238
A. V. Pomogaeva and A. Y. Timoshkin
open oligomers are 4.8, 4.7, and 4.5 eV. Energies of the lowest singlet states are 6.1,
5.7, and 4.3 eV for the closed Al, Ga, and In binary compounds, respectively. The
corresponding values for the open oligomers are 4.8, 4.7, and 3.8 eV. These values
are larger than the corresponding B3LYP data by about 0.2–0.3 eV.
The intensity of the absorption for the open rods is significantly smaller than the
one for the closed rods. Two absorption peaks are distinguished in the spectra. These
two intensive peaks are separated by about 40 nm for Ga and Al and by about 50 nm
for In in case of open oligomers and only by 13–18 nm in case of closed oligomers.
In case of [CH 3 InNH] 10 , these two peaks are essentially merged into a single, wide
absorption band.
For all binary compounds, HOMO/HOMO−1 → LUMO transition makes the
greatest contribution (more than 90%) to the lowest spin-singlet state S1. For open
(CH 3 ) 3 [CH 3 MNH] 9 H 3 oligomers, oscillator strengths of excitation into the S1
state are very small: 0.012, 0.004, and 0.001 for M=In, Ga, and Al, respectively.
Open oligomers have also low-lying S2 state in the immediate vicinity of S1
state (it differs by about 1–4 nm from the S1 state). For all oligomers, S2 state
is predominantly (~99%) the HOMO−2 → LUMO transition; the S2 oscillator
strengths are 0.048, 0.013, and 0.003 for In, Ga, and Al open oligomers, respectively.
The absorption band with the largest intensity for the each compound is
associated with a group of states which have more complex composition. Spinsinglet states with the largest oscillator strength for the closed oligomers are
fully symmetric (A 1 ) and for Ga and In binary compounds correspond mostly to
HOMO−4 → LUMO excitation (more than 90%). In case of [CH 3 AlNH] 10 , main
contribution to the state is HOMO−7 → LUMO (50%) and HOMO−6 → LUMO
(19%). These transitions occur from the delocalized MOs to the end-localized
LUMO.
For open oligomers, two adjacent states, which are very close in energy (they are
indistinguishable in Fig. 6.19), have the largest oscillator strengths: 0.067 and 0.063
for In, 0.052 and 0.038 for Ga, and 0.030 and 0.017 for Al. One of these states has
the largest contributions from HOMO−2 → LUMO+3, HOMO−1 → LUMO+1,
and HOMO → LUMO+2 transitions. The composition of the other adjacent state
is more complex, and it is different for Al, Ga, and In oligomers.
Experimental spectra obtained for solid powder [RGaNH] n compounds R=H
(Fig. 6.20a) and R=CH 3 (Fig. 6.20b) also reveal absorption in the ultraviolet range
[119]. The absorption maximum for [CH 3 GaNH]n powder is about 280 nm, i.e.,
red-shifted compared with our calculated spectra (Fig. 6.19). It is not clear what
kinds of oligomers, open or closed, are dominant in the powder samples studied
experimentally [119]. The range of lengths of oligomers in experimental samples is
also unknown. Taking into account red shift in excitation energies for the oligomers
upon elongation, it was suggested [119] that oligomers of intermediate length
(n=10–12) are prevail in the experimentally studied samples.
A. V. Pomogaeva and A. Y. Timoshkin
open oligomers are 4.8, 4.7, and 4.5 eV. Energies of the lowest singlet states are 6.1,
5.7, and 4.3 eV for the closed Al, Ga, and In binary compounds, respectively. The
corresponding values for the open oligomers are 4.8, 4.7, and 3.8 eV. These values
are larger than the corresponding B3LYP data by about 0.2–0.3 eV.
The intensity of the absorption for the open rods is significantly smaller than the
one for the closed rods. Two absorption peaks are distinguished in the spectra. These
two intensive peaks are separated by about 40 nm for Ga and Al and by about 50 nm
for In in case of open oligomers and only by 13–18 nm in case of closed oligomers.
In case of [CH 3 InNH] 10 , these two peaks are essentially merged into a single, wide
absorption band.
For all binary compounds, HOMO/HOMO−1 → LUMO transition makes the
greatest contribution (more than 90%) to the lowest spin-singlet state S1. For open
(CH 3 ) 3 [CH 3 MNH] 9 H 3 oligomers, oscillator strengths of excitation into the S1
state are very small: 0.012, 0.004, and 0.001 for M=In, Ga, and Al, respectively.
Open oligomers have also low-lying S2 state in the immediate vicinity of S1
state (it differs by about 1–4 nm from the S1 state). For all oligomers, S2 state
is predominantly (~99%) the HOMO−2 → LUMO transition; the S2 oscillator
strengths are 0.048, 0.013, and 0.003 for In, Ga, and Al open oligomers, respectively.
The absorption band with the largest intensity for the each compound is
associated with a group of states which have more complex composition. Spinsinglet states with the largest oscillator strength for the closed oligomers are
fully symmetric (A 1 ) and for Ga and In binary compounds correspond mostly to
HOMO−4 → LUMO excitation (more than 90%). In case of [CH 3 AlNH] 10 , main
contribution to the state is HOMO−7 → LUMO (50%) and HOMO−6 → LUMO
(19%). These transitions occur from the delocalized MOs to the end-localized
LUMO.
For open oligomers, two adjacent states, which are very close in energy (they are
indistinguishable in Fig. 6.19), have the largest oscillator strengths: 0.067 and 0.063
for In, 0.052 and 0.038 for Ga, and 0.030 and 0.017 for Al. One of these states has
the largest contributions from HOMO−2 → LUMO+3, HOMO−1 → LUMO+1,
and HOMO → LUMO+2 transitions. The composition of the other adjacent state
is more complex, and it is different for Al, Ga, and In oligomers.
Experimental spectra obtained for solid powder [RGaNH] n compounds R=H
(Fig. 6.20a) and R=CH 3 (Fig. 6.20b) also reveal absorption in the ultraviolet range
[119]. The absorption maximum for [CH 3 GaNH]n powder is about 280 nm, i.e.,
red-shifted compared with our calculated spectra (Fig. 6.19). It is not clear what
kinds of oligomers, open or closed, are dominant in the powder samples studied
experimentally [119]. The range of lengths of oligomers in experimental samples is
also unknown. Taking into account red shift in excitation energies for the oligomers
upon elongation, it was suggested [119] that oligomers of intermediate length
(n=10–12) are prevail in the experimentally studied samples.