The solve
nt a
nd temperature depe
nde
nce of the phototautomerizatio
n of 1-methyl-2-(2'-hydroxyphe
nyl)be
nzimidazole (
4) a
nd the
novel compou
nds 2-(4'-ami
no-2'-hydroxyphe
nyl)be
nzimidazole (
1), 2-(4'-
N,
N-diethylami
no-2'-hydroxyphe
nyl)be
nzimidazole (
2), a
nd 1-methyl-2-(4'-
N,
N-diethylami
no-2'-hydroxyphe
nyl)be
nzimidazole (
3), together with the grou
nd-state rotamerism a
nd tautomerism of these
new compou
nds,have bee
n studied by UV-vis absorptio
n spectroscopy a
nd steady-state a
nd time-resolved fluoresce
ncespectroscopy. A solve
nt-modulated rotameric a
nd tautomeric equilibrium is observed i
n the grou
nd state for
1,
2, a
nd
3. I
n cyclohexa
ne, these compou
nds mai
nly exist as a pla
nar sy
n normal form, with the hydroxylgroup hydroge
n-bo
nded to the be
nzimidazole N3. I
n etha
nol, the sy
n form is i
n equilibrium with its pla
nara
nti rotamer (for
1 a
nd
2), with the phe
nyl ri
ng rotated 180
ntities/deg.gif"> about the C2-C1' bo
nd a
nd with a
no
npla
narrotamer for compou
nd
3. I
n aqueous solutio
n, a tautomeric equilibrium is established betwee
n the a
nti
normalform (or the
no
npla
nar rotamer for
3) a
nd the tautomer (with the hydroxyl proto
n tra
nsferred to thebe
nzimidazole N3). The sy
n normal form of these compou
nds u
ndergoes i
n all the solve
nts a
n excited-statei
ntramolecular proto
n-tra
nsfer process from the hydroxyl group to the be
nzimidazole N3 to yield the excitedtautomer. The tautomer fluoresce
nce qua
ntum yield of
2,
3, a
nd
4 shows a temperature-, polarity-, a
nd viscosity-depe
nde
nt radiatio
nless deactivatio
n, co
nnected with a large-amplitude co
nformatio
nal motio
n. We co
ncludethat this excited-state co
nformatio
nal cha
nge experie
nced by the tautomer is associated with a
n i
ntramolecularcharge tra
nsfer from the deproto
nated dialkylami
nophe
nol or phe
nol (do
nor) to the proto
nated be
nzimidazole(acceptor), affordi
ng a
no
nfluoresce
nt charge-tra
nsfer tautomer. Therefore, these compou
nds u
ndergo a
n excited-state i
ntramolecular coupled proto
n- a
nd charge-tra
nsfer process.