sql check if the datetime in between 2 datetime

I have a  query and What I trying to do is to exclude the people that are taken in this time frame, but it still showing me a person where this person should be excluded

	SELECT DISTINCT U.userKey
		, U.firstName
	    , U.lastName
	    , U.email
	FROM users U
		
	WHERE U.enable = 'Y' and u.userKey=36749
		and  not EXISTS (
			SELECT su.instructorKey
			FROM Session S 
			INNER JOIN SessionUnit SU  ON S.SessionKey = SU.SessionKey
			inner join sessionMap SM  on SM.sessionKey= S.sessionKey
			WHERE
			 su.instructorKey = U.userKey
AND
(
'2015-09-01 08:01:00' between SU.sessionStart and SU.sessionEnd
OR '2015-09-01 09:59:00' between SU.sessionStart and SU.sessionEnd
OR SU.sessionStart between '2015-09-01 08:01:00' AND '2015-09-01 09:59:00'
OR SU.sessionEnd between '2015-09-01 08:01:00' AND '2015-09-01 09:59:00'
) 

) 

order by   U.lastName

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