Well, you already nearly wrote the answer yourself!
for ...
... do something
if condition
break; %stop the loop
end
end
So, in your case, probably:
for l = 1:6
c11(l) = opt11{l}
if l>1 && c11(l) <= c11(l-1)
break;
end
end
Of course, the above could be just written without the loop:
c11 = cell2mat(opt11(1:6));
c11 = c11(logical(cumprod(diff(c11) > 0)));
