assembly: Advances and Applications - page 309

t Bu

CO

t Bu

OC

OC CO

Cl Re CO

Br Re CO

N N

N N

OC CO

O O

O N

Re CO OC Re N O

O R

OC O O

CO

OR

RO

R

S

11

S S S

R

RO

9

OC

O

O CO

N O

O N OC Re

Re C O OC RO N

N B r

O

O CO

OC

OC
OC

Re N N Re CO

N N

CO

Br
R = C 6 H 13 , a ; C 12 H 25 , b

OC

OR

OC Re Cl

t Bu

t Bu

OC CO

8

exhibits two broad
absorption bands at 329 and 422 nm (see Fig. 10.1A) [41,42]. The
lowest energy band is assigned to a mixture of MLCT and

The absorption spectrum of triangle 9a

π→π

*

excited
states. The room temperature emission spectrum of triangle

9 in

ClCH Cl (DCE) exhibits structured features with maxima at 499

and 476 nm for

CH

2 2

9a and 512 and 470 nm for 9b . The low-temperature

emission spectrum of displays more distinct structure with

bands at 463, 488, 520, and 549 nm with an average spacing of 1188
cm

9a

− 1

in 2-methyltetrahydrofuran. These observations illustrate
that the dual emission bands are actually a vibronic structure form
of the excited state. A very minor blue shift when going from

the solution to a rigid glass also supports the assignment that the
emission occurs from a ligand-based

π→π

*

π→π

*

excited state [28,29].
The extremely short lifetime (ca. 360 ps) and relatively high-
energy emission from = 0.032) indicate that the emission is

apparently ligand-localized singlet

9a (

F
em

*

π→π

excited state. However, the

∼

8 -1

π→π

*

fast radiative decay rate ( excited

state localized on the corresponding bridging ligand implies that
the fluorescence is still able to compete with the other nonradiative
decay processes. The folding motions of the hexyloxyl chains in

k 10 s ) from the singlet

r

are
believed to be responsible for the rapid vibrational relaxation from
the

9a

3

MLCT excited state. In fact, except for the vibronic structures
observed at high-energy positions, a very weak emission centered
at 602 nm with lifetime 163

µ

s was also observed at 77 K glass. This

MLCT transition on the basis of the absence
of vibrational structure, although some involvement of triplet

band is assigned as a

3

*

π→π

character cannot be completely ruled out. The photolysis into the
π−π

9
resulted in red shift of the two absorption bands at 329 and 406

*

transition at 313 nm of highly strained triangular structure