To imp
rove the selectivity of sensing, the the
rmodynamics of the complex fo
rmation of some calix[4]a
renehosts with neut
ral phenol guests was studied in ca
rbon tet
rachlo
ride as nonpola
r solvent. The molecula
r shapeof calixa
renes was va
ried by the selective functionalization with
tBu and O-CH
2-Ph (O-benzyl) o
r OP
rg
roups at the uppe
r and lowe
r rim,
respectively. To va
ry the elect
ron density on the guest's a
romatic
rings,the pa
rent phenol was functionalized in the pa
ra position with elect
ron-withd
rawing Cl, as well as H, andelect
ron-
releasing CH
3 and
tBu g
roups. To study the inte
raction between calixa
rene and the guests, PL andquantum-chemical methods we
re applied. The
results
revealed an ove
rall 1:1 complex stoichiomet
ry exceptfo
r the pa
rent dibenzyloxycalix[4]a
rene, whe
re 1:2 host-guest stoichiomet
ries we
re obse
rved i
rrespective ofthe quality of phenol. In the latte
r case, the complex fo
rmation shows a self-switched cha
racte
r: the fi
rstphenol molecule is included in the calixa
rene cavity, and only afte
rwa
rd, a second guest molecule is boundby the two benzyloxy a
romatics. Although the enthalpy change p
redicts st
rong inte
raction between the hostand the guest, the Gibbs f
ree ene
rgy change of the complex fo
rmation is small,
resulting in a
relatively lowcomplex stability. The solvent-
relaxation measu
rements suppo
rt that the unexpected ent
ropy change could bethe consequence of the
reo
rientation of solvent molecules a
round the calixa
rene building block. The
reo
rientationis assisted by dispe
rsive fo
rces between solute and solvent molecules. IR and RAMAN analysis of thecomplexes exclude a conside
rable pa
rticipation of the phenolic OH g
roup in the stabilization of the complex.This
result is in ag
reement with ea
rlie
r findings whe
re dete
rministic
role of
rs/pi.gif" BORDER=0 >-
rs/pi.gif" BORDER=0 > inte
raction in the complexstability was assumed.