6 Group 13–15 Needle-Shaped Oligomers and Nanorods: Structures. . .
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lowest singlet states have lower energy values for the open rods, which lead to a
red shift in the absorption spectra of open oligomers comparing with closed one
oligomers.
The predicted profound difference in electronic properties between open and
closed rod-shaped oligomers can help to identify the type of termination of the
oligomeric rod in the experimental studies.
We also tracked the effects of changing the metal atom and substituents on terminal groups on the electronic properties of the Ga-N-based nanorods. Substitution
of Ga atoms by Al moderately increases energy gaps for both closed and open
oligomers. Substitution of Ga by In atoms effectively reduces the energy gap in both
cases. However, for closed oligomers, the metal substitution is the most efficient in
positions closer to the metal-terminated end of the rod, while for open oligomers
the metal substitution near the amido-terminated end of the rod has a predominant
effect.
Substitution of H atoms by CH 3 groups on metal centers does not lead to a
qualitative difference in electronic and structural properties of the [RGaNH] 3n+1
oligomers but results in a constant shift in quantitative characteristics, such as
decrease of HOMO-LUMO gap by 0.6 eV for closed and by 0.7 eV for open
oligomers with respect to values for the hydrogen-substituted rods.
It was found that for the open oligomers, the molecular orbital energy values
of end-localized states are highly sensitive to a particular terminal group. This,
along with a variation of the length of the oligomer, allows adjusting the HOMOLUMO gap to cover a wide spectral range. For example, the elongation of
F 3 [HGaNH] 3n (CH 3 ) 3 from n = 3–38 reduces the HOMO-LUMO gap from 6.5 to
2.9 eV, the elongation of (CH 3 ) 3 [HGaNH] 3n F 3 from n = 3–38 reduces the HOMO–
LUMO gap from 5.2 to 0.9 eV, and the change from F 3 [HGaNH] 3n (CH 3 ) 3 to
(CH 3 ) 3 [HGaNH] 3n F 3 reduces the HOMO-LUMO gap from 2.9 to 0.9 eV in the
case of n = 38 and from 6.5 to 5.2 eV in the case of n = 3.
For the long oligomers, where the influence of the substituents at the opposite
end of the rod is negligible, HOMO-LUMO gap values decrease in the order
F > CF 3 > H > Me for the substituents on the Ga atom. The order of substituents on
the nitrogen atom, which contributes to the corresponding reduction of the gap, is
as follows: F < CF 3 < H < CH 3 . Qualitatively, this trend is observed both for open
and closed oligomers, but in the latter case, the influence of the substituents on band
gap energy values is much less pronounced.
The HOMO-LUMO transition in long open oligomers requires a relatively small
energy change (which could be as low as 0.9 eV for X=CH 3 , Y=F in oligomers of
circa 10 nm of length) but involves an intermolecular end-to-end charge transfer. In
contrast, in closed oligomers the transitions occur only at the nitrogen-terminated
end of the oligomer and require a large energy change (at least 6.3 eV for n = 38,
X=CH 3 , Y=F). Overall, we conclude that the type of termination (open or closed)
has the largest effect on the electronic properties of [HGaNH] 3n oligomers. The
elongation and variation of substituents on the terminal groups have comparable
effects on the HOMO-LUMO gap and can be effectively combined for fine tuning
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lowest singlet states have lower energy values for the open rods, which lead to a
red shift in the absorption spectra of open oligomers comparing with closed one
oligomers.
The predicted profound difference in electronic properties between open and
closed rod-shaped oligomers can help to identify the type of termination of the
oligomeric rod in the experimental studies.
We also tracked the effects of changing the metal atom and substituents on terminal groups on the electronic properties of the Ga-N-based nanorods. Substitution
of Ga atoms by Al moderately increases energy gaps for both closed and open
oligomers. Substitution of Ga by In atoms effectively reduces the energy gap in both
cases. However, for closed oligomers, the metal substitution is the most efficient in
positions closer to the metal-terminated end of the rod, while for open oligomers
the metal substitution near the amido-terminated end of the rod has a predominant
effect.
Substitution of H atoms by CH 3 groups on metal centers does not lead to a
qualitative difference in electronic and structural properties of the [RGaNH] 3n+1
oligomers but results in a constant shift in quantitative characteristics, such as
decrease of HOMO-LUMO gap by 0.6 eV for closed and by 0.7 eV for open
oligomers with respect to values for the hydrogen-substituted rods.
It was found that for the open oligomers, the molecular orbital energy values
of end-localized states are highly sensitive to a particular terminal group. This,
along with a variation of the length of the oligomer, allows adjusting the HOMOLUMO gap to cover a wide spectral range. For example, the elongation of
F 3 [HGaNH] 3n (CH 3 ) 3 from n = 3–38 reduces the HOMO-LUMO gap from 6.5 to
2.9 eV, the elongation of (CH 3 ) 3 [HGaNH] 3n F 3 from n = 3–38 reduces the HOMO–
LUMO gap from 5.2 to 0.9 eV, and the change from F 3 [HGaNH] 3n (CH 3 ) 3 to
(CH 3 ) 3 [HGaNH] 3n F 3 reduces the HOMO-LUMO gap from 2.9 to 0.9 eV in the
case of n = 38 and from 6.5 to 5.2 eV in the case of n = 3.
For the long oligomers, where the influence of the substituents at the opposite
end of the rod is negligible, HOMO-LUMO gap values decrease in the order
F > CF 3 > H > Me for the substituents on the Ga atom. The order of substituents on
the nitrogen atom, which contributes to the corresponding reduction of the gap, is
as follows: F < CF 3 < H < CH 3 . Qualitatively, this trend is observed both for open
and closed oligomers, but in the latter case, the influence of the substituents on band
gap energy values is much less pronounced.
The HOMO-LUMO transition in long open oligomers requires a relatively small
energy change (which could be as low as 0.9 eV for X=CH 3 , Y=F in oligomers of
circa 10 nm of length) but involves an intermolecular end-to-end charge transfer. In
contrast, in closed oligomers the transitions occur only at the nitrogen-terminated
end of the oligomer and require a large energy change (at least 6.3 eV for n = 38,
X=CH 3 , Y=F). Overall, we conclude that the type of termination (open or closed)
has the largest effect on the electronic properties of [HGaNH] 3n oligomers. The
elongation and variation of substituents on the terminal groups have comparable
effects on the HOMO-LUMO gap and can be effectively combined for fine tuning