J Fluoresc
character of the emission. The wave function of this state
can be expressed by the equation below:
dimethylamino and methoxy derivatives have lower
quantum yield. The lowest triplet state of these com-
plexes, where the emission comes from, is quantum
mechanical mixture of MLCT and LC states. Contribu-
tion of each of these states in that mixture defines the
nature of the phosphorescent emission. That contribution
is determined by the mixing coefficient α and depends
on extend of spin-orbital coupling, configuration inter-
actions and energy difference between states. In this
study we find that phosphorescent properties of the
complexes depend on temperature. As a reason for this
we point out change in nature of the emitting state. We
suggest transition from 3LC to 3MLCT dominant char-
acter of the T1 state going from room temperature to
77 K. We make this conclusion after study of the
phosphorescent spectra of the complexes in solutions
with different polarity.
pffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢁ
ꢁ
ꢁ
ꢁ
ꢀ
1
3
ΨT
¼
1 ꢀ a2 LC þ a MLCTi
1
α is mixing coefficient and can be approximated with the
formula:
ꢁ
ꢁ
ꢁ
ꢁ
ꢂ
ꢀ
LC HSO 1MLCT
3
a ¼
ΔE
ꢁ
ꢁ
ꢁ
ꢂ
ꢀ
where LC HSO 1MLCT is the spin-orbital coupling ele-
ment, characterizing the strength of spin-orbit coupling be-
tween 3LC and 1MLCT transitions. Note that the value of α
also includes contributions from spin-orbit coupling be-
tween the 1MLCT and 3MLCT states, and between the
3
ꢁ
3MLCT and LC states [25].
3
Usually, the lowest excited state inherits almost all photo-
physical properties (except oscillator strength) of the state
with the biggest mixing coefficient. In this case we consider
3MLCT as the lowest excited state, nevertheless, at room
Acknowledgments This study was funded by the Bulgarian Ministry
of Education and Science, National Science Fund, Project “National
Centre for Advanced Materials (UNION). Modul 2 Advanced materi-
als for medical and pharmaceutical applications”, ДЦВП 02/2 от
29.12.2009.
3
temperature the emission has (π,π*) character due to ad-
mixture of the 3LC into wave-function of the emitting state.
One can see the batochromic shift in emission spectra of the
complexes with increase of the solvent polarity (Fig. 6).
This is typical behavior for emitters with (π,π*) character
of the excited state. At low temperature, the shift is hypso-
chromic with increase of the solvent polarity. We explain
this effect consider change in character of the emitting state.
It is well known that one of the main mechanisms for
quantum mechanical mixing of the states is trough vibra-
tional coupling. At low temperature when the vibrations are
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3
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1
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Studding this series of four heteroleptic Ir(III) com-
plexes we define the effect of the substituent into cyclo-
metalated benzothiazole moiety of the Ir complex. As
these complexes are 3(π,π*) emitters they are sensitive
to extend of the conjugated π-system. In this way π
electron-donating substituents shift the emission band to
low energies. Moreover, flexibility of these groups is
crucial for phosphorescent quantum yield. We find that